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The process of punching a soft material uses a hallow punch with beveled and sharpened edges that is driven into the material. The backing material is usually a soft metal or wood.

I am working on such a process and have a few questions for the community. I am punching 3 mm discs of thickness 0.2 mm from a elastomer material with tensile modulus of 220 MPa and a max tensile strength of 35 MPa. The disc remains within the hallow punch and is extracted afterwards.

I have measured the force required to punch using a force gauge and found that 140 N was required to punch and retain the disk.

The end process will use a pneumatic cylinder to drive the punch.

1) Is the punch force calculation the following - "Punch Force = perimeter x thickness x shear stress"? Can the shear stress be equated to the materials tensile strength? Using the above, I get a punch force of 66 N. This is well below what is measured using the gauge. Why?

2) What is the effect of a blunt or dull blade on the punch force? Hows does the blades increase in contact area influence the punch force (shear)?

3) In the above example, what is the relationship between the normal force/pressure to the punch force?

4) How would you select a backing material? Softer than the punch's material (HSS Steel), but not too soft as to deform when struck by the punch?

Thanks.

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  • $\begingroup$ Does your theoretical calculation include the real thickness of the cutting edge as it getss thicker or is it based on a theoretical blade infinitely thin? $\endgroup$
    – Solar Mike
    Aug 18 '18 at 21:58
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    $\begingroup$ Also, why not drive the cutter down with a cam? And you need a hard surface that will not deform as it absorbs will flex or compress away... $\endgroup$
    – Solar Mike
    Aug 18 '18 at 21:59
  • $\begingroup$ Hi Mike, I would like to understand the effect of the punch's leading edge area (i.e. the blades area). Would you say that for an infinitely thin punch, the punch force of 66 N would apply? However, in practice the forces are 2 x this due to other effects such as blade dullness and the effects described by Chris. $\endgroup$
    – Aon
    Aug 18 '18 at 22:33
  • $\begingroup$ Between my comments and Chris' answer you should be able to figure it out... $\endgroup$
    – Solar Mike
    Aug 18 '18 at 22:41
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The force requires will indeed depends on the sharpness of the tool and indeed its overall geometry.

This is complicated by the fact that, as the punch blade as finite thickness you are also pushing material out of the way and have friction between the tool and the sides of the cut.

Equally 'sharpness' isn't really a very well defined property.

Cutting a soft material with a wad punch is a somewhat different situation from punching say sheet metal where you are shearing the material. Resistance to cutting with a blade isn't straightforwardly related to tensile strength. Rate effects may be significant too. So I don't think that formula you quote is very appropriate in this context.

There are a few sensible options for the backing board, hard rubber, particle board, end-grain hardwood, polypropylene etc, whatever you use you are probably going to want to be able to change it fairly frequently.

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