In article , DoN.
Nichols wrote:

On 2015-05-27, Bob Gentry wrote:
The AAMCO 7" shaper has a 'feature' I don't understand. Maybe one (or
more) of you guys has an answer.

All the machine tool feed collars I have used (not really all that
many) have been graduated in some even numbered manner. Fifty marks,
sixty marks, two hundred, etc.

Sure -- convenient leadscrew pitches make for nice increments
on the collars.

The graduated collar on the tool slide of this unit is different.

There is the usual '0' then the fifth mark is '5', then '10', etc.
This goes to the '60' mark.
Then there is one tick mark (two spaces) and then the orig. '0'

This actually _does_ result in a measured 62thou. advance per rev.

The same as my Rockwell/Delta 7" shaper (a descendant of the
AAMCO).

What this means is that they cheaped out and used some 16 TPI
shaft (not even an Acme thread, but a more common V-thread) as the feed
screw here. I've been tempted to try making something more reasonable.
But a 10 TPI would be way too coarse for that small a diameter. (Wrong,
after looking and measuring it, it is 3/8" diameter, not the smaller
diameter that I though that I remembered). 20 TPI could get you 0.050"
per revolution, but if you make it Acme, it results in problems finding
a tap to cut that. You would have to make some more Acme threaded stock
using drill rod, cut flutes, and harden it properly to have a chance at
tapping the nut. So -- I haven't yet been motivated to do this. :-)

One reason to avoid threads too coarse for a given diameter is that the
nut may spin on the leadscrew under pressure, making it difficult to
machine parts with any accuracy.

The issue is the coefficient of friction of nut on screw while well
oiled, and the angle the screw thread makes with the screw axis. If
this angle is too large for the coefficient of friction, the nut will
spin relative to the screw, rather than remaining where put.

Joe Gwinn