OpenSCAD

A cuckoo clock wouldn’t be very interesting if something didn’t pop out at the top of the hour. The obvious choice here is for the Chestburster to act as the cuckoo. This requires some form of linear motion to attain. I briefly entertained the idea of buying a linear actuator but couldn’t find anything that was the right speed and price.

So the alternative was using regular rotary motors and designing a rotary-to-linear motion converter. Luckily I have a whole drawer full of cheap DC motors from my work on OpenADR. Next, linear motion is a well-documented subject so designing a converter couldn’t be that hard, right?

As it turns out, it can. I am by no means a mechanical engineer. My degree and job are in computer/software engineering, so I had a great deal of trouble designing a linear mechanism I was satisfied with. I also wanted to avoid complex control electronics, so I wanted a design that produced reciprocating motion. This would let me to use a basic MOSFET to control the motor in a single direction along with a limit switch to detect a full stroke from the cuckoo.

Naturally, I chose to use a piston mechanism, most commonly used in cars to convert linear motion to rotary motion. The motor turns a linkage arm which then forces the piston (in my case the Chestburster) up and down a guide channel. The direction the motor is spinning doesn’t matter and a limit switch at the bottom of the guide would detect when the piston is fully retracted.

I got as far as designing the whole mechanism for 3D printing before deciding to scrap the it. The large number of linkages added unnecessary complexity and I was worried about the arms bending or snapping. Additionally, with the length of the guide added to the lengths of the arms, the whole mechanism was too long to fit in the depth of a human sternum and therefore the mannequin.

My next thought was to use a Scotch yoke mechanism. It’s much simpler, shorter, and wouldn’t require multiple axles to be added for assembly. The red part at the bottom of the design is a telescoping tube assembly to help reduce the depth of the design. I got as far as printing and assembling the mechanism (minus the telescoping tubes). Unfortunately, the design broke pretty much right out of the gate. The slot in the yoke requires low friction to work properly and the friction of the PLA on PLA proved too much. The axle going in the slot snapped off far too easily.

So with my previous two attempts failing, I decided to eschew reciprocating motion for something simpler. I have a few spare L9110s motor controllers and can afford to use one if it makes the mechanical design easier. So I opted for a rack and pinion system. This is essentially just a linear gear being driven by a rotary gear. By mounting the motors on the carriage within four racks, simple and constrained linear motion is achieved. This also allows for the depth of the mechanism to be equal to the height of the Chestburster. There are also plenty of rack and pinions available on Thingiverse and so I found one to use for my system.

With the previous version of the chassis in a working state, I only made two changes for version 0.3. The first change was a major one. As I mentioned previously, I was still a little unhappy with the ground clearance on the last version of the chassis. It ran well on hardwood and tile floors, but tended to get caught on the metal transition strips. It also still had some trouble on the medium-pile carpet in my apartment.

Increasing ground clearance required some significant changes to the chassis design due to the way I was connecting the motor. In my last revision of the chassis (0.1 to 0.2), all I had to do to increase the ground clearance was lower the motor mount so it was closer to the chassis base. However, since I already moved down the motor almost as far as it could go in the last revision, I didn’t have any more room to do the same here! Alternatively I could have just increased the diameter of the wheel, but I was concerned about the motors not having enough torque to move the robot.

The only option left was to no longer directly drive the wheels from the motors and instead use gears. Using gears makes it possible to offset the motor from the base of the chassis but still maintain a strong connection to the wheels. Another benefit is that it’s possible to increase the torque traveling to the wheels by sacrificing speed.

	module gear(num_teeth)
	{
	num_slices = 20;
	//rotate_angle = ((360 / num_teeth) / 2);
	rotate_angle = 0;
	union()
	{
	for (zScale = [–1, 1])
	{
	scale([1, 1, zScale])
	linear_extrude(height=5, center=false, convexity=10, twist=rotate_angle, slices=(num_slices / 2))
	{
	import (file = str("Gear_", num_teeth, ".dxf"), layer = "", origin = 0);
	}
	}
	}
	}

view raw

Gear.scad

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To design the gears, I used a gear generator site to generate a 16-tooth and 8-tooth gear DXF file. Using OpenSCAD’s import function, I imported the DXF files and then projected them linearly to create the 3D gear object.

For the small gear, I subtracted the motor shaft out of the 3D object so it could mounted to the motor.

I merged the large gear with the wheel object so that the wheel could be easily driven. I’m now using a 2mm steel axle to mount the wheel and gear combo.

By slightly repositioning the motor, I was able to move the gears into place so the wheel was properly driven. By mounting the 8-tooth gear to the motor and the 16-tooth gear to the wheel, the wheel now sees a 2x increase in torque at the cost of running at 0.5x the speed. Additionally, with the wheel no longer directly mounted to the motor, I was able move the wheel axle lower. This allowed the wheel diameter to be decreased from 50mm to 40mm while still increasing the overall ground clearance from 7.5mm to 15mm.

I did the above calculations for the force and speed on version 0.2 of the chassis as well as the new force and speed based on the motor specs I pulled from Sparkfun’s version of this motor.

Another part of the chassis that had to change in order to increase ground clearance was the caster. As shown above, version 0.2 had a hole in the chassis to make room for a semi-spherical caster wheel directly mounted to the chassis floor. Doubling the ground clearance, however, would have necessitated the caster, and by extension the hole, increase to a much larger size.

To avoid this, I made the caster entirely separate from the chassis. With two mounts, a 2mm steel axle, and a ellipsoid wheel, the caster no longer needs large holes in the chassis and frees up some internal space. I’m a little concerned that these new casters won’t be able to handle the transitions between carpet and hardwood well, due to their smaller size, but I can always revert to using a hole in the chassis and make them much larger.

The second change I made to the chassis was a minor one. In my mind, the eventual modules that will go with the navigation chassis will be plug and play, meaning no need for screwing or unscrewing them just to swap modules. To accomplish this I knew I needed some sort of mounting method inherent in the 3D design of the chassis.

I anticipate some sort of USB or 0.1″ header connector for the method of keeping the modules in place and electrically connected, but for helping to guide the module into I added guide rails to the left and right side of the inside wall of the chassis. These rails will make it easy to properly align the modules and will also keep the module vertically stable.

Keith Elliott's Personal Site

Alien Clock: Cuckoo Mechanisms

OpenADR: Navigation Chassis v0.3