On Sat, 19 Mar 2011 22:33:39 +0000, Cydrome Leader wrote:

I finally attached the thread cutting kit to a sherline lathe. The kit
came with a 60 degree carbide cutter.

Even with 1/4" brass rods, that broke in probably minutes, followed by
the spare I had.

the cuts looked like **** too. They weren't really shiny and the "feel"
from the handcrank on the headstock/gear train didn't seem right. I was
only cutting 2 to 3 mils per pass.

I decided to gring my own with 1/4" HSS steel blanks on a Tormek knife
sharpener.

That worked much much better. The cuts look nice and shiny.

So what's the deal with the ugly finish I was getting with carbide? I
think I know why the tips snapped off, but the finish part I don't
understand. A mild steel rod looked awful with the carbide cutter too.
It looked like there was more tearing or smearing than cutting going on.

Make sure any cutting edge is sharp sharp. Not just "marketing sharp".
You want it scary sharp. Even touching it up with a bench grinder is
probably going to get it sharper than it was when you got it. If I
really care about a cut I rough the tool out on a grinder, but then I
hone it by hand on a stone.

If you were trying for something like a #5-40 thread, then be aware that
the thread is really deep compared to the material diameter, and that's
going to steal lot of strength from the piece. Consequently, your piece
will start bending away from the tool, and Really Bad things will
happen. I wouldn't try that deep a thread without using a center in the
tailstock. (actually for that sort of thing I use a die!) The finer the
thread is in proportion to the OD of the piece the less trouble you're
going to have doing the cut.

Make sure the tool height is correct -- you want to be bang on the center
of the work; get it off and your tool won't work right.

--
http://www.wescottdesign.com