Friday, 3 October 2008

Spurred into action

Forrest's success at milling spur gears with Tommelise 2.0 got me wondering how well RepRap can make them with Fused Filament Fabrication. An obvious test case are the two gears on Ian Adkins' alternative extruder design. These are laser cut, but look just about achievable with FFF. I couldn't find a picture of Ian's, these are a similar design by Vik Olliver.

Here is how they came out :-

The infill does not meet the toothed edge very well but they seem very strong never the less. They mesh well even though the involute profile seems a bit rounded off.

In an attempt to get the infill to work better I tried running my machine with 0.3mm filament, 0.24mm layers. I didn't make the nozzle hole smaller, I just changed one number in my software so the filament is being stretched finer. The result was interesting :-

The infill fills the teeth and is also very flat. The hole for the motor shaft has come out smaller and the teeth are a bit asymmetrical. They lean clockwise, the direction the outline is laid down in. I think this is because I am extruding 0.3mm filament through a 0.5mm hole so the point it exits from can move about but 0.2mm. I expect using a 0.3mm hole will solve that.

I also had a go at making a smaller version of my screw top pot with 0.3mm filament. I used the same g-code, I just scaled all the coordinates by 0.3/0.5.

It came out well and the thread still works.

With the finer filament it takes about 3 times longer to make anything so I upped the speed from 16mm/s to 32mm/s. That had an unfortunate side effect: The wall of the pot is quite thin so after the outline and inner wall have been extruded, the infill is a very narrow zig-zag. That makes it a high frequency movement. Speeding it up made it more like an audio vibration which seems to resonate the frame of my machine, shaking anything loose off the desk.

HydraRaptor growling from Nop Head on Vimeo.


  1. Thanks for answering that question about FFF and gears. Why so many teeth for a 2:1 ratio?

    I REALLY, REALLY like those screw top pots you're making.

  2. Why so many teeth for a 2:1 ratio?

    I don't really know anything about gear theory, you would have to ask Ian, but I think they do look more like gears you see around than the very low tooth count versions which accentuate the involute profile.

    I think the standard metric mod gear system has the number of teeth proportional to the diameter. It may be the same as with threads: when you buy them in you need standards, but when you make your own, standards no longer make any sense.

    I could imaging lots of fine teeth give less backlash, but it doesn't matter much for this application.

    I REALLY, REALLY like those screw top pots you're making.

    Well you can't make them with a milling machine, so you will have to get back to the true path of additive fabrication ;-)

  3. Oh I've never left the additive path. I just don't consider it to be the whole gospel any more. :-)

  4. Oh it will be, but maybe not in our lifetimes, but certainly sometime before the singularity we will have molecular level additive fabbing that will take in elements and spit out cell phones.

  5. I'm not holding my breath for that. Eyeballing those gears that Ian did and then going back to my involute profile gear script I get things that look like that when I do a pressure angle of 30 degrees for those kinds of tooth counts. I wonder if that's what is going on?

  6. Btw, I'm thinking about using a Sarrus linkage instead of guide rails for Tommelise 3.0. :-D

  7. > Btw, I'm thinking about using a Sarrus linkage instead of guide rails for Tommelise 3.0. :-D

    I'll be interested to see how that goes, will it need multiple 'folds' to give a decent distance of travel compared to it's size?
    Will extra joints add significantly to the slop/inaccuracy?

    I had a play with building a Lipkin linkage a while a go, It was incredible seeing a motor produce a straight line movement with only linkages. I also love that accurate straight lines can be produced with almost sticks and pins level technology. The downside is it requires a lot of space for a small movement.

    With either linkage there is the issue of non-linearity when driven from the pivot. Will you be driving your setup from one of the pivots or using it only as a guide with something like threaded rod for the positioning?

  8. "I'm thinking about using a Sarrus linkage"

    That will be interesting to follow, as is making your own steppers.