Sunday, September 18, 2011

Hot Punching

I like to design projects which use hot punching as a technique. There is a little bit of a learning curve to climb before it gets easy but when mastered it is fun to do and the swell around the hole looks nice.

In the earliest days of my metal working experience I visited a smith who owned a historic shop and he offered some advice based on his 30 years of working. I still remember some of those things and one was to keep my punches sharp. I really didn’t understand the significance of that at the time but I became a believer through practice.

Usually I try to set up punching work when I have an assistant to either strike or to hold. Most of the holes I punch are 5/16” or 3/8” round by design. With holes smaller than 5/16” there is more risk of the small diameter punch drawing so much heat it reaches forging temperature and deforms. With holes larger than 3/8” a lot more force is required to create the hole.

If hole position must be very precise I will make a light mark and check the position and make adjustments and eventually set a shallow depression which can be seen of felt at punching heat.

A six pound sledge is my tool of choice for striking. I try to avoid placing the hot workpiece on the anvil until the punch is ready to place so the anvil doesn’t suck heat away prematurely. When the punch is in proper position, vertical and still it is time for a firm blow with the sledge. The punch is rotated about 90Âș while the hammer is lifted. The the punch is held steady and struck again once of twice until it is felt to “bottom out” against the anvil. Then the punch is cooled in the quench tub.

If the work piece is turned over the darker, cooler, circle is visible if the punch sunk deep enough to form a very thin area which will be the slug. This is where it gets interesting to me. If the punch face is flat and the edge sharp and the workpiece is cool enough when the punch is centered on the dark circle and struck the slug will fall out. In other words, if every thing is just right, it works perfectly.

If the punch is not sharp it acts like a fuller and moves metal back and forth without creating a stress riser perimeter which will break in shear. If the workpiece is too hot the slug mass perimeter will stretch and not shear. With enough practice this gets easy but, for me, never uninteresting. I just notice smaller and smaller details.

It seems like a trivial thing to rotate the round punch but it is an important detail if the hole is to turn out close to perfectly round. It’s sort of hard to dress a round punch so that it is perfectly round. Usually it is a little bit oval. If that is the case the hole will still be round if the punch is rotated while the workpiece is at a forging heat. If the punch is too oval it will be too difficult to twist so there is a limit to the our-of-round which is tolerable.

The body of the punch has a slight taper so the finished hole may have a very slight hour glass shape and each hole may not be quite the same diameter. Finishing holes with carefully made drifts corrects those imperfections.

I own a couple of square handled punches but have rarely used them to punch holes. I don't use many carriage bolts or make many square passthroughs. If I need a square hole I usually make a round one and use the square punch as a drift. Or can it really be called a drift if the tool doesn’t pass all the way through? Maybe it's a mandrel. It could be an opening punch but that sounds like a segue to boxing.

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