Presses-and-Equipment-for-SheetMetal-Dies

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Presses and Equipment for Sheet Metal dies 
POWER PRESS TYPES 
The types of power presses available for metal-cutting and forming 
operations are varied, the selection depending upon the type of operation. 
Not all types of presses will be described because of space limitations. The 
basic types of presses and press mechanisms will be described to give the 
beginner the necessary background for designing press tooling. 
Presses are classified by (1) type of frame, (2) source of 
power, (3) method of actuation of slides, (4) number of slides incorporated, 
and (5) intended use. Most presses are not classified by only category one but 
several. For example, a straight-side press may be mechanically or 
hydraulically driven and may be either single or double acting. 
 Classification by frame type: The frame of a press is fabricated by 
casting or by welding heavy steel plates. Cast frames are quite stable and 
rigid but expensive. Cast frame construction also has the advantage of placing 
a mass of material where it is needed most. Welded frames are generally 
less expensive and are more resistant to shock loading because of the greater 
toughness of steel plate. 
The general classification by frame includes the gap frame and the 
straight side. The gap frame is cut back below the ram to form the shape of a 
letter C. This allows feeding a strip from the side. Some gap-frame presses 
have an open back to permit strip feeding from front to back or ejection of 
finished parts out the back. Gap-frame presses are manufactured with solid 
frames fixed in a vertical or inclined position. Others are manufactured with a 
separate frame mounted in a base, which allows the frame to be inclined at 
an angle in three different positions. 
The reason for inclining the press is to allow parts to fall through the 
open back by gravity. The three-position inclinable press is frequently 
referred to as an open-back inclinable (OBI) press (see Fig. 3-1). Solid gap-
frame presses are obtainable in higher tonnages than inclinable ones because 
of the rigid base and solid construction. 
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The OBI press is the most common press in use today. It ranges from 
a small 1-ton bench press to floor presses rated up to 150 tons. Its main use 
is for blanking and piercing operations on relatively small work pieces, 
although bending, forming, and drawing operations can also be done. 
Fig, 3-2 shows the major components of an OBI press, as follows: 
1) A rectangular bed, the stationary and usually horizontal part of the 
press, serving as a table to which a holster plate is mounted. 
2) A bolster plate, consisting of a flat steel plate from 50 mm. to 125 
mm. thick, secured to the press for locating and supporting the die 
assembly. 
3) The ram, sometimes called the slide, which reciprocates within the 
press frame and to which the punch or upper-die assembly is 
fastened. 
4) A knockout, consisting of a crossbar through a slot in the ram that 
contacts a pin in the die to eject the work piece. 
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5) The flywheel, which absorbs energy from the motor continuously and 
delivers its stored energy to the work piece intermittently, making it 
possible to use a smaller motor. 
6) The pitman, consisting of a connecting rod to convey motion and 
pressure from the main shaft or eccentric to the press slide. 
 
 
Fig. 3-3 Single action straight side eccentric shaft mechanical press. 
 
The straight slide press incorporates a slide or ram, which travels up 
and down between two straight sides or housing and commonly used for large 
and heavy work. The size of the press is limited to some extent because 
reduce the working area. However the frame construction does permit large 
bed areas and longer strokes. The drive mechanism is generally located 
above the bed, The straight slide press incorporates a slide or ram, which 
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travels up and down between two straight sides or housing and commonly 
used for large and heavy work. The size of the press is limited to some extent 
because reduce the working area. However the frame construction does 
permit large bed areas and longer strokes. The drive mechanism is generally 
located above the bed, although under drive presses may be obtained with 
the drive mechanism located below the bed. Straight side presses are 
classified as single, two or four point suspension, depending upon the number 
of connection between the slide and the main drive shaft. Fig, 3-3 shows a 
typical straight slide press. 
Classification by source of power: The great majority of presses 
receive their power mechanically or hydraulically. A few manually operated 
presses are hand operated through levers or screws, but they are hardly 
suited for high production. 
Mechanical presses use a flywheel driven system to obtain ram 
movement. The heavy flywheel absorbs energy from the motor 
continuously and delivers its stored energy to the work piece 
intermittently. The motor returns the flywheel to operating speed between 
strokes. The permission slowdown of the flywheel during the work period is 
about 7 to 10 percent in nongeared presses and 10 to 20 percent in 
geared presses. The flywheel is attached directly to the main shaft of the 
press (non geared), or, it is connected to the main shaft by a gear train. 
Nongear drives are used on presses of low tonnage and short strokes. The 
number of strokes per minute on nongear drives is usually quite high. 
 
 
Gear driven presses transmit the energy of the flywheel through a single or 
double reduction gear. The single reduction gear drive is suited for heavier 
blanking operations or shallow drawing. The double-gear drive is used on large, 
heavy presses where it is necessary to move large amounts of mass at slower 
speeds. The double reduction greatly reduces the strokes per minute without 
reducing the flywheel speed. 
 
Basic types of mechanical press drives 
a) Nongeared 
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b) Single geared 
c) Single reduction gear 
d)Double reduction 
 
 
 Fig. 3-5 Typical double action hydraulic press with a Die cushion 
 
Hydraulic presses have a large cylinder and piston, coupled to a hydraulic pump. 
The piston and press ram are one unit. The tonnage capacity depends upon the 
cross-sectional area of the piston (or pistons) and the pressure developed by the 
pump. The cylinder is double acting in order to move the ram in either direction. 
The advantage of a hydraulic press is that it can exert its full tonnage at any 
position of the ram stroke. In addition, the stroke can be varied to any length 
within the limits of the hydraulic-cylinder travel. The speed and pressure are also 
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constant throughout the entire stroke Fig. 3-5 shows a typical hydraulically driven 
press. 
Classification by method of slide actuation: The flywheel of a press drives the 
main shaft, which in turn changes the rotary motion of the flywheel into the linear 
motion of the slide or ram. This is generally accomplished by incorporating 
crankpins or eccentrics into the main drive shaft, as shown in Fig, 3-4. The 
number of points of suspension of the slide determines the number of throws or 
eccentrics on the main shaft. Points of suspension are places where pressure is 
transmitted by connection to the slide. Press connections, called connecting rods 
or pitman, are usually adjustable in length so that the shut height of the press can 
be varied. 
The most common driving device is the crankshaft, although many 
newer presses use the eccentric for ram movement. The main advantage 
of the eccentric is that it offers more surface areafor bearing support for 
the pitman and main disadvantage is its limitations on the length of 
stroke. Therefore, presses having longer strokes generally utilize the 
crankshaft. 
In addition to eccentrics and crankpins, cams, toggles, rack and 
pinions, screws, and knuckles actuate slides. Space does not permit 
discussion of these mechanisms in this text. Information may be obtained 
by referring to the various die-design and press handbook. 
3.1.4 Classification by the number of slides incorporated: The 
number of slides incorporated in a single press is called the action, i.e. the 
number of rams or slides on the press. Thus a single-action press has one 
slide. A double-action press has two slides, an inner and an outer slide 
(see Fig. 3-5). This type of press is generally used for drawing operations 
during which the outer slide carries the blank holder and the inner slide 
carries the punch. The outer, or blank-holder, slide, which usually has a 
shorter stroke than the inner, or punch-holder slide, dwells to hold the 
blank while the inner slide continues to descend, carrying the draw punch 
to perform the drawing operation. 
A triple-action press is the same as a double-action with the 
addition of a third ram, located in the press bed, which moves upward in 
the bed soon after the other two rams descend. All three-slide movements 
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are properly synchronized for triple-action drawing, redrawing, and 
forming. 
 GENERAL PRESS INFORMATION 
The tool designer must know certain fundamentals of press 
operation before he can successfully design press tooling. 
Press tonnage: The tonnage of a press is the force that the press ram is 
able to exert safely. Press slides exert forces greater than the rated 
tonnage because of the built-in safety factor, but this is not a license to 
overload. 
The tonnage of hydraulic presses is the piston area multiplied by 
the oil pressure in the cylinder. Changing the oil pressure varies the 
tonnage. The tonnage of mechanical presses is determined by the size of 
the bearings for the crankshaft or eccentric and is approximately equal to 
the shear strength of the crankshaft metal multiplied by the area of the 
crankshaft bearings. The tonnage of a mechanical press is always given 
when the slide is near the bottom of its stroke because it is greatest at this 
point. 
Stroke: The stroke of a press is the reciprocating motion of a press 
slide, usually specified as the number of inches between terminal points of 
the motion. The stroke is constant on a mechanical press but adjustable on 
a hydraulic press. 
Shutheight: The shut height of a press is the distance from the 
top of the bed to the bottom of the slide with the stroke down and the 
adjustment up. The thickness of the bolster plate must always be taken 
into consideration when determining the maximum die height. The shut 
height of the die must be equal to or less than the shut height of the 
press. The shut height of a press is always given with the adjustment up. 
Lowering the adjustment of the slide may decrease the opening of the 
press from the shut height down, but it does not increase the shut height. 
Thus the shut height of a die must not be greater than the shut height of 
the press. It may be less, because lowering the adjustment can reduce the 
die opening in the press. 
Die space:Die space is the area available for mounting dies in the press. 
 
