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.
Reply to
JimL
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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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Reply to
John Stevenson
Thanks, but that is 1 mm per rev. of the gear blank or 40 revs of the hob. Still too fast? Jim.
Reply to
JimL
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".
HTH
JG
Reply to
JG
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
Reply to
JimL

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