Change from External to Internal teeth

If I have a drawing of a tooth profile (accurate, involute) in AutoCAD of an external spur gear, is there any way to make this the profile for an internal gear? I don’t know anything about AutoCAD but theoretically If you “flipped” it using the pitch diameter as a “curved axis” upon which to “flip” would that work? If so, how do I actually do that in AutoCAD? Thanks.

EDIT: I was just looking at some pictures of internal gears, and now I’m wondering, is it the exact same profile with no flipping? Like a “negative” in a sense. Like if I took a spur gear and put in in a bucket of plaster, and then it hardened and I pulled it out, would I have an internal gear of correct dimension/shape?

No - you would not end up with a gear of the proper tooth shape, pitch diameter, nor clearance.

You’d be better off using a single tooth in the proper orentation, translating it to where it’s pitch line meets the pitch crcle of the mating gear(s), then rotating instances by the proper amount - I.E. 360/(number of teeth)

Sorry about the double post, I was going to delete one but it was late last night and I forgot. Anyhow, now that I know they are not the same, is there any way I can derive the correct internal profile from the external profile I have? I can’t draw a proper single tooth. See, I need the drawing to be involute so I can take this drawing somewhere to be manufactured (edm, perhaps laser, etc.). I have a program (Geargen) that will make an involute external gear in a dxf, but not internal. Can I somehow convert it in Inventor or AutoCAD? Or are there any other programs that will make the drawing?

I found an involute internal gear on cbliss, but it does not seem to show anything when I open it. All the features are grey-ed out.

Is there any way to make a gear drawing good enough to cut in AutoCAD?

www.bostongear.com has CAD files for all the gears they sell which includes internal gears as big as 16 pitch. Click on Boost Spec 2 on their website (you will need to register).

This is an add on program for auto-cad, it saved us countless work when we needed a rack for the window motor. NameBright - Coming Soon
comes with a 15 day trial, after that 69.95 it does internal, external, sprockets, racks, and right angle. Very easy to use Good luck

That is because the End of Part marker has been moved to the top of the browser to save space, as detailed here. Move the End of Part marker to the bottom of the Model Browser to make the features active and save the part.

If you use a decent involute profile in Inventor, you should be able to export a dxf from a .idw of it to AutoCAD or a CNC program to cut.

Thanks for the help, that worked. Now, I’m wondering if the involute internal gear here Power is good enough to actually cut one from. Would you happen to know?

I had two bad experiences with first library parts and machining. I would start with knowing what you want and making the part yourself. The profile of the jidico motor’s gear and the van door motor are both off enough to make rework necessary. Good luck

If you’re refering to Ole Germer’s Involute Gear, I think it’s accuracy is as good as you’re likely to get. However, I haven’t fab’ed any parts from it myself.

I still use a lisp file that I found many years ago for AutoCAD. Do a search for Gear.lsp on the Net and you should find what you need. I have modified mine over the years, and it works great. I send the data to MasterCAM or Gibbs and they cut well on our Fadal machine.

If you can generate the proper involute curve for an external tooth gear of the required pitch, then you are nearly done. The tooth form for an internal gear is the inverse of the external involute (in fact, my old copy of The Handbook of Gears gives the formulae for determining gear tooth thickness at any point on the addendum or dedendum for both external and internal gears, and they are identical save for the note that when calculating for internal gears “it is the tooth space that is calculated and the internal gear tooth thickness is obtained by a subtraction from the circular pitch at that radius.”).

-dave