"Karl Townsend" wrote in message
...
On Sat, 19 Mar 2011 23:37:11 -0400, "Tom Gardner" w@w wrote:


"Cydrome Leader" wrote in message
...
Jim Wilkins wrote:
On Mar 19, 6:33?pm, 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.

Carbide is a matrix of grains glued together, like concrete, and it
doesn't take as sharp an edge as HSS. I doubt you'll ever need carbide

I though they were giant crystal or something like that. The "like
concrete" part makes sense.

on a lathe with so little power anyway. It's likely that if the
material was that difficult to cut the lathe would twist away when you
tried to force a carbide bit into the work.

High Speed Steel will cut stainless steel and Grade 8 hardened bolts,
it just dulls quickly.

Does HSS leave nicer finish when sharp than carbide on these materials?
Considering tiny pieces of the tips broke off the carbide cutters I
have,
I can't really test this anymore.

I much prefer HSS to carbide for a lot of jobs and especially threading.
You have to
have rigidity squared to use carbide well and horsepower too. Great for
removing a
lot of material quickly or production jobs. I've got a 7.5 HP Reed
Prentice with the
rigidity and HP and only use carbide 25% of the time.

NOT, repeat NOT for the original poster. I recently bought carbide
threading inserts for my CHNC lathe. WHAT AN IMPROVEMENT! On a machine
built for it, these are great. Of course, I never thread at less than
500 RPM on larger diameters and into the 1000s if going under one
inch. Don't know if the reflexes could ever be fast enough on a manual
machine.

They couldn't. g

For the OP, what Harold said. And don't forget to radius the tip of your HSS
threading tool. The correct amount for each thread pitch is published in the
manuals, and you can make a close approximation with a good loupe and a
caliper set to the right width.

First, though, just get the darned thing cutting right with a HSS tool, even
if you leave the tip sharp to start with. It's tricky on a very small lathe.
Follow Harold's recommendations.

--
Ed Huntress