# Correction of Thickness and Depth of Roughing Hob for Roughing Straight-Sided Spline Shafts

March 13, 2013
Two alternative formulae References for calculation

Our thoughtful contributor sends the following insight:

“Sometimes in the process of milling process we change their thickness for technological reasons. We can do it because the roughing hob tooth profile is trapezoidal and there is no difficulty in correcting it by regrinding the hob tooth thickness and depth on the thread-grinding machine.

“When we change the tooth hob thickness and depth, we change the roughing thickness spline for technological reasons.

“In practice, the roughing splines include grinding allowances that usually vary by the amount A=0.1 – 0.15 mm (0.004-0.006 inch), so there is no influence on other theoretical elements of the spline shafts roughing.

“When the thickness of a roughing spline increases by amount A, the tooth thickness of the roughing hob is reduced by amount S3 on the thread-grinding machine.

"Below we present some formulas for A.S.I.A.

Formula A.S.I.A. – 1

“When the thickness of a roughing spline decreases by amount A and S3 is known as S3=S2-S1, the tooth profile of the roughing hob is corrected for height or depth. This tooth is ground down on a thread-grinding or grinding machine by an amount L (Fig.2). The addendum depth height H of tooth profile from pitch line being kept constant, tooth thickness increases from S1 to S2.

Formula A.S.I.A. - 2

“The tool designer can easily calculate with help from reference books the design of a roughing hob for roughing spline straight-sided shafts.

1. M – angle of trapezia tooth roughing hob.
2. S1, S2 thickness of tooth.
3. D – pitch circle diameter spline shaft (take from the books)
4. B1, B2 – spline roughing thickness.
5.  H – addendum of tooth roughing hob.