Re: More Barber #12 hobber questions


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Posted by BillS on October 26, 2009 at 00:56:07:

In Reply to: More Barber #12 hobber questions posted by Jim Pollock on October 25, 2009 at 21:08:42:

The loose key/bevel gear needs to be fixed, and I think the backlash at the flywheel could be reduced as well. Don't know if .010 backlash for the bevels is excessive, but it sounds about right to me. Cutter drive components are going to have backlash by design, and the flywheel is there to help smooth out impacting hob cutter action on the drive components. It also smooths cutter rotation some, but there must be some backlash in the cutter/flywheel gear set. Actually, to cut some large angles, you might need to remove the flywheel, and this seldom causes a problem with tooth finish. Why? read on...

Check your outboard cutter arbor bearing for any up/down play. When you tighten the outboard bearing, push it against the arbor as tightly as you can before snugging down the two nuts. This is important to take any endplay out of the cutter arbor.

As I recall, you said there was no up/down play in the cutter arbor but you want to check that again to be sure, with the outboard bearing removed. Two things here: When the tapered arbor shaft bearing is not worn, there should be very, very little up/down motion. This is controlled by good taper fit and proper thrust washer adjustment. Also, with a high viscosity oil or grease in the taper bearing, there should be enough drag on shaft rotation to dampen or prevent the cutter from "bouncing" rotationally in the cut. When fully assembled, you should not be able to easily rock the shaft back and forth by hand.

There are other possible causes of scalloping of teeth. Mainly, the workpiece drive worm must be properly adjusted for very small backlash and endplay, but this was covered in the earlier thread. Also table side play must be adjusted to prevent table twisting due to cutting forces. That was covered as well.

Any adjustment to remove play should be checked over the full range of movement, particularly with regard to gibs. Also, removing backlash will cause more frictional heating, and the subsequent possibility of extreme heat and seizing. Adjustments of non-zero backlash components should always be monitored for excessive heat build up. But no heat usually means that the adjustment is slack. Zero backlash components (e.g. roller /ball bearing) are more forgiving of close fit than bronze and steel. I suspect your no 12 is full of "bronze and steel".

In a nutshell, the cutter is impacting the workpiece like a jackhammer at a frequency set by cutter speed and number of gashes. To cut smoothly, there can be no (or very little) play in the machine, cutter table, or work arbor. Anything that can move will, and these unwanted micro movements usually create finish problems.

Are you cutting helical? Cutting a high helix LH with RH cutter (or RH with LH cutter) can cause severe finish problems as well as possible cutter breakage. Tight control of worm/wormgear backlash is especially important here.

For climb hobbing add feed screw backlash to the headache - this can be a real hob cutter breaker. Ironically, climb hobbing usually improves finish - read on...

I have a theory about conventional hobbing and scallops. As the cutter tooth exits the cut, cutter drive force suddenly releases and cutter rotation accelerates. This acceleration is permitted by drive backlash. This additional rotation before the next tooth will shift the next tooth position slightly axially at the worse time - while it is at full depth. At full depth and slightly out of sync, how could it not "scallop" the finish tooth surface before beginning the normal cut? Once in the cut, the cutter drive is once more loaded, backlash is removed and the tooth shifts back into correct position. Of course, the process is not precise and thus produces an irregular pattern of scallops.

By contrast, Climb hobbing takes the max impact when cutter tooth enters at the OD. Cutting force diminishes gradually as it exits at the root. Seems to me that any gradual unloading of the drive as the cutter tooth exits is less likely to cause the cutter rotation to "spring ahead" so the next tooth should be in its correct position for the next impact.

Climb hobbing is trying to pull the hob table into the workpiece. If the feed screw has much backlash at all, this can break the cutter, or at least make some horrible sounds!




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