Turning brake drums

This job really threw the brakes on all the fun!

A friend of mine contacted me a few days ago about turning a couple brake drums off of an equipment trailer. They're big honking things - 14" in diameter and every bit of 40 lbs each.

Tailstock adapter seen here.

My Logan only swings about 10", so it was out. Luckily, I have access to a 16" lathe in my shop at work. Unfortunately, the drums' geometry didn't allow for them to fit in the lathe without some specialized tooling - which I was not in the mood to make.

What I ended up doing, was making an extension for the tailstock quill. It has a male MT4 taper on one end, and a female MT4 socket on the other. It pushes the gauge line of the tailstock out about 4 inches or so.

For anyone that's wondering, the tapers on this adapter were cut with the compound slide on this lathe. I chucked up a drill chuck with a MT4 taper, and trammed the compound against it until I had less than .0005 variation along the length of the taper. It ended up working a lot better than I thought it would!

A shot of setup. That's an 8" chuck for a sense of scale.

This adapter allowed me to chuck the drum on the hubcap bore portion, and support the other side with a bullnose center in the seal bore. The extension gave me enough space between the tailstock and the drum for the carriage to move, but not enough room to do one complete pass inside the drum - doh!

I ended up having to cut about 2 inches worth to the finished diameter. Once it was clean, I'd move the boring bar out farther and cut the remaining 1 1/2 or so. I was able to blend the two cuts pretty well. Because of the protrusion of the boring bar (≈5.5 L/D), I clamped a square aluminum bar to the back of the boring bar. This is a trick I learned from a now retired, never-touched-a-cnc, machinist. This trick works wonders at getting a little extra usable length out of a boring bar without scapping your workpiece.

After all the setup and figurin', the boring operation itself went fairly smoothly... once I got past the learning curve of turning a brake drum. The thing is effectively a giant bell - so it resonates like a mother if you even look at it wrong.

In the middle of a cut. Note the Shop-Vac
sucking up the rust and asbestos dust.

To reduce chatter, I wrapped some rubber sheeting around the drum, and slowed the lathe down to as slow as it would turn - 45 rpm. The slow speed worked wonders at reducing chatter, but it also meant that each cut was unbearably slow! (approximately 2 inches long at .011/rev at 45 rpm = about 4 minutes per pass)

In interesting observation I made while cutting these parts: They cut a whole lot better when they're cold! They were about 35°F when they were first put on the lathe (stored in my truck). When I'd bring one in and chuck it up, I could get away with running it at 90 rpm. Once it started to approach room temperature - I'd have to slow it back down. I don't know what the mechanism is that drives that (if anyone out there knows, please let me know!), but it was interesting.

The surface finish turned out pretty good considering the precarious setup, the material (unidentified iron, based on the chips), and the cutting speed (160 fpm with carbide).

Now that its all said and done, I'm not sure if I'd do another brake drum - at least not for anything short of about 35 bucks a pop.  I have between 4 and 5 hours in these four brake drums. If another set ever comes up, they should go a little bit smoother now that I know whats up.

One about to come off the lathe.

Hopefully, if anyone out there is contemplating do anything like this, you'll know what not to do... or to run away.

Thanks for reading!