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Introduction to Die Cutting Structure 
 THE FUNDAMENTALS OF DIE-CUTTING OPERATIONS 
 While there are many die-cutting operations, some of which are very 
complex, they can all be reduced to the following simple fundamentals. 
 
 
Fig. 4-1 Drop-through Blanking Die Fig. 4-2 Piercing Die Assembly 
 Plain blanking: Fig. 4-1 shows a simple operation of this type. The 
material used is called the stock and is generally a ferrous or nonferrous 
strip. During the working stroke the punch goes through the material, and 
on the return stroke the material is lifted with the punch and is removed 
by the stripper plate. The stop pin is a gage for the operator. In practice, 
he feeds the stock by hand and locates the holes to be punched as shown. 
The part that is removed from the strip is always the work piece (blank) in 
a blanking operation. 
4.1.2 Piercing: This operation consists of simple hole punching. It differs 
from blanking in that the punching (or material cut from stock) is the scrap and 
the strip is the work piece. Piercing is nearly always accompanied by a blanking 
operation before, after, or at the same time. Fig. 4-2 shows a typical piercing 
die assembly. 
4.1.3 Lancing: This is a combined bending and cutting operation along a 
line in the work material. No metal is cut free during a lancing operation. 
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The punch is designed to cut on two or three sides and bend along the 
fourth side. Fig. 4-3 and 4-4 show the principle of the lancing operation. 
 
Fig. 4-3 lancing action Fig. 4-4 Strip lanced for free metal for 
forming 
 
4.1.4 Cutting off and parting: A cutoff operation separates the work 
material along a straight line in a single-line cut (Fig. 4-5). When the 
operation separates the work material along a straight line cut in a double-
line cut, it is known as parting (Fig. 4-6) . Cutting off to separate the work 
piece from the scrap strip. Cutting off and parting usually occur in the final 
stages of a progressive die. Cutting off is also used to chop up the scrap 
strip skeleton as it leaves the die. This makes the scrap much easier to 
handle. Fig. 4-7 shows the basic principles of cutting off and parting. 
 
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Fig. 4-5 Cutoff action Fig. 4-7 Layout for making blanks by 
Cutoff 
 
4.1.5 Notching: This operation removes metal from either or both edges 
of the strip. Notching serves to shape the outer contours of the workspace 
in a progressive die or to remove excess metal before a drawing or 
forming operation in a progressive die. The removal of excess metal allows 
the metal to flow or from without interference from excess metal on the 
sides. Fig. 4-8 shows a typical example of notching 
 
Fig. 4-8 Notching Fig. 4-9 Shaving 
 
4.1.6 Shaving: Shaving is a secondary operation, usually following 
punching, in which the surface of the previously cut edge is finished smoothly 
to accurate dimensions. The excess metal is removed much as a chip is 
formed with a metal-cutting tool. There is very little clearance (close to zero) 
between the punch and die, and only a thin section of the edge is removed 
from the edge of the work piece. Fig. 4-9 described the shaving operation. 
 
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Fig. 4-10 Trimming a horizontal flange 
4.1.7 Trimming: This operation removes the distorted excess metal from 
drawn or formed parts and metal that has been needed in a previous 
operation. It also provides a smooth edge. Fig. 4-10 shows tooling for 
Trimming a horizontal flange on a drawn shell in a separate operation. 
After scrap from a sufficient number of trimmed shells has accumulated, 
the piece of scrap at the bottom is severed at each stroke of the press by 
scrap cutter shown in this figure and falls clear. 
 CUTTING ACTION IN PUNCH AND DIE OPERATIONS 
The cutting action that occurs in blanking or piercing is quite similar 
to that of chip formation ahead of a cutting tool. The punch contacts the 
work material supported by the die and a pressure buildup occurs. When 
the elastic limit of the work material is exceeded, the material begins to 
flow plastically (plastic deformation). The punch penetrates thework 
material, and the blank, or slug, is displaced into the die opening a 
corresponding amount. A radius is formed on the top edge of the hole and 
the bottom edge of the slug, or blank, as shown in Fig. 4-11a. The radius 
is often referred to as rollover and its magnitude depends upon the 
ductility of the work material. Compression of the slug material against 
the walls of the die opening burnishes a portion of the edge of the blank, 
as shown in Fig. 4-11b. At the same time, the plastic flow pulls the 
material around the punch, causing a corresponding burnished area in the 
work material. Further continuation of the punching pressure then starts 
fractures at the cutting edge of the punch and die (see Fig. 4-11c). Under 
ideal cutting conditions, the fractures will meet and the remaining portion 
of the slug edge will be broken away. A slight tensile burr will be formed 
along the top edge of the slug edge will be broken away. A slight tensile 
burr will be formed along the top edge of the slug and the bottom edge of 
the work material (see Fig. 4-11d). 
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Fig. 4-11 Cutting action progression when blanking 
and piercing metal 
 
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Fig. 4-12 Characteristic appearance of edges of parts produced 
by piercing and blanking 
 Fig. 4-12 Shows the characteristic appearance of the edges of parts 
produced by blanking and piercing operations in detail. The edge radius 
(or rollover) is produced during the initial stage of plastic deformation. 
The edge radius is more pronounced with soft materials. 
The highly burnished band is the result of the material’s being forced 
against the walls of the punch and die and rubbing during the final stages 
of plastic deformation. The sum of the edge radius depth and the 
burnished depth is referred to as penetration, i.e., the distance the punch 
penetrates into the work material before fracture occurs. Penetration is 
usually expressed as a percent of material thickness, and it depends upon 
the properties of the work material. As the work material becomes 
harder, the percent of penetration decreases. For this reason, harder 
materials have less deformation and burnished area. 
The remaining portion of the cut is the fractured area, or break. The 
angle of the fractured area is the breakout angle. The tensile burr is 
adjacent to the break. The burr side of blank or slug is toward the punch, 
and the burr side of the work material is toward the die opening. 
 Die Clearance: Clearance is defined as the intentional space between the 
punch cutting edge and die cutting edge. Clearance is always expressed as the 
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amount of clearance per side. Theoretically, clearance is necessary to allow the 
fractures to meet when break occurs, as shown in Fig. 4-13. The amount of 
clearance depends upon the kind, thickness and hardness of the work material. 
 
Excessive clearance allows a large edge radius (rollover) and 
excessive plastic deformation. The edges of the material tend to be drawn 
or pulled in the direction of the working force, and the break is not 
smooth. Large burrs are present at the break edge. 
 
Fig. 4-13 The effect of clearance (a) too little clearance: fracture do 
not meet(b) Correct clearance: fracture do meet 
 
Insufficient cutting clearance caused the fractures to miss and 
prevents a clean break, as shown in Fig. 4-13a. A partial break occurs, 
and a secondary break connects the original or main fractures. This is 
often referred to as secondary shear. The secondary break does not allow 
separation of the material without interference, and a second burnished 
ring is formed, as shown in Fig. 4-13b. The burnished ring may appear as 
a slight step around the outside edge of the blank or around the inside 
edge of the hole. Insufficient clearance increase pressure on the punch 
and die edge and has a marked effect on die life. 
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Authorities disagree on the correct amount of clearance for a given 
work piece material. One reference may recommend 6 percent of stock 
material thickness per side, while another may recommend 12 percent for 
the same material. The difference is probably in the end result each is 
striving for. The designer should consider the application of the pierced or 
blanked work piece. When the purpose is only to make a hole, as in the 
case of structural steel, wide clearances may be used to increase die life. 
Blanked work pieces that assemble as an integral part of a mechanism 
require tighter clearances. Fig. 4-14 shows various edges for 
stampings with a description and use of each. Note that the 
recommended clearance varies from 2 to 21 percent for mild steel. 
The diameter of the blank or pierced hole is determined by 
measurement of the burnished area. Since the burnished area on the 
blank is produced by the walls of the die, the diameter of the blank will be 
the same as the diameter of the die (disregarding a slight expansion after 
the blank is pushed from the die). The same principle applies to the 
diameter of the pierced hole. The burnished area in the hole is caused by 
the punch; thus the diameter of the pierced hole will be the same as the 
punch. Therefore, die clearance is either placed on the punch or the die, 
depending upon whether the pierced hole or the blank will be the desired 
work piece. If the blank is to become the work piece, the die diameter is 
made to the work piece size and the punch is reduced in size an amount 
equal to the die clearance. If the pierced hole is to become part of the 
work piece, the punch is made to the correct hole size and the die 
opening is made oversize an amount equal to the die clearance. In simple 
terms, the die controls blank s ize and the punch controls hole size 
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Angular clearance is necessary to prevent backpressure caused by 
blank or slug buildup especially when the punches or die block are fragile. 
Recommended angular clearance varies from ¼ to 2 per side, depending 
upon the material and the shape of the work piece. Soft materials and 
heavy-gage materials require greater angular clearance. Larger angular 
clearance may be necessary for small and fragile punches. 
 
