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Unread 01-31-2017, 09:25 AM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Sperkowsky View Post
You can also cut polycarbonate with a xacto knife. The point is not just that it's possible to cut the material. The point is the ease of doing it.
I can see the point that you are making. My experience has been that aluminum is also very easy to cut, especially when using any sort of power tool with the appropriate blade.

If you're using a router, aluminum should be chipped at about the same rate as PC because aluminum can be machined at around 2x the SFM at about 1/2x the chipload per tooth.
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Unread 01-31-2017, 09:52 AM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Knufire View Post
See the original intake from the 111 2012 robot or the 1678 2014 robot for examples of polycarbonate intakes.

The idea here is that you want a material that is tough, not strong. While strength is a measure of the amount of stress a material can take before failure, toughness is a measure of how much energy the material can absorb before failure, and is a mixture of the strength and ductility of the material. You want the polycarbonate intake to deflect when it is hit instead of breaking, but this naturally means that your intake won't be stiff. On the other hand, you can have a very stiff aluminum intake that will always be in the same spot relative to your robot frame, but you will then have to build it to be strong/tough enough to withstand impact.
+1

Definitely take a look at their After the Game video on their 2012 intake. Stacking up lexan plates will give you strength and flexibility and are incredibly easy to make replacements of if you break them and you can stack up more plates to add strength and rigidity.
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Unread 01-31-2017, 10:03 AM
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Re: Polycarbonate vs. Aluminum

I think the important thing to realize here is that both materials will work and the best material for you depends on availability and your ability to work with the material.

Try not to fall into the trap of choosing a material because it had the most supportive comments on CD.
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Unread 01-31-2017, 10:09 AM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by JamesCH95 View Post
To play devils advocate: I can replace every instance of PC in your post with aluminum and everything would still be true. I say this owning my own CNC router that I routinely cut aluminum with and having cut plenty of aluminum with a jig saw.
You can definitely get away with a much crappier CNC router when cutting polycarb. Not every CNC router is good with aluminum, but I've yet to find one that isn't good with polycarb.

It's certainly a material that is for the most part easier to work with.

---

I like polycarb intakes. I like them to be a bit stiffer than 1678's 2014 intake (that violent shaking back and forth gives me the heebie jeebies even though it was fine), so I like to use either Vex's 2x1 polycarbonate tubing or McMaster's 1x1 tubing for some structure. It certainly can be strong enough to do the job - a rough rule of thumb is to double the thickness of the equivalent aluminum part.

Aluminum can work too, it just likes to stay bent and doesn't absorb impact energy in the same way.
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Unread 01-31-2017, 11:47 AM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Chris is me View Post
I like polycarb intakes. I like them to be a bit stiffer than 1678's 2014 intake (that violent shaking back and forth gives me the heebie jeebies even though it was fine)
A lot of people were unnerved by the swaying from them. If asked about it I would grab one and bend it over 45 degrees and let go to demonstrate my confidence in them.
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Unread 01-31-2017, 12:34 PM
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Re: Polycarbonate vs. Aluminum

My team (3357) switched out our aluminum shooter deck/intake aluminum for polycarb in the middle of the season because we had a small crack in it. We also did it because we wanted to decrease weight. It worked well and was able to bend without cracking.
Here's a video of us with it: https://m.youtube.com/watch?v=o-mdnlaoFbs
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Unread 01-31-2017, 01:13 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by RoboChair View Post
A lot of people were unnerved by the swaying from them. If asked about it I would grab one and bend it over 45 degrees and let go to demonstrate my confidence in them.
Floppy Intakes unite?

I don't understand why teams don't take advantage of material properties like that more often.
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Unread 01-31-2017, 01:16 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by PayneTrain View Post
How did those VP mounts hold up?
We didn't have issues with it, besides being difficult to assemble.

The key was 2 bearings in the VP mount tube (one on each side of the tube), and then a third bearing on the other polycarb arm. This meant that, even as the arms twisted, the C-C for the chain was held constant. But since the polycarb arm is flexy, the intake shaft isn't over-constrained, even with three bearings (as it would be if all three bearings were held constant relative to each other).

Hope this makes sense.

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Unread 01-31-2017, 01:29 PM
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Re: Polycarbonate vs. Aluminum

To play devil's advocate: consider for this year is the potential violations of R03 with a highly compliant mechanism. An intake that can bend over 45 sideways is great for avoiding damage, but may not meet R03 to an inspector's satisfaction.

