Tuesday, January 26, 2010


Getting there with herringbone rack and pinion

I finally hit a break in my day job and after sleeping the clock around beginning Friday afternoon was able to get back to my Reprap work. I'd been working with herringbone rack and pinion design and had begun writing Art of Illusion scripts to generate this kind of technology. I'd done most of the rack script a few weeks ago, but I needed to be able to design herringbone gears a bit more efficiently.

Finally, on Sunday the scripts for the racks and gears began to come together. After a considerable amount of feeling around I found that I could reliably print an 8 mm radius, 12 toothed gear. Connecting such a pinion directly to a 1.8 degree step NEMA 17 gives me a 0.25 mm/ step on the axis without microstepping. I then designed a 32 toothed gear which let me get that resolution down to 0.094 mm/step. You can see the layout here...

The NEMA 17 turns the 12 toothed pinion at the top of the picture.  It turns the 32 toothed gear which shares an axle with a second 12 toothed pinion which engages the rack.  It's simple, easy and quick to print and doesn't backlash if you apply just a slight bit of compression to the gear train.  My next task is to design a printable axis assembly to seat that assembly.  My goal is to get rid of the skateboard bearings, too.

The scripts are in a lot better shape, but they're still not really ready for prime time.

with a microstepping control board can't you get the resolution you need without the additional gears?

if 1.8deg = .3mm then a 1/4 microstepper could give you less than the .1mm you are shooting for.
"gives me a 0.25 mm/ step on the axis without microstepping. I then designed a 32 toothed gear which let me get that resolution down to 0.94 mm/step."

Uh, what? 0.25 mm/step *down to* 0.94 mm/step?

I'm confused. a quarter mm per step seems smaller than almost a mm per step. Decimal point in the wrong place?

Or it's possible I'm just an idiot and don't understand.
It was a typo. I meant to say 0.094 mm.
have you considered connecting a worm gear to a pinion? It might require a wear in period and a way to tighten it.

It would give you a very high gear ratio and extreme precision.
Yeah, I have. I haven't figured out how to write a script to describe a worm gear assembly, however, and for sure I haven't booked time to actually write and test the script to describe one.

I'm going to play with my herringbone scripts for a while and see if I can make them do what I want. :-)
An appropriately designed worm could be applied directly to a rack... One could even be designed to meet the rack at an angle so that the stepper would mount alongside (diagonal to) the rack. It might be hard to get the backlash out without driving up the friction losses though.
i like this development this is a good tool to be working on, i think this is worth the time.

i am interested in seeing if this could work for worm gears too.
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