Question on punching

[Archie Zietman]

Hello.
I just spent an hour punching as many holes of different sizes as i could in 1" by 1/4" flatstock. Looking at them now, even the best of the holes have ever so slightly ragged insides to them where the "biscuit" got whacked out. I crowned the edges of my punches a good bit, maybe too much on the smaller punches, would this leave a raggedy inside to punched holes?

thanks,
Archie

[Blacksmith Jim]

Someone with more experience, etc., may come along and say something else.. But I don't think you want your punches crowned at the edges at all. You want them sharp so they can cut through the metal.

[Frosty]

A punch shears through the steel and there will be roughness in the resulting hole. If you want smooth, punch undersize and drift to dimension.

And yes, sharp flat or better yet, hollow ground works better than crowned let alone pointed.

Frosty

[Ian]

Archie,
I was told that punches should have the edges of the face slightly radiused so as not to be square and sharp, because the metal actually flows around the head of the punch as you drive it in. A square edge on the face can leave a rough hole when you punch out the biscuit.
Driving the punch too far through on one side can also cause problems, when you come to turn the bar over and punch the thinner side it can stretch more before it shears, you've got to get the first side about 2/3 of the way through, then flip the bar. If you choose the right heat for the final tap the biscuit shears off, not stretches and gives like warm toffee if that makes sense.
Finally using a drift to size the hole will give you the clean smooth finish you want.

[Rob Browne]

I understood that drifting was part of the procedure. You punched the original hole then drifted to size. The the finish is always good and controlled.

Another thing to use to make really neat holes is a punching plate. Just a bit of plate steel with holes of varying sizes in it that is placed over the pritchel hole to help stop the distortion of your work while drifting and punching.

Edited November 17, 2008 by rmcpb

[Sam Thompson]

The correct temperature has always seemed important to me. It's difficult to knock the biscuit out if you have to take a heat when you turn over. If you need two heats, take the second during punching the first side and cool the punch so that there is a pronounced black (cool) spot on the bottom where the steel has been squeezed against the face of the anvil when you turn over. Cooling the punch and knocking through the hole need to be done as quickly as possible, before the biscuit warms up.