Cutting weight from large aluminum part

on our robot we have 2 large panels (roughly 17x28 with a total area of
87.6 in³) made out of 1/4in 6061 aluminium. Each side currently weighs 8.5lbs. how do we get this downt to about 5.5 without ruining the structal integrity of the panels.

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You will have to show some pictures, and possibly describe how it is loaded. It is likely that there is a lot of material that can be removed by either drilling or milling.

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Replace them with 1/2" plywood.

But seriously…that’s one way to do it. Without seeing the robot design, it’s impossible to give you more specific details.

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MrForbes is correct about the relative mass density of plywood vs. aluminum. A good grade of plywood (Baltic Birch or Sande) at 1/2 inch thickness will be very strong and stable, but not as strong as 1/4" aluminum. However, if you pocket the aluminum without close attention to where it is loaded by forces from the mechanism or other parts of your robot, it could end up being weaker at a critical point than the plywood replacement would have been.

It sounds like you might have something very similar to us. We have two pretty large panels as well, although ours are made out of 1/2" polycarb, which has a density of close to 1/2 that of 6061 aluminum. According to the CAD, each panel weighs about 8.25lbs.

We were initially considering pocketing it significantly. Especially on the right side of the image above, we would be able to remove a significant amount. However, for balance purposes, we would want to remove material from the left instead (we have more weight on that side of the robot). We ended up compromising by using the non-ball side of these panels to mount electronics, instead of having another panel of some sort to do so on. So, drop a little weight from that removed panel and keep the weight here - it ended up working out for us, but your situation may be different.

In the picture above, you can consider how the top left bearing hole supports our intake, and the type of stress it might be under from the weight of the intake and any impacts that may occur. Three of the remaining 5 bearing holes put stress when the ball moves through the system - the ball pushes against the bottom and the wheels on the shafts for those holes. On the bottom, we have a thinner polycarb sheet backed by churros bolted into these panels, so ultimately you end up pushing against the bearing holes and the churro mounting holes.

All that said, we could pocket it if we really needed to, but we’re underweight (by a significant amount), so we didn’t bother!

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This is nearly identical to our side panels, our other option is using 3/8 lexan but at 28 in tall we thought that might be too flexible. I was mainly wandering how to figueout where to put the pockets and keep redgidity.
Here is a link wo the onshape document.
https://cad.onshape.com/documents/4521b1d35feab34d3f4b0818/w/0df8b707b67cc6bbf833a712/e/57cba3e644b75add4ee3169f?renderMode=0&uiState=6209727917af790a85b65711

You’d probably be fine going down to 1/4" polycarbonate, given your setup is similar to Jon’s. That’s what we’d run it at. Honestly 1/4" is pretty much the thickest stuff we buy and use.

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If my kids presented me with this Onshape document and a need to cut weight, here’s what I’d do.

  1. Find a local machine shop that can bend 1/8" aluminum. Maybe they’ll sponsor us, maybe I’ll have to pay them, maybe I’ll need to go get some root beers for the machinists in the back. Whatever.
  2. Tell my CAD team to switch the thickness to 1/8" and design in a bent flange along that back spine, and depending on the sponsor’s capabilities maybe other places too.
  3. Probably refactor the hood bits while we’re at it; if they can be flanged as well (or just have things like Keystone 4337 brackets or Thrifty brackets riveted on) and we just have rivet or bolt holes instead of long slits along most of the height, it’ll make the vertical plates even stronger.
  4. Since I’m adding flanges on the back, I might design a large flat gusset to tie the plates together instead of using a bunch of churro. Fiddle with distances appropriately, and it could also serve as the vertical back panel in the middle there to eliminate a part.

Polycarbonate would also almost certainly do the trick; I’m influenced by past experiences with sheetmetal. Whichever route you choose, I hope you get it dialed in!

if you leave 1/2" of material around every hole and connecting all the holes, and 1/2" wide strips wherever the mounting holes are more than 2" or 3" apart, it will probably be rigid enough.

What equipment do you have available to machine it with? and I’m assuming you don’t want to lose the time/money/effort you have already put into making the parts.

it is going to be professionally water-jet.