Internal and external gear tooth profiles

[DPittman]

I have a book on gears coming and I expect it will explain my questions but I thought I would ask here as there is some great experience and know how.

So if you ground a fly cutter tool to cut an external gear, would that flycutter tool be suitable to use as a broaching tool on an eternal gear of the same module/DP?

If the answer is too big and long and hairy to explain here don't worry, I will eventually find the info elsewhere I think. There just are some people here that can explain things very succinctly.

 

[Johnwa]

I’m pretty sure the answer is no. I think that the shape of the internal broach has to be the same as the external gear “tooth”. The flycutter cuts the spaces between the external teeth

 

[DPittman]

I’m pretty sure the answer is no. I think that the shape of the internal broach has to be the same as the external gear “tooth”. The flycutter cuts the spaces between the external teeth

So the flycutter is basically the same profile at external gear tooth (I think) because gears mesh together...what I can't seem to understand is, is the internal gear tooth any different? Because you use an external gear to mesh with the internal gear and all is fine??

 

[Johnwa]

No, the flycutter is the same shape as the space between the teeth. It has a concave shape while the teeth are convex.

There’s some diagrams on this page that illustrate the differences.
https://khkgears.net/new/gear_knowledge/gear_technical_reference/calculation_gear_dimensions.html


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[DPittman]

No, the flycutter is the same shape as the space between the teeth. It has a concave shape while the teeth are convex.

There’s some diagrams on this page that illustrate the differences.
https://khkgears.net/new/gear_knowledge/gear_technical_reference/calculation_gear_dimensions.html


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Ok thanks for that link and explanation I will have to read it thoroughly and see if any of it can sink into my thick noggin.

 

[PeterT]

The quick answer is making internal gear profiles for ring gears requires a different profile than the module cutter profile used to make an outside (spur gear). Your book will probably talk about this. I've seen methods to make broach profiles that are 'pretty close' but that's a relative term depending on your application. You might also run across references relating to clock gears but generally they are different tooth profiles. They can get away with things the mechanical world cannot which is why involute profiles are preferred for transmitting any kind of load. Think of our mechanical gears as more rolling on one another with a single tangent line, not really sliding on one another as we visualize 'meshing' . And then there is clearance & compensation & other factors that goes into the profile based on size & other parameters.
https://engineering.stackexchange.com/questions/13633/internal-teeth-involute-gear-pitch-circles

Having said this I have seen what I think are perfectly functioning ring gears & racks that were EDM-d but that's another level of sophistication and money. This guy did his planetary cam gear that way. I bought my ring gear. Clearly he wins the tool envy award hands down LOL.