 
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Fig. 4-15 The use of angular clearance 
 Stripping: The forces that cause the blank to grip inside the die 
walls also cause the stock material to grip around the punch. The stock 
material will rise as the press ram is raised unless some means of 
stripping the stock material from the punch is provided. Fig. 4-15 shows 
the two basic types of stripping device. 
 
 
Fig. 4-15 Basic types of stripping devices (a) Fixed type and 
(b) Spring loaded type 
The amount of pressure required to strip the stock material from the 
punch varies from 5 to 20 percent of the cutting-force requirements. 
This is only a rough estimate, as many variables affect stripping 
pressure. For example some materials cling more than others. Thicker 
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materials require more stripping force because more material is in 
contact with the punch. Holes punched close to the strip edge do not 
require as much stripping force because there is less backing and the 
metal can give. Punches with polished sidewalls tend to strip easier 
than those with rough surfaces. More force is also required to strip 
punches that are close together. 
 Cutting forces: The force required to penetrate the stock material 
with the punch is the cutting force. If the die contains more than one 
punch that penetrates t he stock material simultaneously, the cutting 
force for that die is the sum of the forces for each punch. Knowledge of 
cutting forces is important in orderto prevent overloading the press or 
failure to use it to capacity. 
The formula for determining cutting forces takes into account the 
thickness of the stock material, the perimeter of the cut edge and the 
shear strength of the stock material. The shear strength of the stock 
material is the force necessary to sever 1 sq. in. of the material by 
direct shearing action. 
It sometimes becomes necessary to reduce cutting forces to 
prevent press overloading. One method of reducing cutting forces is to 
step punch lengths, as shown in Fig. 4-16. Punches or groups of punches 
progressively become shorter by about one stock-material thickness. A 
second method is to grind the face of the punch or die at a small shear 
angle with the horizontal. This has the effect of reducing the area in shear 
at any one time. Shear also reduces shock to the press and smoothes out 
the cutting operation. The shear angle chosen should provide a change in 
punch length of from 1 to 1 ½ times the stock thickness. Shear that is 
equal to or greater than the stock thickness is called full shear. Cutting 
forces are reduced by approximately 30 percent when full shear is applied 
 
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Methods of reducing cutting forces: 
a) Stepping punches 
b) Single shear on punch 
c) Single shear on die 
d) Double shear on punches 
e) Double shear on punches 
 f) Convex & concave shear 
 
 
Scrap – Strip Trip Layout For Blanking 
In designing parts to be blanked from strip material, economical stock 
utilization is of high importance. The goal should be at least 75 per cent 
utilization. A very simple scrap-strip layout is shown in Fig. 6-1. 
SCRAP ALLOWANCE 
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A scrap-strip layout having insufficient stock between the blank and the strip 
edge, and between blanks, will result in a weakened strip, subject to breakage 
and thereby causing misfeeds. Such troubles will cause unnecessary die 
maintenance owing to partial cuts, which defect the punches, resulting in nicked 
edges. The following formulas are used in calculating scrap-strip dimensions for all 
strips over 0.8 mm. thick. 
t = specified thickness of the material 
 B = 1.25 t when C is less than 64 mm 
 B = 1.5 t when C is 64 mm or longer 
 C = L + B, or lead of the die 
Example: A rectangular part, to be blanked from 1.5 mm thick steel 
(Manufacturers Standard) is 10 X 27 mm. If the scrap strip is developed as in Fig. 
6-2, the solution is 
 t = 1.5 mm 
 B = 1.25 X 1.5 = 1.875 mm 
 C = 10 + 1.875 = 11.875 mm 
 W = 27 + 3.75 = 30.75 mm 
Nearest commercial stock is 32 mm. Therefore, the distance B will equal 
2.3mm. This is acceptable since it exceeds minimum requirements. 
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 Minimum Scrap-Strip Allowance: If the material to be blanked is 0.6 
mm thick or less, the formulas above should not be used. Instead, dimension B is 
to be as follows: 
 Strip width W Dimension B 
 0 - 75 mm 1.3 mm 
 76 – 150 mm 2.4 mm 
 150 – 300 mm 3.2 mm 
 Other Scrap-Strip Allowance Applications: 
Figure 6-3, 6-4 and 6-5 illustrates special allowances for one-pass layouts: 
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View D. For layouts with sharp corners of blanks adjacent, B = 1.25 t. 
Fig. 6-4 Allowances for one-pass layouts. 
 
 Percentage of Stock Used: If the area of the part is divided by the area 
of the scrap strip used, the result will be the percentage of stock used. 
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 If A = total area of strip used to produce a single blanked 
part, then 
 A = CW (Fig. 3-35), and a = area of the part = LH. 
 If C = 11.5 mm and W = 32 mm then A = 11.5 X 32 = 368 
mm² 
 If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6 
mm² 
 Percentage of stock used: 
a 256.5 
 = = 70% approx. 
A 368 
 EVOLUTION OF A BLANKING DIE 
In the planning of a die, the examination of the part print immediately 
determines the shape and size of both punch and die as well as the working area 
of the die set. 
 
 Die Set Selection 
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A commercially available standardized two-post die set with 150 mm overall 
dimensions side-to-side and front-to-back allows the available 76 mm. wide stock 
to be fed through it. It is large enough for mounting the blanking punch on the 
upper shoe (with the die mounted on the lower shoe) for producing the blank 
shown in Fig. 6-6, since the guideposts can be supplied in lengths of from 100 to 
225 mm. 
Since the stock, in this case was available only in a width of 76 mm the length 
of the blanked portions extended across the stock left a distance between the 
edges of the stock and the ends of the blank of 6 mm or twice the stock 
thickness; this allowance is satisfactory for the 3.2 mm stock. 
 Die Block Design 
By the usual „rule-of thumb‟ method previously described, die block thickness 
(of tool steel) should be a minimum of 20 mm for a blanking perimeter up to 75 
mm and 25 mm for a perimeter between 75 and 100 mm. For longer perimeters, 
die block thickness should be 32 mm. Die blocks are seldom thinner than 22 mm 
finished thickness to allow for grinding and for blind screw holes. Since the 
perimeter of the blank is approximately 178 mm a die block thickness of 38 mm 
was specified, including a 6 mm grinding allowance. 
There should be a margin of 32 mm around the opening in the die block; its 
specified size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap 
screws and dia. 10 mm dowels are located at the corners 20 mm from the edges 
of the block. 
The wall of the die opening is straight for a distance of 3.2 mm (stock 
thickness); below this portion or the straight, an angular clearance of 1½° allows 
the blank to drop through the die block without jamming. The dimensions of the 
die opening are the same as that of the blank; those of the punch are smaller by 
the clearance (6 per cent of stock thickness, or 2 mm), which result in the 
production of blanks to print (and die) size. 
The top of the die was ground off a distance equal to stock thickness (Fig. 6-7) 
with the result that shearing of the stock starts at the ends of the die and 
progresses towards the center of the die, and less blanking pressure is required 
than if the top of the die where flat. 
 Punch Design 
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The shouldered punch (57 mm) long is held against a 6 mm thick hardened 
steel backup plate by a punch plate 20 mm thick) which is screwed and doweled 
to the upper shoe. The shut height of the die can be accommodated by a 32-ton 
(JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For 
the conditions of this case study, shear strength S = 42 kg/mm², blanked 
perimeter length L = 178 mm approx. and thickness T = 3.2 mm. 
From the equation P = SLT 
The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons. 
This value is well below the 32-ton capacity of the selected 
press. 
 The shut height (Fig. 6-7) is 178 mm less the 1.6 mm 
travel of the punch into the die cavity. 
print immediately determines the shape and size of both punch and 
die as well as the working area of the die set. 
 
