Posted by Ron V on October 03, 2008 at 21:53:15:
In Reply to: Re: Tangential fly hobbing posted by BillS on October 03, 2008 at 20:35:01:
Fly cutting will only generate the correct form with tangential movement. Without this movement, bearing will be more pronounced at the outside edges of the contact pattern on the gear.
With smaller pitches, the fly cutter geometry generally is that of a rack having straight sides. There are several worm tooth profiles but generally the form is of an involute helicoid with a convex rather than a straight profile. With larger pitches this convex profile begins to become quite pronounced so skilled gear cutters will generally try to replicate this on the fly cutters they produce.
Reasons for tangentially feeding a tapered hob:
If the number of starts on the hob is the same as the worm and the hob O.D. is only fractionally larger than the worm O.D.; then the most accurate form possible is generated on the blank.
Finish is of higher quality with less obvious generation flats being produced than the radial plunging method.
The taper eases the cutting forces being generated when first entering the blank and also reduces the "hammering" effect usually seen as each start comes into contact with the blank.
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