Post-006
Jan. 17th, 2022

Pixar Lamp video part 3: machining some bearings!

Puff the magic dragon is no match for the N51 waterjet.

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In the previous post we made a whole new linear rail — for our Pixar Lamp to jump up and down on, but we had an issue with the bearings that allows it to slide along the rail. We used zip ties which were too loose and made the rail not as effective or smooth as we wanted. Today we're going to do two things, first we want to design a new linear bearing that slides on the string easier, and we want to machine this bearing, install it in our lamp, and test its effectiveness. Second, we might try adding the bungees into the system. 

We have the following design requirements:

  • The bearings must be both easy to machine and install

  • we want to be able to remove the lamp from the rail easily for calibration 

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We came up with two ideas and drew them out, the first involved taking a block, drilling a 1/16th inch hole vertically through it, drilling some tapped M5 holes, and cutting it in half with a bandsaw. This seemed too complex to machine, but the advantage was we would only need 2x-4x of them depending on how tall we made it.

The second idea, is the one I finally decided to do. And that was to take a piece of stock and waterjet out some squares with rounded corners. Then drill a slot in the top face and two M5 screw holes in the horizontal face. The spacing of these holes would match the spacing on the old linear bearings so we can use the same mounting holes. A better picture is below. And it'll make more sense in the pictures when we machine and install them.

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So we're going to water jet out 6 of these "blanks" with slots already placed in them. And then drill out the holes on the mill, and tap them upstairs in D-Lab which has a bunch of metric taps. We'll use some jigs to align the part on the mill but the part doesn't have to be super precise because the lamp itself has "slots" not holes which means we can shift the bearings left to right to get the 1/16th inch slots for the cable to align. The important part is the cable stays vertical and isn't stretched/bent in any direction. Because of the small size of the part, we may drill the holes by hand but ensure their horizontal alignment is OK. The next step was to waterjet the little blanks.

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While water-jetting the blanks, we ran into a few problems. If we tried using tabs to keep the pieces from falling through into the bottom of the waterjet, the tabs were too big and the piece was hard to remove. Bridges between the individual pieces also didn't work because the bridges were too long. So we came up with another strategy.

Every time the waterjet got close to finishing a cut we would panic, slam the pause button, and grab the result before it fell into the depths of the waterjet to be lost forever. As the solar car aero lead Daniela Vallejo once said, losing a part to the waterjet creates an emotion we can only describe as "profound sadness." We thankfully didn't lose any this time so we went up to D-Lab to commence the tapping and drilling. The old bearing blocks were M5x0.8mm taps so we shall use those.

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So we would have used a mill to drill the holes but it wasn't super convenient at the time and we thought we could get adequate results for this experimental part using "aggressive eyeballing." Essentially I drew a sharpie mark down the middle and lined the drill bit up with the center of the sharpie mark knowing the holes should be close to the fillets based on the CAD. We drilled them and we tapped them. The holes lined up well enough at a first glance and after tapping them and comparing them to the previous linear bearings and based on the size of the slots on the lamp, we think they'll work well but of course we'll test them tonight. 

The last thing to think about is lubricant. And many metal-on-metal sliding systems use graphite lubricant. Essentially you take graphite power and coat the surface with it, and the power acts as a lubricant. We will see if the system needs this after we test it, we might just shave a pencil, because you really don't need anything more complex.

#pixar #bearings #linear-motion #machining