On Fri, Mar 05, 2010 at 01:23:46PM +0000, Andy Pugh wrote: > I am thinking of making a faster rotary axis using an ER32 collet > holder I have on a 3/4" ground shaft and some taper roller bearings. I > would drive that with a spare stepper I have, at about 10:1 ratio. (or > one of the little servos) > I can't believe that there are very large rotating forces on a gear > during hobbing, I think it is probably largely balanced.
The proof of that seems to be in the success of free-wheel hobbing. In MEW 78 [1], a gashed blank is fed into a hob, without any form of synchronisation. i.e. The forces must be "restorative", not disruptive, in order to spontaneously push a gashed blank into synchronisation. The photograph of the finished gear is not large enough to show if it is more than "useable", which is all that he claims. I also have a photograph around here somewhere, of a wormwheel being free-wheel hobbed with a tap held in the lathe chuck. In MEW 75, another author used a couple of CMOS chips for the programmable divider between the spindle encoder and stepper driver, to select the number of teeth. He describes no problems with cutting forces, other than cutting to the tooth depth in three passes in harder steel. Otherwise he just sets the depth, and cuts the teeth in one pass, in an ungashed blank. That setup has the hob running on a mandrel between lathe headstock and tailstock. The stepper-driven spindle holding the gear blank is mounted on a vertical slide on the cross-slide. From the photographs, that makes the feed perpendicular to the hob axis, which seems to me to ignore the helix angle. How that creates a spur gear with proper gaps, is not clear to me. On the other hand, preferring the quiet running of helical gears to the whine of spur gears, I have run the above setup in my mind, with (a virtual) EMC advancing the phase of the gear blank as it is fed across the rotating hob. If the rate of phase advance matches the helix angle, then the blank should come out the other side as a helical gear, I believe. (And both blank and feed are perpendicular to the hob axis. What could be simpler?) Checking that with another thought experiment, we run the helical gear on a rotating matching worm. As the gear is slid back and forth on its axis, its rotation advances and retards in accord with the helix angle. Hmmm ... where can I dig up physical examples quickest, to try it out? Incidentally, if choosing to generate the tooth profile, using a straight (no helix) hob, then that slow process can be accelerated by making a three or four tooth hob. It cuts the prior iteration on the previous tooth and the next iteration on the next tooth, speeding up the process. (Or providing a cleaner tooth form for a given number of iterations.) There was an article on that in MEW 107. Hopefully some of that is useful, Andy. Erik [1] www.model-engineer.co.uk says it is putting 130 back issues on line, but will be charging a subscription. -- Wisdom is one of the few things that looks bigger the further away it is. - Terry Pratchett, _Witches Abroad_ ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval Try the new software tools for yourself. Speed compiling, find bugs proactively, and fine-tune applications for parallel performance. See why Intel Parallel Studio got high marks during beta. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users