...
The box itself was laser cut from 1/4" Baltic birch plywood. While we explored using a pattern from makercase.com, we ultimately decided to place the interlocking fingers ourselves, for greater control. This allowed for better control over kerf adjustment, which would have been hardcoded in the makercase sketches we could get. Care was taken to ensure proper calibration of the laser kerf for the interlocking fingers, oversizing the positive/male components to accommodate what the laser burns away. Before cutting, test fingers with different amounts of kerf were printed, and their tight fit verified. We have repeated this process enough times that we have mastered it, and can quickly churn out a new box with a perfect fit quite quickly.
The lid and the "topper" fixed top piece were exceptions, in that they were 1/8" plywood. This was done to both reduce the moving mass, and because these parts are largely cosmetic.
The functional inner walls with slots cut for the curved slider were laser cut from 1/4" acrylic. Early versions used 1/8", but this proved not to be stiff enough. These parts included interlocking fingers to attach to the bottom and walls (and therefore more kerf adjustments), and also included cutouts to precisely locate the gears and the back slider rod holders.
The slider for the curved slot was a 3mm steel rod that extends across the width of the box, acting as a cross beam. It was designed to extend about 1mm beyond the acrylic walls on each side to prevent it from leaving the slot. In practice, we have not seen it derail, so this seems to be sufficient.
The two-link part crank is made of: a 3D printed PETG pivot attached through the acrylic by a steel pin to a gear on the other side, a 3mm steel rod that is attached with adhesive to a hole in the pivot, and a 3D printed PETG bearing sleeve that receives the rod and attaches to the front cross beam rod with a revolute joint.
The straight slider on for the back edge of the wall lid is a 3mm steel rod that is attached via pivot to the back of the lid. It . The rod is held in place by two 3D printed bearings bushings of PETG (which we found to have lower friction than PLA), which are precisely located by the laser cut holes. The rod is kept a few millimeters from the wall, so that there is clearance for the front-edge pin, but we minimized this distance as much as possible to reduce twisting. A steel cross beam across the back of the lid keeps the two sides in sync and reduces unwanted torsion by netting the twisting force.
The lid is attached via 3D printed PLA pivots to the front and back cross beams. It's Their only functional purpose is to keep the separation between the front and back edge sliders, as the left and right sides are attached more soundly by the cross beams.
The gears are all 3D printed in PLA, because PETG wasn't available for us. They followed a template provided by the RMD course in previous years, adjusted for our needs. . Initially, we used the preprogrammed dimensions and only changed the number of teeth. Looking at the equations more closely, we realized the pressure angle was 25°. This was rather large and led to increased slipping. We reduced this angle to 20° for all of our gears to allow for better contact and force transmission.
The 28-tooth gear at the crank pivot is attached via 3x3 square steel pin to the pivot holder. This is driven by a 16-tooth gear that is attached to the end of the drive shaft. The drive shaft is a 3x3mm square steel rod going the width of the box. Around the middle of the rod is a 40-tooth spur gear, which accepts input from the 30-tooth pinion gear which is attached to the motor.
These gears, and their sizes, are mostly chosen based on translating rotation from place to place, rather than force/velocity choices. For example, the drive shaft has to clear the body of the servo to reach both sides. The original design intended a false floor in the box, and the drive shaft would hide under that, necessitating the two-gear arrangement on each end to bring power up to the crank. Even though we were mostly focused on relocating power, there is a modest 28/16 * 40/30 = 2.33 gear ratio to increase torque. This also means we will use about 210 degrees of servo rotation to cause our approximately 90 degrees of crank rotation.
The pinion gear is 3D printed with an integrated servo horn cutout, which wraps the provided servo horn very closely, allowing for low-backlash transmissionprovided servo horn very closely and screws onto it, allowing for low-backlash transmission. Unfortunately the servo horn that was available had a very long arm, which required paying attention to clearance as it rotated.
The motor is held in place by a 3D printed housing, which is both glued and screwed to the floor. The housing clamps the motor using a nut and bolt, keeping it steady.
...
The buttons and switches were cheap commercial parts from an electronics starter kit. But each was augmented with a 3D printed holder that allowed us to mount each of them in a drilled hole in the front and back of the box.
