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#162 Problems in Punching and their Countermeasures (1) Punching Burrs

Category : Die Design
June21, 2013

In common punching operations, the cut surface is formed in the material in the sequence of droop → shear cross-section → fracture cross-section → burr, as shown in Fig. 1.

Fig. 1

When a soft steel plate (SPC steel) which is used most frequently in press forming is cut with a standard clearance, the length of the shear cross-section becomes about 1/3 of the material plate thickness, and the height of the burr becomes about 0.03 to 0.05. In case this amount of burrs is considered to be within normal tolerable range, this was called "return" thereby distinguishing it from abnormalities.

However, due to the product liability laws in recent years, even a small burr is considered to be a cause of scratch injury to hands or fingers, and it is being required more and more frequently that the burrs be removed by surface flattening.

Burrs should be made extremely small. Depending on the material being formed, the manner in which burrs appear differs. Burrs can occur easily in the case of soft materials because their elongation is large. Therefore, the punching clearance is made small. Since hard materials can break easily (brittle), there is no problem even if the clearance is made larger (the punching force becomes smaller as the clearance becomes larger). In the case of brasses, increasing the amount of zinc makes the elongation smaller and the material more brittle. The amount of zinc is controlled as a countermeasure in punching.

Burrs become small when an appropriate clearance is used. Therefore, the clearance is set so that it becomes uniform for the shape being punched. This is the basic attitude when preparing a punching die. However, even if a uniform clearance is set, there are shapes in which burrs appear quickly. As shown in Fig. 2, this is pronounced at the apexes of sharp projection or recess parts, which are followed by corners that do not have rounding.

Fig. 2

The cause of this is the chipping of the corners of the punch or the die. Chipping occurs due to combined effects of the shape and the punching conditions. Even when an appropriate clearance is used, the punching conditions are bad in the corner parts (the punching conditions will be similar to when the clearance is small), and the cut surface will not be normal. The countermeasure is to make sure that the punching conditions are the same as in the straight line parts. There are two methods for this, one of which is to round the corners (corners with an R of 1/2 of the material plate thickness or more), and the other is to increase the clearance.

Burrs are also generated due to the relationship between the punch and the die. This is shown in Fig. 3.

Fig. 3

Lopsided and twisted clearances are the commonly found phenomena.

The checking of the clearance of a punching die can be made by punching a paper or vinyl sheet. A paper or vinyl sheet is placed on the die, the punch is stopped just when it has entered into the die by a very small amount, and the condition of punching of the paper or vinyl sheet is inspected. The parts that have been cut neatly are the parts where the clearance is small. The clearance is large where the fibers of the paper are remaining uncut or where the vinyl sheet has been cut so that it is stretched. Adjust so that the cutting is uniform.

Apart from the above, there are other causes of burr generation. The above are only the typical causes of burrs.

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