Last month we discussed the formulas for determining the proper amount of hob shift. To continue with this train of thought, I felt we should talk about determining the proper direction of hob shift.

Although in most cases the direction that you shift the hob will be relatively unimportant to the job, there are cases where consideration of the direction is worthwhile. If, for instance, the finish of the gears to be cut is of primary importance, shifting should always be done so that in each new position the worn teeth on the hob do the roughing and the sharp teeth do the finishing. If tool life is of prime importance, then the opposite is true.

The following rules apply regardless of the type of cut being taken, climb or conventional.

With the hob rotating clockwise:

• Using a right hand hob with tooth finish of primary importance, shift the hob from back to front.
• Using a right hand hob with tool life of primary importance, shift the hob from front to back.
• Using a left hand hob with tooth finish of primary importance, shift the hob from front to back.
• Using a left hand hob with tool life of primary importance, shift the hob from back to front.

With the hob rotating counterclockwise: simply reverse the above recommendations. If you have any questions concerning these formulas please feel free to contact me at rpi@repair-parts-inc.com.