repeat drive mini - mounting plates
the gearbox and motor
The gearbox and motor of choice for this project is the Repeat Drive Mini - Mk2. The bolt pattern for mounting the gearbox to the plate requires two M2 screws spaced 10.5mm apart. A hole in the center needs to be made large enough to allow the 3mm shaft and the 6mm shaft collar to pass though.
The center hole needs to be large enough to pass the 6mm collar through but small enough to not interfere with the countersunk screw holes. M2 Flathead screws generally have a head diameter of 4mm. 10.5 bolt spacing minus the 4mm for the counter sink radius would require the center hole to be 6.5mm. It will not matter if the hole is any larger than this because the heads of the screw would still interfere if that was the case.
The motors on the 4 gearboxes I purchased all have the wires exiting the motor perpendicular to the bolt holes. If I align the gearbox mount holes vertically on the mounting plates the motor wires will exit horizontally, allowing for shorter frame heights.
Height can be a concern for robots, with this in mind the mounting plate should be as short as reasonably possible. A 2mm padding around the gearbox should allow for plenty of room, this sets the height of the mounting plate at 20mm.
plate material and thickness
Carbon fiber because… duh…
Flat head screws will be utilized for mounting the plate to the frame and the gearbox to the plate. The plate will need to be be thick enough to be countersunk without the fastener bottoming out. The largest screw in the assembly will be a M3, which needs at least 1.8mm of material to be counter sunk flush. 2mm carbon fiber plate should work for this task.
plate to gearbox screw
Face mount M2 holes in the gearbox can accept roughly 3.7mm of thread before the tip of the mounting screw starts to interfere with the internal carrier plates. Since the planned thickness of the mounting plate is 2mm, a maximum screw length of 5.7mm can be used. A M2 flathead cap screw that is 5mm long should be a suitable choice.
The clearance hole for an M2 screw is 2.4mm diameter. A 90 degree counter sunk hole will ensure that the screw heads will not interfere with the hub or wheel.
The price of these screws is $8.51 for 25 and 2 are required, which results in a price of $0.68 per mount assembly.
These M2 screws weigh approximately 0.15g each for a total of 0.3g per mount assembly.
plate to plastic frame screw
The choice for mounting the carbon fiber plate to the frame is fairly subjective. There are many variables that need to be taken into account when deciding what fastener is correct, such as:
- Frame material type
- Exposed or armored wheels
- Can the fastener be used on other assemblies within the bot to cut down on unique component count
A M3 Flat Head x 12mm long plastite screw is a fairly generic choice in my opinion and should work for a variety of different situations. McMaster didn’t have a steel version of these available, I settled for stainless steel.
The clearance hole for a M3 screw is 3.4mm in diameter. Just as with the previous M2 screw, the hole will be countersunk for clearance.
The screw holes should be mounted at least 3mm away from the gearbox radially. I will mount them an additional 1mm away for extra clearance. Utilizing the same offset from the exterior profile of the plate should also work.
since a 4mm radial offset is being utilized from the screw center, the width of the panel will not be an integer since it will be tangentially aligned to the screw hole. I will arbitrarily round the width up to the next even integer, this will make the number easier to recall when designing the mating components.
The price of these screws is $13.70 for 100 and 4 are required, which results in a price of $0.55 per mount assembly.
These M3 screws weigh approximately 0.42g each for a total of 1.28g per mount assembly.
designing the part in cad
With current prices for carbon fiber, these mounts would cost around $0.32 per mount. The 2mm carbon fiber mounting plate should weigh 1.5g
weight and cost
Carbon Fiber Mounting Plate
M2 x 5mm FHCS
M3 x 12mm Plastite
These prices do not account for tax, shipping, manufacturing, or over-purchased products.