Rotary Broaching a Keyway on the Lathe with simple materials you have in the shop

**Crusty** · Feb 2, 2020, 01:11 PM

Here's mine that I made from an adjustable tool holder from Ebay. The cutting tool is a 1/4" lathe tool shaped to cut on the end and it makes 1/4" square holes. The front cap flange of the tool holder has been machined 1º off of perpendicular and installed shoved all the way to the outside of its adjustment range, and if a rod is inserted into the bronze bearing cup and the assembly rotated in a lathe chuck the rod appears to make an X shape in the air. The tool is cutoff so that its cutting edge is located as close as I can get it to the crossover of the X seen before.

When chucked in either the mill or the lathe, as the chuck rotates one angular position is a little closer to the work than all the rest so as that position comes to each corner it pushes the tool a little deeper into the work and by successively cutting each corner the square hole progresses into the work. At some point, determined by the relief ground into the tool, the tool shank begins contacting the entry hole and so it can progress no further into the work. That's why you see commercially made tools with long reliefs ground into them so that the tool can go deeper but it's also taking significant meat from the tool core making it weaker.

It takes a lot of force from the machine and a generous amount of lube is required but the broaching goes surprisingly fast at high spindle rpms. The operation leaves inward turned flower petal looking chips in the hole and those are easily broken out with a pointed tool later. I made a steel carrier for that tool which the cup bearing rotates around and if I want a different size hole then I have to make a carrier for that different sized tool, but so far making 1/4" square holes deep enough to get a ratchet into has been enough for me.

**JRock** · Feb 2, 2020, 02:55 PM

Is that a turret tool? I think I have a couple of them around, wondering what I'd do with'em. That's great!.
Love to see the guts.

**Crusty** · Feb 2, 2020, 03:59 PM

I think it originally was. They can still be found on Ebay for a reasonable price and it saved me a bunch of work.

It's really dirt simple. I just flycut a 1º off perpendicular surface along the long axis of the tool holder cap flange, then machined a bronze cup bearing with a top flange that fits the tool holder cap bore and a similar looking tool carrier out of steel which fits the cup and is bored to hold the square lathe tool by its corners. The bearing is only as long as the tool holder cap is deep and the tool carrier doesn't quite bottom out in the bearing so that the broaching force is felt by the bearing flange and the carrier flange.

The slickest part of the build for me was chucking the lathe tool in the 4 jaw and then mounting a Dremel with a cutoff wheel onto my tool post and when it was brought edge on against the spinning work it ground a nice hemispherical depression in the end of the lathe tool to provide rake for the cutting edges. It looked like it was done in an actual machine shop.

Good luck in your search for where you put those tool holders.

**JRock** · Feb 2, 2020, 06:28 PM

Very cool. That CutOff wheel is a great idea.
I know exactly, approximately, which pile it's in

**Crusty** · Feb 2, 2020, 06:46 PM

You just think you know which pile. I'd bet a mustard covered donut that it'll eventually be found elsewhere. That's how it goes here - I'm about to order another pair of vice grips because I can't find mine and I've looked everywhere 3 times (everywhere except where they are).