Machining

Machining

I have been building a caster set for a Takahashi EM400 mount. These are a very expensive, high end product, and because the first customer that asked for a set wanted brass compression rings to protect the finish on the legs, I indulged him--and all subsequent customers.

What is a brass compression ring? Instead of having a screw directly squeezing down on a piece of metal, the screw squeezes down on a brass ring that sits in a channel. A detailed description of how it works is here. And here's a picture of one of these with the brass compression ring visible:



Anyway, how do you cut one of those channels in which the brass compression ring sits? The brass can't move up or down, so you have to cut this channel a little ways into the sleeve. Until today, I have done this with a tool that I constructed that slides into the boring tool hole in the tool holder of my lathe. (Pictures and description are here.) This is called a reverse rake boring tool holder. (If I had known what it was called, I might have bought it.)

The problem is that there is only a single cutting point on the tool in the holder, so you get one beautiful right angle, when you really need two, like this cross section shows:



One way to do this is to cut the near end of the channel, then take the cutting tool out of the holder, turn it around, and then cut the far side of the channel. This way you get two right angles--with a bit of hassle.

Anyway, today I decided that I would take a 1/4" cutting tool, and instead of a single cutting point, I would grind it so that it is a right angle, producing a 1/4" cutting edge. Now, to cut a 1/2" wide channel, I only have to turn it into the near end, then crank the lead screw 1/4" to produce the far end of the channel.

Obviously, because the cutting surface is larger, you have to be a less ambitious about how much you try to cut at once--take off relatively thin layers, and don't advance the lead screw too quickly, or the workpiece will pull right out of the chuck and go flying! And I wouldn't try cutting steel or even aluminum using this approach--but for Delrin, it works okay. It doesn't produce as beautiful a finish as a single point of contact, and I would not be surprised to see the cutting tool wear out quickly. But it does produce a very neat channel without having to reconfigure the boring tool holder.