hobbing problems

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Posted by Michael Kelly on November 27, 1999 at 17:38:46:

I am hobbing a spur gear, (16tooth, 8 d.p., 20p.a.) on an older Barber Coleman # 12 which has a double overarm for support. The Material being hobbed is 4140 heat treated to Rockwel 28 - 32 "C" SCALE. I will admit i do have some runout in the arbor aproximately .001 and cutter runout of .0005" I have hobbed one piece with 3 passes to come to correct wire size. The feed I am useing is .0075" and hob speed is aprox 220 rpm. I do not have the necessary change gears to slow down the cutter any more than this . I am conventional hobbing.
the reccomended feeds and speeds are 90 hob speed and .050 feed (aprox) my question is this 1) why would the runout be .002 tir on one piece and .006 to .009 on another assuming the blanks are good and nothing has changed in the runout of cutter & arbor. Material on the cutter is not marked on it i assume it is like m2 it is made by itw and whole depth is .270" should
should i lower my cutter on the first pass to .135 depth and finish with another .135 cut or hob first pass with a .260 depth of cut and finish with a .010 finish cut? The process is quite long and there is no money in the job right now unless i can speed up the feeds & speeds and take more than three passes. I need to increase my cutting time per piece as well as runout being within .004 tir. i know i can get the piece within .002 tir because i have done it on the setup pieces. after the setup is to depth , i cut one piece at .0075 feed and still have excessive runout. any suggestions would be appreciated

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