 
 Die Sheet Selection 
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A commercially available standardized two-post die set with 150 mm 
overall dimensions side-to-side and front-to-back allows the available 76 mm. 
wide stock to be fed through it. It is large enough for mounting the blanking 
punch on the upper shoe (with the die mounted on the lower shoe) for producing 
the blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of 
from 100 to 225 mm. 
Since the stock, in this case was available only in a width of 76 mm thelength of the blanked portions extended across the stock left a distance between 
the edges of the stock and the ends of the blank of 6 mm or twice the stock 
thickness; this allowance is satisfactory for the 3.2 mm stock. 
 Die Block Design 
By the usual „rule-of thumb‟ method previously described, die block 
thickness (of tool steel) should be a minimum of 20 mm for a blanking perimeter 
up to 75 mm and 25 mm for a perimeter between 75 and 100 mm. For longer 
perimeters, die block thickness should be 32 mm. Die blocks are seldom thinner 
than 22 mm finished thickness to allow for grinding and for blind screw holes. 
Since the perimeter of the blank is approximately 178 mm a die block thickness of 
38 mm was specified, including a 6 mm grinding allowance. 
There should be a margin of 32 mm around the opening in the die block; 
its specified size of 150 x 150 mm allows a margin of 45 mm in which four M10 
cap screws and dia. 10 mm dowels are located at the corners 20 mm from the 
edges of the block. 
 The wall of the die opening is straight for a distance of 3.2 mm (stock 
thickness); below this portion or the straight, an angular clearance of 1½° allows 
the blank to drop through the die block without jamming. The dimensions of the 
die opening are the same as that of the blank; those of the punch are smaller by 
the clearance (6 per cent of stock thickness, or 2 mm), which result in the 
production of blanks to print (and die) size. 
 The top of the die was ground off a distance equal to stock thickness (Fig. 
6-7) with the result that shearing of the stock starts at the ends of the die and 
progresses towards the center of the die, and less blanking pressure is required 
than if the top of the die where flat. 
Punch Design 
 27 
The shouldered punch (57 mm) long is held against a 6 mm thick hardened 
steel backup plate by a punch plate 20 mm thick) which is screwed and doweled 
to the upper shoe. The shut height of the die can be accommodated by a 32-ton 
(JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For 
the conditions of this case study, shear strength S = 42 kg/mm², blanked 
perimeter length L = 178 mm approx. and thickness T = 3.2 mm. 
From the equation P = SLT 
The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons. 
This value is well below the 32-ton capacity of the selected 
press. 
 The shut height (Fig. 6-7) is 178 mm less the 1.6 mm 
travel of the punch into the die cavity. 
 Stripper Design 
 The stripper that was designed is of the fixed type with a channel or slot 
having a height equal to 1.5 times stock thickness and a width of 80 mm to allow 
for variations in the stock width of 75 mm. The same screws that hold the die 
block to the lower shoe fasten the stripper to the top of the die block. 
 If, instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a 
spring-loaded stripper would firmly hold the stock down on top of the die block 
and could, to some extent, flatten out wrinkles and waves in it. 
 
 28 
A spring-loaded stripper should clamp the stock until the punch is 
withdrawn from the stock. The pressure that strips the stock from the 
punch on the upstroke is difficult to evaluate exactly. A formula frequently 
used is 
 Ps = 2.5 x L x t kgs. 
 
 Where Ps = stripping pressure, in kgs. 
 L = perimeter of cut, in mm. 
 t = stock thickness, in mm. 
 Spring design is beyond the scope of this book; die spring 
data are available in the catalogues of spring manufacturers. 
 Stock Stops 
 The pin stop pressed in the die block is the simplest method for stopping 
the hand-fed strip. The right-hand edge of the blanked opening is pushed against 
the pin before descent of the ram and the blanking of the next blank. The 4-8 mm 
depth of the stripper slot allows the edge of the blanked opening to ride over the 
pin and to engage the right-hand edge of every successive opening. 
 The design of various types of stops adapted for manual and automatic 
feeding is covered in a preceding discussion. 
 
A spring-loaded stripper should clamp the stock until the punch is 
withdrawn from the stock. The pressure that strips the stock from the punch on 
the upstroke is difficult to evaluate exactly. A formula frequently used is 
 Ps = 2.5 x L x t kgs. 
 
 Where Ps = stripping pressure, in kgs. 
 L = perimeter of cut, in mm. 
 29 
 t = stock thickness, in mm. 
 Spring design is beyond the scope of this book; die spring 
data are available in the catalogues of spring manufacturers. 
 Stock Stops 
 The pin stop pressed in the die block is the simplest method for stopping 
the hand-fed strip. The right-hand edge of the blanked opening is pushed against 
the pin before descent of the ram and the blanking of the next blank. The 4-8 mm 
depth of the stripper slot allows the edge of the blanked opening to ride over the 
pin and to engage the right-hand edge of every successive opening. 
EVOLUTION OF A PROGRESSIVE BLANKING DIE 
 Figure 6-8 gives the blanked dimensions of a linkage case cover of cold 
rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts 
made at one setup, with the possibility of three or four runs per year. 
 
 30 
Step 1, Part Specification 
1. The production is of medium class; therefore a second-class die will 
be used. 
2. Tolerances required: Except for location of the slots, all 
dimensions are in fractions. The slot locations, though specified in 
decimals, are not very close. Thus a compound die is not 
necessary; a two or three-station progressive die will be adequate. 
3. Type of press to be used: Available for this production are presses 
of 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or 
200 mm. 
4. Thickness of material: Specified as 32 mm standard cold rolled 
steel. 
Step 2, Scrap-Strip Development 
 From the production requirements, a single-row strip will 
suffice. After several trials, the scrap strip shown in Fig. 6-9 was 
decided upon. Owing to the closeness of the holes it was decided to 
make a four-station die. 
 
The scrap strip would be fed into the first finger stop, and the center hole would 
be pierced. The strip would then be moved in to the second finger stop, and the 
two holes would then be pierced. At the third stage and third finger stop, a pilot 
 31 
would locate the strip and the four corner holes would then be pierced. At the 
fourth and final stage, a piloted blanking punch would blank out the finished part. 
 
Step 3, Press Tonnage 
 It is now in order to determine the amount of pressure needed. Only the 
actual blanking in the fourth stage need be calculated, since the work in the first 
three stages will be done by stepped punches. 
 From Table, the shear strength S of cold rolled steel is 40 kgs/mm². The 
length L of the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut 
(stock thickness t) equals 3.2 mm. From the equation P = S L t 
 P = 40 kgs./mm² x 240 mm x 3.2 mm = 30,720 kgs. 
Or 30.7 tons. 
 This tonnage is greater than can be handled by the available presses. To 
lower the pressure, shear is ground on the blanking punch to reduce the needed 
pressure by on third. This, ? x 30.7 = 30.7 - 10.2 = 20.5 tons. A punch press of 
25-ton capacity would do, but there is reported available only a 30-ton press with 
a 190 mm shut height and a 50 mm stroke. This press is selected. The bolster 
plate is found to be 300 mm deep, 140 mm from centerline of ram to back edge 
of bolster, and 600 mm wide. Shank diameteris 64 mm. 
Step 4, Calculation of the Die 
(a) The die. The perimeter of the cut equals 240 mm and therefore the 
thickness of the die must be 25 nm. The width of our scarp-strip opening is 60 
mm with 32 mm extra material on each side of the opening, it will be 60 mm + 64 
mm = 124 mm or 130 mm width. The distance from the left side of the opening in 
stage 4 to the edge of the opening in stage 1 equals 3 C + 30 + 6 = 192 + 30 + 6 
= 228 mm and plus 62 mm = 290 mm or 296 mm long. Therefore the die should 
be 2.5 x 130 x 296 mm long. 
(b) The die plate. As a means of filling in between the die and the die shoe, a 
die plate of machinery steel is used. To secure the die plate to the die shoe M12 
cap screws and dowels are used. A minimum of twice the size of the cap screw for 
the distance from the edge of the die to the edge of the die plate is needed, which 
will equal 25 mm. Twice this distance = 50 mm and 50 mm added to the size of 
the die will result in a die plate of 25 x 180 x 346 mm. Figure 6-10 shows the die 
 32 
and die plate fitted together, and with the holes, which show the sharpening 
portion and the relief portion. 
 
