View Full Version : How do I make this part?

12-05-2015, 01:52 AM
The two cylinders need to be axle-like and will be pressed (or at least friction fit) into bearings.
The inner diameter of the crescent is 25mm; the outer diameter is 37mm.
The crescent thickness is 3mm (or 3.175) and the block/cube thickness is 6mm to match the hinge pins/axles.
(Actually, the entire thing could be 6mm worst case -- or I could make it like that, and then mill down the center bit)

Given that the two cylinders turn like hinge pins/axles (this is for steering of a direct-driven wheel) I presume this should be steel, not aluminum, too.

Maybe I can put some flat into a lathe, turn the two sides, and then clamp it in a vise and mill the crescent?
I'm having a bit of a problem figuring out exactly how to hold it in a lathe though. Four-jaw chuck, and actually start with a wide piece (as if the crescent was a circle) for indication/balance?

Or mill it all out, including cutting cylinders with circular motion, and just turn it once for surface/precision/polish at the end? There's not much to hold onto if I do that...


Here's how I'd imagine it holding onto the motor.
Note that this all goes on the inside of a wheel that has an inner diameter of 85mm, so I don't have a lot of clearance to play with.


12-05-2015, 02:53 AM
The part could probably be forged or cast as a single piece, but that would not really be a diy project for most people. Assuming these are for the bigger pololu gearmotors where the stall torque is no more than an AX-18, then steel is still probably not necessary and aluminum-to-aluminum brazing kits/rods from mcmaster are super inexpensive.

Maybe turn the pins from square aluminum stock on a lathe leaving the cube portion that meets the half-ring intact, then mill out the notch to mate to the half-ring (as I'm guessing the rendering is unintentionally showing)? After the half-rings are made, then braze two pins to each half-ring and finally cut the mounting holes in the completed assemblies? For making the half-rings, maybe mill a full ring from 3mm aluminum plate and slice it down the middle to get two properly sized half-rings? Or turn thick wall aluminum tubing to the proper size then cut off in 3mm slices for less waste than milling from plate?

Or maybe just buy the machined pololu brackets for 37mm gearmotors, mill 3mm holes in the flat sides aligned with the output shaft, and then press 6mm rods/pins with a step turned down to 3mm?

12-05-2015, 01:21 PM
Those are some ideas! I had previously considered welding, but couldn't think of how to get the necessary tolerance in making the pins co-linear. Brazing sounds like a better option (which I've never done, but understand the theory of, so that's most of the battle, right? :-)

Another option that came to me last night was to split this in two T-shaped brackets instead. Those would be easier to deal with on a lathe as there would be less stick-out and less ungainly shape to hold on to.

Also, there is a fourth axis on that there CNC mill at the TechShop, so I wonder if I can actually stick some flat bar into the rotating table (put on sideways) and mill/turn down to size while also milling the bracket bit. Especially when using a tailstock, this ought to be good enough, and has the benefit of using only processes I'm already comfortable with.

The high-level idea is to figure out how to make the four-wheel-drive-and-steer I built for Money Pit II actually work with more precision -- the cheap/small ball joints I had to use, especially for steering linkage, plus the non-rigid mount to the hobby servo, make them slop up to 5 degrees, which makes it hard to go straight... With a simpler suspension (not double-A-frame) with only one degree of freedom, holding the motor/wheel assemblies, that ought to be easier. Assuming I can fit the entire assembly inside the 85mm clearance of the inner diameter of the hub -- another problem I want to solve is making the pivot axis be within the wheel, not outside the wheel like on my current suspension.


The flat plate gives a thrust bearing/washer something to rest/push on. Add a ball bearing and a shaft collar and it might work!

So, "doing it right": thrust bearing + ball bearing.
"Doing it cheap": oilite washer + bushing.

I wonder if I can get away with only one of these, suspending the motor from above? I wonder what kind of bend rigidity I'd get from the pin going into a bearing. Perhaps a pin bearing would be better for that... or double ball bearings. At the risk of making the stack too tall...

12-05-2015, 05:06 PM
Another option is to mount the wheel to the motor and make a bracket that moves all the motor mounts to the exterior of the wheel directly above the center of the tire. Basically, any of the pololu mounts connected to an inverted ?-bracket that has a vertical pin directly above the center of the tire for all connections to the suspension and steering. I know I've seen it before in large moving and construction equipment requiring high maneuverability (potential for 360+ steering) while not requiring large suspension travel at high speeds. Sort of like the steered caster system in the PR2, but using a single wheel.

12-06-2015, 12:59 AM
Hmm, I have trouble imagining what you're describing.

Unless you're describing extending the axle of the motor very far, so that I have more clearance within the inner hub of the wheel?

The current Money Pit already does a bit of this -- it moves the suspension out beside the wheel, using dual A-frames, but the pivot point for the wheel is then two inches inside the center of the wheel, which makes for unnecessary torque on an already not-too-rigid steering linkage.

Another option I've considered is using bevel gears, and have the motor sticking straight up, inside the wheel -- another variant on "long axles."

12-06-2015, 01:17 PM
Basically, a 'C' bracket that connects the motor mounts inside the wheel to a (keyed) pin directly above the center of the tire. There are several ways to add suspension and steering to a simple vertical pin above the center point of the tire.

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The suspension I think might be easiest would be placed entirely on the keyed shaft, with the stack consisting of: 'C' bracket top surface, spring, flanged bushing, bottom of chassis, 3D printed timing belt pulley or herringbone gear vertically constrained on keyed pin, another flanged bushing, top of chassis (or just a bracket to hold the bushing), then a shaft collar or snap ring to prevent the keyed pin from launching out the bottom of the chassis when lifted off the ground. The printed pulley or gear would be slip fit with a plastic key to match the keyway of the pin; as the pin slides vertically through the pulley/gear as part of the suspension, the keyway ensures the pin's rotation always matches the pulley/gear connected to the steering actuator. If you make sure the keyed pins are always bonded to the 'C' brackets in the same orientation, and all the motor connections use a connector instead of directly soldered, then the entire wheel assembly would be easily interchangeable. Just unplug the motor, remove the shaft collar or snap ring, then slide the entire assembly out the bottom of the chassis.

Hex rod might be a better choice than a keyed pin, since there would be more surfaces involved in rotationally constraining the shaft to the printed pulley/gear.

If you wanted to get super fancy, then using a hex tube instead of the keyed pin would permit a drive shaft contained inside the tube to transmit power to the wheel from a motor rigidly mounted in the chassis. Several shafts and bevel gears would have to be added to the 'C' bracket and would need a second printed pulley/gear to connect the internal drive shaft to the motor in the chassis.

12-06-2015, 04:57 PM
a (keyed) pin directly above the center of the tire

Ah! Got it.

If you wanted to get super fancy

Yeah ... no. :-)

12-06-2015, 11:24 PM
So, first, four axis machining is delightfully convenient :-)


Second, the C-bracket that pulls the turn point up above the wheel: I worry that that would put too much torque in the front/back direction, especially when hitting curbs at speed (which has killed motors before ...)

I'd be totally happy using an AX12 as the totality of the turning part of the suspension, and using a sprung arm to put that on in turn.


12-18-2015, 11:45 PM
Okay, this may work.

I have some needle roller thrust bearings coming in, too. If you squint, you can see that I milled a cutout to rest it at around the horn.
Also, I'm going to have to mount a plate on the horn face of the servo for the bearing to rest against.