On Sat, Mar 06, 2010 at 12:31:04PM +0000, Andy Pugh wrote: > On 6 March 2010 07:58, Erik Christiansen <dva...@internode.on.net> wrote: > > 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. > > I don't think it does. I was intending to set the vertical slide at an > angle and use coordinated motion in X and Z so that the blank moves > along it's true axis.
Ah, yes, of course. (Maybe if I have "coordinated motion" tattooed inside my eyelids, I won't keep forgetting that option.) Yes, if the gear helix doesn't match the hob helix angle, then the vertical slide needs to be rotated. > > 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?) > > I am not sure. It depends in if a helical gear is conceptually a gear > with the teeth rotated on the surface, or a stack of infinitely thin > gears with a pitch difference between each. > In this picture: > http://school.mech.uwa.edu.au/~dwright/DANotes/gears/photos/BrownHobbing.jpeg > The hob axis seems to be tilted to match the gear helix angle, rather > than the hob helix angle. The hob must always cut at the gear helix angle, so that alone determines the hob helix positioning. The hob axis then ends up where the hob helix puts it. There does not appear to be any options there. (According to the grindings of my mental gears, anyway.) Looking for some support on this, I found a simpler way to hob helical gears: Just crank the hob over some more: http://en.wikipedia.org/wiki/Hobbing They insist on feeding "parallel to the blank's axis of rotation", and cite a supporting reference. I can't see that working, though, except for free-wheel hobbing. > I think that in either case you ideally want to match the hob helix > angle to cut a true gear form. For the feed angle, yes. But if you use coordinate motion, you've solved that without effort. If my changing the gearblank's phase, while feeding it across the hob, is equivalent to your coordinate motion to simulate skewed feed, then it should work as well. Since a helical gear meshed with a worm must either move with your coordinate motion, or rotate, to pass its teeth across the worm, I think they're equivalent. (OK, your method is probably easier in gcode, so I'd go with it too.) > I did discuss this with my dad (50 years a gearbox machinist then > designer then service manager) but he seemed unable to grasp that the > feeds and drives are trivial with CNC, but the axis geometry is less > so) What he knows is easy for him. What we haven't done yet is hard for us. Erik -- If you understand what you're doing, you're not learning anything. - A. L. ------------------------------------------------------------------------------ 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
