On Tue, 15 Feb 2005 11:06:59 -0800, Tim Wescott
wrote:

F. George McDuffee wrote:

Posted to both RCM and AMCNC

About dividing heads / rotary tables.

snip

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 ]


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

Etc.

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


Jim

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 ]

F. George McDuffee wrote:

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 ]


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

Etc.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

80/63 = 1.26984 .01% error

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 (

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.

F. George McDuffee wrote:
SNIP

We are in the process of scrounging a
large enough piece of 6061 T6 to make a 127-tooth gear to allow
metric threading.

SNIP

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

--
Steve Walker
(remove wallet to reply)

On Tue, 15 Feb 2005 18:36:51 +0000, wrote:
snip
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
Jim
snip
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.

snip
on dividing worm gear ratios
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.
....................
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.
.......................
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.

GmcD

snip
We are in the process of scrounging a
large enough piece of 6061 T6 to make a 127-tooth gear to allow
metric threading.
snip
What size are you looking for? I have some hunks, free if you want them.
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
http://www.mousebar.com/fhome.html ? Claim is that it as strong
(or stronger) than 1020 steel. Prices look good. Any down sides
or tricks to machining?

GmcD

Whaaat?

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.

Pete