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"Correct" gear cutter does not match gear profile (What did I do wrong?)

ecosta

Aluminum
Joined
Apr 1, 2012
Location
MA
Rookie question, I haven't touched gearing since high school, and even then I didn't quite understand it. I have to make a new gear for my Bridgeport shaper head. I did the math to find out what cutter I need, but when the cutter arrived, the profile was different than the gear. It's much smaller and not quite the same shape. This is a NOS cutter I got from eBay.

Here's my math:

It has (had) 13 teeth, with and OD of .75"

13 teeth +2 / .750
15 / .750 = 20DP

13 teeth = a #8 cutter (12-13 teeth)

So I ordered a #8 cutter with a 20DP, 14.5 pressure angle, but it's not right for the profile of the gear.

The only things I can think of is I did my math wrong? But I've double checked and thought everything was correct. OR it's because the gear cutter says BEVEL on it? Is the cutter I ordered for a bevel gear, not an involute gear?

I know the gear is hard to read on one side, it's does say 8 20DP BEVEL 14.5 pressure angle
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sfriedberg

Diamond
Joined
Oct 14, 2010
Location
Oregon, USA
Yes, a big part of your problem is that is a cutter for bevel gears. There are several people here who will say there's no such thing as a bevel gear cutter, but I've seen shelves of WW2-vintage old stock gear cutters at Sam Mesher in Portland. About all that was left was odd-number diametral pitches and bevel gear cutters.

Additionally, it is not unusual at all for pinions and other special gears to have non-standard tooth addenda and dedenda (tooth height plus and minus relative to the pitch diameter). Looks like that pinion had extra clearance at the roots, similar to what a true generated profile might look like from a mating tooth with large addendum.

[Added in edit] You may be able to make use of that bevel gear cutter, if you can figure out how much the tooth side profiles have been moved toward each other. Instead of taking one cutting pass with the cutter profile centered on the gear space centerline, take two passes, each offset slightly relative to the gear space centerline. That should produce a correct width gear space with proper tooth profiles. ISTR that bevel cutters were often made for a minor face diameter that was 1/3 of the pitch diameter, but that memory could be wrong and that cutter might not comply with that practice even if I remember correctly. You still may have to relieve the roots of the spaces to make room for the mating gear.
 

Mark Rand

Diamond
Joined
Jul 9, 2007
Location
UK Rugby Warwickshire
The other problem is that the cutter has a pressure angle of 14 1/2° and the original pinion will be 20°. The teeth on the pinion are waisted/undercut because they were cut with a hobbing machine. This also implies that the teeth probably weren't cut with a modified addendum, because that is used to avoid the undercut teeth.The Brown&Sharpe type of cutter that you have will never produce an undercut tooth, it can't if you think about it...

The undercut isn't a problem. The incorrect pressure angle will make the pinion run roughly and wear rapidly.

20° B&S type gear cutters are available, but they aren't cheap and will be as common as rocking horse sh!t on EBay.


BostonGear.com don't carry stock gears down to 13T, but the price for a custom one might be cheaper than the alternative. The alternative is to get a gear cutter for the mating gear and make a new pair from scratch with a 14.5°PA. I did this with the apron gearbox for my Hardinge HLV, but that was because every single gear was wrecked and the whole lot needed remanufacturing!
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Not an expert (plenty here though)....

However, I think I see two other issues. First, are you sure of the match as far as pressure angle? At the visibly needed depth, it sure does not look parallel to the tooth form. Might be if pulled out much farther, but.....

Second, small tooth count pinions have toothspace shapes that a form cutter is not capable of making. You can kinda make the part, but it may not work as well as a hobbed gear.

Maybe if you gash it to the needed depth and width, then do offset passes as described by the previous poster.

I see another person was typing when I was.
 

Dan from Oakland

Titanium
Joined
Sep 15, 2005
Location
Oakland, CA
A lot of the above replies are spot on. Bevel involute cutters are produced thin so that they can be offset to thin the bevel gear teeth by side shifting the cutter. Suppose you could do the same offsetting to cut your pinion as is noted above. For your shaper head drive, this may not be the best solution, as there are limits as to how accurate the resulting tooth profile will be. Thought Zahnrad has made these in the past?
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
PM member Zahnrad Kopf has made these pinions in the past. They are excellent. It is worthwhile to wait for him to make another run and buy one. Then, promise yourself you will never forget to tighten the stroke length adjusting nut before using the head.

The shaper head is usable with the pinion broken. The pinion just makes adjustment easier. If you play with the adjustment a while you will figure how to adjust the stroke without the pinion.
 

ecosta

Aluminum
Joined
Apr 1, 2012
Location
MA
Thanks guys,

I returned the bevel cutter, and was able to locate a regular one. Aside from the width being too small, the form of the bevel cutter looked like it matched the original. It looks a little off in the picture, but that's because of the undercut. In person the 14.5 PA looks correct. I'll see if the new cutter I ordered looks correct, if not I'll have to reassess, or just use the original pinion as is with the broken teeth.

I did contact Zahnrad, he had one left one I messaged him but unfortunately I just couldn't justify the asking price. The replacements he makes look incredible, just out of my price range.

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ecosta

Aluminum
Joined
Apr 1, 2012
Location
MA
A standard #8 with a 20DP was correct, and I was able to make a new pinion for the shaper head
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Erich

Cast Iron
Joined
Apr 4, 2017
Prediction: Gear is not going to mate nicely with mating gear. Your gear does not have the undercut the original gear had. I bet you will get binding when you try to turn the gears.
 

ecosta

Aluminum
Joined
Apr 1, 2012
Location
MA
Prediction: Gear is not going to mate nicely with mating gear. Your gear does not have the undercut the original gear had. I bet you will get binding when you try to turn the gears.
Just reassembled, works fine....
It's only for adjusting the cam, it does not rotate constantly

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