Gear Hobbing Qustions

Greetings- I have just finished a gear hobbing attachment for my non
universal horizontal mill. The arbor is driven by a gear train, and I had to
fabricate a small table to obtain the lead angle offset of 1.7 degrees.
I am using a 16 DP, 14-1/2 PA hob with straight rows of teeth. Hob is about
6 cm. in dia and 7.5 cm. long. I am cutting spur gears.
I am having problems with gear tooth thinning. I also see rough machined
surfaces .
I feed by hand, probably 1.0 mm. per revolution of a 40 tooth gear blank
while the mill spindle is running at 375 RPM , cutting aluminum.
I get better results by not feeding continiously- just feed one mm, wait for
the blank to revolve once then feed again.
Would appreciate any comments, suggestions as I am new to hobbing. My gears
cut with single involute cutters look much better. Thanks, Jim.
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1.0mm per rev ? Far too fast, as you say if you wait for 1 rev then feed it's better. The cutter needs a rev to form the teeth.
-- Regards,
John Stevenson Nottingham, England.
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John Stevenson
Thanks, but that is 1 mm per rev. of the gear blank or 40 revs of the hob. Still too fast? Jim.
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from "JimL"
You may be telling me something here that hasn't occured to me because I am hobbing very thin gears (1 to 4mm) and much smaller teeth (0.5MOD). I don't make any adjustment to allow for the helix angle of the Hob so if I am right, then this could be the cause of your tooth thinning.
Even in aluminium that feed is high - I would aim for 0.5mm.
What you haven't said is how deep your cuts are. I consequently assume that you are cutting to full depth in one pass. On a 16DP gear that would be 0.15" (assuming a dedendum of 1.4*DP) in which case I would take three cuts 0.08", 0.05" and 0.02".
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Thanks, JG- . I now understand John's statement on the feed rate of blank into cutter. I make three passes for the depth about like you suggest. My blanks are 13 mm. thick. I have never tried not offsetting my table. It would be great if I did not have to as I could eliminate my extra table. I suspect most of the thinning is do to excessive feed rate (horiz.) as John said. More trials coming up. Jim
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