Gear cutter doesn't match profile of gear (what did I do wrong?)

Ecosta777

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Sorry for the poor title, I wasn't sure how to word it in the short space.

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.

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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The current Chinese cutters are numbered backwards. in their system, a #8 is 12-13 tooth. However, the old standard and what American cutters use is #8 is 135 tooth and larger. You would need a #1 to cut 12-13 tooth. The markings on that cutter appear to be old, meaning it's likely for 135 tooth.
 
The current Chinese cutters are numbered backwards. in their system, a #8 is 12-13 tooth. However, the old standard and what American cutters use is #8 is 135 tooth and larger. You would need a #1 to cut 12-13 tooth. The markings on that cutter appear to be old, meaning it's likely for 135 tooth.
Machineries Handbook, 1968...

Yes, should have added cutter was a NOS off eBay
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Maybe the old gear isn't made to specifications possibly causing part of the problem. The shape of the teeth
looks incorrect to my eye....
 
Edit. Read fail. I thought the broken gear was one cut with the new cutter.
 
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I agree with cathead - that original gear ain't right. How well does it mesh with the other gears in the shaper gearing? It may have been replaced before with one cut with the wrong cutter. If so, it's time to get things right.
 
Cutters marled bevel will have narrower profile than those for normal straight cut gears, that is because the bevel cutter is made to fit the inner (small) end of the teeth; bottom line, it is clearly the wrong cutter.
 
The original gear was correct.....
I got a regular #8 cutter with a 20DP, and that was the right profile. Just finished the new pinion over the weekend it just doesn't have the undercut at the bottom of the tooth like the original, but it fits perfect on the rack.
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