Shearing_UFRGS

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In Die Supervision of Shearing Process 
 
A. Richter, J. H. Corrêa de Souza, L. Schaeffer 
Universidade Federal do Rio Grande do Sul - Laboratório de Transformação Mecânica, 
Porto Alegre, Brazil 
 
 
Abstract 
 
All sheet-metal parts are exposed to at least one shearing operation, even if it is only to 
separate the blank from the strip or to trim the border of the finished part. Therefore the 
shearing process is one of the most important processes in the manufacturing of sheet-metal 
products. In many cases is a basic requirement for the part to leave the process entirely 
finished. Additional processes like tearing of the burr, planification and so on have to be 
avoided to reduce the number of operations and the costs of production. Shearing processes, 
which serve for the production of parts are generally executed by tools which are driven by 
presses which make a simple linear movement. This implies the use of process supervision, 
which is able to measure the load on the tools and the displacement of the active parts of these 
tools. Systems of this type were designed and used for the supervision of different cold- and 
sheet-metal-forming processes. The paper describes some experiences, which were made in the 
supervision of the shearing process. 
 
Key-words: shearing, quality, supervision 
 
 
1 Introduction 
 
Modern production requires constant supervision of all factors, which have influence on the 
product quality. With high production rates, the quality of the resulting part in each production 
step depends on the domination of the process: this means the perfect state of the tools, the 
constant properties of the material and the use of an adequate tool-machine which offers high 
accuracy at high productivity [1,2]. 
 
In shearing operations the quality of the fabricated parts is principally determined by the edge 
of the parts. The burr, the sheared and the broken zone are important indicators for whether or 
not the process was realized under optimum conditions or not. Figure 1 shows a section of a 
cut edge with the denomination of the relevant parameters. 
 
In many products the presence of burr interferes in following production steps like stapling, 
painting and other mounting or handling operations. In products, which are intended to be used 
without further processing, it is to be determined whether the presence of burr acceptable. 
Burr free shearing depends on the die clearance and the state of the cutting edge. In many 
cases an expensive precautionous maintenance of the tools is used to guarantee the perfect 
conditions of the tools and the process. The process supervision demonstrated in this paper is 
intended to reduce precautionous maintenance by the permanent monitoring of the shearing 
process. This type of supervision allows an automatic judgement of the tool state without the 
examination of the fabricated parts and the disassembly of the tool [3,4]. 
 
b
c
d
e
a
f
g
 
 
Figure 1: Sectional view of a cut edge: a,b) rounded zone, c) sheared zone, 
d) broken zone, e) burr, f) deflection, g) sheet thickness. 
 
 
2 Equipment used and experimental set-up 
 
2.1 The shearing tool and its instrumentation 
 
Research was carried out using a shearing tool, which was mounted in a 40t press and a 
process supervision system. The shearing tool executes a circular closed cut with a diameter of 
25 mm. It is mounted in a two post, three plate die set. The two guide-posts are fixed in the 
lower plate, with the intermediate and the upper plate guided by bushes. The punch is fixed on 
the upper plate, and the die in the lower plate. The intermediate plate carries the blankholder 
and serves as guide for the punch. Punch and die are fixed with ball lock retainers, which allow 
quick exchanges without loosing tool alignment. The blankholder pressure and the extraction 
of the punch are executed with the force of two helicoidal springs (Figure 2) [5]. 
 
 
 
 
 
Figure 2: Photo of the instrumented shearing tool which was used for the research. 
 
The design allows the utilization of various types of blankholder springs, such as elastomer 
coil springs or conical spring washers. The tool is completely decoupled from the press, which 
only supplies the force for the cutting process and for the compression of the springs, to 
eliminate deflection of the posts and misalignment of punch and die. The return of the tool 
after the process is executed by the force of two smaller helicoidal springs that elevate the 
movable parts to the initial position. 
 
The forces to be measured by the sensors installed in the tool are the punching force, i.e., the 
cutting force, and the blankholder force, i.e., the force that acts over the blank area due to the 
springs before punching. The punching force sensor is placed directly over the punch, as 
shown in fig. 3-a, and the blankholder force sensor under one of the tool springs, as shown fig. 
3-b, therefore measuring one half of the total force transmitted to the blankholder by the 
springs. As fig. 3 shows, both transducers are designed to be inserted into the tool plates, with 
their dimensions and locations inside the tool defined by the forces to be measured and by the 
tool components that will transmit these forces. 
 
The tool is equipped with a resistive displacement transducer, which is placed at the right side, 
out of the post. fig 3-c. The displacement transducer measures the relative movement between 
the punch and the die. 
 
 
 
Figure 3: Position of the load cells for the force measurement 
and the displacement transducer [5]. 
1) punch, 2) die, 3) punch retainer, 4) die retainer, 5) shock plate of the punch load cell, 
6) die shock plate, 7) punch guide, 8) blankholder, 9) blankholder spring, 10) return spring, 
11) return spring guide, 12) guide post, 13) upper plate, 14) intermediate plate, 
15) lower plate, 16) base plate, 17) and 18) bushes 
 
The sensors are connected to a process supervision system, which is described below. 
 
 
2.2 Process supervision 
 
The process supervision system is shown in figure 4. It allows the registration and supervision 
of up to five force channels, which can be evaluated and displayed in diagrams force vs. time 
or force vs. displacement. The system consists of a microprocessor based, easy to operate 
central unit, which accommodates amplifiers, analogue to digital converters and the 
microprocessor. Its front panel is occupied by the display and some controls. The rear panel 
contains the connectors for the sensors, the interface for a printer and an exit, which allows to 
switch off the press in case of detection of an error in the process [6,7]. 
 