Step 5, Calculation of Punches 
 Good practice requires 10 per cent of the metal thickness to be removed 
from the basic dimension of the blanking punch. This same value is used on the 
die opening, since holes are to be pierced in the blank. The clearance rule will be 
applied to the die opening in Stages 1, 2, and 3, and to the punch in Stage 4 (see 
fig6-11). 
For Stage 4: Blank to be 60 mm square, 
Stock thickness = 3.2 mm; 10% = 0.32 mm. 
Punch = 60 – 0.32 = 59.68 mm. 
 Therefore the die opening will equal 60.01 to 60 mm and 
the punch will equal 59.68 to 59.67 mm. 
For Stage 2: Slot to be 8 mm wide by 34 mm long. 
Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm. 
 33 
Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to 
34.00 mm. long. 
 The punch and the die opening will have straight sides for at least 3-
11 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for the 
punches of Stage 2, and also the stepped arrangement of the punches for all 
stages 
 
Step 6, springs 
 A solid stripper plate can be used for this job. 
Step 7, Piloting 
 Figures 6-9 and 6-11 illustrate the arrangement for piloting. In this case it 
is direct piloting. However, if the part did not have a center hole, and the slots 
and other holes were too small, indirect piloting would have to be provided. 
Step 8, Automatic Stops 
 Finger stops will act as stops when a new scrap strip is being inserted but, 
after that, an automatic spring drop stop must be used to halt the scrap strip. 
Figures 6-12 illustrate details of the completed drawing of the die. 
 34 
 
 Die Design 
 WIRE METHOD OF LOCATING THE CENTER OF PRESSURE: 
The center of pressure of a blank contour may be located mathematically, but 
it is a tedious computation. Location of the center of pressure within 12 mm 
of true mathematical location is normally sufficient.A simple procedure 
accurate within such limits is to bend a soft wire to the blank contour. By 
balancing this frame across a pencil, in two coordinates, the intersection of 
the two axes of balance will locate the desired point. As an example of the 
marked influence this factor may have on tool design, a rather unusual blank 
is shown in Fig. 5-1. Here, the center of pressure is near one end of the 
blank, and will require the indicated imbalance in the press tool design. 
 35 
 
Die clearance is the space between the matting members of a die set. Proper 
clearance between cutting edges enables the fractures to meet and the 
fractured portion of the sheared edge has a clean appearance. For optimum 
finish of a cut edge, proper clearance is necessary and is a function of the 
kind, thickness and temper of the work material. 
At Fig. 5-2, which shows clearance C for blanks of a given size, make die to 
size and punch smaller by total clearance 2C. At B, which shows clearance for 
holes of given size, make punch to size and die larger by the amount of the 
total clearance 2C 
 
 36 
The application of the clearance for holes of irregular shape is diagrammed in 
Fig. 5-13; at B the hole will be of punch size, while at A the blank will be of 
the same dimensions as the die. 
 
 DIE BLOCK GENERAL DESIGN: 
Overall dimensions of the die block will be determined by the minimum die 
wall thickness required for strength and by the space needed for mounting 
screws and dowels and for mounting the stripper plate. 
Wall thickness requirements for strength will depend on the thickness of the 
stock to be cut. Sharp corners in the contour may lead to cracking in heat 
treatment, and so require greater wall thickness at such points. 
Two, and only two, dowels should be provided in each block or element that 
requires accurate and permanent positioning. They should be located as far 
apart as possible for maximum locating effect, usually near diagonally 
opposite corners. Two or more screws will be used, depending on the size of 
the element mounted. Screws and dowels are preferably located about 1 ½ 
times their diameters from the outer edges or the blanking contour. 
Die block thickness (see Table 5-1) is governed by the strength necessary to 
resist the cutting forces, and will depend on the type and thickness of the 
material being cut. On very thin materials 13 mm. thickness should be 
 37 
sufficient but, except for temporary tools, finished thickness is seldom less 
than 22 mm., which allows for blind screw holes and also builds up the tool to 
a narrower range of shut height for press room convenience. 
 DIE BLOCK CALCULATIONS: 
Method 1 (“Rule of Thumb”). Assuming a die block of tool steel its thickness 
should be 20 mm. minimums for a blanking perimeter of 75 mm. or less 25 
mm. thick for perimeters between 75 mm. and 250 mm. and 32 mm. thick for 
larger perimeters. There should be a minimum of 32 mm. margins around the 
opening in the die block. 
The die opening should be straight for a maximum of 3 mm; the opening 
should then angle out at ¼ to 1 ½ to the side (draft). The straight sides 
provide for sharpenings of the die; the tapered portion enables the blanks to 
drop through without jamming. 
To secure the die to the die plate or die shoe, the following rules provide 
sound construction: 
1) On die blocks up to 175 mm square, use two M10 cap screws and two 
dowels of dia. 10 mm. 
2) On sections up to 200 mm. square, use three cap screws and two dowels. 
3) For blanking heavy stock, use cap screws and dowels of dia 12-mm. 
diameter. Counterbore the cap screws 3.2 mm. deeper than usual, to 
compensate for die sharpening. 
 
Method 2. This method of calculating the proper size of the die was derived 
from a series of tests, whereby die plates were made increasingly thinner until 
breakage became excessive. From these data the calculation of die thickness 
was divided into four steps: 
1) Die thickness is provisionally selected from Table 5-1. This table takes 
into account the thickness of the stock and its ultimate shear strength (see 
Table 4-1). 
Table 5-1. DIE THICKNESS PER TON OF PRESSURE 
Stock Die Stock Die 
 38 
thickness 
mm. 
thickness 
cm. 
thickness 
mm. 
thickness 
cm. * 
0.2
5 
0.5
0 
0.7
5 
1.0
0 
1.2
5 
* 
0.
118 
0.
236 
0.
335 
0.
433 
0.
512 
1.5 
1.8 
2.0 
2.3 
2.5 
0.5
90 
0.6
49 
0.7
08 
0.7
48 
0.7
87* For each ton per sq. cm. of shear strength. 
Table 5-2. FACTORS FOR CUTTING EDGES EXCEEDING 50-
mm. 
Cutting perimeter mm. Expansion Factor 
50 to 75 
75 to 150 
150 to 300 
300 to 500 
1.25 
1.5 
1.75 
2.0 
 
2) The following corrections are then made: 
a) The die must never be thinner than 8 mm. to 10 mm. 
b) Data in Table 5-1 apply to small dies, i.e. those with a 
cutting perimeter of 50-mm. or less. For larger dies, the 
thickness listed in Table 5-1 must be multiplied by the 
factors in Table 5-2. 
c) Data in Tables 5-1 and 5-2 are for die members of tool-
steel, properly machined and heat-treated. If a special alloy 
of steel is selected, die thickness can be decreased. 
d) Dies must be adequately supported on a flat die plate or die 
shoes. Thickness data above do not apply if the die is placed 
 39 
over a large dopening or is not adequately supported. 
However, if the die is placed into a shoe, the thickness of 
the member can be Decreased up to 50 percent. 
e) A grinding allowance up to 0.25 to 0.5 mm must be added 
to the calculated die thickness. 
3) The critical distance A, Fig. 5-4, between the cutting edge and 
the die border must be determined. In small dies, A equals 1.5 
to 2 times the die thickness; in larger dies it is 2 to 3 times the 
die thickness. 
Table 5-3 MINIMUM CRITICAL AREA 
 
