"Walter Harley" wrote in message
...
I have a Sherline! Finally broke down and got myself a lathe. Now I am
trying to learn to use it.

I want to turn a rectangular-profile slot approximately 1/32" wide, into
an
acetal (moderately hard plastic) shaft with an OD of 1/2". The slot wants
to be maybe 3/32" deep; that is, OD of the inside of the slot is 5/16".
(In
case this description is unclear, imagine making a grommet.)

What kind of tool should I use to do this? Should I use a parting tool?
Should I try to grind something more or less like a parting tool?

My concern with doing it with a parting tool is that, although it might
work
for this particular problem, I think I'm limited to cutting a slot exactly
the width of the parting tool. It would be nice to learn how to make
slots
of various widths, including fairly skinny ones.

Thanks in advance!

Piece of cake, Walter. Acetal machines beautifully so there should be no
problem with the cut. Grind yourself a parting tool that has relief top to
bottom on the sides and front of the tool, and a slight taper front to
back, then put the radius of the wheel on top, making sure it starts at the
cutting edge and goes positive towards the rear. Finish the tool by honing,
keeping the corners sharp and the front square. The top grind of which I
spoke means the grind will be deeper at the shank than at the tip, where it
cuts, and will give you a small amount of positive rake. That makes the tool
cut quite well and promotes chip flow. I've been grinding parting and
grooving tools that way for years with outstanding results. Make sure the
cutting end is square, not ground to the side, and not grooved like Tim
suggested. He'll come to understand that's not a good idea as soon as he
starts grinding parting tools.

The width of your parting (or grooving) tool should be no wider than the
desired width (duh!), and if it's a little narrower all you have to do is
mike the tool so you know the width, then cut your groove on location and
move over the prescribed amount, using a dial indicator (long travel) the
amount necessary to achieve the desired width. That's how one cuts snap
ring or O ring grooves and other undercuts when you don't have the exact
size tool on hand. With a square ground end you can move from one side to
the other when you're to depth and end up with a very nice bottom cut.
This will all make complete sense to you the first time you try it.

Speed doesn't matter with acetal. The faster the better. You won't hurt
the tool and you damned sure won't have any trouble with the plastic
melting. The faster you run it, the better it machines.

One word of caution. Grinding a parting tool that thin requires a wheel that
has no bounce in it, so make sure your grinding wheel is running very
smoothly. The thin section of tool likes to break off at the shank when it
bounces. You'd also not want to make it very long so it has rigidity in
keeping with machining steel.

If you're lost on my description, I'd be pleased to send you a scan of a
tool so ground. Contact me on the side if you're interested.

Harold