A Square Hole

I needed this: a square hole in the end of a brass rod.

It's for a project on Beck TD which will be described in another post. The hole needed to be rather precisely 0.120 inches square, and the way you usually accomplish that is with a broach. That's a square tool that is pressed into a round hole to make it square.

As it happens, I already had a homemade 0.120" broach that I had used earlier in the project, but it was to punch out a piece of plastic, and it was made of soft "key stock" metal. I tried to press it in a 0.120" round hole in brass, and it bent. But I decided to persevere, and tapped it straight. I fell into a rhythm like this:

1. With a hammer, tap the broach a tiny amount into the hole, and then pull it back out.

2. Straighten the broach again.

3. Repeat 1 and 2.

4. Move to the lathe, and re-drill the hole to remove the metal that the broach bent into the hole.

5. Repeat 1-4 a whole bunch of times.

Finally, the broach would go no deeper, and I had a square hole and a seriously beat-up broach;

I tried it on the speedometer cable on Beck TD, and it fit. It was only much later that I learned that the gap between the workpiece and the shoulder of the cable was a fatal flaw.

Once I figured that out, I started over with much harder metal, called "drill rod." I made a precise broach, and after this photo was taken, I used my acetylene torch to harden it by heating to cherry red and quenching in water.

Here's how you make that broach. You put the workpiece in a collet chuck (an ER32 chuck here, if you care) so that  you can precisely rotate it 90 degrees over and over. Using an end mill, you shave the square shape.

Incidentally, I'm able to put the collet chuck back in the exact same place every time because of the stop clamped to the back jaw of the vise. I made that recently, and am quite pleased with the way it turned out. It certainly saved the day on this project:

At first, I thought I had it kicked, because that hardened broach pressed right into the hole:

But, it wouldn't come out! I tried a variety of techniques of increasing force, finally snapping the broach off in the workpiece. Back to the drawing board.

I got the bright idea of using my mill/drill to align the workpiece and the broach, to allow me to remove it multiple times to clear debris and avoid binding. Later I told Phil Oles of that idea, and he confirmed that I had managed to re-invent a standard technique for doing that task. He also suggested that I make a graduated series of broaches so I didn't have to take such a big bite.

I set out to make four broaches in the sizes 0.100", 0.107", 0.114" and 0.120". I started with the largest one, for the same reason that a trim carpenter starts with the longest piece of trim: if you cut it short, you can use it elsewhere! But somehow I managed to make that first one to the exact size I needed:

When I was done, I had a full set. I hardened the ends, and used a parting tool to put 1, 2, 3 or 4 grooves to keep straight the order they are to be used.

After that, it was just careful work to gradually make the square hole. Worked great, and now it fits as it should:

And now that I have a good set of tools, even if I manage to ruin this piece as I continue to work on it, I can just make another with those hardened broaches. Coming soon, a post that tells what all this effort was for!

UPDATE: Here's the link to that next post...