Use of gear pitch gauges

Rudd · May 12, 2018

I am making a pinion for the feed on an antique DP. It is 1.25" OD, 13 teeth. So, I got a No. 8 12 pitch cutter. It cuts teeth much too wide to match the original. I'm thinking these are not standard teeth. Did they use a lower cutter number - i.e., a 6, a 7? A cutter of a different DP than standard?
Would a gear pitch gauge gauge help me sort this out? It looks like it would only address the DP number.
Thanks

Zahnrad Kopf · May 12, 2018

Not enough info to help you.

Were the milled originally? Were they Shaped? Hobbed? What are the actual specifications? Center Distance? Outside Diameters? Number of Teeth? You should have that info to have begun anything, regardless.

johnoder · May 12, 2018

Some makers of cutters are known to number them backwards. USA cutters used #8 for 12/13 teeth (per my 1914 UTD catalog). The backwards numbered off shore cutters would use #1

The cutter for 12/13 teeth will have a pronounced curve. The cutter for 135 to rack will have hardly any perceptible curve

UTD thought you should have a page of comparative curves #1 to #8 and provided same in their 1914 catalog

Rudd said:
I am making a pinion for the feed on an antique DP. It is 1.25" OD, 13 teeth. So, I got a No. 8 12 pitch cutter. It cuts teeth much too wide to match the original. I'm thinking these are not standard teeth. Did they use a lower cutter number - i.e., a 6, a 7? A cutter of a different DP than standard?
Would a gear pitch gauge gauge help me sort this out? It looks like it would only address the DP number.
Thanks

PeteM · May 12, 2018

You also need to verify the pressure angle of both your gear and cutter. Older gears tend to be 14.5 deg. pressure angle. Newer gears (and cutters) tend to be 20 deg.

Gear tooth gages will come in both these pressure angles (and sometimes others, such as 25 deg.).

Rudd · May 12, 2018

It's 14.5 PA - antique DP - US made.
I wish I knew where I could see such a chart. I did learn that University of Texas at Dallas (UTD) has a machine shop.... :rolleyes5:

At whole depth - 0.180" the tops of the teeth are almost a knife edge.
Is there a way to measure the remaining teeth of the existing pinion and back into the correct cutter?
Zahnrad. is there other information needed that would help nail this down?

Zahnrad Kopf · May 12, 2018

Rudd said:
It's 14.5 PA - antique DP - US made.
I wish I knew where I could see such a chart. I did learn that University of Texas at Dallas (UTD) has a machine shop....
At whole depth - 0.180" the tops of the teeth are almost a knife edge.
Is there a way to measure the remaining teeth of the existing pinion and back into the correct cutter?
Zahnrad. is there other information needed that would help nail this down?

Post or email me the info you gathered and we'll see if we cannot get you set up properly. Offhand, the basic calcs look okay, but you don't state the OD of the original Pinion. So, we don't know if it's the same. Too, it very well could be a Non-Standard Tooth Form. You don't state the Center Distance or the mate's dimensions to be able to discern any abnormal dimensions.

Rudd · May 12, 2018

in first post 1.25" OD. Center distance unknown, all I have is the pinion. It works on the rack cut in the quill of the DP, which is across the country in a woodbutcher's shop. 13 teeth in pinion. Rough measure of the width of tooth "about in the middle" gives me .132 or so - so sounds like 12DP to me. I believe it to be 14.5 PA - based on profiles in Boston Gear catalog. Those indicate to me the issue with the narrow land at the top of teeth would be worse - is this correct? Also antique (1920's?) US Made I could be wrong.

Edit - whole depth seems to be correct - matches on what I cut and the original. .180" WD

I appreciate your help, I'm trying to help someone else out, and I have to go in for surgery the 23rd that is going to leave me worthless for quite a while.

Conrad Hoffman · May 12, 2018

That looks relieved for clearance like many pinions are and must be, and was probably done on a gear shaper. I don't know if it can be done with the numbered cutters, but I'm not a gear guy.

PeteM · May 12, 2018

Doesn't look like a standard 14.5 deg. gear profile, unless I'm misjudging the photo. Seems there's a sort of bulb at the end of the gear tooth.

You might be able to make a single tooth fly cutter to pretty closely match the profile.

Any chance you could buy a pair of standard gears and mate them to the old shafts??

Zahnrad Kopf · May 12, 2018

Are you sure that you don't have an imported mill that is numbered backwards? Does it have the correct number of teeth marked upon it?

L Vanice · May 12, 2018

PeteM said:
...Any chance you could buy a pair of standard gears and mate them to the old shafts??

He said the pinion drives rack teeth cut in the quill of the drill press.

Larry

johnoder · May 12, 2018

Here is what any appropriate standard cutter will do for you profile wise for 13 teeth - with some variation related to PA

This is 48 DP which is of no consequence since the standard profile applies to all DP

It looks like this is not what you want since it does not in any way look like your "roundy" teeth

13 Teeth 12 DP - Google Search

Leg17 · May 12, 2018

Looks like a shop-made form cutter is in order.

L Vanice · May 12, 2018

Rudd said:
in first post 1.25" OD. Center distance unknown, all I have is the pinion. It works on the rack cut in the quill of the DP, which is across the country in a woodbutcher's shop. 13 teeth in pinion. Rough measure of the width of tooth "about in the middle" gives me .132 or so - so sounds like 12DP to me. I believe it to be 14.5 PA - based on profiles in Boston Gear catalog. Those indicate to me the issue with the narrow land at the top of teeth would be worse - is this correct? Also antique (1920's?) US Made I could be wrong.

