Friday, January 28, 2011

Fiery Friday - Making a Center Punch

Welcome back!

I've probably already mentioned this a half a dozen times, but one of the things I love about being a blacksmith is being able to make my own tools. Instead of running up to the local hardware store and buying a cheap screwdriver that the tip will break off within a few months of use, I can just grab some spring steel and forge my own. A few months ago, I decided to change the rear brakes on my car, but for the life of me could not find the special lug key to get that special lug nut off. I called around and nobody had one in stock except for the dealership that was over a half hour away and they wanted $40 for it. Since all I needed was a large glorified torx bit (that's the star shaped one), I decided to make my own. I forged a 1/2" square bar to make it round. Now that it was about the right size and with it at a yellow heat (2200+ degrees), I pounded the end of it into the troublesome lug nut. Basically I just used the lug nut as a die to make impression that I needed. Once the star shaped tipped bar cooled I was able to latch a large pipe wrench onto the bar and crack the lug loose. Crisis diverted...

Today I'm going to show you how I make a center punch for marking steel. Mostly I use mine to mark where I want a hole drilled, but also sometimes to mark a measurement that I need to see after a piece of steel is red hot and any soapstone or silver pencil markings would have been burned off.

First I start with a piece of coil spring that I had lying around my garage. Most "spring" material is 1070 (with .7 % carbon) . It's the carbon that makes it stronger and that's what we need in a tool that will be pounded into other pieces of steel. I really don't know exactly what this is, but I have experimented with it and know from a few tests how it will react to my heat treating methods. We'll get into that later. I've already cut off a piece and straitened it hot.

Since this steel is "stronger" than the mild steel (low carbon) it will be more difficult to forge. What would normally take me one heat to forge in mild steel might take me two to forge with this spring steel.

Once it's good and hot, I start forging about 6" of the end of the spring into a square shape. I could simply leave this round, but I want to add a simple twist to the handle and twisting a piece of round stock, yep you guessed it, it pretty much stays round. So there is no real functional aspect to this twisted handle except perhaps to give a good grip for holding it.

Here is the once round spring forged square and ready for the twist.

After another good heat, I clamp it into my vice and twist about 5" of it. There is no need to take exact measurements here, I'm just going with whatever looks good. 

Once I have the twist the way that I want it, then I taper the tip to a point to form the business end of the center punch.

Here I  knocked the corners off the taper to start forming it round again. When forging tapers, it is always easier to forge them square first and then once you get the angle and size where you want it, forge it round again. (if you want it round)

Besides needing a bit of straightening, it is now fairly round.

To straighten everything up without damaging any of the details I get it hot again and use a wooden mallet and a piece of 2x4.

After it has had a chance to cool slowly I clamp it in my vice and cut it off just before the twist ends. Like I said earlier, I'm not measuring anything, I'm just making it look "right".

I round off that squared sharp end that the band saw left and rough grind the tapered tip at about a 90 degree angle. (just like the store bought ones)

Now to make it really hard.

The next step is the heat treat. Heat it again in the forge to a non magnetic state. Yes, that means heating it until a magnet will not stick to it any longer. I usually go just a bit hotter than that to allow for some cooling from the forge to the oil quench. Once up to temperature, it gets quickly dunked into a coffee can full of transmission fluid. This cools the spring steel quickly, locking all of the atoms into the state they are when it's at nonmagnetic. Now the center punch is very hard, but brittle. You could compare it with a diamond. A diamond can scratch just about anything, but hit it with a hammer and it will shatter to pieces. If I were to hit this center punch with a hammer, it may very well break into ten pieces.

So...... now I need to take some of the brittleness out of it by "tempering" it. To temper it I take a propane torch to the fat end and slowly heat it until the colors run towards the business end. I'm looking to get a darker straw color on the tip and blue or purple everywhere else. The straw color (about 480 degrees) will give me the hardness similar to the edge of a knife and the blue or purple color (about 560 degrees) will make it more like a spring, just like this piece of steel's original purpose. In the picture you can see that the end where the hammer will hit is actually past the purple color (back to normal color) which will make it just like I had never quenched it in oil. I don't want this so hard that it will mark up my hammer. In the middle is a nice dark blue/purple and the tip is a straw color.

As soon as the tip gets to that dark straw color, I quench it into some water to stop it from getting any hotter. Now I have a center punch that is not only hard, but also strong. I can now hit it with a hammer and it will easily mark mild steel and not deform or crack.

Here is the nearly finished center punch. All I need to do is regrind the tip the point I want and it's ready to go. I tested it by punching some mild steel a dozen or so times. There was no sign of any deformation on the sharp tip and it made a nice clean mark in the mild steel.

To not get too technical, I did not get into the details on how to thoroughly heat treat steel (annealing, normalizing, heat soak times, proper critical temperatures, proper quenching oil, etc...), but this simplified method does work well for what I need it to do. Another tool you could make that would follow almost the same heat treating procedure, would be a screw driver. Perhaps I'll leave that one for next week.

Have you ever heard of the phrase "don't get too hot and lose your temper"? That is an old blacksmith phrase. Blacksmiths use tools that have been heat treated and therefore "tempered" to a certain hardness. If you use a tempered tool on a red hot piece of steel and don't continuously cool it in water as you use it, it may get too hot and "lose it's temper". The hardness from the heat treat would be gone and you'd need to heat treat it all over again.

Thanks for coming by and I hope you enjoyed yourself. If you have any questions, please don't hesitate to ask.


Mom said...

As always, I am impressed with your talents.
How long from start to finish did it take to make the punch?

Anonymous said...

thank you for making tempering understandable


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