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I'm trying to reverse engineer a set of spur gears that I have on a model RC car, but they are tiny, so there's a limit to what I can measure with my calipers. My plan is to create a new gear design with a few important changes that will mesh with these existing gears, then have it professionally 3d printed in nylon.

I've used this image as a reference for what data I might want to obtain:

And here's an image of the gears in question:

So far I have these measurements for each gear:

Gear 1:

  • Teeth: 10

  • Pitch Diameter: 5mm

  • Outside Diameter: 6mm

  • Root Diameter: 4mm

Gear 2:

  • Teeth: 22

  • Pitch Diameter: 11mm

  • Outside Diameter: 12mm

  • Root Diameter: 9.5mm

Note: I don't know how to obtain the base diameter, only the root diameter, since the base diameter doesn't seem to be directly measurable. Also, the pitch diameters are a guess, but I'm assuming for simplicity the people who designed them probably picked 5mm and 11mm to go with 10 and 22 teeth respectively. It's the sort of thing I'd probably do. Also it matches with the centre-to-centre measurement of the meshed gears being 8mm, which is the same as (5mm + 11mm)/2.

I'm thinking of using this OpenSCAD script to replicate the gear designs:

https://github.com/openscad/MCAD/blob/dev/gears/involute_gears.scad

Do I have enough data to make a replica of these gears using that script? If not, what else do I need? The script's comments seem to indicate that I need to know the pressure angle, but looking at the diagram I linked, I can only figure that out if I have the base diameters.

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  • $\begingroup$ I wouldn't be at all surprised if Thingiverse or similar sites have models for these gears available for download. So, what's the "changes" you plan for the new gears? Just a change in ratio? $\endgroup$
    – Carl Witthoft
    Commented Oct 18, 2016 at 13:27
  • $\begingroup$ Sort of. This is out of a Basher Rocksta micro rock crawler. There are I think five of these gears stacked up to turn a fast electric motor into an extremely slow crawl. I'd like to eliminate one of those gears to make it about twice as fast as it is now, so I'm going to make a gear with the same ratios only with a big spacing section to remove one of the gears. Anyway, I definitely need to make the design myself, it won't exist anywhere as a standard part. $\endgroup$
    – Excrubulent
    Commented Oct 20, 2016 at 4:08

1 Answer 1

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Given these measurements I think these are standard Module 0.5mm (Mod 0.5) gears, and that alone plus a little web search should let you design for them to mesh correctly, and buy, design or manufacture any further gears that will mesh correctly with them.

Easiest to measure:
Outside diameter / (Tooth count + 2) = 0.5mm.

Useful cross check:
Pitch diameter / (Tooth count) = 0.5mm.

Technically you may have to consider the possibility of incompatible Pressure Angles, but for modern (< 50 year old) injection moulded gears, I suspect that's such a remote possibility as to be non-existent.

I deal with 14.5 degree pressure angle Imperial gears on pre-WW2 lathes but as far as I know everything recent uses 20 degree pressure angles.

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  • $\begingroup$ Great, thanks. I'm only looking for enough information to get on with the job, and I think I've got that. $\endgroup$
    – Excrubulent
    Commented Oct 20, 2016 at 4:09

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