Edit - whole depth seems to be correct - matches on what I cut and the original. .180" WD

I appreciate your help, I'm trying to help someone else out, and I have to go in for surgery the 23rd that is going to leave me worthless for quite a while.

View attachment 228364
View attachment 228365

If that is the original pinion, I suggest the tooth form looks like cycloidal, not involute. Cycloidal gears are used in clocks and watches where the large gear drives the small pinion. It was common in 19th Century American brass movement clocks to make cycloidal pinions with round steel wires inserted in a "lantern" shaped cage. The mating gear only engaged the outer half of the wires, which is, of course, a true semicircle.

Lantern pinion pictures: lantern pinion - Yahoo Image Search Results

So, the pinion gear tooth form needs to be verified from the rack tooth form, straight or curved, and angle if straight, and the pitch, which is easy to measure on a rack. An involute 14.5 degree PA rack will look like an acme thread in profile, with the teeth having straight flanks with a 29 degree included angle.

Larry

Zahnrad Kopf · May 12, 2018

Rudd said:
Rough measure of the width of tooth "about in the middle" gives me .132 or so - so sounds like 12DP to me. I believe it to be 14.5 PA - based on profiles in Boston Gear catalog.

Wait, wait, wait...

"Sounds" like 12DP...???? No... Do us the favor of accurately measuring the original gear again. Please let me know what it is.

Given the supposed age of the drill press, the appearance of the original, and not enough information, I have a sneaky suspicion that you are dealing with a Circular Pitch gear. NOT as Diametral Pitch one. I'm leaning toward a 1/4" Circular Pitch. And after seeing Larry's post up above my own, Im inclined to agree with him. I always forget about Cycloidal gears and his avocation keeps them firmly in mind. :cool:

Rudd · May 12, 2018

OK, Sorry Zahnrad, I was being a bit flippant. 13 teeth, 1.25 OD *is* 12 DP. Pitch tooth width is within spitting distance of the value from the Boston Gear catalog for 12DP. 0.132 roughly measured v. 0.1309 from Boston Gear.
Cutter is import, marked Z12-13.

By "Cycloidal" - is that the same as epicycloidal, which I have read about but never seen? It kind of makes sense in that the quill would drive the pinion when you let go of the DP handle and the spring retracts the quill.
Given that this thing is ultra-low RPM, I'm wondering if a different involute cutter wouldn't get it close enough - see the last photo attached.
I'm really wondering if this is some special profile Buffalo Forge came up with for DP's. I am verifying age of machine with Owner.
Owner says age unknown but is in 1929 Catalog

Tip of cutter is just shy of root of thread when curved "shoulders" hit OD of part.

Cut I got.

If the cutter didn't sweep out so much at the curve, I think it would do it. See red line drawn in. Would this be a lower number cutter? i.e., a 7, a 6?

EPAIII · May 12, 2018

Boston Gear had a couple of pages in their catalog that illustrated the tooth profiles for a range of pitches for both 20deg and 14.5deg gear tooth. They were actual size for each pitch so you could just lay a gear over one or in a meshing position. I found them very helpful in IDing gears.

They seem to have been taken over by another company and I can not find that catalog any longer. I had downloaded it, but the two folders where I had it were both empty. Can they erase things on my hard drive?

But if you can find a similar set of drawings, perhaps they would help.

PeteM · May 13, 2018

EPAIII said:
, , ,. I had downloaded it, but the two folders where I had it were both empty. Can they erase things on my hard drive? . ..

Could be they only let you download an HTML link. But even in that case there should still be that dead link left in your folder , , ,

Erich · May 13, 2018

A 14.5 degree PA gear with only 13 Teeth will be severely undercut. If I remember my college gear classes, any teeth under 32 has some amount of under cutting.
The involute curve is defined from a Base circle. (The circle the string unwinds from in the descriptions of the involute in all the text books. The involute is undefined inside the base circle.
For a 14.5 degree gear with 32Teeth, the base circle coincides with the root diameter of the gear. At smaller tooth numbers the base circle is larger in diameter. That means part of the side of the tooth does not have an involute shape. In fact the tooth has to be scooped out to make room for the tip of the mating gear. Mating with a rack requires the most amount of scooping out (Called Undercutting).
Undercutting is bad for two reasons. First the pinion tooth is not as strong. Duh. Second, for part of the tooth engagement, the action is not conjugate (fancy word that describes how involute gears transmit uniform angular motion)That means the action is not smooth so gears will be noisy, wear poorly and not transmit uniform motion.
All issues except strength that really don't matter to a drill press.

Dan from Oakland · May 13, 2018

Adding to what Erich posted above ;
Pretty sure what you are seeing here is normal undercut in a pinion with a low number of teeth. PA is most likely 14.5 and its probably an involute not cycloid. In this case, the undercut comes up to almost the pitch diameter and the part was most likely hobbed, as a space cutter could not form the undercut for obvious reasons. My guess is it was not cut on a gear shaper, as the additional depth of a shaper depth cutter (2.25/DP) would only increase the amount of undercut and make the tooth even weaker than it is. Checking the base pitch is the best way to determine the actual measurements or the quick and dirty way (which most folks can't do) is to simply roll the sample part with a few different hobs but. . . .

Use of gear pitch gauges

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