This month I'd like to discuss hob nomenclature. These are commonly used terms when describing a Hob.

Active Length: Also called the "cutting face width", it is the axial length of the hob's relieved portion. Note that this length does not include the partial teeth typically at each end of the hob.

Flute: A straight or helical longitudinal groove that forms the tooth face of one row of teeth and the back of the preceding row. As the number of flutes increases, the number of cutting teeth also increases. Given the same feeds and speeds, a hob with more flutes generates a smoother profile. The flute also provides space for chip clearance so too many flutes in a given hob can cause chip and finish problems.

Hob Length: The hob's overall length. Longer length hobs tend to extend tool life.

Hub: A qualifying surface at each end of a shell type hob, which the manufacturer provides for checking diametral and face runout.

Keyway: A slot through which the turning force is transmitted to the hob. It may be a longitudinal slot through the bore or a transverse slot across the hub face, in which case it's called a clutch keyway.

Lead: The axial advance of a thread during one complete revolution.

Lead Angle: The angle between the tangent to a helix and a plane of rotation.

Outside Diameter: The diameter of the cylinder that contains the tops of the cutting edges of the teeth.

Pitch Diameter: The diameter of the pitch cylinder, which is a reference cylinder for deriving the hob's design elements, such as lead, lead angle and tooth thickness.

Thread Helix: The helix of the hob thread in the pitch cylinder.

Tooth Face: The surface against which the chips impinge.

This month is Thanksgiving and all of us here at Repair Parts, Inc. and Re-New Machine & Maintenance would like to wish all of you a very Happy Thanksgiving.