Gear cutting question

Wino1442

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Hi all..... I have to replace a power-feed gear on my lathe. I've made a few practice runs at making a gear before I attempt to make the actual gear and have a question. The original gear has 24 teeth, is 16DP and has a 14.5° PA. Doing the math, which I'm pretty sure I did right, tells me that the diameter of the blank should be 1.625" , but the original gear measures 1.595".
So the question is: Do I need to make adjustments to the depth of cut because of the difference between original gear diameter and what the math says the diameter should be?
 

benmychree

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It would be of interest to determine if the old gear is undersize on the OD and if the pitch diameter is correct, perhaps check the size of the root diameter and see if it is undersize as well as the OD, if this is true, you would cut to standard depth going from the undersize OD. If the root diameter is standard, make the blank to the standard OD and cut to standard depth.
Having said that, it is not entirely unheard of to modify pitch diameters (and subsequently OD and root Dia. to suit some special circumstance in the gear train. Where is this gear in the feed train?
 

Wino1442

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Thanks for the response....the gear is for the power cross-feed on a Craftsman/Atlas 12" lathe.

I'm wondering if I make the new gear the same diameter as the old gear and use a depth of cut of .135" would the new gear have the correct tooth form because, I believe, that depth of cut would be based on a 1.625 diameter gear and not the 1.595" of the old gear.
 
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AGCB97

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In my experience, if the diameter of the blank is not according to the number of teeth and other parameters the last tooth cut will be long or short. No good!
Aaron
 

benmychree

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In my experience, if the diameter of the blank is not according to the number of teeth and other parameters the last tooth cut will be long or short. No good!
Aaron
I do not understand what you mean about the last tooth being long or short, If I do understand, the only way to have that happen would be by an indexing error in counting holes in the index plate, something that has happened all too often before I got a genuine B&S dividing head with the graduation on the sector arms.
 

benmychree

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The only thing that happens when teeth are cut too deep is that the chordal thickness is reduced and the root diameter is also reduced. Are we talking about the same thing?
 

AGCB97

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If everything is correct including DOC but the diameter of the blank is too small, you advance your indexer by the proper amount each time but when you get to the end, the last tooth overlaps to some amount the 1st cut tooth. It took me a while to understand this also. It is because you are advancing the indexer by the amount that WOULD if the diameter was correct end up at the proper spot for the last tooth. But you can not fit the same number of correctly sized teeth on 2 different diameters. That's what I meant by long or short (tooth distance not tooth height.

As I said in post #4 "in my experience"

Respectfully
Aaron
 

benmychree

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I agree that you cannot end up with the same number of correctly sized teeth on an undersized blank, but as I said, with this condition and equal spacing, you just end up with somewhat smaller chordal thickness; in a gear sized as above the difference in tooth thickness would be quite small, and result in slightly more backlash in the gear train. I have occasionally measured chordal thickness in accurately sized blanks cut to full depth, and found them to be thinner than called for, and for most purposes, it seems to make no difference.
Also respectfully
John
 

Wino1442

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Aaron....so what's the fix? Do you increase or decrease the DOC by the difference between the "correct" diameter and the actual diameter of the blank?
 

AGCB97

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I don't know. Might have to do some study in machinery's handbook looking not at the tables but at the formulas. I've never had to make an unconventional gear (if there is such a thing) my 'experience' came when I was sloppy when making a gear blank and thought 'what's the difference if it's a little off'. I found out!
Aaron
 

cathead

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If it were me, I would make a standard 24 tooth 16DP 14.5PA gear since the original had those specifications.
 

Lo-Fi

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The correct term for this is "profile shifted". It's a common practice; either to make the teeth larger at the root for strength, or to compensate for the centre distance not being quire right for the two gears in question if they were standard profile.
Explained brilliantly here:

I've never cut a profile shifted gear myself, but it should be easy enough to dig out some info on doing so with milling cutters. You'll need to cut the blank the correct diameter as well as get the DOC right.
 

AGCB97

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Or maybe as @cathead said cut the standard gear and then if necessary take up to .030 off the OD to match. Even at that you'd only loose .015 of tooth height.
Aaron
 
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