Calculating lead for a helical gear

**99Panhard** · 10-24-2020, 01:11 PM

I would think there must be a formula... It's a DP8 gear, 20 teeth, helix angle 20 degrees. I confess to being a bit math challenged so I've probably found the answer already but simply don't recognize it.

**Jim Christie** · 10-24-2020, 02:45 PM

**99Panhard** · 10-24-2020, 02:52 PM

I have the B&S book and the Cincinnati book and I'm sure they explain what I should do but I'm having a difficult time understanding it...if I keep reading it over, sooner or later it will start to make sense. I always struggle with these computations until I've seen it done.

**Jim Christie** · 10-24-2020, 03:08 PM

I also find it difficult to translate what's in the book into practice with things like that.
If you have an existing gear and can make the setup with it then when you follow the tooth path with a dial indicator it should give you a zero reading right across tooth if you are set up right although if there is wear on the sample then it may be hard to tell.
Jim
I have no other easy tricks to offer .

**johnoder** · 10-24-2020, 09:23 PM

Might be helpful - but be prepared for some deep wading

In the second image go to the line about 2/3 of the way down the page to find a formula for "L"

Pn is for Pitch normal, which is your 8DP - and the value immediately to the right of that is the sin of your 20 degrees

The value ABOVE the line is "pi" times your number of teeth

$helical-math-.jpg$

**Limy Sami** · 10-25-2020, 06:17 AM

The Oracle has spoken

Thank you John.

**99Panhard** · 10-25-2020, 07:18 AM

Thanks John..I am beginning to understand how to do it.

jp

**johnoder** · 10-25-2020, 07:38 AM

Originally Posted by Limy Sami

The Oracle has spoken

Thank you John.

Aw come on fellas (and of course the gals) - its all written down for us. Not sure about first image - but second is of course from Earle Buckingham - who has at least three volumes on the subject of gears

And Joe I come up with 22.9635" lead. See if you agree.

You do know that if you had the OTHER gear tooth count AND a center distance - you could use the first image to see how close the helix angle actually was to 20 degrees - using the first image. A way to fine tune the helix angle and lead - so important if one is trying to make a mate to something old

**ramsay1** · 10-25-2020, 09:55 AM

And when you set up your lead box. you will probably find that this particular lead falls between two options...The experience I had with Kearney and Trecker....Not perfect but better than nothing.. Cheers; Ramsay 1

**EmanuelGoldstein** · 10-25-2020, 10:08 AM

Originally Posted by ramsay1

And when you set up your lead box. you will probably find that this particular lead falls between two options...The experience I had with Kearney and Trecker....Not perfect but better than nothing.. Cheers; Ramsay 1

If you are going to heat treat after, head to the too-tight side. They usually unwind a bit in heat treat.

**Limy Sami** · 10-25-2020, 02:27 PM

Originally Posted by johnoder

Aw come on fellas (and of course the gals) - its all written down for us. Not sure about first image - but second is of course from Earle Buckingham - who has at least three volumes on the subject of gears

And Joe I come up with 22.9635" lead. See if you agree.

You do know that if you had the OTHER gear tooth count AND a center distance - you could use the first image to see how close the helix angle actually was to 20 degrees - using the first image. A way to fine tune the helix angle and lead - so important if one is trying to make a mate to something old

Yes John, but you know where to find it, and more importantly can explain it to guys like me, when plain textbooks etc etc don't.

**99Panhard** · 10-25-2020, 04:15 PM

Originally Posted by johnoder

You do know that if you had the OTHER gear tooth count AND a center distance - you could use the first image to see how close the helix angle actually was to 20 degrees - using the first image. A way to fine tune the helix angle and lead - so important if one is trying to make a mate to something old

I wish I did have it but that would require dismantling the engine it's on - and that is in Florida. The car is in remarkable unrestored condition - never having been apart since it was built. The cotter pins are still in the nuts. All the fasteners have ALAM threads. These appear to be the predecessors to SAE threads but aren't the same for all sizes. The 1/2" bolts are 1/2-18 and several of the other sizes I've only seen in pre-WWI catalogs.

I am pretty sure that the mating gear has 40 teeth - since it drives one of the two camshafts (It's a T head engine) and the magneto drive gear has to have 1/2 the teeth of the cam gear. Unfortunately, this gear drives the generator and the water pump rather than the mag so it could be different but the exploded views I've seen of the engine suggest otherwise...I will, however go back and check that.

There won't be any need to heat treat. It's actually under very little load, runs in a constant oil bath and I would prefer it be softer than the mating gear if only because if one is going to wear this is the one it should be.

**99Panhard** · 10-25-2020, 04:22 PM

Originally Posted by ramsay1

And when you set up your lead box. you will probably find that this particular lead falls between two options...The experience I had with Kearney and Trecker....Not perfect but better than nothing.. Cheers; Ramsay 1

Which it does.. it falls between 22.93 and 23.04

jp

**99Panhard** · 10-25-2020, 06:19 PM

I'm getting closer to understanding this but I still haven't come up with the same number John did. I think the problem is the sine of the 20-degree helix. I'm using this chart from the B&S book on gearing but what does the first column signify? It isn't explained in the book...

**rwilliams** · 10-25-2020, 06:37 PM

If I understand the question, I think the that first column is minutes so you can get more refined than whole degrees.

Bob

**johnoder** · 10-25-2020, 06:37 PM

Extreme upper left corner .34202 is the sine of 20 degrees and you multiply that by 8.

OR .34202 TIMES 8 = 2.73616

You divide that result that into 20 teeth times "pI". and 20 times pi = 3.14159 X 20 = 62.8319

and 62.8319 divided by 2.73616 = 22.9635"

Originally Posted by 99Panhard

I'm getting closer to understanding this but I still haven't come up with the same number John did. I think the problem is the sine of the 20-degree helix. I'm using this chart from the B&S book on gearing but what does the first column signify? It isn't explained in the book...

**99Panhard** · 10-25-2020, 07:02 PM

EUREKA! Now I get it!

Thanks again John...

**Screwmachine** · 10-25-2020, 07:05 PM

I remember needing to make a rack to match a helical gear. I was chatting with a guy named Heinz in the U.S. who had a long history with Dixi and a large handful of other Euro makers. Mikron rep. When I mentioned blueing the teeth of my cylindrical gear to find the helix angle he laughed so hard- and this was at the wedding reception of my toolmaker friend-; looking back I laugh too. I did blue those teeth, and did some estimation, and made a rack that worked. But this was effectively an adjuster for a microscope, no real power involved. As a watchmaker who's done a shit ton of gearing in prototyping the echos of Heinz's laugh ring loudly in my ears every time I try to hoke things. And 99.9 % of my shit is spur gears.

**ramsay1** · 10-25-2020, 07:53 PM

Originally Posted by 99Panhard

Which it does.. it falls between 22.93 and 23.04

jp

Exactly what happened when I cut the helical gear for my Kearney and Trecker std his speed milling attachment.. The tooth contact is not true across the entire tooth but it is plenty good enough and relatively quiet in operation. Cheers; Ramsay 1

**johnoder** · 10-28-2020, 09:09 AM

Wondering..

If it was in fact 8DP normal pitch, possibly Van Keuren's magic shown here would yield a pitch diameter using regular old .216 dia. (spur gear wires) wires in order to have something closer than eyeball for helix angle calcs