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_
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