D. P. R. Chibikart: Well, I did the hard part.

Thursday, May 24th, 2012 @ 3:20 | D.P.R. Chibikart, Project Build Reports

The first step of the Great Proletariat Revolution of Chibikart will actually be sort of bourgeoisie. I’m running a few waterjet experiments to determine the best tolerance for parts – since while I can totally just throw the stock files out there, it’s not very useful directly. With laser cutters, which cut “on the line”, slots are automatically made bigger and tabs made smaller. Waterjets, though, make an attempt to compensate for the kerf of the stream – which is usually 0.030″ or more for larger machines, and changes with distance of the nozzle from the surface, wear of the nozzle, and other factors.

For instance, to get 1/4″ thick aluminum tabs and slots in parts to fit without sanding and filing, I have to change the nozzle offset from 0.014″, the default setting, to 0.012″. The machine I most frequently patronize does not have taper-free cutting accessories, so the extra 0.002″ per cut is just due to the stream changing shape. Using a commercial service would mean the offset is not under my control, so I’d have to design extra loose – just how loose is something I’m trying to work out.

Additionally, piercing and traversing (non-cutting movement) all factor into machine runtime and hence expense. One thing I’m trying is combining multiple parts into a single “metapart”:

Those are some of the brake parts combined into one profile. For small parts, this also prevents them from falling into the tank – while I’m sure shops will take care of this on their own, it’s one of those things which will make their lives easier.

This operation is actually not easy – I have to make the little tabs as a separate part (all of them at once using the other parts as adaptive references), then put them into a 2D drawing which I then edit in AutoCAD to get rid of unnecessary lines. It’s kind of a pain to change things, since AutoCAD imported drawings aren’t really parametric.

Here’s an example panel which Big Blue Saw‘s rendering engine made of the panel of 1/8″ parts:

The quoted cost of this panel was $138. This is actually an incredibly good deal. Let’s say that I buy a panel of 24 x 24″ 1/8″ 6061 aluminum from a reasonably priced metal dealer – that’s about $40 including shipping, typically (not counting surplus deals and random sellers as possible sources because it’s too nondeterministic). The only really “public” machine on campus is the MIT Hobby Shop, which charges $2 per minute of run time if you’re a student and $3/min for funded/departmental project (this, too, is an incredibly good deal at $120-$180/hr). This panel would take at least half an hour to cut

So if I actually had to buy everything, even cutting on campus is already exceeding what Simon can provide me without the hassle of dealing with a big panel of metal and working in between machine maintenance – since being a public machine, it is prone to n00b damage.

Let’s face it – waterjetting is expensive as fuck if you actually have to pay for it and do it in small quantities.

I wanted to get the riskiest part out of the way first, and that was the set of sprockets and hubs (and the whole wheel assembly in general). These parts were all cut using standard offset – no magic was applied.

The sprockets in particular are actually not raw generated profiles. I “profile shifted” them inwards by 0.005″ during modeling in order to make sure they come out either on-size or even slightly loose. Historically my waterjet sprockets need a significant amount of ‘running in’ because the extra material from the taper would cause the rollers to not seat completely. That is something I can not assume others will be able to (or have the patience) to do, so it is a good example of the kind of pre-compensation that is going into these parts.

After breaking the sprues apart, I wrapped a #25 chain around it and… hey, it works. The fit is pretty snug, indicating there can be even more shift if needed. I’m having a hard time believing that the thing is cutting nearly 5 thousandths out of spec – this alone might make the experiment invalid as no commercial machine being revenue-employed will be allowed to run that far out of whack.

This is one example “nice instruction picture” which might make it into the final Instructable. Yes, there is tapping involved. Start practicing now.

And another! Chamfering your sprockets by spinning them haphazardly in a drill press while attached with a 1/2″-bolt to the chuck. If there is one Instructable which could get thrown out for encouraging dangerous behavior…

Here are the completed test wheels. This took less than an hour, and is actually the most complex manual fabrication job on the whole thing – save for wiring. Everything else ought to be plug-and-play or assembly only, as far as I can tell.

The tools involved were a drill press, cordless drill, tap (and handle), 1/2″ (to clear out the sprocket bore), #29, and #42 drill bits, metal file, and some 4-40 x 1/2″ cap screws. Oh, and a 1/2″-13 bolt.

At this moment, I’m out of 1/8″ aluminum to finish the rest of the 1/8″ frame bits, so I’m going to experiment next on the critical 1/4″ parts such as the steering knuckle blocks. I may be able to assemble the brake lever soon, too, but it depends on what 1/8″ aluminum I can scrap together. I should have more metal coming later this week or very early next week.




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    4 Responses to “D. P. R. Chibikart: Well, I did the hard part.”

    1. LB-Projects Says:

      your work is absolutely amazing, I really enjoy reading this documentation.

    2. jafoca Says:

      Following along intently – can’t wait to see how this progresses. I really may try to build depending on final BOM cost.

    3. the chuxxor Says:

      Rest assured that it will cost somewhere between $750 and 4 figures…

    4. jafoca Says:

      Should be possible. Need to see what random stuff I can ebay off to raise some funds.