As an experiment, now that I've modified the AlibreCAD Python script to use Module or Pitch Diameter and learned how to cut internal spline teeth I created two parts with double the original harmonic drive: 140T and 142T with 0.5module teeth.
The cup ID dropped to 65mm from 80mm for the 1.25module 70T. I've printed it out of PLA again since the yellow I'm using is easier to look at for details compared to the shiny PETG black. Printing the cup went well with clean teeth. The ring gear is still finishing and I'm not impressed. For whatever reason it's not looking good. As for profiles, once you go above 135T the standard gear cutter wheels are marked 135 to infinity where infinity means an infinite pitch diameter also known as a rack profile. So if one wants to use standard gear cutter tooling the number of teeth in the cup/gear assembly must be over 135. To test that theory I recreated the original 70/72T design using AlibreCAD which then generated the involute tooth profile which we've been told isn't as effective as pure triangle or slight sine curve on the tooth slope. Now where before the cup/gear combination would bottom out before making full side contact this one is really tight. Maybe too tight. But mix the two with the cup made from the original STEP file profile and the AlibreCAD ring and the fit is perfect. More testing on that when I find some more 3mm screws of the proper length. Meanwhile once the 3D print of the 0.5 module ring gear is complete I'll see if it's possible to clean up the teeth. The thing is that the teeth are smaller. So the cup doesn't have to flex nearly as much as it does with the 1.25module cup. Perhaps if the 1.25module cup also had 140T and the larger diameter it also wouldn't flex as sharply. And therefore less chance of cracking. > -----Original Message----- > From: Chris Albertson [mailto:albertson.ch...@gmail.com] > Sent: December-20-21 10:22 AM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Harmonic Drive > > On Fri, Dec 17, 2021 at 6:05 PM Sam Sokolik <samco...@gmail.com> wrote: > > > Hmm - seems we are making things a bit more complicated than they need to > > be.. Why didn't the simple flex gear work ok? You need to make the > > system with fine enough splines that the flexure is at a minimum... You > > can lower the backlash to a minimum by applying a small amount of > > pressure to the system. > > > > The problem with small splines is that he is making this with printed > plastic. There is a minimum feature size that works. Seriously you want > to stay above module 0.5 and bigger if you can. Mod 1.0 is more > realistic There are two reasons. (1) plastic is not very strong unless > you make the parts big and (2) the printer makes parts with dimensional > tolerances of about 0.4 mm so if your parts have 1mm features you can > expect 50% dimensional errors. That said, modulo 1.0 gears work really > well. Smaler ones have a short life. > > Finally ANY flexing at all is the death of printed parts. They fail by the > failure of interlayer adhesion. Think of printed plastic like it is soft > pine wood. When making gears from yellow pine, the direction of the grain > matters a lot and no one would think of making module 0.5 gears with pine. > > I've been able to build an entire CNC conversion kit for a mini-mill from > printed plastic. The stuff is VERY rigid and strong if you make the parts > large enough > > > > > > > > > > > sam > > > > On Fri, Dec 17, 2021 at 7:46 PM John Dammeyer <jo...@autoartisans.com> > > wrote: > > > > > Alright. So it spins really freely but keeping the planetary cluster > > > centred is a bit of an issue. So the attached photo shows roughly (very > > > roughly) what I had in mind. > > > > > > With the right coupler between the motor and the gear cluster the > > > planetary assembly should stay in the same place. There really isn't any > > > axial load. There is a place for a 40mmx19mm bearing that I happen to > > have > > > on hand. > > > > > > But what about the driven ring gear? In one of the model shops today I > > > saw a package of 100 5.5mm stainless steel balls they sell for putting > > into > > > paint bottles to help mix them up. > > > > > > Seemed and ideal size for a bearing race between the two ring gears. > > Kind > > > of like what Gene H. stated he did on his setup. (Pictures?) So the > > Blue > > > Gear is fixed. The Green Rotates. The lazy susan like bearing between > > > handles axial pressure in the direction of the blue gear. But really the > > > only thing holding up the green ring gear is the planetary assembly. > > > > > > Say we wanted to mount a face plate or chuck onto the green gear. I'm > > > open to ideas on how to stabilize that. Sketches would be great. > > > > > > Methinks a part that surrounds the blue gear and extends over the green > > > and they also have a bearing race between. Or we make a similar bearing > > > mount to the rear one for the front with the gear cluster holding this in > > > place extended out to the bearing. Now the faceplate is attached to > > this. > > > But a lot more axial twist on the small bearing in the middle. > > > > > > Thanks > > > John > > > > > > _______________________________________________ > > > Emc-users mailing list > > > Emc-users@lists.sourceforge.net > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
