Harold and Susan Vordos wrote:
"Jim Stewart" wrote in message
...

I'm turning 2 flywheels for a working model
IC engine and I've not been able to get the
quality of finish that I'd like.

The flywheels are about 1" thick and 4" in
diameter. I was working on the face of one
last night. The problem is the small digs
the cutting tool occasionally makes. I can
polish out the overall surface to a mirror
finish with successively finer grades of
sandpaper, but I cannot remove the digs with
the coarsest grade of sandpaper I have.

Can anyone suggest a tool shape and feed that
might give me a better finish. My tool is
slightly rounded with the recommmended angles
for steel.


Jim,
You're experiencing the "joy" of machining what must be mild steel, or low
carbon steel. Without the addition of an element for free machining, that's
the nature of the stuff. The only way you can get around the tearing is
to take a cut that is deep enough, and also run fast enough. That requires
the use of carbide, and depth may have to be more than .025" in order for
the material to machine without tearing. Surface finish then comes out with
a high luster instead of a dull appearance. The alternative is to run
with high speed, fair amount of positive rake, with the cut well lubricated,
and accept the finish that you get, polishing progressively after machining
until you've removed the intermittent tearing that occurs.

If you want to avoid this condition, use other materials. The materials
available for production machines avoid that problem by the addition of lead
or other elements, or in the case of stressproof, the material undergoes
considerable cold working, improving its properties immeasurably. It
generally machines without tearing.
For free machining, you can choose from several alloys of mild steel----some
of which are 12L14, 1213,1215,1117, &11L17. Stressproof, while free
machining, has a higher carbon content and can be heat treated. It would
not be classified as a mild steel as a result, but a medium carbon steel.

If, by chance, your flywheel is cast gray iron, there has to be a problem
with your tool geometry. It's not known for tearing, due to the nature of
the material.

Yup, it's mild steel. I thought it was a good deal
at $.60/lb, but maybe not now...

The frustrating thing is that the tool will cut
beautifully for maybe a tenth of an inch and then
for no reason dig in. I was hoping that there was
some magical tool geometry that might help.

I don't think I can take a .025" cut and hold the
tolerance that I want, but I will do some tests
on a piece of scrap. I will also play with a greater
positive rake and diamond stoning the tool.

Thanks for the advise.