 
 
 
 
Figure 4: View of the procontrol system which was used for the supervision 
of the shearing process [6]. 
 
The supervision is based on the fact that differences in the process, which result in differences 
in the product quality, can be seen in its load vs. displacement curve, also called the process 
signature. The perfect function of systems of this type depends on the reproducibility of the 
process [8,9] 
 
Figure 5 shows the curve of a perfect shearing process, which was executed with a punch and 
die with sharp cutting edges at the required clearance of 5%. The other three curves in this 
diagram show curves of processes which were executed under non ideal conditions: errors in 
the die clearance and a chamfer at the cutting edge of the punch which serves to simulate a 
wear situation [5]. 
 
All these curves were measured during the cutting of specimen of an austenitic stainless steel-
sheet (ASTM 304, X 5 CrNi 18 10, W.-Nr. 1.4301). The samples had a thickness of 1,0 mm. 
The beginning of the measurement was triggered by the displacement signal. 
 
The supervision is conducted as the comparison of the signature of a process, which was 
executed under perfect conditions with the curve of every new process. Therefore the curve of 
a perfect process has to be measured and saved for reference inthe system. The comparison is 
then realized by the use of upper and lower limit curves which have to defined by the user 
(figure 6). The crossing of these limit curves causes an error message and the switch off of the 
press. The error has to be confirmed by the user before the production can be restarted. 
 
0
2
4
6
8
10
12
14
16
18
20
0,00 0,50 1,00 1,50 2,00 2,50 3,00
displacement in mm
fo
rc
e 
in
 k
N
 5% clearance
 10% clearance
 20% clearance
 worn punch
 
 
Figure 5: Load vs. displacement curve of a shearing process which was executed under 
perfect conditions and other processes which show different errors. 
 
 
0
2
4
6
8
10
12
14
16
18
20
0,00 0,50 1,00 1,50 2,00 2,50 3,00
displacement in mm
fo
rc
e 
in
 k
N
 reference curve
 upper an lower warning limit
 upper and lower error limit
 switch-off limit
 
 
Figure 6: Load vs. displacement curve of a shearing process with error-limit curves. 
 
 
3 Error recognition 
 
The quality of the sheared surface is principally influenced by the die clearance and by the 
wear situation of the tools. Some examples of curves, which were measured with 
combinations of punches and dies with different die clearances are shown in figure 5. This 
figure also demonstrates an example of a shearing operation, which was executed with a worn 
punch. All of these errors result in a higher burr a thinner sheared and a wider broken part. 
 
The function of the error recognition is shown in figure 7. This figure shows the reference 
curve of a process, which was executed under perfect conditions (sharp cutting edges at punch 
and die and 5% die clearance), the limits for error and warning and the curve of a process, 
which was executed with a tool with a worn punch and a sharp die. The die clearance was 5%. 
 
0
2
4
6
8
10
12
14
16
18
20
0,00 0,50 1,00 1,50 2,00 2,50 3,00
displacement in mm
fo
rc
e 
in
 k
N
process curve
hits error limits
 
 
Figure 7: Error detection in a shearing process. 
 
It can be seen that the longer flow of the material, which is caused by the worn cutting edge 
results in a lengthening of the curve and causes a pass of the error limits. This results in an 
error message and the switch off of the tool machine. 
 
 
4 Conclusions 
 
The results of the process measurements show that it is possible to detect and recognize 
different tool states by the evaluation of the process signature. This allows the use of the same 
concept of supervision system, which is already recognised for other sheet-metal-forming 
processes. Future research has to show the limits in the detection especially of small errors, 
which have little influence on the shearing force. 
 
5 References 
 
[1] Brüninghaus, G., Prozeßüberwachung an Stanzautomaten, Blech Rohre Profile 40 
(1993) 7/8 pp. 580 – 584 
 
[2] Klutz, H.-J., Messen und Regeln im Stanzprozeß, VDI-Z-Special Blechbearbeitung 
October 1993, pp 14 - 21 
 
[3] König, W., Herres, W. U., Werkstückfehler anhand meßbarer Prozeßkenngrößen 
erkennen, Industrie-Anzeiger 34 (1988) pp. 31 – 33 
 
[4] Breitling, J.; Pfeiffer, B.; Altan, T. e Siegert, K. Process Control in Blanking, Journal 
of Materials Processing Technology 71 (1997), pp 187 - 192 
 
[5] Richter A., Souza, J.H.C., Schaeffer, L., Ferramenta Instrumentada para Pesquisas 
em Processos de Cisalhamento de Chapas, Proceedings of the III National Conference 
of Sheet Metal Forming, Porto Alegre - Brazil, October 24th - 27th 2000, pp 60 - 67 
 
[6] Richter, A., Liebig, H. P., Controle de Qualidade em Processos de Estampagem, 
Proceedings of the 55th Annual Congress of the Brazilian Association of Metalurgy and 
Materials. Rio de Janeiro July 24th - 28th 2000 
 
[7] Liebig, H.P.; Bober, J.; Richter, A., Prozeßüberwachung in der Druckfügetechnik, 
Bänder Bleche Rohre 37 (1996) 9, pp 24 - 28 
 
[8] Blümel, K. W.; Hartmann, G. e Lübeck, P. Reproduzierbarkeit des Umformprozesses, 
Stahl und Eisen 108 (1988) Nr. 25,26, pp 1237 - 1241 
 
[9] Richter, A., Möglichkeiten und Grenzen der Prozeßüberwachung zur 
Qualitätssicherung beim Fügen durch Umformen Doctors Thesis, Technische 
Universität Hamburg-Harburg 1997