Monday, August 28, 2006

Grinding a lathe boring tool

I am building a flash hider for an AK-style rifle from mild steel. It is a slip-on sleeve design with set screws. I haven't decided on the port geometry, but I have no mill or end mills to cut slots so it may end up being more of a muzzle brake than a flash hider.

The largest drill bit I have is .5". However, the female hole accepting the muzzle has to be .550" in diameter. Instead of going out and buying bigger drill bits or boring bars and holders, I decided to try making my own from what I had lying around.

So last night I ground a boring tool from a 5/16" tool steel blank. After the grinding dust settled, I was left with this:

My main concern was that the tool shaft is very hard but brittle so might break easily during use. I imagine that boring bars are somewhat flexable to reduce breakage (Although this this would cause more deflection, but there must be some balance) So I kept to .002" cuts max. I used a fair amount of cutting oil as well. There was no chatter and the tool cut smoothly. It was pretty exciting.

Now I see the value of carriage stop - when boring it is difficult to tell how far the tool has gone, as it is inside the workpeice. If it goes to far, especially after multiple passes, the tool with dig in and most likely snap. I dropped a cylinder of copper onto the v-groove on the ways to act as a tell-tale. It worked, and I disengaged the half-nut just before I reached it, going the extra mm or so by hand with the carriage handwheel, and "felt" for the start of chatter.

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