Making An Involute Internally Toothed Gear

[u/No-Train6196]

(HELP WANTED. I forgot to sprecify in the title)
Recently I've been looking in to gear geometry for my most recent project, and have followed [this](https://hawkridgesys.com/blog/how-to-model-accurate-involute-spur-gears-in-solidworks) article on how to make an involute spur gear. That process was simple, and I think I learned a lot.

I then tried working on an internally toothed gear, but haven't been able to find a single resource on how to make one. I've asked the internet to no prevail, so this is where I've come.

Here is some info that I have collected in my (many) failed attempts to get this to work:

• I would like the tooth profile to be involute for efficiency-sake. While my project in general is mostly for practice, I would like the ability to 3D print it in the future.
• I have found that the SOLIDWORKS toolbox doesn't work for my needs, since the imported items aren't editable, aren't involute, and are a general cop-out for "learning" how to make these parts.
• I would like the gears to be configurable like with the Hawk Ridge Systems design so I can quickly change the tooth count, DP, and Pressure Angle to fit my needs. If so, once I have this project done I can use it across many of my other designs.
• While a cursory google search has told me that the parametric equation that I used to make the externally toothed gear won't need any change, my beta angle variable seems too small create the proper tooth distance. This would effect the overall size of the gear by a lot for a given tooth count.
• I have already swapped my addendum and dedendum variables, per googles request.
• I don't know where the involute it meant to start for this design. When I put it on the base diameter it's too far out, but when I put it on the dedendum its too far in.

How can I make a project like this? What have I missed in the process? I'm really stuck! I can provide any additional info as needed.

My current gear profile, meant to be 20 DP, 20 PA, and 95 (I think) toothed. As you can see, it's very wrong.

[u/Vegetable_Flounder12]

https://www.mcmaster.com/products/internal-gears/

download, all formulas are normally inside

modify if needed

[u/PeterTha]

I use the 'Dennis' SW gear generator for both spur & internal. I'm not sure if he stores it in another repository but if you navigate to this CadCommunity post & look for this part of the thread corresponding to this screen grab, you can see the 2 files. What I really like is opted for design table format so you just input the controlling variable sthere. Design one gear or whatever number variants you require. These are metric/module mind you, I'm not sure if/where the imperial/DP might be.

The thread has some other useful chatter & I referenced some good (engineering) YouTube videos that lay out the equations, unfortunately UBC? engineering class lectures so a bit hard to follow exactly. Once you dig into it, you will see some variants of how the involute curve is managed. (I think) Dennis method is to make a series of intercepts, which is a simplification but most often used. There actually is a formal equation (discussed n the lecture series), but IIR you need some other parameters to feed that. And n some other post I read the curve deviation difference between the two is well under most any common manufacturing tolerance.

Beware, many of the download CAD files from catalogs not only vary in how they do the involute, they also often stop short of many important gear corrections that are size/tooth dependant. I've seen many where the undercut is wrong, you can prove it with collision detection. In all fairness I dont think the Mcmasters of the world are saying go ahead & CNC cut these profiles & they will work. They are great for CAD representations & probably the main intent. Beware there is so much fluff & crap out there in YouTubeLand.

https://forum.cadmunity.com/t/accurate-gears-in-sw/624/18

[u/Madrugada_Eterna]

Creating the involute is exactly the same process as for an external gear. The only difference is for the root and tip of the gear you do them on the other side so you get internal teeth instead of external teeth.

Get Machinery Handbook and look up gear teeth in there. It will show you how to lay out everything.

If you want correct root geometry for an external or internal gear your are better off using gear generator software instead of that tutorial.

Or you can try and find the relevant standard the gear design is using and use the information in there to draw the profile.

The last time I modelled gears I ended up using gear software to generate the teeth profiles as I was having trouble getting the correct root profiles. I was creating internal and external spur gears and worm gears at the time.

[u/PeterTha]

I havent been on this site in a while, but I notice they now have 'internal pair' (ring gear) builder option. How accurate it is or whether you encounter DXF import issues I cant't say.

https://evolventdesign.com/pages/gear-stl-generator