Gear hobbing vs gear shaping: which process for which gear

Hob external spur and helical gears, splined shafts and worm wheels whenever the hob has room to run out at the end of the cut. Shape internal gears, gears that sit close to a shoulder or flange, and cluster gears where two gears share a shaft. Broach internal splines and keyways when the volume justifies a broach.

A hob is a cutting tool shaped like a worm whose thread is interrupted by gashes, forming rows of cutting teeth. Hob and blank rotate in a fixed ratio, like a worm meshing with a worm wheel: with a single-start hob, the blank turns by one tooth pitch for every revolution of the hob. While the hob feeds along the workpiece axis across the face width, its teeth make a series of overlapping cuts whose envelope forms the involute flanks — the profile is generated continuously. For helical gears the hob is swivelled to the helix angle plus or minus the hob lead angle (plus when gear and hob have opposite hands, minus when they have the same hand; for spur gears, the lead angle alone), and the blank receives an additional differential rotation linked to the axial feed.

Because the cut is continuous, hobbing is the most productive method for external gears. On a CNC hobbing machine the hob and workpiece spindles are coupled electronically, so the number of teeth and the helix angle are set in the program rather than with change gears. Module and pressure angle are fixed by the hob: a different module needs a different hob.

A shaper cutter looks like a gear with cutting edges. It reciprocates along the tooth direction while cutter and workpiece rotate together, generating the profile stroke by stroke. On the return stroke the cutter retracts slightly so it does not rub.

Shaping is slower than hobbing, but the cutter only needs a short run-out — a small relief groove is enough. That is why it can cut internal gears, gears next to a shoulder and the second gear of a cluster, none of which a hob can reach. The method is often called "Fellows" shaping after Edwin R. Fellows, who invented the gear shaper and founded the Fellows Gear Shaper Company.

We cut gears up to Ø260 mm and module 6 on six-axis CNC hobbing centres (SMG SH-260, S&T Dynamics GHO-200), shape internal and shouldered gears by the Fellows method on three Lorenz universal hobbing and shaping machines, and broach keyways and splines. Blanks are turned in-house; hardness and dimensions are measured in our own laboratory, and gear accuracy is measured with the CMM gear module and a contour measuring device. See gear manufacturing.