Introduction: DIY Planetary Gearbox for Nema17 Stepper Motor
I wanted to build a small and light gearbox for a stepper motor (used as excuse: for building a light extruder for a 3D printer - didn't trust the bowden setup, and the scavenged driver might not have enough torque for a direct drive - real cause: playing around with the lasercutter :-)
I decided to use POM (polyoxymethylen), since it's quite strong, self lubricant and you can cut it with a lasercutter... if you have a good ventilation: Burning POM produces formaldehyde, not the best product for your health. So using a CNC mill might be more healthy.
On the other hand, we have now a three stage filtering system especially for formaldhyde, and a just fine tuned laser cutter - perfect for fine teeth.
I used apart from the laser cutter:
- POM (i used three different thicknesses, 2,3 and 4mm to get a really small and light version, but using only one (4mm) thickness is also possible)
- 3* bearings 3mm inner diameter, 7mm outer diameter, 3mm height
- 1* bearing 5mm inner diameter, 10mm outer diameter, 4mm height
- 3* 3mm hexagon socket head screw 12mm long, with 12 washer and 3 nuts
- 1* 5mm hexagon socket head screw 30mm long (but trimmed down) and 3 nuts (small height)
- 2* 3mm hexgon socket head screw 40mm long with 2 washers
- NEMA17 stepper driver (5mm diameter axis)
- MK8 drivegear for 5mm axis
Step 1: Design
The design was made with inkscape. First i drew the outer dimensions depending on the NEMA17 stepper motor. Then i played around with the gear extension (extension -> render -> gears) to get a gear which fits into this outer dimension (especially not covering the four holes for the screws). The alignment tool (center on object on another) was my best friend :-)
After a while i decided to go with a circular pitch of 2mm and the basic pressure angle of 20. 60 teeth allows a nice conversion ratio (nicely dividable by 4,5,6...) and fits well into the housing. I was a little bit worried that the teeth might be to small, but with the 4mm POM it seems to be strong enough.
Next I made the small gear for the stepper motor: Fewer teeth result in a higher conversion ratio. With 4.8mm diameter hole (for press-fitting it on the 5mm stepper driver axis) less teeth than 12 seems to be not a good idea, i still wonder why this works - there is far less than 1mm material left. But again, POM seems to be a nice material...
After this i just needed the three gears between the inner and outer gear: Just aligned booth gears in the middle of the housing, then played around with the number of teeth until i found a gear which fits nicely between booth gears - there might be of course a more scientific way: Something like the middle gear and two gears together should have the same diameter as the outer gear. In each case the diameter is directly proportional to the number of gears... Anyways, in this case, 24 teeth was the magic number found by trial and error (and 24*2+12 seems to be exaxtly 60, but that might only coincidence ^^). A 7mm hole for the bearings completed each of this gears.
The resulting gear ratio according to wikipedia would be then 1/( 1 + number of teeth of the outer gear/ number of teeth of the inner gear), that would be 1/(1+ 60/12) = 1/6, a little bit to good, maybe a bigger inner gear would be better.
Holding this three gears together is done with two circular plates with 3mm holes for each of the gears and a center hole for the stepper axis on one side and the outgoing axis. The later is fixed with a inserted nut, therefore one of these circles has a corresponding hexagon in nut-size in the middle. At the beginning i thought about holding the nut in place with two circular plates below and above and a 5mm hole in the middle (such that the nut can't fall out of its hole), but that wasn't necessary. But that's why there is a ring of 6 holes instead of three, the unused three were for mounting the plates together.
The three holes where made by first aligning the gears as they are mounted later, then aligning one 3mm hole on top of the axis of one of these gear. After that the circular plate together with a duplicated 3mm hole can be grouped, rotated by 120° and then ungrouped.
After this all shapes can be lasercutted (the housings with additional circles inside for cutting out the free space inside). Either with different thicknesses as i did (i even cutted the axis of the stepper driver for a smaller housing and less weight), or just one thickness at all.
Step 2: Start Building
The first thing to do is checking if the 3mm washers are small enough for the small bearings (should only press on the inner ring of the bearing, not the outer one). Mine were not. The solution was staple a bunch of them on a screw, press everything together with a nut, mount it in poor men's lathe (a.k.a. power drill) and sand them down to the desired diameter.
Then insert the bearings and one 5mm nut in the corresponding gears and disc. Since the walls are not 90° because of the laser they might only be inserted from one side, but should be therefore also nice press-fitting.
The bigger bearing is pressed into the top layer.
Step 3: Mounting the Gears
In the next step, insert the 12mm screws in the disc. On each screw belongs first a normal washer, then a smaller one (the ones sanded down for the bearing), than the gears with the bearing. The small gear for the stepper axis is inserted in the middle of the three outer gears and should be directly above the 5mm nut. Followed again by a smaller washer and a normal washer on top of each 3mm screw. A second disc for more stability is added, and three nuts hold everything together.
Step 4: Stacking on the Stepper
Next: Stapling layer after layer on top of the stepper driver. The top side of the layer with the gear ring should be on one level with the end of the stepper axis. If this is not at least almost possible to achieve (e.g. by using only 4mm layers), just add some washers as sort of additional layers.
Thereafter gear block with the inner gear could be pressed onto the stepper axis.
More layers of the housing (and one circular layer on top of the inner gears to cover the screw heads) are then added. The resulting housing should be at least one 5mm nut height higher than the inner gears with it's top circular layer, because such a nut will be also mounted inside the housing.
Step 5: Cover & Outgoing Axis
A drivegear is mounted on the 5mm screw which is used as outgoing axis (If you want to use it as an extruder). Two nuts additionally fixate the drivegear (and are used for a bigger distance, but might at least one might be removed later, instead the screw will be shortened).
The screw is then inserted in the bearing in the top layer and fixated with another nut.
After this action, the whole top layer is mounted on the top of the gearbox by screwing it into the nut in the inner gear circle.
The remaining part is just mounting gearbox and stepper together with the two remaing long 3mm screws (on diagonal edges, remove the two corresponding screws of the stepper motor before.)
Step 6: Result
Result: It works and seems to be quite robust so far.
Now i just need to add the 3D printed direct drive extruder on top of the gearbox, and one of the the other layers should be extended to add a fan below to cool the hotend. And some kind of mounting system would be nice... But the POM gears work so well that i think i don't need the gearbox, might be better to just shrink down the geared Extruder.
But at least i have a working gearbox :-)