Friday, March 19, 2010

No peel, no warp, no backlash



I got Slice and Dice working well enough to let me do some serious R&D. After a side excursion with partial STLs of hands and naked ladies, I got back to work on the herringbone rack and pinion technology.

My first exercise was to see if I could print single herringbone pinions without having a lot of meltdown problems. Skeinforge had a lot of problems in that regard. In fact, Slice and Dice let me do that.


I printed it pretty much solid for strength and paused it for a couple of minutes midway through the print and let it cool down for a few minutes.  No problems.  This one was printed with 0.25 layers.

Next came the rack.  I decided to use the exercise last night to press the limits of Slice and Dice.  I figured that I ought to be able to print a 250 mm diagonal layout rack.  That let me find a half dozen limits bugs in the S&D code and get them cleared away.  I also decided to see if I could print a useful 0.1 mm layer.

In fact, I could.  Here we are about halfway through the print.



It took about an hour, by the way.


Here it is, complete.



And a closeup of the teeth.


For some reason the extruder is not shutting off between layers.  While that is easy enough to clean up with side cutters, I'll be diving into my code to see if I can sort that out in the next few days.

The pinion gear mates with the rack with no backlash.


I didn't make much of an effort to clean up the pinion which you can see in this outdoor pic.


I've found that the colour rendering is always better outdoors.

Finally, for those of you who haven't a sense of scale looking at the Rapman print table.


My next task will be to figure out how to put the pinion on a  printed, extended shaft so that it can be secured from both sides and mate with the short axled NEMA 17 that will drive it.  Then it will be a matter of integrating my Pololu Allegro 4983 microstepping driver board into my I2C bus and driving the thing from my Microchip 18F4550 uC board.

7 comments:

Cinmay said...

This looks really promising. It's really fun to follow your progress. Keep up the good work. Hope to be able to test this all out soon.

Rhys Jones said...

Sweet! If there is one thing I don't like about Mendel its the ridiculous price of the belts.

paul said...

Sweet indeed. Is it easier to print another gear on the same body as the herringbone and then a gear for the stepper to mate with it, rather than an extended shaft, or is that just stupid? (Or could a stepper with an appropriate gear drive the top of this one?)

Forrest Higgs said...

Well, if it's stupid then I'm stupid because that about what I've come to the conclusion needs doing. :-D

allen said...

Just a though, but if you run into problems with your gears binding (not sure how much thrust they are subjected to...), put a channel in between the herring sides, and one down the center of the gear. We use some similar gears at work, and have a channel in the center to prevent lubricant binding (it will cause hydraulic lock if you have liquid lubricant...), but I would imagine that you would get some friction from the points of the gear and the pinion intersecting in the center that might be detrimental.

Forrest Higgs said...

allen: When I researched herringbone gears, I noticed that the many had the channel that you are talking about and wondered what purpose it served. It's great to hear from someone who has had personal experience with the technology who actually knows.

My understanding of things right now is that I will not be looking at a lot of lateral or axial thrust in the assemblies. That understanding may change, however, when I get to the point where I put herringbone racks and pinions into operation rather than just printing them.

Seriously, I appreciate the tip. I'll be looking out for that and may well be going back to revise my gear and rack scripts. :-)

Alan S. Blue said...

In the last paragraph you discuss your next task. If I'm understanding correctly, I've seen a clever trick for removable yet tightly-mated plastic gears on a round shaft.

Picture three parts:
1) Your gear. But open the central hole to the largest feasible diameter, then have a couple (as many as possible) gear-teeth intrude somewhat on the axial void.

Parts 2 & 3 go on opposite sides of the gear, pinching it between them.

Part 2 is the tricky piece. It's a washer with a threaded split tube on one side. The tube is just large enough to allow part 2 to slide easily on the shaft. The washer has gear-teeth to match the internal teeth of your main gear.

Part 3 is essentially a wingnut/washer combination. It engages the split threaded tube of part 2 and compresses it tightly against the shaft.

The grip of the split tube on the shaft can exceed the grip you'd get from just resin-casting plastic right onto the shaft in the first place.

Getting the thread just right is tricky, but part 2 & 3 would work for most any gear you're willing to put the 'interior keying teeth' on.