FRC Belly Pan CAD Tutorial

I didn’t realize how contagious our robot was - it’s still in quarantine.
Fortunately, no one else on the team caught the virus.

Hopefully I will get access this summer - I will follow up with pictures when I can.

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@john3928 I gained access to the practice robot.
Here is a picture showing one of the mounting blocks holding a SPARKMAX. There is a second block on the other end of the SPARKMAX, but it’s difficult to take a picture showing both blocks. Make sure you use thread locker to prevent the screws from backing out!

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We did a similar thing using 3D prints to mount the SPARK MAXs so that they could be mounted onto the same hole pattern as a Talon SRX/Victor SP:

The mounts looked like this:

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You’re making a false comparison - you are comparing a lightened metal belly pan to an unlightened metal belly pan. You should be comparing it to an unlightened plywood or composite belly pan. The weight is much more comparable.

I am saying it is often a waste of manufacturing time or resources to lighten metal for a belly pan versus using a strong quality plywood or composite and not lightening it.

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That is another approach.
Two comments -
First, you may want to check if the SPARKMAX is able to slide longitudinally - I see the lateral and vertical constraint with the block and the zip ties, but not the last degree of freedom.
Second, while heat may not be an issue, by inserting the 3D printed part between the motor drive and underlying structure you may not be able to dissipate heat.
We mounted our drives to aluminum structure pieces.

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Here is a fair comparison. This compares 6mm Baltic Birch versus .09" and .125" 6061. For the .09" 6061 I used a 53% lightening factor, since this is what @Daniel_Kassab acheived on their robot.

Belly Pan W (in) Belly Pan L (in) Belly Pan A (in^2) Weight (lbs) Lightening % Lightened Weight (lbs)
25 30 750 6.6 0.53 3.1
25 30 750 9.1 0.6 3.7
25 30 750 4.6 0% 4.6
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So it actually seems baltic birch is heavier than I realized for our 2020 belly (23"x32") pan going 6mm baltic birch would have weighed 4 - 4…5ib (varies based on different density numbers) assuming no holes or cutouts. While our pocketed aluminum belly pan (0.9" thickness) was 2.4ib or 6.5ib without pockets or cutouts for 63% weight savings. So it seems baltic birch belly-pans weigh about 40% more although this could vary depending on how aggressive pocketing is.

Two things to consider:

  1. Daniel’s belly pan had gearbox cutouts in it that would be trivial to implement in plywood and were responsible for a large portion of the material reduction, so the comparison isn’t entirely fair. I’d say a weight closer to 80% of the plywood weight is accurate, so like 4ish lbs.

  2. Of course a lightened aluminum pan is optimal, when you don’t consider resources and manufacturing time. My point is mostly that the extra ounces, maybe extra pound, can often be worth the decrease in design and manufacturing resources within the context of build season. One pound down low isn’t really going to kill you.

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Also, the increased thickness of plywood will mean it has much higher out of plane stiffness for mounting components. No wobble. Plus, no worries about your lightening pattern potentially giving poor shear stiffness.

If someone wants the ultimate, a nomex/carbon-fiber bellypan sounds fun. :slight_smile:

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@GraveFurball how’s that tutorial coming along?

While I wasn’t too involved in the electronics board when I was on 3494, I wouldn’t say that making it out of carbon fiber is much of an advantage over wood, thin plastic, or thin metal. You get all the disadvantages of carbon fiber while not taking advantage of the benefits enough for it to be worth it.

Yeah, yeah. The fact that we didn’t do our electronics board out of carbon this year kind of threw a wrench in things.

@ThaddeusMaximus 3/4 years I was on 3494 we did a carbon fiber/nomex belly pan. While it is very light and relatively strong, it can be problematic because it is difficult to machine (and thereby modify when we inevitably miss something in the CAD). The layups were also multiday affairs, which can put you behind quickly if you’re not careful. IMO, carbon works much better as a talking point for the judges than a functional material for the robot, except for in niche cases where you need the strength and don’t have the weight to back it up (see our ramps in 2018).

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Every year I have been on 3200, we have used 1/8" or 1/4" polycarbonate sheet for our bellypans. A little pricey, but it’s strong and very easy to machine. Works well for electronics pans too!

Plastics could be very good in belly/brainpans. Some good cheaper options would be Polypropylene (PolyPro, PP) and PET-G. Both can take substantial abuse and are laser cuttable (even if the edges are a bit gross) and you can even get perforated PP sheets.

For sake of completeness: the other downside of carbon is its low impact resistance. Not a concern for a bellypan (except in 2016) but since carbon is very stiff, it doesn’t flex much when it takes load, so has little room to absorb energy. (Kevlar and fiberglass modifications can help with this as can intelligent shape design.) Hence why if you watch a modern F1 crash… everything just kinda blows up quickly and you never see something as simple as a bent component.

CF is not the paragon of materials; it solves common but specific problems. :slight_smile: