"Glenn" wrote in :

Dan,
Your last two posts make a lot of sense to me. Thanks for the
illumination. If I am following this correctly the tool radius would
limit the feed rate as, if you exceed the radius in feed rate you
begin to make threads and a rough finish.

Right. Also the trailing edge of the tool will be cutting. Proper tool
geometry isn't really designed for that.

I tend to leave my feed
rate set at .0042" and adjust the depth of cut and speed to get "happy
sounds" from the lathe I can see from what you have said that I
should be trying to up that feed rate to reduce tool travel time and
wear. The QC box on my lathe has feeds listed from .0078" to .0042"
so I have always stayed below .01" for feed but it looks like I could
go up to 100 TPI on the threading chart and have a reasonable feed
rate for a tool with 1/64" radius. Am I close to correct here?

Yes. Just remember the higher the feed rate the greater the turning forces
are. As a rule of thumb I would also limit the feed rate to about half of
the radius. A 1/32" radius at a feed rate of .01 IPR should give you right
around a 125 Ra finish in a proper set up on a good machine. A .01 IPR feed
rate with a 1/64" tool nose radius will give you around a 225 Ra finish. I
think that's too rough, and too much of the nose is buried into the cut.

You also need to consider the shape of the tool tip. An 80 degree tool can
take a higher feed than can a 35 degree tool tip. When roughing it's also
better to have a positive lead on the tool if possible. But often a square
shoulder is required on the part being turned so you have to use a 0 degree
lead tool or a slight negative lead of 3-5 degrees. Try not to use negative
lead tool for roughing. They aren't very good at it.

--

Dan