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)?

Good morning

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

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

Thank You Jason!

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.

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

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

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.

.

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