Contributor Mark Grizotsky has developed another timely insight:

“When tool-designers develop roughing hobs for milling roughing straight-side spline shifts, they must remember this about the wear of the teeth of these hobs: it appears on the tooth at the time of milling on the profile of the trapezia of the tooth, on the front cutting face edge, and on the rear peripheral relief side.

“A hob is a very expensive cutting tool, and it must be expected to work a long time, with a sawing action that is demanding.

“We need to calculate the wear and thickness of the tooth’s trapezoidal profile, making allowance for the roughing spline.

“After developing the tooth profile we need to make a control of the profile of the roughing spline, graphic checks. We use tracing paper or CAD to check if the picture of the process is correct. We can adjust the thickness of the tool of the hob for wear, and again make a control drawing.

“As you can see, this job is very complicated.

“The size of wear tooth hob allowance minus 0.005 mm must be added to the thickness of the tooth, and we need to control thickness of tooth and thickness of spline by using a comparator or optical microscope. Allowance of the spline and thickness of spline must be checked very carefully.

“If control after comparator shows that we need to make an improvement to the tooth and spline, we need to regrind the tooth hob by following the drawing of the roughing hob. For a large quantity regrinding on the front cutting face edge we need to have a long place on the outside diameter of the hob. This tool design must to be calculated after the last regrinding, and the tooth must be strong and calculated according to book strength of materials.

“When you regrind a trapezoidal profile of teeth, you will need to verify (and account) for the geometric formula of elements of trapezia. (See sketch).

Where:
a/b = sin M formula A.S.I.A. – 3.
And:
c/b = tan M formula A.S.I.A. – 4.
If M = 16 degree sin = 0.27564
and tan = 0.28675
it means that b/a = 1/0.27564 = 3.63, and
it means that wear b on the diameter
b = 3.63 x a — formula A.S.I.A – 5
when a-wear on the side trapezia.

“Angle M is angle of trapezia of tooth of roughing hob. If grinding a=0.1 mm wear on the side of trapezia, that b = 0.363 mm wear on top of tooth on the outside diameter.

“Grinding goes to the inside the hob, and the hob also must have enough material to keep constant dedendum.”