 Critical distance A must not less than 1.5 to 2 times die 
thickness. 
 The critical area between the die hole and the die border 
must be checked against minimum values in Table 3-4 and 
die thickness B corrected if necessary. 
4) Finally, the die thickness must be checked against the empirical 
rule that the cross-sectional area A x B (Fig. 5-4) must bear a 
certain minimum relationship to the impact pressure for a die 
put on a flat base. In Table 5-3 impact pressure equals 
thickness times the perimeter of the cut times ultimate shearing 
strength. If the die height, as calculated by steps 1 and 2, does 
 40 
not give sufficient area for the critical distance A (Fig. 5-4), the 
die thickness must be increased accordingly. 
With the die block size determined, the exact size of the die 
opening can now be determined. Assuming a clearance of 
approximately 10 percent of the metal thickness, and by the rule-of-
thumb method. 
 Metal thickness = 1.6 x 10% = 0.16 mm. 
If the finished die opening is 25 mm. dia., then add 0.16 mm., 
giving 26.16 ± 0.025 mm. If the blank were made according to size, 
the clearance would be applied to the punch. 
 PUNCH DIMENSIONING: 
The determination of punch dimension has been generally 
based on practical experience. When the diameter of a pierced round 
hole equals stock thickness, the unit compressive stress on the punch 
is four times the unit shear stress on the cut area of the stock, from 
the formula. 
The diameter of most holes are greater than stock thickness; a 
value for the ratio d / t of 1.1 is recommended. The maximum 
allowable length of a punch can be calculated from the formula. 
 This is not to say that holes having diameters less than stock 
thickness cannot be successfully punched. The punching of such holes can be 
facilitated by: 
1. Punch steels of high compressive strengths 
2. Greater than average clearances 
3. Optimum punch alignment, finish, and rigidity 
4. Shear on punches or dies or both 
5. Prevention of stock slippage 
6. Optimum stripper design 
 
The determination of punch dimension has been generally based on practical 
experience. When the diameter of a pierced round hole equals stock 
thickness, the unit compressive stress on the punch is four times the unit 
shear stress on the cut area of the stock, from the 
 METHODS OF PUNCH SUPPORT: 
 41 
Following figure presents a number of methods to support punches to meet 
various production requirements: 
 
 
 
 42 
 
 43 
 
 44 
 
 
STOCK STOPS: 
 Finger Stops: 
In its simplest form, a stock stop may be a pin or small block, against which 
an edge of the previously blanked opening is pushed after each stroke of the 
press. With sufficient clearance in the stock channel, the stock is momentarily 
lifted by its clinging to the punch, and is thus released from the stop. Figure 
6-23 shows an adjustable type of solid block stop which can be moved along a 
 45 
support bar in increments up to 25 mm to allow various stock lengths to be 
cut off. 
A starting stop, used to position stock as it is initially fed to a die, is shown in 
Fig. 5-23, view A. Mounted on the stripper plate, it incorporates a latch, which 
is pushed inward by the operator until its shoulder (1) contacts the stripper 
plate. The latch is held in to engage the edge of the incoming stock; the first 
die operation is completed, and the latch is released. 
 
 Automatic Stops: 
The starting stops shown at view B, mounted between the die 
shoe and die block, upwardly actuates a stop plunger to initially 
position the incoming stock. Compression springs return the manually 
operated lever after the first die operation is completed. 
Trigger stops incorporated pivoted latches (1, Fig. 5-24, views A 
and B). At the ram‟s descent, these latches are moved out of the 
blanked-out stock area by actuating pins, 2. On the ascent of the ram, 
springs, 3, control the lateral movement of the latch (equal to the side 
relief) which rides on the surface of the advancing stock, and drops 
into the blanked area to rest against the cut edge of the cut-out area. 
 
 46 
 
When feeding the stock strip from one stage to another, some method must 
be used to correctly locate and stop the strip. Automatic stops (trigger stops) 
register the strip at the final die station. They differ from finger stops in that 
they stop the strip automatically, the operator having only to keep the strip 
pushed against the stop in its travel through the die. A typical automatic stop 
designs shown in figure 5-24. In this lever end is raised by the trip screw as 
punch descends and cut the blank. On the return stroke end of lever drops 
and lever end would drop it former position if it were not for the endwise 
action o the lever, which causes the lever end to drop onto the top surface of 
the stock instead of into blank space. The mounting of the finger on the pivot 
is loose enough to allow for this endwise movement. 
When feeding the stock strip from one stage to another, some method must 
be used to correctly locate and stop the strip. 
 47 
Automatic stops (trigger stops) register the strip at the final die station. They 
differ from finger stops in that they stop the strip automatically, the operator 
having only to keep the strip pushed against the stop in its travel through the 
die. 
 
 
 
 48 
 
 Cropping: 
An unusual stop that requires no moving mechanisms is known 
as the French stop or Cropping as shown in fig. 5-26. It operates on 
the principle of cutting a shoulder in the edge of the stock strip, which 
acts as stop. A strip wider than necessary is inserted into the strip 
channel until it contacts the shoulder built into back gage. The first hit 
of the press performs the first station operation and at the same time 
punches from the side of the strip a section of metal equal to the 
length of the pitch. This operation leaves a shoulder in the side of the 
strip. The strip is then advanced until the strip shoulder contacts the 
shoulder on the back gage on the return strokeof the ram. 
 
 49 
 
 
The advantage of the cropping is its accuracy and speed of 
operation. It is especially well suited to the light-gage materials that 
are easily distorted when pushed or pulled against a stop pin. Its main 
disadvantages are the extra cost of tooling and extra stock scrap. 
 PILOTS: 
Since pilot breakage can result in the production of inaccurate parts and the 
jamming or breaking of die elements, pilots should be made of 
good tool-steel, heat-treated for maximum toughness and to 
hardness of Rock-well C57 to 60. 
 
Press-fit Pilots: 
 
Press-fit pilots (Figs. 5-28 and 5-29, view C), which may out of the punch 
holder, are not recommended for high-speed dies but are often used in low-
speed dies. For holes 20 mm in diameter or larger, the pilot may be held 
 50 
by a socket-head screw shown at B. A typical press-fit type is shown at C. 
Pilots of less than 6 mm diameter may be headed and secured by a socket 
setscrew, as shown at D. 
Indirect Pilots: 
Designs of pilots that enter previously pierced holes in the strip as shown in 
Fig. 5-27. This practice provides more support under the strip. It helps to 
locate the pitch and prevent distortion. 
Spring-loaded pilots: 
Spring-loaded pilots should be used for stock exceeding 1.5 mm thickness 
sheet as shown in Fig. 5-30. This allows the pilot to retract in case of misfeed. 
Tapered slug-clearance holes through the die and lower shoe should be 
provided, since indirect pilots generally pierce the strip during a misfeed. 
Spring-loaded pilots are not necessary on thinner material because the pilot 
will pierce the strip rather than break in the event of misfeed. In this case 
tapered slug-clearance holes through the die and lower shoe should be 
provided. 
Misfeed detector: 
Fig. 5-31 shows a precision misfeed detector used in a manner similar to that 
of a pilot. The detector senses out-of-register position of stock and actuates a 
switch to cut off the electric power to the press. 
 
 51 
 
 
 
 52 
 STRIPPERS: 
 Strippers are of two types, fixed or spring-operated. The primary 
function of either type is to strip the work piece from a cutting or non-cutting 
punch or die. A stripper that forces a part out of a die may also be called a 
knockout, an inside stripper, or an ejector. Besides its primary function, a 
stripper may also hold down or clamp, position, or guide the sheet, strip, or 
work piece. 
 Fixed strippers: 
The stripper is usually of the same width and length as the die block. 
In the simpler dies, the stripper may be fastened with the same screws 
and dowels that fasten the die block, and the screw heads will be 
counter bored into the stripper. In more complex tools and with 
sectional die blocks the die block screws will usually be inverted, and 
the stripper fastener will be independent. Following fig. 5-32 is will 
make clear picture of fixed type stripper plate. 
 
The stripper thickness must be sufficient to withstand the force required to 
strip the stock from the punch, plus whatever is required for the stock strip 
channel. Except for very heavy tools or large blank areas, the thickness 
required for screw head counter bores, in the range of 10 to 16 mm will be 
sufficient. 
 53 
The height of the stock strip channel should be at least 1½ times the stock 
thickness. This height should be increased if the stock is to be lifted over a 
fixed pin stop. The channel width should be the width of the stock strip, plus 
adequate clearance to allow for variations in the width of the strip as cut, as 
follows: 
 Stock thickness mm Add to strip width in mm 
 Up to 1 2.0 
 1 to 2 2.4 
 2 to 3 2.8 
 Above 3 3.2 
 If the stripper length has been extended on the feed end for better 
stock guidance, a sheet metal plate should be fastened to the underside of the 
projecting stripper to support the stock. This plate should extend slightly in 
for convenience in inserting the strip. The entry edges of the channel should 
be beveled for the same reason. 
 
