Friday, June 20, 2014

Guest Post: Making Center and Prick Punches

The best prick & center punches you'll ever use

by Firehand

(Editor's note: Firehand is a blogger and a part-time blacksmith. His previous article, An Introduction to Blacksmithing, may be found here.)

Or at least equal to the best.

I've made punches and chisels out of a lot of stuff; some worked pretty well, some not so much. The best have been made out of Star drills.

If you've never had to use one, be glad. These are how masons cut holes in stone before power tools and suitable bits were available. You can still find them new, or at flea markets, and they're excellent steel for the purpose, and can be found in a number of diameters.

Materials Needed

First, you'll need:
  • Star drills
  • Cutting torch (propane or acetylene will do)
  • Hacksaw
  • Some way to grind to shape.

Crafting Your Punches

Decide how long you want your punch to be and mark where you'll need to cut.

1) You're probably going to have to heat that area enough to anneal it (get it hot enough to 'soften' it by removing the hardness set up by the original heat-treating). Check it with the hacksaw; if it's too hard to cut or will only barely cut, get out the torch.

In a place where the light is a bit dim, start heating that area. You don't have to get it to a bright red heat, just a barely-visible red (which is why the dim light). Turn the piece as you heat, when you've got that color just set it aside to cool. Or if you have another area to anneal, do it now.

2) Once cool, clamp it in a vise and cut off.

You'll have to grind two bevels: a long one leading up to the actual point, then the point itself. I've got a belt/disc sander and use it. And here's how to keep the taper nice and even:

3) Chuck it in the drill.

If the end you'll be hitting with the hammer is sharp-cornered, you'll want to round it off a bit first, then turn the piece around to cut the bevels.

4) Hold it at a shallow angle and start cutting.

The nice thing about making this is you don't have to worry about keeping it from getting too hot, as you'll be hardening and tempering it later.

5) When you have the main bevel cut, you can shape the point.

The difference between a prick punch and a center punch is the angle: a prick punch has a longer, sharper taper; you use it to carefully mark the exact spot to drill with just a light tap with the hammer. A center punch has a shorter, wider taper; you use it with a suitable whack to make that starting point deep and wide enough that the bit won't walk when you start drilling.

6) Adjust your angle to suit, and cut the point.


Once that's done, you can heat-treat it. You'll need the torch and a can with some light oil in it; motor oil will work fine. I've heard of people using corn or olive oil when they didn't have something better. (No pictures for this part of the process; I couldn't take them because my hands were full.)

7) Start heating just behind the point; you want that end hardened for a good 1/4" or so behind the point, that way when it does get dull you can just sharpen it (unless you overheat it, in which case go back to the start). Turn it and watch carefully for the colors to change; in this case you want a medium red/cherry red evenly in that last quarter- to three-eighths inch, just as the 'shadow' disappears (as described in the blacksmithing post). As soon as you get that color, stick it point-first into the oil and swirl it around to cool.

8) Once completely cool you can check it with a file; with most of the drills I've used, when fully hardened the file will just skate off without biting. If you've got more than one to do, harden the other one while the first cools completely.

9) Clean off the oil and lightly sand the bevels - by machine or hand, either works - so you can see the tempering colors. And yes, you must clean the oil off first; if you have some on the shined surface it'll start burning when it gets hot and make the colors hard to see.

Do this in a place with good light, but not in direct sun, so you can see the colors change. Start heating - low flame, a good inch or so back from the point - turning the piece to keep it evenly heating. You'll first see the yellow appear around the heated area and start marching down (slowly, if you use a low flame; use a high flame and it'll move fast, sometimes too fast). Since this is a tool for a direct strike that has to be hard enough to cleanly cut into steel as well as any other metal you might be using, watch it closely: the sequence is faint yellow/darker yellow/bronze, and I'd suggest stopping it at darker yellow or light bronze by dipping the point - just that last half-inch or so - into the oil and holding it there about ten seconds, then lowering the punch most of the way in. The reason for doing it this way is that if you only cool the point and then take it out, there might be enough residual heat in the area you used the torch on to transfer down the piece and reheat the point, possibly enough to mess up your just-finished temper.

Here's the punch right after tempering. Note that between the oil from the quench and then washing it off, the colors are a bit darker than they were at the time it went into the oil to stop the temper.

10) That's it. Clean the oil off and give it a try; your punches should now be hard enough to stay sharp after marking.

I made two from that drill bit:  prick punch on the left, center punch on the right.

You can make chisels with this exact process, except instead of  grinding a point, you grind a flat bevel on each side, and then the edge bevel. Hardening is the same, but tempering depends on what you're using it on; for harder steel you might want to take it to a darker bronze.

Also, for a wider chisel, on the star end you can grind off two opposing arms, then use the other two as the main bevels; grind the end back to square, then grind the cutting bevels.

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