On Thu, 27 Oct 2005 22:21:11 -0500, Jon Elson
wrote:

Eric R Snow wrote:
On Thu, 27 Oct 2005 20:23:48 GMT, Jim Flanagan
wrote:


Hello..
I wanted to ask anyone if they have attempted to do this on their
lathe. I only have a metal lathe and want to cut a flat archimedes
spiral onto a 1/8" thick piece of lucite. The spiral, fairly coarse,
would be something on the order of 6 turns/inch. The diameter of the
plastic would be about 3.5".

In any case, I could use some pointers as to how and efficently
accomplish this. Thanks for any advice you may offer.

-jim

It sounds like you want to cut a spiral on the face of a disc. Others
have responded with methods for cutting the OD of the part which would
give you a helix. If you are needing to cut a spiral then a manual
lathe isn't the best way for this because they are set up for
threading parallel with the Z axis, not the X axis. I have cut scrolls
Most newer lathes have "power crossfeed". As well as the half nuts
on the leadscrew for threading (Z axis), they have a keyway in the
leadscrew that drives a power take-off arrangement that turns the
crossfeed (X axis). Some lathes have different feed ratios on
the crossfeed, so you have to compensate in the gearing. The Atlas,
for instance, has an 8 TPI threading screw and a 10 TPI crossfeed screw.
So, they ingeniously put a 20:16 tooth gear set in the apron so the
feeds come out the same for both axes of motion. But other models
may need a different setting when using the crossfeed.

One complication is there won't be a threading dial for the crossfeed,
so you might want to just leave the crossfeed engaged throughout the
entire operation, and back the spindle up to wind the crossfeed back to
the start of the thread. If you disengage the power crossfeed,
it likely won't start at the right alignment between the feed and the
spindle when you re-engage it.

Jon
Jon,
I know that most lathes have a powered X axis. But the gear ratio is
really high. On one lathe in my shop the X axis advances about .014"
on the diameter for each revolution of the grooved rod that supplies
power to both axes. This guy needs about .320" on the diameter. So if
I was to use the power feed the grooved rod would need to spin 23
times as fast as the spindle. With the gears in my lathe I can't do
this.
ERS