Posted by BillS on December 16, 2012 at 14:47:02:
In Reply to: spline positioning posted by cranky on December 15, 2012 at 18:42:11:
If you can mount each part in the same alignment position to the workpiece holder (e.g. using a keyway on the part or mill a pocket for the eccentric), then it is simply a matter of "timing" the holder/part to the machine.
With workpiece mounted, advance or retard the workpiece by stopping the machine, disengage an index gear, and manually rotate index shaft to synchronize work and cutter. Mesh index gears and run to see if the cutter is properly synced with workpiece. Nice thing about this adjustment is that you do it with power off, so you can bring cutter tooth (or space) up under workpiece and view it statically near the feature you want in alignment.
Selecting the hob tooth closest to the part can be done this way:
(1) bring cutter close but under part where teeth will be cut.
(2) Run machine and place a sheet of paper between cutter and part.
(3) Carefully close the distance between cutter and part.
(4) When paper is decidedly pulled through by cutter, you should have .003/.004 between part OD and highest hob tooth. With machine off, manually rotate machine main drive to bring that cutter tooth back to "paper" separation. If the hob tooth is visibly not centered on a radial of the part, then loosen and shift cutter axially to center hob tooth to part. Mark this hob tooth as your "Full Depth Tooth".
Knowing which cutter tooth will be cutting at full depth lets you directly align a part tooth space to your feature.
If you need to align feature to a part tooth centerline, look at it this way. On a single lead hob cutter, the cutter teeth 180 deg from the "full depth" hob tooth will form the tooth top, so you can align a part tooth centerline to the feature by manually pulling machine drive to rotate cutter 180 deg around. This should visibly align a hob tooth space to a part radial.
Sighting along hob tooth (or hob space) can only approximate a best alignment. If after cutting first piece, you should know what direction to shift workpiece rotation to improve alignment. You never want to make adjustment under power! If you inadvertantly power up while index gears are disengaged, then you must start all over.
Once you are satisfied with feature and tooth alignment, everything should stay in sync
IF YOU DON'T SHIFT THE CUTTER AXIALLY. For instance, machines that automatically
shift cutter axially to distribute cutter wear will loose sync.
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