Gears can be expensive. Custom gears with weight relief can be even more expensive. 2D fabrication processes can be used to significantly cut these costs if you’re willing to put in the time and deal with a bit of extra prep/cleanup.
There are plenty of vendors out there that’ll sell you a gear, but more often than not the gear you can buy isn’t exactly what you’re after. Similarly, places like https://www.rushgears.com/ will make custom gears to order, but at a steep price, particularly at the low quantities most projects will need. Maybe if the loads are light you could 3D print the gears. Given the choice between “not right”, “too expensive”, and “wrong material” there’s got to be another option, right?
This is the spot I was in when looking to do a custom gearbox for a 30lb combat robot. I was able to source a few gears that were “right” but to get the full reduction at the strength, weight, and footprint required I would need to go custom.
Using 2D fabrication (waterjet and laser cutting) I was able to get gears cut to spec at a much more affordable price via https://www.bigbluesaw.com/.
For my gears I opted for laser cutting from AR500 plate. I also opted to go with a stacked gear approach to minimize the impacts of taper and HAZ due to the cutting process.
Two stages in this gearbox would be done via stacked, laser cut gears. The smaller gears were cut from ⅛” plate and the larger gears were cut from 3/16” plate. For each stage of gear you stack three small gears and align them with two large gears. The reason for doing this is to ensure that there isn’t a situation where a single stacked plate is only engaged with another single stacked plate. This minimizes the risk of one of the stacked gears taking on the entire load passing through that stage of the gearbox and reduces the chances of the binding that could result from one gear wearing faster than the others. I also opted for hex shafting as it would better spread the forces out than a single keyway, reduces sharp corners (the cutouts in the gears have rounded features at each corner) and avoids creating a thin section near the keyway that would be a likely failure point.
The first step in gearbox assembly was match marking the gears so I could easily assemble each stage without having to determine which of the 12 possible orientations was the correct one for installation. For this I set each plate on a common shaft and used a green paint pen to mark the teeth.
As you can see looking at the above image, some of the teeth look a bit rough. This is illustrative of one of the downsides to this fabrication method – As features get more detailed and plate thickness goes up the surface quality of the cut will tend to go down. Luckily, this is a case of things looking worse than they are.
With the gears marked it was time to make the shafts and spacers for the gearbox. (Along with a few other shafts used for the build)
With the shafts made it’s test fit time.
Here you can see how the differing plate thicknesses force all gears to be engaged instead of allowing for the possibility of only one gear being engaged during use. The gaps you see between the plates can be closed up with additional shims if necessary.
At this point the gearbox was ready for run-in using valve grinding compound to smooth out the rough surfaces on the gear teeth.
Once the gears were running smoothly the gearbox was disassembled and cleaned to remove the valve grinding compound.
After a full cleaning it was time to reassemble and lubricate the gearbox.
With that all done a quick hand check showed that things were running smoothly and were ready for use.
Notes and Lessons Learned
- At 16p, 3/16” thick plate resulted in aesthetically poor teeth, but a bit of finishing work got them running well
- Taper could quickly become an issue at higher plate thicknesses
- Using low taper waterjet cutting may allow for full thickness gears with no HAZ and should be considered if the project budget allows it
- Using hex shafting required some additional work on the front end but should simplify maintenance
- The ability to design weight relief into the gear profiles is a massive benefit when dealing with tight weight limits
- Good gear tooth profiles are important, as is modifying the profiles to accommodate the fabrication method. Kerfs and beam/jet radius need to be accounted for in your profiles and should be designed into the part so you’ve got better control of it.
- There are plenty of sources out there for gear tooth profiles that you can use as a baseline, https://geargenerator.com/, https://evolventdesign.com/pages/spur-gear-generator, and https://www.engineersedge.com/calculators/spur_gear_calculator_and_generator_15506.htm are just some of them
