Posted by BillS on September 16, 2009 at 21:21:19:
In Reply to: Re: barber colman #12 gear hobber questions posted by Jim Pollock on September 15, 2009 at 20:45:16:
Right you are - the flywheel is on the faster driving shaft, not the hob cutter shaft. The cutter shaft (large) gear meshes with a long face driving gear to allow the cutter slide to move about the length of a cutter.
Tightening the flywheel should make a difference, since being loose on the shaft is almost like there is no effect from it.
You can expect a large amount of backlash in the cutter drive train depending on machine wear. As mentioned by David, taking a heavier (one-pass) cut will load up the backlash and usually improve finish as well. Running the hob at higher spindle speed will probably improve smooth running and finish, but you want to be careful cutting 8620 at a high speed. Use a good cutting oil and keep checking for hob wear.
Looking at the index worm/wormgear again -
With the indicator at the 3" gear diameter, the backlash you are seeing is probably just between gear and worm. 0.002 isn't a lot, but imagine that this play can result in tooth "scallops" .002 deep.
It may take considerably more torque to cause the worm to move vertically in its housing (assuming there is play between the worm and its upper thrust bearing).
If you look at the upper part of the worm shaft, there should be a collar that "jam" locks the underlying thrust adjuster. Both are on top of the worm housing. If you observe end play in the worm, then loosening the locking collar allows you to adjust the end play out.
If you are cutting a spur gear, the cutter table should be equal to the hob cutter angle. This ideally provides the cutter teeth with equal side clearances. It doesn't have to be exactly set, and for some situations you might try moving it 5 - 10 minutes from ideal in the direction that will oppose the direction of workpiece rotation.
Also, put a dial on the advancing side of the table, to "feel" any table jitter while the machine is in the cut. If you feel the table jumping or bumping, there is slack which can allow the table to rotate slightly as it loads and unloads in the cut. To stop this requires rescraping the machine ways. Simply adjusting the gib will probably not do it, since the worn-in saddle must be eliminated.
Bad finish in the cut is often due to a number of things. Fixing each source of slack or play will produce some improvement
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