The main reason I can see of using a rounded punch is to avoid the possibility of cracks / risers when using a punch with sharp corners. As can be seen at about 4.30. You use hot metal to avoid them developing but I can see the situation where 'a few more bows won't harm' causing harm.
Thank you for the reply you make a good point, I find for this application a small radius on the corners is all you need to avoid a stress riser, about a 16th of an inch, you have to also consider when overlapping two cuts those radii create sharp corners as well (protrusions) that can lead to a cold shut, like anything else it’s a balancing act. the radius in the corners doesn’t add to the cutting pressure that much cutting pressure is just a matter of the periphery of the punch. The difference between the two punches I used in the video are their overall periphery. The smaller periphery equates to less cutting pressure, therefore less displacement, causing a taper which leads to the burr which leads to mechanical removal of the burr, it’s a compounding problem. Time is money as they say and it’s put most limited resource.
The secret to cleanly punched holes or slots is primarily the temperature. Most beginners punch the slug out at too high a heat, so the material doesn’t shear off cleanly, leaving an ugly burr.
I think the technical difference between the two punches is more akin to light hammer blows acting on the surface of the workpiece. I can post a follow up showing the punched hole prior to shearing the plug, the “old” punch leaves a tapered hole that doesn’t support the shearing action and results in the rag. That’s just my hypothesis anyway…thank you for your valid points as well, as I did in the video I try to shear my plugs out with the residual heat.
Bravo and awesome tip- thanks for sharing! Would like to ask if your 1" drift was 1" you drew out or if you upset a 7/8 piece and tapered??? Subscribed!