All Most wires and components were soldered in place, without use of a breadboard or PCB. This was done to conserve space for this simple circuit, but it does increase the mess of wires. Except the connections to the Arduino, which are and reduce points of failure, since the circuit was so simple. However the connections to the Arduino could not be soldered without ruining the school's Arduino, and are therefore prone to falling out. Without using solder, we didn't have a better way to make this interface reliable.
...
Next, we inserted the two 16-tooth gears on the outside of the functional walls keeping the inside of the box free from clutter (aside from the electronics, which in a final product would be much more compact and hidden under a false floor). These two gears are joined axially by a square rod that is also attached to the 40-tooth spur gear in the middle of the box. This is the main means of power transmission and how we were able to power two mechanisms with one motor. After pressing these gears snug to the walls, we inserted the two 24-tooth gears that would end up being the output gears connected to Link 1 on the inside of the box. To actually connect the round metal rod that is Link 1 to the gear, we had to 3D print a holder. This holder did not directly connect to the 24-tooth gear, instead, they were axially joined with another square rod to allow for one-to-one torque transmission. It should also be noted that initially , we joined these two components (the gear and the Link 1 holder) with a 3D printed integral connector, however, the forces produced sheared that connection. Upgrading our designs to slide onto and share the metal rod was a necessary improvement. Link 1 was then pressed into the holder with glue for extra measure.
...
To once again reduce the friction, we added the only lubricant we managed to obtain, petroleum jelly, to the sliders and the gears where needed. After this, we positioned the motor that we fit into a 3D printed holder next to the spur gear. We aligned the pinion and spur and then screwed the motor mount into the floor of our box. This was a secure connection and we thought would allow for a smooth transmission of power between the two gears. Unfortunately, after completing the box, we noticed the main square steel shaft was bending away from the motor due to the force as the gears turned which led to the gears skipping. This would have been easily correctable if we knew to buy a stiffer shaft or could have reinforced the one we had. Regardless, once we mounted the motor to the box, we put the electronics together and aligned the button and switch holes of our walls. We finally joined our final walls together and glued the “topper” piece on, finishing the box. , finishing the box. We added a velvet cover to the bottom of the box to cover the ground-down nubs of the motor housing screws that were protruding – they were the only screws available.
It is important to mention that we went through scores of 3D printed parts testing tolerances, changing designs, altering dimensions, and finding the perfect fit. We eventually realized the best method was to print the part with a slightly tighter fit than perfect and sand and file until perfect. This proved to be the most efficient as the 3D printing needed to be less precise and the end result was always exactly what we wanted. Every piece of our box was sanded or filed to either allow for a tight, no glue needed fit or to minimize friction while maintaining secure, stable motion. On the acrylic slots alone, we sanded from 320 grit up to 3000 grit to help minimize friction. While we wanted to use aluminum for these functional walls, the time requirements limited what we could feasibly accomplish as neither of us had any experience with the CNC machines or other methods of smoothly and accurately cutting the holes and slots we needed. Another important change we made was to the CAD models of the gears. Initially, we used the preprogrammed dimensions and only changed the number of teeth, however, looking at the equations more closely, we realized the pressure angle was 25°. This was rather large and led to increased slipping. We reduced this angle to 20° for all of our gears to allow for better contact and force transmission.
| Multimedia | ||||||
|---|---|---|---|---|---|---|
|
...
Buttons
Instead, we opted for two user buttons. One would open the box while the user holds the button, and one would close the box while the user holds the button. This is not as friendly a UX as a single-press open/close toggle button, but it was what we could achieve without position control. These buttons were 3D printed to wrap around a cheap push button from a starter electronics kit. An extending plunger was adhered to the button's surface, so that it would protrude from the side of the box when mounted from the inside.
Each button was connected to 3.3V source from the Arduino on one end, and to both a digital Arduino input pin (we used 7 and 8) as well as a 10k Ohm pull-down resistor to ground on the other end. The pull-down keeps the voltage at the input pin low, instead of floating in an unpredictable way. The 10kOhm 10k Ohm resistor is large enough that the voltage drop across the input pin is still ~3near 3.3V when the button is pressed.
...