Blanking Clearance and Punch Velocity Effects on The Sheared Edge Characteristic in Micro-Blanking of Commercially Pure Copper Sheet

This study aims to identify the influences between clearance and punch velocity on the part edge quality of blanked parts. Experiments have been conducted using material copper, punch-die clearance and punch velocity variations. In order to determine the reachable punch-die clearance and punch velocity required for blanking. The quality of the part-edge characteristics shows that higher punch velocity and decreases clearance value can improve the part-edge quality, resulting in smaller burr height and rollover, and a larger shear zone. Furthermore, it could be observed that the part-edge quality improvement when blanking with high punch velocity is much more distinct for stele than for copper. According to blanking theory, this improvement was expected because copper have much higher heat conduction coefficients. Therefore, the heat dissipates faster and the desired stress relief effect does not take place to the same degree as for stele. Copyright © 2017Journal of Mechanical Engineering Science and Technology All rights reserved


I. Introduction.
Micro-components have been widely used in many industry clusters such as electronics, healthcare, aerospace, biomedicine and automobiles [1].The miniaturization and weight reduction of various industrial products have become advanced, and higher accuracy and quality are strongly demanded for the parts needed for constructing these products [2].
One of the popular and highly-in-demand forming processes is shearing.Shearinghas been defined as a process employed in the manufacturing of metal parts with a specific design from sheetmetal stock and includes a wide variety of operations such as punching, blanking, embossing, bending, flanging and coining [3].The process of micro-components is performed by not only shearing, but also by EDM, laser, and etching [4].
The blanking process has long been studied by several researchers for a wide variety of materials or workpieces as well as tool conditions to improve the quality of blanking products, especially the quality of the material surfaceon the micro-hole fabrication by mechanical punching process [5].The accuracy of the machine from the blanking process is evaluated by the surface quality and dimension accuration.At the same time, the main factors that affect the quality of the blanked edge, according to [6], [7], [3] and [8]are clearance punch-die and punch velocity.
The high-volume production of micro-components should be the main goal for the design of micro-manufacturing [9].However, the blanking process is expected to be widely used, because of its high productivity and machining stability.This study discussesoptimization of clearance and the potential punch velocity as process parameters to influence product quality of tested material.

A. Micro Punch CNC MachineTools
One of the products developed is Micro Punch CNC Machine.Micro Punch CNC Machine is a combination of press dies machine with CNC (Computer Numerically Controlled) driving system as shown in figures 1 (a).Press dies can produce a part with uniform quality with minimum machining time.The working principle of press dies is using penetration between punch and d The alignment accuracy of punch and die Micro Punch CNC machine up to 10 μm with the maximum force of 1663 N. In addition, the blanking process on press dies system has several advantages, including it can produce a uniform sheared edge quality and size products with lower cost in the manufacturing of mass products.

B. Materials
The specimen used for the blanking process were strips of commercially pure copper sheet (99.6%) with thickness of 500 µm.Punch tool using High Speed Steel (HSS) materials type Nachi Standard with a Rockwell C hardness of 64 HRC (figure 1 (b))while the die using Mild Steel (figure 1 (c)).Mechanical properties of experimental setupare shown in Table 1.

C. Force Calculation
The equation to measure the total force required to blanking the specimen(copper) with circular section is as follows.
(1) Known that shear strength ( ) of the material is 0.7 times the tensile strength (TS) of 210 MPa.The obtained ( ) is 147 MPa.It can be obtained that the force (P) needed to blanking the specimen with t: 500 µm is 182.3N. Based on the calculation of the force required for punching process as compare to optimal punching machines, it can be concluded that the maximum force of 1663 N may allow blanking process on the specimenwith the required maximum force of 182.3 N.

D. Punch-die Clearance
Clearances used for this study were 2.5, 5, 7.5 and 10%. Figure 2 describes clearance punch and die used in this study.
The calculation of punch size variations based on the following formula.(2) where d p is the external diameter of the punch, d d is the internal diameter of the die, and t is the thickness of the specimen.The punch diameter variations result are shown in table 2 below.

E. Testing Parameters and Measurement Method
The experiment was conducted at room temperature.Punch velocity (ʋ) of Micro Punch CNC Machine used 100, 500, 2000 and 2600 mm/min.In order to complete the accurate analysis of the blanking process there are performed four times to obtain the most valid data, and follow with clearance variation.The accuracy alignment measurement uses "Dino-Lite Edge AM 4515 T5" camera, featured with an LED light source that is located on the side of the punch serves as illumination during alignment process.The alignment between the punch and die is very important before doing the experiment, aimed to minimize the tool wear and to obtain a same sheared edge on the circumference of a circle.
The amount of shear zone from section of the slug were obtained using an optical microscope camera (Type Olympus C-35AD-4).To observe the validity of shear zones each slug has been measured at 5 different locations around the circumference (see figure 4).

III. Results and discussion.
Table 1 shows that shear zone increases according to the increasing of punch velocity and decreasing the clearance value.At the highest speed (2600 mm/min) with the 2.5% clearance the shear zone up to 71.58%.Consequently, the sheared surface is smoother and exhibits less burr formation.At low speed (100 mm/min) with the same clearance value indicates shear zone value of 58.42%.At the clearance value of 10% does not show the surface of the shear zone at all.Edge quality can be improved with increasing punch velocity, speeds may be as high as 2000 to 2600 mm/min.In Figure 4 (c), (d) sheared edges can undergo severe cold working due to the high shear strains involved.Figure 5 shows at 5% clearance the surface of the shear zone has a proportion below 50%.This is because the clearance value ratio is too large for copper sheet thickness of 500 μm, thuspart edge characteristic for the burr height and rollover will have a high enough proportion.
The study with clearance 7.5% (figure 6) was only 1 sample test at a speed of 2600 mm / min which has a smooth surface, the other velocity samples do not show any sheared edge, the surface looks rough and dull.
It can be noted from figure 7 that the shear zone is null for 10% clearance in all punch velocity value.As the clearance value increases the zone of deformation becomes larger and the sheared edge becomes rougher.The sheet tends to be pulled into the clearance region, and the perimeter or edges of the shear zone become rougher.This present Research conducted in accordance with research that has been done previously by [4], [10] and [11]increasing clearancevaluebetween punch-die has an effect: a) rollover increase; b); small shear zone; c) bigger ruptureand; d) increasing burr.With increased on clearance, the deformation area becomes larger and the shearing zone becomes rough.A rough shearing zone can be accepted as a product, but continued operation may be needed to make the blanked edge smoother, this may increase production costs.

IV. Conclusion
A good clearance design not only increasing the quality of product manufactured, but also reduces burr height.Influence clearance and potential punch velocity in this research showed good agreement with [4].The ideal outcome of part-edge characteristics is having small rollover and burr and having at least 75% of shear zone.This can be achieved with clearance value of 2.5%.

Table 1 .
Mechanical properties of experimental setup

Copper sheet) Punch tool (High Speed Steel) Die (Mild Steel)
Fig. 2. Punch and die Clearance Fig. 3. Form errors on sheared specimen, t: sheet thickness,te: rollover, ts: shear zone and hb: burr height

Table 2 .
Variations in diameter punch against clearance

Table 3 .
The shear zone results of the experimental measurement for commercially pure copper sheet