"John" wrote in message
...
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.




The carbide was not ground for cutting the brass. You need a honed edge
and high rake angle brass. You are better off using HSS because you can
grind the proper angle on the tool. You could order inserts designed for
cutting brass too.

John

Positive rake for brass is trouble, due to hogging. That's especially true
on a small, light duty machine.

Beyond that, threading tools should not have rake unless the rake angle is
compensated in the included angle, and then you are limited to the
questionable performance you'll achieve. The best policy is to keep rake
at 0 degrees when chasing threads, and to feed by the compound, which would
be set at 29 -1/2 degrees. I prefer 29 degrees, to insure the back side of
the thread cleans up.

Using a carbide tool for brass after it has seen steel is a huge mistake.
The keen edge that is necessary for the tool to cut well will have long
since been dulled, yielding poor performance. Brass tends to act like a
bearing, and will readily float a tool that is slightly dull, yielding
inconsistent results.

A person with little to no experience in chasing threads is never wise to
experiment with carbide. It takes almost nothing to chip the fine tip of a
carbide threading tool, which, once chipped, will perform poorly, if at all.
Stick with HSS unless you have more than a good reason to go to carbide,
then insure that you use the proper grade. That often spells the
difference between success and total failure.

Harold