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Basic Gear Hobbing Query

Brannigan

Plastic
Joined
Jan 8, 2018
I've been running an old Barber Colman #3 to cut splines into brass shafts, using a 48DP serration hob to cut 18 teeth. All working perfect.

Now, I have a job to cut 36 teeth into the same diameter shaft. Might this be as simple as adjusting the change gear to cut 36 teeth, with the same hob, to double the number of revolutions? Obviously the teeth will be cut to half depth. Or would this ruin the piece?

Wondering if I would have to cut 18 teeth, then shift the slide or indexing of the shaft to cut another set of 18 teeth inbetween the first lot.

I'm new to hobbing and just trying to figure this out. Have been quoted £500 + VAT for a 96 serration hob so would clearly like to work out a cheaper route.

Thanks for your thoughts!
 

adh2000

Titanium
Joined
Dec 21, 2005
Location
Waukesha, WI
I’m no expert for sure but it seems obvious that if you are going to cut a different number of teeth into the same diameter shaft you will need a different hob. One hob will cut one tooth shape. Change the blank diameter and you must change the tooth count.


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Brannigan

Plastic
Joined
Jan 8, 2018
I’m no expert for sure but it seems obvious that if you are going to cut a different number of teeth into the same diameter shaft you will need a different hob. One hob will cut one tooth shape. Change the blank diameter and you must change the tooth count.


Sent from my iPhone using Tapatalk

Realise that this would apply for involute gear teeth, though the tooth shape of serrations, in this case, is exactly the same.
 

Dan from Oakland

Titanium
Joined
Sep 15, 2005
Location
Oakland, CA
If you are going to cut twice the # of teeth on the same diameter shaft, your tooth form will need to have ½ the circular pitch of your current setup. You are correct that you will need a 96 DP hob for the new job. Basically, the circular pitch x #of teeth= pitch diameter of the part- its simple geometry- no way around that. As far as the cost of the hob, welcome to the world of gear cutting, although that does seem a little on the high side. Call Ash Gear- even with shipping I would imagine you can save some money.
 

TGTool

Titanium
Joined
Sep 22, 2006
Location
Stillwater, Oklahoma
And check on possibly just renting the cutter. Some years back we worked with a shop that cut some gears, but when we needed something they weren't tooled for, they just rented the cutter. Of course the rental cost was in our bill, but it beat being charged full price for tooling for a on-off.
 

David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
18 serrations plus 2 = 20
20 / 48 = 0.416" dia. Is this your o.d.?

36 serrations plus 2 = 38
38 / .416 = 91.2 dp cutter, not 96. (38 / 96 = 0.396" dia.)

Unless it is your pitch diameter that is most important. I know, I'm nitpicking.
 
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Joined
Apr 14, 2018
Location
Airstrip One, Oceania
18 serrations plus 2 = 20
20 / 48 = 0.416 dia. Is this your o.d.?
Depends on what he is talking about. Serrations are not the same thing as involute teeth. There's no standard for serrations, they can be anything.

So, if one wanted to cut these teeth, one would need more information. But the pitch question will be the same, it's hard to cut 10 tpi with a 5 tpi hob :)
 

David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
Depends on what he is talking about. Serrations are not the same thing as involute teeth. There's no standard for serrations, they can be anything.

...to cut splines into brass shafts, using a 48DP serration hob...

So, if one wanted to cut these teeth, one would need more information. But the pitch question will be the same, it's hard to cut 10 tpi with a 5 tpi hob :)

Yea, not enough info. I'm just saying that a 96dp cutter backed out to a larger dia will change the pressure angle of the splines, maybe in not a good way.
 
Joined
Apr 14, 2018
Location
Airstrip One, Oceania
Yea, not enough info. I'm just saying that a 96dp cutter backed out to a larger dia will change the pressure angle of the splines, maybe in not a good way.
You're assuming that since he used the word splines, he meant splines. But he also said "serrations" which is an entirely different kettle of fish. Those can be anything the designer wanted.

So if one were to obtain a cutter for this, the first thing is to determine what he's actually talking about.

Been here, gone through this, a few (hundred) times ...

One actually needs the info off the print or second choice, off the end of the hob he's already using and some kind of check that this second part does yes indeedy need the same teeth as the first part.
 

David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
You're assuming that since he used the word splines, he meant splines. But he also said "serrations" which is an entirely different kettle of fish. Those can be anything the designer wanted.

So if one were to obtain a cutter for this, the first thing is to determine what he's actually talking about.

I agree!

Been here, gone through this, a few (hundred) times ...

One actually needs the info off the print or second choice, off the end of the hob he's already using and some kind of check that this second part does yes indeedy need the same teeth as the first part.

We had a guy come into the shop to have a shaft made. He said that the shoulder lengths had to be exact.

When he came back to pick it up, he pulled out a carpenter's folding wood rule and measured it. He said that the lengths looked good.
 
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David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
36 serrations cut on a .416 diameter blank with a 91.2 dp hob. I'll do a long addendum one on the same dia. with a 96 dp, then overlay the two profiles.



This is the profile difference between 91.2 and 96 dp cut to my guessed diameter of .416". I guessed at the hob's form, too:

172135502.png
 
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David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
Now, I have a job to cut 36 teeth into the same diameter shaft. Might this be as simple as adjusting the change gear to cut 36 teeth, with the same hob, to double the number of revolutions? Obviously the teeth will be cut to half depth. Or would this ruin the piece?

Wouldn't work.

Wondering if I would have to cut 18 teeth, then shift the slide or indexing of the shaft to cut another set of 18 teeth in between the first lot.

Maybe. Look at this:


In this case, the 48dp cutter is working beyond its intended pitch diameter, so the tooth-space angles are exaggerated.


What it's for will determine if it will work (I would buy the right hob).
 
Joined
Apr 14, 2018
Location
Airstrip One, Oceania
I think he left the building :) Had to laugh at the "Made in USA" part, you may just as well have printed Star Cutter. That's the only US one left, I think. Fellows gone, Barber-Colman gone, ITW gone, Cleveland gone, that's all I can think of ... and Star was the one you went to for cheaper unground hobs.

With the prices they get and the ridiculous delivery times, you'd think some small shop would get into that field. Hint hint.
 

David_M

Hot Rolled
Joined
Sep 30, 2014
Location
Midway, GA, USA
At least the video was made the USA :D. I put it there because the wagon-wheel effect made it look like it was revolving backwards. I'll add gashing next time :).
 
Joined
Apr 14, 2018
Location
Airstrip One, Oceania
You're going to have a hell of a time sharpening that hob, hippy :) Front faces of the teeth have to be in a straight line longitudinally. Sometimes they are at an angle but always straight enough so you can run a grinding wheel across the face.

Sharpener is usually a saucer type wheel grinding on the slanted portion of the wheel. Unless the hob is designed with rake, front face of the teeth has to be on a line that passes through the center of the bore.

Also, the relief makes the tooth deeper at the back than the front. Maybe your model is like that and I am just looking from the wrong side, but imagine a relieving lathe - as the hob blank rotates forward, the cutting tool plunges front to back making the tooth deeper at the trailing edge. It's the same shape but pushed back and down.

Back to the old drawing board :) Nice first attempt tho.
 








 
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