5.9.2 Spring-operated strippers: 
 54 
Where spring-operated strippers are used as shown in fig. 5-33, the force 
required for stripping is 35000 times cut perimeter times the stock thickness. 
It may be as high as 20 per cent of the blanking force, which will determine 
the number and type of springs required. The highest of these values should 
be used. 
 Selection of stripper springs: 
Die springs are designed to resist fatigue failure under severe service conditions. They are 
available in medium, medium-heavy and heavy-duty grades, with corresponding permissible 
deflections ranging from 50 to 30 percent of free length. The number of springs for which space is 
available and the total required force will determine which grade is required. The required travel 
plus the preload deflection will be the total deflection, and will determine the length of spring 
required to stay within allowable percentage of deflection limits. As the punch is re-sharpened, 
deflections will increase, and should also be allowed for. 
 To retain the stripper against the necessary preload of the springs, and 
to guide the stripper in its travel, a special type of shoulder screw known as a 
stripper bolt is used. 
 Choice of the method of applying springs to stripper plates depends on 
the required pressure, space limitations, shape of the die and the nature of 
the work, and production requirements. The stripping pressure may be from 5 
to 20 percent of the cutting pressure. The amount of pressure needed to hold 
thin firmly while it being cut must also be considered when selecting springs. 
With so many variable factors involved, exact results cannot be expected from 
formulas, although they may be useful as guides. The “Metal Handbook” gives 
the following formula for stripping force: 
L = KA 
 
Where 
L = Stripping force 
K = Stripping 
constant, Kg / cm.2 
A = Area of cut 
surface, cm.2 
Approx. values for K, as determined by experiment: 105 for sheet 
metal thinner than 1.6 mm. when cut is near an edge or near a 
preceding cut; 150 for other cuts in sheet metal thinner than 1.6 
mm; and 210 for sheet metal more than 1.6 mm. thick. 
 55 
 (stock 
thickness x 
length of cut) 
 
 Die springs are available in various service grades with corresponding 
permissible deflections ranging from 25 to 50 percent of free length. For 
example, DME Spring catalogue lists four load ratings. Based upon max. (See 
fig. 5-34) 
50% deflection = medium pressure (MP) 
37% deflection = medium high pressure (MHP) 
30% deflection = high pressure (HP) 
25% deflection = extra high pressure (XMP) 
An important feature of these springs is that they are similar 
dimensioned thus are interchangeable. The steps in selecting die 
springs are as follows: 
1) Estimate the stripping force required according to formula. 
2) Determine the amount of space available for spring mounting. 
3) Select the max. allowable number of springs, which will fit into 
the available space and total required force, will determine 
which grade of spring is required. 
4) Determine the deflection. The required travel plus preload 
deflection will be the total deflection and will determine the 
length of spring required to stop within the allowable 
percentage of deflection limits. Allowance should be made for 
punch sharpening, which will increase deflections over a period 
of time. 
Selecta spring from the lowest (greatest deflection) load rating series from 
the table. It is important that the spring not be compressed beyond the 
specific limit for the highest-pressure spring of corresponding length and 
 56 
diameter. Then if the springs prove too “light” for the stripping force required, 
each one is replaced with the next higher rated springs. This will provide more 
stripping pressure without the need changing spring pockets, support rods 
and the like. 
 
 
 KNOCKOUTS 
Since the cut blank will be retained, by friction, in the die block, some means 
of ejecting on the ram upstroke must be provided. A knockout assembly 
consists of a plate, a push rod and a retaining collar. The plate is a loose fit 
with the die opening contour, and moves upward as the blank is cut. Attached 
to the plate, usually by rivets, is a heavy pushrod, which slides in a hole in the 
shank of the die set. This rod projects above the shank, and a collar retains 
and limits the stroke of the assembly. Now the assembly of the ram stroke, a 
knockout bar in the press will contact the pushrod and eject the blank. 
It is essential that the means of retaining the knockout assembly be 
secure, since serious damage would otherwise occur. 
 In the ejection of parts positive knockouts offer the following 
advantages over spring strippers where the part shape and the die selections 
allow their use: 
 57 
1) Automatic part disposal; the blank, ejected near the top of the ram 
stroke, can be blown to the back of the press, or the press may be 
inclined and the same result obtained. 
2) Lower die cost; knockouts are generally of lower cost than spring 
strippers. 
3) Positive action; knockouts do not stick as spring strippers 
occasionally do. 
4) Lower pressure requirements, since there are no heavy springs to 
be compressed during the ram descent. 
 
Fig. 5-35 shows a plain inverted compound die, is of the simplest type. 
It consists of an actuating plunger, knockout plate and a stop collar 
doweled to the plunger. Shedder D consists of a shouldered pin 
backed by a spring, which is confined by setscrew 
 
 58 
Scrap – Strip Trip Layout For Blanking 
In designing parts to be blanked from strip material, economical stock 
utilization is of high importance. The goal should be at least 75 per cent 
utilization. A very simple scrap-strip layout is shown in Fig. 6-1. 
 
 SCRAP ALLOWANCE 
 A scrap-strip layout having insufficient stock between the blank and 
the strip edge, and between blanks, will result in a weakened strip, subject to 
breakage and thereby causing misfeeds. Such troubles will cause unnecessary 
die maintenance owing to partial cuts, which defect the punches, resulting in 
nicked edges. The following formulas are used in calculating scrap-strip 
dimensions for all strips over 0.8 mm. thick. 
t = specified thickness of the material 
 B = 1.25 t when C is less than 64 mm 
 B = 1.5 t when C is 64 mm or longer 
 C = L + B, or lead of the die 
 59 
Example: A rectangular part, to be blanked from 1.5 mm thick 
steel (Manufacturers Standard) is 10 X 27 mm. If the scrap 
strip is developed as in Fig. 6-2, the solution is 
 t = 1.5 mm 
 B = 1.25 X 1.5 = 1.875 mm 
 C = 10 + 1.875 = 11.875 mm 
 W = 27 + 3.75 = 30.75 mm 
 Nearest commercial stock is 32 mm. Therefore, the 
distance B will equal 2.3mm. This is acceptable since it exceeds 
minimum requirements. 
 
 
 Minimum Scrap-Strip Allowance: If the material to be blanked is 
0.6 mm thick or less, the formulas above should not be used. Instead, 
dimension B is to be as follows: 
 Strip width W Dimension B 
 0 - 75 mm 1.3 mm 
 76 – 150 mm 2.4 mm 
 60 
 150 – 300 mm 3.2 mm 
 Over 300 mm 4.0 mm 
 Other Scrap-Strip Allowance Applications: 
Figure 6-3, 6-4 and 6-5 illustrates special allowances for one-
pass layouts: 
 
View D. For layouts with sharp corners of blanks adjacent, B = 
1.25 t. 
Fig. 6-4 Allowances for one-pass layouts. 
 61 
 
 Percentage of Stock Used: If the area of the part is divided by the 
area of the scrap strip used, the result will be the percentage of stock used. 
 If A = total area of strip used to produce a single 
blanked part, then 
 A = CW (Fig. 3-35), and a = area of the part = LH. 
 If C = 11.5 mm and W = 32 mm then A = 11.5 X 32 = 
368 mm² 
 If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 = 
256.6 mm² 
 Percentage of stock used: 
a 256.5 
 = = 70% approx. 
A 368 
 
 
 
 62 
EVOLUTION OF A BLANKING DIE 
 In the planning of a die, the examination of the part print immediately 
determines the shape and size of both punch and die as well as the working 
area of the die set. 
 
