Thursday, September 14, 2006

Threading on a lathe

Finally (successfully) cut some threads on my lathe! They are 24 tpi on a peice of aluminum.. and some random diameter since I am just trying to get the process down.

The hardest thing to figure out was the gearbox changes to drive the leadscrew at the correct speed. Since this is a mini-lathe there aren't any of the nice gear selector levers or anything. So I poped off the gearbox cover and fiddled with it until I figured it all out.

The first time I tried I wasn't using the threading indicator, I was just trying to back up the leadscrew between each pass so as not to lose my position. The tracking was off - I suspect that changing the leadscew direction offsets the alignment. So I just turned down the botched threads and started over at a slightly reduced diameter.

This time I actually used the threading dial indicator, which works perfectly! Duh. I put a light line on the peice, stopped the carriage, pulled back on the crosslide to clear the peice, cranked the carriage back to past the end, set the tool .001 deeper, and engaged the leadscrew at the designated mark, 1-8 in this case. The tool tracked perfectly! I repeated 5 times, and ended up with this:



I didn't make the tool sharp enough so I ended up with squarish threads, but the boint is, it worked! By the time I was done I was shaking from excitement. I don't have a center gauge yet so its hard to grind the tool at 60 degrees, but I'll get one before I try any serious work. The next logical step is internal threading, which should not be harder save grinding the tool. Here are all the tools I've grinded so far:



From left to right: A cartridge rim cutter, a parting tool, a ~60 degree external threading tool, a boring tool and a double-sided roughing/finishing tool. An external threading tool would look a lot like the boring tool but be thinner to provide relief for the little cutting "finger" that would need to be ground at 60 degrees. It seems like it may be the hardest tool to grind of all :)

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