SAAZOR – Hob technology and PVD coatings for highest demands

Heavy-Duty-Skiving Hobs

Heavy-Duty-Skiving Hobs

Heavy-Duty-Skiving Hobs

General

Skiving hobs are tools for finishing precut and hardened or tempered gears ranging in hardness between 58 to 64 HRc. That way a much higher precision can be achieved with the premilled gear geometry, distorted from the hardening process. Wether there is some additional honing or grinding following this process depends on the application.
The great advantage of skiving instead of tooth flank grinding lays in the elimination of almost all runout and spacing errors within just one pass. Under certain conditions, the rough grinding process can, for more economical reasons, be replaced by skiving hobs. The most outstanding feature on these hobs is the negative rake angle which can, depending on module or application, lay between –16 to –30 degrees.
This angle allows a skiving cut on the flanks, but the gear tip and root area will not be touched at this time. Thats why a protuberance hob is used during the roughing process in order to ensure enough clearence for the tip of the skiving hob. Overcutting the gear head with the hob should be avoided; chamfering skiving hobs should only be used in rare instances.

Cutting Conditions

Due to the raise in pressure during the hobs initial contact, only climb hobbing should be used. The cutting speed depends on the module and the hardness of the work piece. It is between 60 and 140 m/min for “wet cutting”. The axial feed rate is based on the desired accuracy and is between 1 and 4 mm per work piece revolution.

Wear

Normal wear is evident by the rounding of the cutting edge. Depending on the application it should be 0,03 to 0,1 mm. The maximum permissible wear depends on the chip thickness and is recognisable when the first signs of spalling appear. An indication for excessive wear can be seen in a chipping of the cutting edges.
Even small feed rates or a too small chip thickness by sizing can be reasonable for this flat chipping of the cutting edge into the face of the tooth. It occurs because of the increasing pressure at the rounded flanks.

Sharpening

The preservation of the cutting edge strength is an unconditional requirement for proper functionallity. During the deep grind sharpening processs one has to ensure the absolute integrity of the diamond wheel and proper flow of coolant in order to avoid thermal damage of the sensitive carbide. The corresponding data for the sharpening offset can be found in a list, delivered with the skiving hob. Technical information upon request.