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Sunday, February 11, 2007

Parting School

While I wait for UPS to deliver my Grizzly order, I decided to practice parting this morning. I learned a few things.
A sharp tool is important. After grinding a fresh edge on my parting tool, I checked what I had with the 10x eye loupe in the photo. Lots of burrs and a wire on the top edge. The diamond stone took care of that, and gave me a keen edge. I bought the stone, which isn't really a stone at all but a flat steel, with diamond grit fixed to it's surface somehow, for sharpening woodworking tools. It stays flat, instead of wearing out in the center, so what you sharpen gets a straight edge instead of a curved one. It's at least ten years old, and hasn't worn out. It works great on tool bits. We'll see how diamond grit works on carbide when the Grizzly order gets here.
Parting tool height is Critical. The steel cylinder standing on end is a tool height gauge. It's just a piece of steel turned round and faced off to the precise height (or as precise as you can make it) of your lathe's spindle center. To use it, stand the gauge on your cross slide bed, rotate your tool rest around till the tip of your tool is adjacent to it, and then adjust tool height till the cutting edge of the tool is flush with the top of the gauge. Maybe some people can "eyeball" tool height, but I can't.
If you're fortunate enough to have a Quick Change Tool Post (QCTP) tool height adjustment is easy. Just turn the knurled nut. Those of us that are QCTP challenged must resort to using shims. Go to Sears and buy a set of feeler gauges. They're cheap. Take the gauges out of the holder, and you've got shims of all sizes.
Tool height should be "on center". That's what works best with my lathe. Maybe something else will work for you. Try on center first. See what happens.
Machining speed counts. It's different for different materials. I dragged out some charts and found I was going way too slow. My parting tool was pushing the metal out of the way instead of cutting it. For a fixed rotational speed, in rpm, cutting speed in surface feet per minute decreases with decreasing work piece diameter. On a large workpiece, you may have to increase rotational speed as the parting tool cuts it's way into the work.
Finally, my text book says use cutting fluid. I'm using kerosene, because that's what I have. It's not the right stuff. I've got more research to do. I'll let you know what I settle on when I do.

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