Practical Ideas: A Further, Simplified Method for Producing Hobs

May 18, 2010
How to convert two radius-arc side profile teeth into trapezoidal profiles

Mark Grizotsky of Thornhill, Ontario, writes with a new approach for producing hobs:

“My idea is to convert two radius-arc side profile teeth into trapezoidal profiles that have the same pitch for roughing straight-sided spline shafts on the gear milling machine. The trapezoidal profile substitutes for the radius profile, which is very complicated to calculate and produce.

“We start from our hypothesis that at least one equal angle of a trapezoidal profile can provide a parallel allowance on the width of a roughing straight-sided spline shaft, and the needed sizes of allowance by standard.

“We found that these two angles are the angle of trapezia inscribed in the two arc-radius teeth of the hob. The angle and two radii can be determined by a computer program. The angles are located between the vertical line and the sides of trapezia, forming two equal angles.

“This angle (shown in the diagram) is independent from the outside diameters of the spline. The angle, shown in the diagram, is a very economical way of producing a trapezoidal profile of hob by grinding, via abrasive wheel on a Matrix grinding machine. For radius profile grinding we would need more expensive equipment and highly qualified machinists.

“The trapezoidal profile also is necessary in emergency cases when we need to produce hobs in a very short time. As a precaution, I make a test cut before actually producing the roughing straight-sided spline shafts (see table).”