Posted to both RCM and AMCNC

About dividing heads / rotary tables.

A few weeks ago I posted a general question about one-off gear
cutting to both RMC and AMCNC for information my craft machining
class could use. Many valuable suggestions were received, both
on-list and direct.

We have completed our first gear, a 45 tooth, 1.00-m/m module
change gear that expanded the thread capabilities of our Emco
Compact 10 lathes. This proved to be a rewarding project for
both myself and my students in that it provided a useful project
to apply/practice their lay-out, lathe turning, milling, boring
and tool grinding skills. We are in the process of scrounging a
large enough piece of 6061 T6 to make a 127-tooth gear to allow
metric threading.

A general question was asked about how the gear ratios for
dividing heads were selected. The older/larger heads seem to use
a 40:1 ratio, the newer moderate duty heads such as the Myford
seem to use 60:1 and most of the rotary tables, with or without
dividing plates, use 90:1.

I can understand that it would be desirable to have a ratio with
a common factor with 360 degrees, however it would appear that
even larger ratios such as 120:1 would be even better for lighter
home shop projects.

Does anyone have comments, observations, suggestions, etc. they
would care to make about the ideal dividing head worm gear ratio?
How about comments, etc. on the Myford (open) style of dividing
head [see http://www.lautard.com/Divheadphotos.html ]