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The ROBOT must remain constrained to the maximum inspected volume at all times during the MATCH
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Originally Posted by Ty Tremblay View Post
I think the important thing to realize here is that both materials will work and the best material for you depends on availability and your ability to work with the material.

Try not to fall into the trap of choosing a material because it had the most supportive comments on CD.
Exactly, you found the words I couldn't.
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Unread 01-31-2017, 01:46 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Michael Corsetto View Post
The key was 2 bearings in the VP mount tube (one on each side of the tube), and then a third bearing on the other polycarb arm. This meant that, even as the arms twisted, the C-C for the chain was held constant. But since the polycarb arm is flexy, the intake shaft isn't over-constrained, even with three bearings (as it would be if all three bearings were held constant relative to each other).
Just to go on a tangent here, but any time you have a long shaft like this (>2 feet between supports), you can sometimes get away with more than two bearings per shaft since the span is so long. It's not something that can be only done with polycarbonate or anything like that, especially if 2 of your 3 bearings are in pockets milled into the same tube. Just another fun fact to add to the conversation here.
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Unread 01-31-2017, 01:56 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Chris is me View Post
Just to go on a tangent here, but any time you have a long shaft like this (>2 feet between supports), you can sometimes get away with more than two bearings per shaft since the span is so long. It's not something that can be only done with polycarbonate or anything like that, especially if 2 of your 3 bearings are in pockets milled into the same tube. Just another fun fact to add to the conversation here.
Confirmed. Here are 4x bearings on the same shaft!

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Unread 01-31-2017, 01:56 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Chris is me View Post
Just to go on a tangent here, but any time you have a long shaft like this (>2 feet between supports), you can sometimes get away with more than two bearings per shaft since the span is so long. It's not something that can be only done with polycarbonate or anything like that, especially if 2 of your 3 bearings are in pockets milled into the same tube. Just another fun fact to add to the conversation here.
Very true, key word being "sometimes"
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Unread 01-31-2017, 02:01 PM
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Re: Polycarbonate vs. Aluminum

This is a pretty significant tangent, but it's not worth making a new thread over. What actually happens when you overconstrain a shaft? Does it automatically bind? Is it somewhat more prone to binding? Is the resistance (load) substantially increased? I know not to do it, but I don't know that actual effects of doing it.
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Unread 01-31-2017, 02:20 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by Cothron Theiss View Post
This is a pretty significant tangent, but it's not worth making a new thread over. What actually happens when you overconstrain a shaft? Does it automatically bind? Is it somewhat more prone to binding? Is the resistance (load) substantially increased? I know not to do it, but I don't know that actual effects of doing it.
If you have a significant gap between bearings and good manufacturing/assembly tolerances (as in James' example above), you won't see much negative impact to performance.

Some potential effects:

1) You won't be able to install the shaft. This is an easy problem to spot. You have three bearings, try to slide the shaft in, and no dice.

2) You figure out a way to install the shaft, with a axially mis-aligned set of three bearings (maybe you constrained the bearings to a rigid member AFTER installing the shaft). In this scenario:

2a) The shaft rotates, but with additional load due to the shaft constantly bending/additional radial load on bearing. This inefficiency can sometimes be hard to spot right away, but can suck power from your system

2b) The whole thing seizes up and won't move. I've seen this happen, especially when the torque applied to the shaft is fairly weak (obviously, since the system is more susceptible to inefficiencies).

Like Chris said, do it right, and you won't have issues. But people don't always do it right, so it is important to understand where things can go wrong.

Best,

-Mike
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Unread 01-31-2017, 03:04 PM
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Re: Polycarbonate vs. Aluminum

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Originally Posted by JamesCH95 View Post
I have only heard of teams having success with hot-bending PC, but I would be very interested to see good cold-forming results.
We commonly bend polycarb on a break to 90 degrees and have never had a failure. Not that it *can't*, of course--but thus far it hasn't.

As to the OP, we generally treat 1/8" and 1/16" polycarbonate as if it's indestructible in terms of impact damage, and it has yet to let us down.

I've been tempted to prototype a "sheet metal robot" made from poly instead of aluminum and take it to an off-season competition, just to see how well it would stand up to full gameplay. ...maybe some day.
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