Fastening Polycarbonate

Our team is planning on using polycarb for the body of our power cell transfer system but have almost no experience with the material. Anybody have any advice or handy hardware tips to help out with this?

Sure, but what to use probably depends a lot on your design. How thick is the plastic? Is there any metal framework to attach it to? If you use thinner material, less than about 0.015", it’s kind of flimsy. Thicker than 0.250" is mostly rigid.

Do you have any sketches or cad model of what you’re trying to do?

Polycarbonate cuts extremely easy on a wide array of machines. Cuts well on bandsaws, cnc routers, table saw, etc. It is very easy to machine, and you can cut it at really fast speeds. However, Do not use a Dremel or other similar tool to cut the material, it melts and turns into a mess that is a pain to clean up.

Bolts or rivets are fine, just no Superglue

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Assuming the polycarb is less than a quarter inch thick, you’re going to want a metal frame backing it, as it will be increasingly floppy the thinner it gets. An aluminum tube structure to provide the rigidity and using pop rivets to attach the polycarb to it works reasonably well. Think of the structure as providing the “ribs” and the polycarb is stretched on top of it.

If you have a CNC router, you can build very rigid structures from 3mm (1/8") polycarbonate sheets by using the same type of interlocking tabs and fastening screws that the wooden Makerbots used:

No aluminum backing necessary, as the three dimensional structure provides for increased stiffness.

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Our team runs typically 80/20 cross members (or 2x1 aluminum, although sliders don’t work for this - just bolts), and we use T-nuts. T-nuts are basically drop in sliders that you can use special 1/4’’ allen heads to fasten the polycarb tightly to 8020. But because of the youngs modulus (I believe it is), it is very flexible and wobbly, so make sure to properly support it if this piece is structural.

It also sounds unprofessional, and to some teams it may be looked down upon, but zip ties get the job done as well - quarter inch holes overlapping or collinear works. Good luck!

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Username does not check out haha

To expand on this, through-bolt connections work best, don’t try to thread into it.

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Bolts or rivets for attaching. We cut thicker stuff on the bandsaw, it works really well, or when needed with a CNC. Thinner stuff we typically score and break (Use a knife to make a grove into it, several passes with the knife from one end to the other. Then bend away from the grove. You’ll hear a loud snap, and it’ll come out in two pieces with a nice straight line!), first by lining it up on our break so we can get a straight line scoring it, then flipping it over to bend it and break it along the score.

You want to stick away from chemicals and polycarb, in general - as a plastic, it’s more prone to degrading from exposure to chemicals than aluminum is. Specifically, stay away from locktite. If you get locktite on polycarb, it’ll cause it to crack and break! (They do make a polycarb-friendly locktite, but that’s a product that most teams likely don’t have).

Concur that a screw or rivet or zip-tie through a hole is great for fastening polycarb to a more rigid structure or other sheets of polycarb. If you use rivets for polycarb-to-polycarb, put a rivet washer behind; don’t count on the plastic to hold the expanded rivet end.

I have also cut polycarb with aviation snips. Use the ones with angled blades so there is enough space for both sides of the material being cut to clear the pivot point.
You can also cut it with a jig saw, at low speed so it tears chunks out rather than melts its way through.

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In my opinion, the best example of this type of structure I have seen is from your team. I was lucky enough to get one of the intake versions from 2018:

This is where this part was on the bot.

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If you plan on cutting it and using a large piece structurally, try to refrain from scoring it with a scribe, as deep scratches can be very detrimental, especially if you plan on holding compression on balls. Try marking cuts with sharpie or a similar marker.

Also, pretty important, try using flat head screws with a countersink or a bolt with a decent sized washer (I prefer the latter). Either works fine, but regardless of your thickness or size, never try to attach via direct contact with the bolt head when avoidable.

This is a rather interesting statement to make. I would like to make some counter-points.

Did you know that countersinking instead of using a straight thru-hole adds more stress and makes an appreciably weaker connection? [sauce]

Flat washers are (almost) ubiquitously unused by any serious machine OEM. They can result in some bad loads being applied to their parent material, and unless they are carefully chosen they can cause a bolted joint to be weaker than one without a washer from effects of stress concentration and adding an inconsistent amount of friction to the joint. [more sauce] They can even cause joint loosening without the nut spinning off!