 Die Set Selection 
A commercially available standardized two-post die set with 150 mm overall 
dimensions side-to-side and front-to-back allows the available 76 mm. wide 
stock to be fed through it. It is large enough for mounting the blanking punch 
on the upper shoe (with the die mounted on the lower shoe) for producing the 
blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of 
from 100 to 225 mm. 
Since the stock, in this case was available only in a width of 76 mm the length 
of the blanked portions extended across the stock left a distance between the 
edges of the stock and the ends of the blank of 6 mm or twice the stock 
thickness; this allowance is satisfactory for the 3.2 mm stock. 
 Die Block Design 
 By the usual „rule-of thumb‟ method previously described, die block 
thickness (of tool steel) should be a minimum of 20 mm for a blanking 
 63 
perimeter up to 75 mm and 25 mm for a perimeter between 75 and 100 mm. 
For longer perimeters, die block thickness should be 32 mm. Die blocks are 
seldom thinner than 22 mm finished thickness to allow for grinding and for 
blind screw holes. Since the perimeter of the blank is approximately 178 mm 
a die block thickness of 38 mm was specified, including a 6 mm grinding 
allowance. 
 There should be a margin of 32 mm around the opening in the die 
block; its specified size of 150 x 150 mm allows a margin of 45 mm in which 
four M10 cap screws and dia. 10 mm dowels are located at the corners 20 
mm from the edges of the block. 
 The wall of the die opening is straight for a distance of 3.2 mm (stock 
thickness); below this portion or the straight, an angular clearance of 1½° 
allows the blank to drop through the die block without jamming. The 
dimensions of the die opening are the same as that of the blank; those of the 
punch are smaller by the clearance (6 per cent of stock thickness, or 2 mm), 
which result in the production of blanks to print (and die) size. 
 The top of the die was ground off a distance equal to stock thickness 
(Fig. 6-7) with the result that shearing of the stock starts at the ends of the 
die and progresses towards the center of the die, and less blanking pressure 
is required than if the top of the die where flat. 
 Punch Design 
 The shouldered punch (57 mm) long is held against a 6 mm thick 
hardened steel backup plate by a punch plate 20 mm thick) which is screwed 
and doweled to the upper shoe. The shut height of the die can be 
accommodated by a 32-ton (JIC Standard) open-back inclinable press, leaving 
a shut height of 240 mm. For the conditions of this case study, shear strength 
S = 42 kg/mm², blanked perimeter length L = 178 mm approx. and thickness 
T = 3.2 mm. 
From the equation P = SLT 
The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92tons. 
 64 
This value is well below the 32-ton capacity of the 
selected press. 
 The shut height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the 
punch into the die cavity. 
 Stripper Design 
 The stripper that was designed is of the fixed type with a channel or 
slot having a height equal to 1.5 times stock thickness and a width of 80 mm 
to allow for variations in the stock width of 75 mm. The same screws that 
hold the die block to the lower shoe fasten the stripper to the top of the die 
block. 
 If, instead of 3.2 mm stock, thin (0.8 mm) stock were 
to be blanked, a spring-loaded stripper would firmly hold the 
stock down on top of the die block and could, to some extent, 
flatten out wrinkles and waves in it. 
 
A spring-loaded stripper should clamp the stock until the punch is 
withdrawn from the stock. The pressure that strips the stock from the 
punch on the upstroke is difficult to evaluate exactly. A formula frequently 
used is 
 Ps = 2.5 x L x t kgs. 
 
 65 
 Where Ps = stripping pressure, in kgs. 
 L = perimeter of cut, in mm. 
 t = stock thickness, in mm. 
 Spring design is beyond the scope of this book; die 
spring data are available in the catalogues of spring 
manufacturers. 
 Stock Stops 
 The pin stop pressed in the die block is the simplest method for 
stopping the hand-fed strip. The right-hand edge of the blanked opening is 
pushed against the pin before descent of the ram and the blanking of the next 
blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked 
opening to ride over the pin and to engage the right-hand edge of every 
successive opening. 
 The design of various types of stops adapted for manual and automatic 
feeding is covered in a preceding discussion. 
EVOLUTION OF A PROGRESSIVE BLANKING DIE 
 Figure 6-8 gives the blanked dimensions of a linkage case cover of cold 
rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts 
made at one setup, with the possibility of three or four runs per year. 
 66 
 
Step 1, Part Specification 
1. The production is of medium class; therefore a second-class die 
will be used. 
2. Tolerances required: Except for location of the slots, all 
dimensions are in fractions. The slot locations, though specified 
in decimals, are not very close. Thus a compound die is not 
necessary; a two or three-station progressive die will be 
adequate. 
3. Type of press to be used: Available for this production are 
presses of 5-ton, 8-ton, or 10-ton capacity, with a shut height 
of 175 or 200 mm. 
4. Thickness of material: Specified as 32 mm standard cold rolled 
steel. 
 
 67 
Step 2, Scrap-Strip Development 
 From the production requirements, a single-row strip will suffice. After 
several trials, the scrap strip shown in Fig. 6-9 was decided upon. Owing to 
the closeness of the holes it was decided to make a four-station die. 
 
The scrap strip would be fed into the first finger stop, and the center hole 
would be pierced. The strip would then be moved in to the second finger 
stop, and the two holes would then be pierced. At the third stage and third 
finger stop, a pilot would locate the strip and the four corner holes would then 
be pierced. At the fourth and final stage, a piloted blanking punch would blank 
out the finished part. 
Step 3, Press Tonnage 
 It is now in order to determine the amount of pressure needed. Only 
the actual blanking in the fourth stage need be calculated, since the work in 
the first three stages will be done by stepped punches. 
 From Table, the shear strength S of cold rolled steel is 40 kgs/mm². 
The length L of the blanked perimeter equals 60 x 4 = 240 mm. The depth of 
cut (stock thickness t) equals 3.2 mm. From the equation P = S L t 
 68 
 P = 40 kgs./mm² x 240 mm x 3.2 mm = 30,720 
kgs. Or 30.7 tons. 
 This tonnage is greater than can be handled by the available presses. 
To lower the pressure, shear is ground on the blanking punch to reduce the 
needed pressure by on third. This, ? x 30.7 = 30.7 - 10.2 = 20.5 tons. A 
punch press of 25-ton capacity would do, but there is reported available only 
a 30-ton press with a 190 mm shut height and a 50 mm stroke. This press is 
selected. The bolster plate is found to be 300 mm deep, 140 mm from 
centerline of ram to back edge of bolster, and 600 mm wide. Shank diameter 
is 64 mm. 
Step 4, Calculation of the Die 
(a) The die. The perimeter of the cut equals 240 mm and therefore the 
thickness of the die must be 25 nm. The width of our scarp-strip opening is 60 
mm with 32 mm extra material on each side of the opening, it will be 60 mm 
+ 64 mm = 124 mm or 130 mm width. The distance from the left side of the 
opening in stage 4 to the edge of the opening in stage 1 equals 3 C + 30 + 6 
= 192 + 30 + 6 = 228 mm and plus 62 mm = 290 mm or 296 mm long. 
Therefore the die should be 2.5 x 130 x 296 mm long. 
(b) The die plate. As a means of filling in between the die and the die shoe, 
a die plate of machinery steel is used. To secure the die plate to the die shoe 
M12 cap screws and dowels are used. A minimum of twice the size of the cap 
screw for the distance from the edge of the die to the edge of the die plate is 
needed, which will equal 25 mm. Twice this distance = 50 mm and 50 mm 
added to the size of the die will result in a die plate of 25 x 180 x 346 mm. 
Figure 6-10 shows the die and die plate fitted together, and with the holes, 
which show the sharpening portion and the relief portion. 
 69 
 
Step 5, Calculation of Punches 
 Good practice requires 10 per cent of the metal thickness to be 
removed from the basic dimension of the blanking punch. This same value is 
used on the die opening, since holes are to be pierced in the blank. The 
clearance rule will be applied to the die opening in Stages 1, 2, and 3, and to 
the punch in Stage 4 (see fig6-11). 
For Stage 4: Blank to be 60 mm square, 
Stock thickness = 3.2 mm; 10% = 0.32 mm. 
Punch = 60 – 0.32 = 59.68 mm. 
 Therefore the die opening will equal 60.01 to 60 mm and 
the punch will equal 59.68 to 59.67 mm. 
For Stage 2: Slot to be 8 mm wide by 34 mm long. 
Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm. 
Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to 
34.00 mm. long. 
 70 
 The punch and the die opening will have straight sides for at least 
3-11 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for 
the punches of Stage 2, and also the stepped arrangement of the punches for 
all stages 
 
Step 6, springs 
A solid stripper plate can be used for this job. 
Step 7, Piloting 
 Figures 6-9 and 6-11 illustrate the arrangement for piloting. In this 
case it is direct piloting. However, if the part did not have a center hole, and 
the slots and other holes were too small, indirect piloting would have to be 
provided. 
Step 8, Automatic Stops 
 Finger stops will act as stops when a new scrap strip is being inserted 
but, after that, an automatic spring drop stop must be used to halt the scrap 
strip. Figures 6-12 illustrate details of the completed drawing of the die. 
 71 
 
Bending, Forming And Drawing Dies 
 BENDING DIES 
 Bending is the uniform straining of material, usually flat sheet or strip 
metal, around a straight axis, which lies in the neutral plane and normal to 
the lengthwise direction of the sheet or strip. Metal flow takes place within the 
plastic range of the metal, so that the bend retains a permanent set after 
removal of the applied stress. The