Manufacturing Tolerance of Lexan Sheet

At the VEX World Championship, our team failed inspection as our lexan was measured to be 0.01" thicker than the 0.0625" thickness limit. However, the lexan we purchased (from a local hardware store) was sold as 1/16" lexan. This was most likely caused by normal manufacturing tolerance of the lexan, causing it to be slightly thicker than listed size. However, the inspectors said that we needed to buy new lexan, causing our team to waste our first 5 hours at worlds tracking down a team that had lexan, a team with tin snips we could borrow, and a team with a drill we could borrow, and then carefully retracing each piece onto the other team’s lexan and re-cutting and drilling all the necessary holes.

Our school’s other team at Worlds used lexan cut from the exact same sheet as ours, but passed inspection.

Is there some leeway in the limit for lexan thickness? Or are teams expected to bring calipers to the hardware store they buy lexan from, and carefully check the thickness of each sheet they buy to ensure it is not 0.005" too thick?

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This would be good to post on the QA once it comes out for next year.

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Personally, I’d question how they were measuring. What were they using to measure? Calipers? Micrometer? A scale? The fact that your sister team passed says there was something not quite right here. If I had to bet, I’d say you were likely in compliance. It seems there should have been some more G2 applied to this situation.

Manufacturing for this type of stuff is typically very good, every manufacturer I could find was within +/-0.005". The rules state thicknesses up to 0.070", which leaves quite a bit of head room over the standard 1/16" sheets.

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Inspectors almost failed 2158R for the same reason, but we passed by saying that callipers have a margin of error.

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The error is almost certainly the human using them. It’s a deceptively hard tool to master and use reliably.

Yeah, but its generally not a good idea to tell the guy with the yellow “green” zipties that he doesnt know how to use callipers.

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Seems better than failing inspection and not competing.

In the future, I would bring your receipts or proof-of-purchase documents to dispute their claim. If it was advertised as 1/16", and they measured something else, it’ll be way easier to blame the company and get through inspection.

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A receipt won’t help if they can’t measure it properly. Just because you have a receipt for 1/16" plastic doesn’t mean that is what is on the robot.

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I agree, especially considering the problems teams faced at Worlds when they tried to dispute calls with referees (in some divisions).

If you can fix a problem without accusing any volunteers of being wrong (and while still not saying anything that’s not true yourself), that’s probably the way to go.

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If the job is being done incorrectly, they need to know so it can be done correctly. I’m not saying be mean about it and shame the volunteer that may have never used a caliper before. Something like “I believe the material in question is within spec, can we please have the head ref or inspector take a look” would suffice. Polite, to the point, and everybody wins. You get your zip tie, and the inspector has learned something which will help every other team.

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Now that I think about it, that makes sense.

Ideally, every inspector would be willing to accept disagreements, and realize that sometimes they may be wrong and the team may be right. I assume this is true for many inspectors already, and I hope it would be true for everyone.

However, I imagine there are some cases where even a calm disagreement could cause an inspector (or any volunteer) to get upset with a team, especially the team implies that they think the inspector is wrong, or if the inspector is running short on time.

In these cases, you’d need to decide whether you’d rather risk the consequences of offending the inspector, or risk the consequences of letting the inspector continue making the mistake.

Getting the inspector’s mistake corrected will help other teams if you are proven right, but in some cases (hopefully rare), it could make the inspector more resistant to future disputes, especially if your team is involved.

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Inspectors are mostly volunteers, not intimately familiar with VRC rules, and with different levels of experience and training. If they were told in the morning meeting that they should be checking for a number of things, there is no guarantee that they all will end up with the same interpretation of the rules, or how strict they should be applying them.

If I was a student, honestly using what I believe is a legal material, and an inspector was flagging it, then my main argument would be to say that it had passed numerous inspections before and all inspectors accepted it to be legal. Some adults may be more inclined to assert their authority over students, but not against the judgement of other, presumably, adult inspectors. Former students are the best inspectors by the way.

My take on this is that, when I am inspecting the robots, it is very easy to see if a deviation from the strict standard gives the team competitive advantage or not or if they are trying to intentionally cheat or honestly thought it is a legal piece.

If extra 5%-10% thickness doesn’t give them some advantage, like being less bendy, where it is very important to be structurally rigid, then I don’t care, especially if it is a young less experienced team.

On the other hand, if one of the top very experienced teams come with sharp corners or out of spec dimensions, I will have no problem flagging them even for minor violations. If it is their 5th year they should know better.

Of course, if I know that it is going to take more than 10-15 min for them to fix and may cause them to miss important matches, then you have to take time to investigate what’s going on and consult with the head ref / head inspector. We don’t want to cause students unnecessary pain if they didn’t intend to cheat.

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