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Monday, May 24, 2010

ELB mid/late May

The first thing you should notice (well, at least if you go to MIT) is that mid/late May is when I should be studying for finals. Hmmm, evil finals or fun machine work... Finals won out eventually, but until then, I put off studying as long as possible to finish these hubs.

Now on to the mill work. I needed to put a round groove into the press-fit surface that would accept a 3mm rollpin. So I used a ball end mill. I had no idea people made these things; they're pretty cool.

I don't have pictures, but during this process I did a tool change on every part in order to drill and power-tap 10-32 grubs screw holes for axle locking. The next step was to flip the part upright and bore the four wire holes. This required an extra long drill because the regular length drill couldn't get the last 3/4" or so.

 Test gauge used to zero the X and Y axis. Mark has a bunch of these.

Really long drill bit. I'm not sure why it looks like I'm drilling into the vice here...weird camera angle I guess.

 I got two finished this day before the shop closed.

The 4 completed hubs. Note the two shorter ones for the rear.

Now I just need to press fit them. Unfortunately, I didn't have all four stators in (grrr...I still don't) yet, so I only did one. First, I had to machine a little tube that would provide a good surface for pressing. You can see it underneath the stator in this picture:

Ta-Da! One finished. 

You can see the rollpin in place in the above picture. That little bugger was hard to get kept bending. Anyways, that's it for now. I'm taking June off, but I'll be back to work on this in July as a side project to teaching high school kids about engineering through an Edgerton Center class. I'm hoping to get the whole board finished by early August! 

Stuff left to do:
2. Wind
3. Solder the motor controller boards
4. Machine the rotors
5. Plastic mill/laser work (hubcaps/spacers/hall effect sensor board)
6. Bore out the tires
7. Assemble a wheel and test it 
8. Mill/waterjet the battery box components, assemble, and test waterproof-ness
9. Find out what Franco did with the radio control
10. Wire everything up
11. Don't kill myself the first time I ride it.

Yikes, that's a lot of work left.

ELB early May

The hubs are the parts that adapt from the axle to the stator. Some people combine the axle and the hub into the same piece (think BWD), but I couldn't do that because longboards have trucks. Trucks are somewhat complicated hunks of metal that allow skate/long/mountain-boards to turn. They also have axles embedded in them meant for non-hub motor wheels, which means I have to have an adapter if I want to use stock trucks. I probably could have made my own, but that would have required a lot of design time and money (large chunks of aluminum are expensive), and I'm a firm believer in "simpler is better" when it comes to engineering. Read the design review for more details on the trucks I used, but for now, here's a picture:

Anyways, back to hubs:

 Front hub design. The rear differs only in length thanks to some stupid brackets welded on the axles I'm using- see the pic of the trucks above.

They are made out of T3-2024 aluminum, which has high shear strength. The hub slips over most of the axle and then threads onto the last half inch. Two grub screws bite into two flat spots ground onto the axle, thus keeping the hub from unscrewing and falling off. The largest diameter surface is a press-fit for the stator, and the groove in that surface is for a rollpin (Ohhh! That's what the round groove in the inner surface of the stators is for...). The thin grooves on the bearing surfaces are for snap rings, i.e. bearing retainers. The four holes that go through the thickest part of the hub are my way of avoiding having to grind a flat spot in the hub under a bearing for wires to route through. The drawback of this plan is that I have to have a massive bearing on the deck-facing side of the motor. Oh well, at least the larger bearing will handle shocks from the horrible Cambridge streets/sidewalks better.

I began by cutting down the stock into pieces slightly longer than the total length of the front hubs. The plan was to make four identical hubs and then cut off some of the end on two of them to make the rear hubs.

 My mess. The second step was to do the boring and threading. Two are finished with that step in this picture.

Mark (the Edgerton Student Shop guy) wanted to use the CNC lathe for the outer surfaces, so I of course agreed (soo much less work and more precise). After about three minutes of CAM, the lathe was ready to go. Check it out:

One of the half finished hubs. S'up Mark.

Four half done! This took maybe 30 minutes. It would have been much quicker if I didn't have to be so careful about not taking too much off of the bearing surface.

Next step was to flip them around and do the other half.

Left: one of the massive 1" x 2" bearings. Top: One hub with the surfaces done.

4 with finished surfaces. You can see the inner threads in this pic.

The next step was cutting snap-ring grooves, one 5/8" and one 1" in each. Mark has an awesome bookcase full of engineering reference guides, and one of them happened to have everything you can imagine about snap-rings in it. He also has some cutting tools specifically designed to cut very very thin grooves.

 So shiny... Note the snap-ring groves.

The next steps were all mill work, and were done later in May.

ELB April

Due to other school projects, I didn't really start building until April. I began by practicing winding the stators.

My winding station. Giant vice with paper for protecting the epoxy. The piece of PVC is how I get leverage.

The stators are GoBrushless 18T, 65mm x 34mm. They have a green epoxy coating for insulation and wire-enamel protection. Unfortunately, that epoxy likes to chip off the tips of the teeth, which means 5-minute to the rescue. So strike two against these stators. What was strike one you ask? The fact that I ordered four of these back in January and STILL haven't gotten the other three in. GRRRRR

It was at this point that I realized that the giant spool of 16AWG wire I bought wasn't going to work because I couldn't fit 15 turns of it on a tooth (I got 14! so close...). So I went to 17AWG:

I tried a bunch of different schemes to get the best possible packing, including that ugly loop sticking out the side. Needless to say I abandoned that idea.

Success! That's 1/2 of 1 phase wound with 15t of 17AWG wire per tooth. These pics were the product of a couple hours of trial and error (including messing around with 16AWG), but now I know how to wind these motors. 
The next step was working on the hubs, probably the most complicated part of this whole project.

Sunday, May 23, 2010

Electric Longboard - Pre-blog work

About the project: While the name of this blog is Rocket Science, it was originally created as a build-log for the Electric Longboard (ELB) project. Inspired by the hub motor stylings of the BWD Scooter, one of Shane's builds, and Charles' many builds, the ELB is a 4WD longboard with custom in-wheel hub motors. I came up with the general idea in November and completed the bulk of the design work over IAP, MIT's awesome-we-don't-have-real-classes-in-January period. I turned the project into a personal (and later, a duo with Franco M.) research project through the Edgerton Center.

Check out the updated design review!