Now, how applicable are the articles and their conclusions to the FRC experience? Well, that’s debatable. Is the solution applicable to FRC with little effort? Absolutely. Using flange-head bolts and nuts, or pan/button head bolts and oversized nuts, is an easy solution that teams can implement.

95 uses button head cap screws and wide-head rivets for plastics typically. We have also threaded plastics (I suggest a minimum of 6 threads of engagement, or at least 3x fastener major diameter, [third sauce] to approach full fastener utilization) with good success. However, plastic threading wears more quickly than aluminum or steel. Re-tightening threads in plastic over and over will fail, eventually.

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I agree with the closing statements, and your argument definitely explains the belly pan failure we had last year…

I still stand by the final statement I made about avoiding contact with bolt heads, though I meant cap heads, but yes, washers and countersinks are definitely not preferable. if alternatives like hex and flanged heads are available then by all means, use those.

I disagree with how relevant the BoltScience paper on steel bolts in steel substrates is to typical FRC joints, in particular FRC joints with steel fasteners in plastic substrates. I think you’re too optimistic about how much preload there is in a typical FRC bolted joint, particularly in a typical FRC joint where steel fasteners are bolting through plastic.

Preload Doesn't Matter in FRC

Bolted joints in industry are designed to hold loads using friction between plies rather than bearing against the bolt body, and often do not include explicit locking features because preload will be high enough for them to stay tight without it. They are made in joints where the substrate and washer and bolt are all the same order of magnitude in strength & stiffness, and factors like the roughness of the zinc are measurable in the preload results, which matter.

Bolted joints in FRC are generally not designed and their preload doesn’t matter. Between bolting through unsupported box sections, plastic creep, and the “what we have in the shop today” bolt sizing, it’s prudent to plan for the bolt to end up in bearing and the joint to not have significant preload. I argue this is why nylon nuts are so popular - positive locking on the joint is a huge help to making robots that work, as mistakes in the design will lead to a joint failing to develop enough pretension in the bolt to stay tight with a standard nut, and even if it was tight - expect plastic creep to allow your joints to loosen as you put your 4-8 hours on the design.

When I evaluate an FRC joint, I’m looking for wide bearing areas between the parts and nylocks so that I can lose >80% of the pretension in a joint and still have my mechanism hold together. The cheap Big Box Hardware Store Approved method is washers, though we’ve slowly been bringing more flange-head and wide-head material in at 841 - mostly so that we can cut part count. One fewer washer is one fewer thing to forget at assembly!

841 uses mostly button head cap screws with our polycarb assemblies, some wide-head 1/4" hardware originally bought for our drive axles, and some rivets. We used #8 vex standoffs to supplement tab-in-slot lasercut pieces with button head screws and no washers two years ago in 1/4 polycarb with surprisingly good results. Most of 148’s lasercut wonders we take inspiration from are held together with #8 button head hardware, thin washers, and nylocks.

Standard-head rivets have worked fine with good design. When we’ve riveted, it’s been into an aluminum angle bracket so that the rivet bulb forms on the aluminum-on-aluminum side of the joint, and oriented to the design load so that the bracket takes most of the load, shear on the rivet holds alignment, and there’s negligible force pulling vertically trying to tear the head through the plastic.

We have not threaded into plastics.

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That’s a reasonable argument. Unfortunately I have never found and literature on ‘plastic panel bolting in short term usage’ literature, so we read what we can and follow the general line of thought.

Washers aren’t going to make your robot explode, they can help load spread. But there are better options.

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Pretty sure we’re actively generating it here on Chief right now, though maybe closer to an “oral tradition” of plastic panel bolting in short term usage rather than “literature” :wink:

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For what it’s worth, we’ve been bolting polycarb on with washers for 10+ years and never seen a failure.

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Now there is a horrifying thought… haha!

Super glue(CA glue) should be fine, Loctite threadlockers are BAD.