Posted by Bill Simpson on January 06, 1999 at 01:05:25:
In Reply to: G & E Gear Cutting Machine posted by Tom Bero on January 05, 1999 at 15:38:41:
Without more information, it is difficult to point at the cause, but here are a few things to consider when teeth are malformed.
A gear hob really needs to be "tight" since many problems in tooth generation can be caused by loose motion in the machine. Also slack, backlash, etc. may not be revealed without exerting large forces similar to those occuring during cutting. Use a dial indicator; don't trust "feel" to find loose motion.
(1) If you are climb hobbing, check for backlash in the table feed screw. Backlash allows the table to "advance" against the non drive side of the feed screw under heavy cutting (first pass) and "relax" against the drive side of the screw on a light (second) cut. This could put a step on the side of a helical gear.
(2) Backlash in the work arbor index worm & wormgear drive should be near zero. There are wearadjustments for worm thrust endplay and worm & gear mesh. Test this with a large torque wrench and a dial indicator out at a large diameter on the work arbor. Should be near zero. Angular backlash in the work spindle can produce a stepped second cut for reasons similar to point (1) above.
(3) Check the hob cutter arbor endplay. This should be near zero. Use a prybar to really try to move the hob, and an indicator to measure end-play.
(4) Look for a sheared or missing key in a change gear, or other gear internal to the machine. Again, apply large torque to expose any shaft/gear slippage that may be there (don't strip a key though.)
Did the step suddenly occur with this job, or has it been a progressively worsening problem?
Does it occur the same way every time? does the step appear to be different near entrance or exit of cut when compared to the middle?
Is there a lot of vibration during cutting? A lot of noise? Any information you can provide may offer more clues to isolate the cause.
Hope this helps,
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