re: mill threading

At 08:17 15/11/00 +0000, you wrote:

>Alan M wrote:
> >What if I setup my rotary table and a tail stock on the Sherline
> >mill, and use a 60deg. conical point (or something else?), by
> >driving the rotary table and the 'X' axis, I should be able to cut
> >threads on the mill!
>
>Yes. Be aware of the low cutting ability of the point of the cutter
>when used this way (low, to no cutting speed) Grinding (or buying) a
>cutter with the point "dubbed off" and made into cutting flutes will
>increase the cut you can take (or the materials you can cut; depends
>on how you look at it)
>
> >one should be able to cut an ACME screw?
>
>Yes. Just use an appropriately shaped cutter.
>
><snip>I haven't sat down to figure out how to do it in CAD/CAM.
>
>Draw a taper(angled line, really) with the x scaled to y by the ratio
>of steps per rotation to steps covered linearly per rotation. So on a
>sherline with 8000 steps per inch, and a rotary table of 1440 steps
>per revolution, to cut ten threads per inch: (we'll just go around
>once)

This technique works very well - I've used it on my Taig CNC mill with a
CNC dividing head - see 2nd photo from the end of this page:

http://www.jeffree.co.uk/Pages/divheadmk2.html

As Ballendo correctly observes, use a conical point engraving cutter with
the tip ground to a sensible (non-zero) diameter - my local supplier will
grind solid carbide cutters to any tip angle and diameter that you specify.
The drawback with this approach is that you can only get within 1/2 cutter
diameter of a shoulder - so preferable to use small diameter (say 1/8")
cutters if this is a problem, or run a die over the thread after milling to
get the last couple of threads cut. This is probably a smart move anyway
to clean up the thread form.

One major drawback - the cutting action produces large quantities of very
fine, needle-sharp swarf - for a week or so after cutting the thread shown
on my web-page I was still feeling the effects of a couple of dozen of
these particles embedded in my hands.

Regards,
Tony