Gear cutting follow-up

With the usual 1/8" pitch leadscrew 63:50 is more accurate and it's also more convenient because it only uses one non standard changewheel.

0.125"x 25.4 x63/50 = 4.0005 mm 0.0125% error


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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

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Reply to
F. George McDuffee

There's more than one way to skin a cat:

127:100 = 1.27 0% error 14:11 = 1.273 0.2% error 19:15 = 1.267 0.3% error 23:18 = 1.278 0.6% error 24:19 = 1.263 0.5% error 33:26 = 1.269 0.1% error


Reply to
Tim Wescott

I guess they are fighting reasonable gear diameter and reasonable tooth size to bear a given load. IE finer pitch =finer teeth= lower load rating or giganto gear. Also on an 8 inch head you would only tend to do a certain size work[meebee 8 inches?] thus defining your tolerance; .1 degree at 4 inch radius is a small number; at 6 inches it is larger etc etc. thus a smaller head can use a coarser pitch with no perceived loss in accuracy.

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We are in the process of scrounging a

What size are you looking for? I have some hunks, free if you want them.

Reply to
Steve Walker

The class decided to go for a 63 tooth gear. Examination of the available index plates and some calculations indicate we can use the 21 hole circle [90:1 gear ratio] . 127 tooth gear would be a real problem. To get exactly we will need a 127 hole index plate, but 49 holes are max on the set-up we have. There is a way to index using two circles to get close but this looks like a real PITA.

Thanks for the on/off list suggestions.

Reply to
F. George McDuffee

on dividing worm gear ratios


...................... Little load for most projects. Worm/Gear used for rotation only, spindle bearings should take any load. Spindle clamped when cutting. From what I can see even 1.00 module / 25 DP worm/gear should be adequate. 120:1 would have a gear about 122 m/m - 4.8 inches in diameter, 180:1 would have have a gear about 182 m/m -

7.1 inches in diameter. On our milling machine the index wheel hangs off the front of the table so there is plenty of room.

Any one built a index head using 180:1 gear ratio and a micro stepper motor? A google search indicates low cost micro stepper motors (c. 60$US) are available with 51,200 steps per rev. With a 180:1 worm this would be a resolution of 9,216,000/rev or

0.000039063 degrees or 0.000000011 seconds per step. Timing circuits to generate a given number of pulses are relatively inexpensive. To generate a 127 tooth gear simply program in 72,567 pulses for each tooth. Eliminates the need for the index plates, sector arms, etc.


Reply to
F. George McDuffee

Thanks for your kind offer. We received a donation of two aluminium 6061 T6 "logs" from some local firms that do aircraft work. one 6 inches in diameter and one 3 inches in diameter. I think we can slice this up like a salami for the gear blanks.

Has anyone used the Fortal aluminum alloy showing up on

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? Claim is that it as strong (or stronger) than 1020 steel. Prices look good. Any down sides or tricks to machining?


Reply to
F. George McDuffee


80/63 = 20*4 / 21*3

You need a 20, 21, and some way for a 3:4 ratio......

15:20, 18:24 etc. Maybe you already have the gears you need. Or maybe a bunch of 20's a 21 and a 15. No need at all to make a 63 tooth gear.


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