Posted by BillS on March 10, 2010 at 17:44:53:
In Reply to: Non Differential Hobbing Seting Teeth When Second Cut posted by bilal on March 10, 2010 at 12:09:11:
Are you cutting helical on non-differential hobber? And want to run a 2nd cut?
I'm going to assume that your answer is yes, cuz I want to take a stab at a possible answer.
This is a very popular topic among job shops using non-differential hobbers to cut helicals, so here goes...
The problem is that the cutter and work piece are out of sync on purpose, and that's why the teeth come out at an angle. If they were in sync, you'd expect a spur gear and making multiple cuts would be no problem.
When you retract the cutter to start again, cutter teeth will not align with workpiece teeth. I'm going to suggest a way to get back into alignment so a 2nd cut may be made.
Raise the workpiece and retract cutter manually to a starting position, safely out of cutting area of teeth.
Engage table feed.
Jog the machine to remove any backlash in table drive caused by retracting the table.
Disengage the feed gears by moving the swing arm on feed change gears out of mesh.
Leave feed lever engaged.
Now you have feed engaged but feed gears are "disabled".
Plan A - Jog the machine to move the workpiece teeth close to alignment with hob teeth. This is the tricky part, since workpiece and cutter are turning. When the machine stops, observe alignment of tooth space with a cutter tooth. You may have to repeat this a number of times until you get a tooth space very close to alignment with a cutter tooth. Turn the motor drive by hand to make small adjustments if necessary.
Plan B - With power off, loosen the workpiece on its arbor, align teeth, and tighten. Make sure that tightening hasn't caused workpiece to rotate. Would this work with 2 or 3 on the arbor? Only if you are very careful!
Plan C - Use plan A or B to get approximate alignment, then loosen cutter slide and shift cutter to final alignment.
How to check alignment, you ask? Place a pin or a straight drill in the tooth space, and slide it toward the cutter tooth. It should be easy to see how close alignment is. Leave enough stock for the 2nd pass so that alignment doesn't have to be super accurate. I'm assuming that the pitch size is large enough to visually check alignment; after all, you're taking 2 cuts, right?
Assuming that the ordeal of aligning has finally paid off, mesh the feed gears again. If you have to move a change gear 1/2 a tooth to get mesh, this should not be a significant source of error.
Set workpiece to expected depth of cut.
Power the machine up. Remember, you already have feed drive engaged - leave it engaged!
Okay, I admit I've never done this, but I don't see why it wouldn't work.
I'll bet there are folks who have found a better faster way to do this.
I'd really like to hear from you - sounds like a good FAQ page topic.