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Hirth Coupling

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I bought some plans from Modelltechnik, maybe my next project, not sure yet.

On a few of his designs he uses a built-up crankshaft method with Hirth couplings to join mid span crankshaft journal segments. These are concentrically retained within journal bearings and axially held together with a cap screw. It looked a bit intimidating, but basically looks like set the dividing head at the prescribed angle & cut across the cylinder face with a vee cutter to prescribed depth. His CAD plans shows the theoretical cut profile: 60-deg vee angle, cut inclination angle, tooth depth from point cylinder face edge & number of teeth. By theoretical I mean the peaks & valleys are defined by the sharp points. Through these links I think I'm able to replicate the cut profile. The teeth appear to mate properly in various section views & I get agreement with the axial inter-mesh distance.

General info

Solidworks + Excel. Shows underlying equations in spreadsheet. Some of the output parameters I can correlate to my 3D model, others I'm not too sure about.

Calculates cut path angle for standard Hirth Joint/Coupling based on given number of teeth and cutting tool profile angle. Generates downloadable OpenSCAD model but I wasnt able to open that file format.

Its real life machining I'm wondering about. The cutter will have some small but defined flat or nose radius. I allowed for that distance/geometry in my CAD model. And the tooth crown should have some defined flat for trough clearance when parts inter-mesh, so factored that distance as well. BTW I'm still preserving the vee cut profile from the theoretical geometry but I guess I'm saying I'm kind of winging it here on the peak & trough details. I'm also not exactly sure how I would measure depth in-situ while machining - kind of like how you would measure a thread to know your pitch diameter is within tolerance.

- Has anyone machined this profile before? (maybe specifically on a model engine crankshaft).

- Does anyone have links to Hirth geometry/equations (ideally for simple minds LOL).

I've ordered a double angle cutter like attached pic just to experiment. Hopefully will do the job?

My cad tester pics

60-deg cutter I ordered.

Roger B:
One of our currently not active members posted a video on this some years ago:

It looks complicated to me  ::)

Thanks, yes I saw that video.
I noticed he cuts the teeth with what looks like a boring bar style of arbor with maybe HSS cutter profile. The multi-tooth cutter I show might bite me when it comes time if I cant get proper clearance for the tool holder etc. but cross that bridge later.


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