Acrylic Gearbox Sideplates

[AndyB]

I've seen teams use delrin, I've seen teams use polycarb. Our team has the resources to use acrylic side plates and I've been thinking about an application for 1/4" or 3/16" acrylic as a gearbox side plate. We have the ability to cut acrylic with a laser and thought it would be the easiest material for us to custom machine.

Acrylic is very very similar to Polycarbonate in everything except for impact strength, as far as I know. A gearbox is a relatively low impact area in my opinion. This makes me wonder, how risky is this scenario?

[vivek16]

I wouldn't do it. It would be very prone to cracking and you don't really want that in a gearbox.

There may not be traditional "impacts" but there would be stress placed on the plates. Although, if laser cut, plexi might not crack as much.

Last year, we put a piece of plexi between two 2x4's and used 4 bolts to hold it on. One of the bolts was slightly misaligned and it managed to create a huge crack in the plexi when we put it in.

I'll stop ranting and let someone with a bit more experience answer your question.

-Vivek

[tennispro9911]

I don't have much experience specifically with gearboxes, but I do with acrylic vs polycarb. Acrylic cracks extremely easily. I wouldn't trust it with a gearbox, but it might work. I know inventor and other CAD programs probably have stress analysis tools, although I'm not sure how good those would be in this case. I would at least make it in CAD and put forces on it at least a few times what you think might possibly be enacted on it during competition.

The good thing with a gearbox is you can make all rounded edges. Make sure you do that, if you go with acrylic. As you probably know, sharp edges are stress points and produce cracks. I've done some laser cutting with 1/4 acrylic, and it isn't that great, and my team avoids acrylic as a rule. However, I don't have enough experience/knowledge to rule it out completely. With virtual testing, input from more experienced members, and real testing, it might or might not work.

[Ryan Dognaux]

Yeah, I'd go with better safe than sorry and use polycarbonate. Something could happen mid-competition and then you'd be kicking yourself for having a gearbox that doesn't function.

[Francis-134]

When you look at the difference in price for such small amounts (maybe a 1 foot by 1 foot section), the peace of mind afforded by Lexan certainly outweighs the additional $5. Who wants to have their $2000 robot not work because you wanted to save a few bucks.

I'm not sure where you get your plastics, but check to see if they have a "scrap" bin where you can buy smaller pieces that may have some imperfections such as missing paper or non-square edges for significantly less money.

[MrForbes]

A local glass shop gave us a bunch of their scraps, including way more 1/4" polycarb than you'd need to make two gearboxes. Ask around! be sure to tell them about FIRST and you might get a new sponsor too.

[AndyB]

It has nothing to do with cost for us. It completely comes down to machinability. You can't cut polycarb with a laser... or at least not the same laser.

We do have CNC mills at our university but it is much more of a hassle to access them ordinarily. And we don't have any engineers on our team that have experience in using them.

But thank you for the advice.

[MrForbes]

Make a pattern with acrylic, use it to make the reals ones from polycarb, using good old manually operated machines!

[Cory]

I wouldn't trust any part on my robot to be made of Acrylic. Doesn't matter if it will never see any load whatsoever.

[Madison]

Originally Posted by AndyB

It has nothing to do with cost for us. It completely comes down to machinability. You can't cut polycarb with a laser... or at least not the same laser.

We do have CNC mills at our university but it is much more of a hassle to access them ordinarily. And we don't have any engineers on our team that have experience in using them.

But thank you for the advice.

What kind of laser is it?

Our laser cutter will cut both polycarbonate and acrylic, among other things, but there are some caveats. The machine can't cleanly cut through anything thicker than about 1/8" polycarbonate before the material begins to melt around the cut. Thinner material cuts quickly but chars. Acrylic, on the other hand, cuts like butter up to 1/2" thicknesses.

If the laser can cut acrylic, there's a pretty good chance it'll be able to cut ABS with little trouble and Delrin with a bit of coercing. Similarly to polycarbonate, Delrin can begin to melt around the cut. We can get through 1/4" Delrin in a single pass, but for anything thicker, we'll usually run multiple low power passes instead of a single, powerful cut.

I would not feel comfortable with acrylic in a gearbox. They see more loading than you realize, I think. CIM motor windings are pretty chunky and rapidly changing their direction throws quite a bit of their weight around, so to speak.

[AndyB]

Originally Posted by M. Krass

What kind of laser is it?

Our laser cutter will cut both polycarbonate and acrylic, among other things, but there are some caveats. The machine can't cleanly cut through anything thicker than about 1/8" polycarbonate before the material begins to melt around the cut. Thinner material cuts quickly but chars.

Acrylic, on the other hand, cuts like butter up to 1/2" thicknesses.

If the laser can cut acrylic, there's a pretty good chance it'll be able to cut ABS with little trouble and Delrin with a bit of coercing. Similarly to polycarbonate, Delrin can begin to melt around the cut. We can get through 1/4" Delrin in a single pass, but for anything thicker, we'll usually run multiple low power passes instead of a single, powerful cut.

I'll look into that. I had no clue you could laser cut Delrin or ABS. Thanks Madison.

[Dick Linn]

If you have a laser, why not make swiss-cheesed aluminum side plates?

[Mike Wittman]

Hi Andy, everyone so far is right. Avoid Acrylic. It is far to brittle to use for a gearbox. In addition, laser cutting will introduce its own stress into the material making it more prone to crack or break. What you gain in structuring your cuts you loose in how the laser affects the material.

I like what squirrel suggested. Make a template and use it to machine your Polycarbonate (PC). I often laser engrave Polycarbonate and then machine to the line. It is also nice for picking or marking center points to drill.

You can cut PC with a laser. You usually need a high wattage laser to do it however, it can be done with multiple passes. It is a very messy process making lots of smoke, leaving lots of residue and also charring the edges of your material, but it can be done.

There is an alternative material you may want to use. It is called PETG. It's properties are somewhere between brittle Acrylic and durable Polycarbonate. The advantage of this material is that it will laser cut cleanly. It can still crack and break, but it is deffinately going to hold up better than Acrylic.

[tennispro9911]

This may or may not be the right place for this question, but for Transmissions, what are people's opinions on the pros of cons of Polycarbonate vs Delrin vs Aluminum. Those seem to be the most popular materials for transmission construction. We are in a similar position as AndyB, except we also would be able to get material waterjetted, which makes it much easier to use polycarb or Al.

[AndyB]

Well, aluminum is the traditional material. It's probably the safest. I got the idea of using a plastic from 254/968. (They use delrin, I believe). The only gearboxes I've seen that use lexan side plates are the AM Toughbox's.