Re: Hirth couplings

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Posted by BillS on September 18, 2005 at 00:36:21:

In Reply to: Re: Hirth couplings posted by Collin Doyle on September 15, 2005 at 21:50:14:

I did a little 'net search and it looks like a Hirth coupling is similar to a curvic coupling, which is a Gleason design. Hirth couplings are used for precision indexing and high torque locking. You see couplings like this inside CNC turning machine tool posts.

I would think that the tooth flanks should contact all along the tooth radials and to be flat or formate in profile. To cut the teeth on a millng machine might require two setups, or three if there is a first roughing cut:
First, mill one tooth flank on each tooth, along a radial to the centerline.
Second, set the path to mill the opposite tooth flank, again setting the path to mill along radials to the center. Each tooth surface, top land, and root must theoretically converge, or vanish at the centerline of the member. The mating member surfaces would also converge at the centerline.

Based on the clearance between root and top surfaces of meshing members, the top and root surfaces would ideally converge on the centerline, but at points displaced by the amount of clearance.

Actually, for small couplings, a 60Deg "V" cutter could make a tooth space in one pass, but the root angle needs to be "dead on" for the number of teeth. Error in root angle is multiplied by two when the members are meshed. I'm thinking of the root angle as the cutter path angle measured off of a perpendicular to the centerline. This would be equal to the amount of tilt of the index head from the table. As number of teeth gets smaller, the root (or tilt) angle gets larger. The top, or face angle would be equal and opposite about the same perpendicular to centerline. So the face would be concave, and the cone containing the root radials would be convex. I don't think there is a pitch line as in mating gears. If you want to assume a pitch line, I guess it would be perpendicular to the centerline. But the cutter path must be parallel to the root angle, not to the pitch line.

The depth of cut, measured at the OD, would be set to give a desired topland width. You don't want a point, since that would give you no clearance when meshed with other member.

Does that make sense??

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