Myford Fine Feed Tumbler Gear

Does anyone have a 12/25 tooth fine-feed tumbler gear supplied by Myford. The gears are 14 1/2 degree PA, 20 DP. I intend to fabricate (hob) one of these compound gears for my ML7, but I would like to know if the 12 tooth pinion has an increased diameter to avoid undercut teeth. The 'normal' diameter for a 20DP, 12 tooth spur gear is 0.70 inches. The 'enlarged' diameter recommended by Machinerys Handbook is 0.762385 inches. The increased centre distance is not a problem because mating gears are always mounted on the change gear quadrant, so centre distance can be adjusted accordingly. Machinerys Handbook also states that 'enlarged diameter' pinions will mesh with 'normal' gears.

I would be grateful if someone could measure the outside diameter of the 12 tooth gear if you have one of these tumbler gears.

Thanks, Jeff

Yo most certainly shouldn’t be worrying about that last 0.000005”.🙂 I would likely not even worry to the nearest 0.001”. 🙂

Make it in Aluminium or other softer metal (even plastic)?

Hi

The blank od for the 12 tooth gear that you have asked about is .746 inch in diameter.

John

Good morning

Just measured the 12T on my S7 and the OD over the teeth = 0.723"

Difficult to know why the difference in measured size of this 12 tooth gear,
in the following post to mine AStroud measured at .723.
As you can see here mine was .746 inch i bought this gear off the Myford
stand at the ME exhibition in the 80's the bore is .375 inch and the
cluster is case hardened.There may be a difference the super 7 and ML7 as
my S7 has a 30 tooth gear and is is 20 deg PA ,the 12 tooth gear will
be 14 1/2 deg PA as is the Myford change wheels .
In any event the limiting factor on the od is the width of the tops of the teeth
if the blank is too large the tops will come to a sharp point ,the gears will still run
but the tops will be weak.

Some useful info here if you are hobbing gears of 20 deg PA.
If you are hobbing the undercut can be left as it is ,to improve the strength of the
tooth at sizes of gears of 20 PA 17 tooth and below the corrections below
produce gears with no undercut.

In practical terms 5 teeth would be about the minimum and that would generally be
cut direct on a shaft as there is little material available to bore a hole in the middle,
eg Align power drive unit for milling machines.

A little more freedom with this is if helical gears are cut as the divide by cos helix angle
increases the gear diameter.

Dimensions for 1 DP for other pitches divide by the dp

eg
20 dp 12 tooth .36/20 =.018" 12 tooth / 20 dp= pitch dia = .600 inch + .018 + addendum .100 inch
blank size .718 .This gives the minimum size with no undercut ,you can of course go larger but this
will eventually produce thin tops to the gears that will require " topping".

TEETH

17 .26
16 .28
15 .30
14 .32
13 .34
----------
12 .36
-------------
11 .38
10 .40
9 .42
8 .44
7 .46
6 .48

John

This is puzzling!

Using the formula from Ivan Law's book on gears, OD = (tooth count+2 ) / DP, so a 20DP gear with an OD of 0.750 would have 13T . (13+2 ) / 20 = 0.750

The gear obviously has 12T, but 0.7465 x 2 = 1.493.

So it looks as if the gear is oversize for a 20DP involute form 12T, unless there was a Zero error.

(12+2 ) / 20 = 0.700

Are there any professional gear specialists out there to offer an explanation?

Howard

It's in the 1st post, small gears are often over size to avoid an undercut particularly if being cut with an involute cutter.

Thank You Jason!

HPC's Technical guide shows how they increase the dia look at "corrected teeth" and you will see why you came up with 13 teeth for that OD,

As Jason said the small tooth count gears are modified with a 'long addendum' to reduce the under-cut and strengthen the gear. The amount of profile shift depends on the tooth size and tooth count.

Thank You Pete.

Are similar tables available for Module Gears?

If so, where can I find them please?

Just in case I ever need tom use them.

Howard

The list in my previous post was slightly corrupted when copying over to
posting.This may be a little clearer.

Dimensions for 1 DP 20 deg PA for other pitches divide by the dp.

20 dp 12 tooth .36/20 =.018" 12 tooth / 20 dp= pitch dia = .600 inch + .018 + addendum .100 inch
blank size .718 .This gives the minimum size with no undercut ,you can of course go larger but this
will eventually produce thin tops to the gears that will require " topping".

Teeth Correction

17           = .26
16           = .28
15           = .30
14           = .32
13           = .34
12           = .36
11           = .38
10           = .40
9             = .42
8             = .44
7             = .46
6             = .48

The same basic rule applies ,for module gear the correction is the tooth number x the
correction eg for a 6 tooth gear of 1 module 1x.48 =.48 added to the pcd of
6 mm = 6.48 mm + the addendum of 2 mm OD = 8.48 mm this is the minimum
blank size to avoid undercutting when hobbing a gear.

There are 3 examples here 1 mod gears . 1 cut at nominal pcd 6mm ,2 at 6.48 mm pcd
and 3rd cut on a 10 mm od ,the first one looks in the photo a little wonky it is the angle
of the camera ,you can clearly see the undercut ,the difference by adding the .48 mm
to the pcd can be easily seen, the last one just shows what happens when the blank
is taken to the maximum .All of these gears will mesh with each other.

Howard's suggestion that the caliper was not zeroed properly is not well thought
out , for if the caliper was not zeroed properly it would only read under size,i would have thought
most shed men would know this.

Getting back to the OP's original question he could use any size from the from the .723 inch measured by
AStroud to the machinery's handbook size of .762 inch ,the only other consideration is the distance
from the root of the tooth to the bore of the hub ,the recommended minimum in a book that i have
is circular pitch divided by 2 would be .078 inch ,my own Myford gear measures at .066 for this
dimension.

Thanks Pete for posting that copy of table 8 .I never had any information on these corrections
for 14.5 PA gears.

John

Edited By John Pace on 19/03/2021 17:58:39

Edited By John Pace on 19/03/2021 17:59:17

Edited By John Pace on 19/03/2021 17:59:55

Gentlemen, thanks to all who contributed, this has been most helpful. It is difficult to understand how two S7 tumbler gears from Myford could have different diameters for the 12 tooth pinions. Without wishing to upset anyone, perhaps Mr Stroud measured his pinion across two diametrically opposite chords instead of across two diametrically opposite teeth. The attached photo illustrates my point.

I will probably use the 0.746 inch dimension as it appears to have produced a suitable gear form. I assume the 'standard' depth-of-cut for the hob is used on a blank of this diameter to produce the desired profile shifted gear.

Regards to all, Jeff

mea culpa, I had measured in situ. I have taken the gear off and remeasured

Thanks for that Mr Stroud, looks like 0.7445/0.7465 is the go!

Here are a couple of useful links for those mined to explore:

**LINK**

http://www.hessmer.org/blog/category/gears/

**LINK**

https://khkgears.net/new/gear_knowledge/abcs_of_gears-b/gear_profile_shift.html

MichaelG.

Hi Michael

Have a look at this gear calculator from Ondrives it only works in module inputs so for 20 DP you would have to input 1.27 mod the interesting thing is when the checkboxes are filled in and calculated the diagram at the bottom of the page changes and shows the the tooth profile.

https://www.mesys.ch/calc/ondrives.fcgi

John

.

Thanks for that, John ... very impressive

For convenient reference: **LINK**

My Bookmarks folder has grown again !

MichaelG.

Edited By Michael Gilligan on 20/03/2021 10:14:40