I need help with plastics
 

I've been planning my summer project and I started getting into material selection. The main thing I'm having trouble with is picking plastics, something I really have no experience with. The dimensions I'm looking for are four 1'/1'/1.5"-2.5" blocks but I'm not adverse to cutting some plastic sheet as I'm going to have to machine it anyway. Some of the factors I'm keeping in mind are
Cost: I'm on a really limited budget. I'm looking at paying no more then $60-$70
Strength: While the material should be reasonably hard I'm more worried about it not being brittle as it will have to deal with a moderate amount of torque.
Machinable: I'm not too worried about this but it should be reasonably easy to work with.
Weight: It has to be decently light weight.

I think that's about it. If it helps I need the plastic for wheels. It won't be used on anything but carpet or tiled floor and at relatively low speeds. Thanks all for any help.

Re: I need help with plastics
 

This is how you get experience. It is smart to ask.

There are MANY options available, well under your budget.

Plastics are rated and compared in many ways, but your application only depends on a few of these characteristics: strengh, weight (density, actually), machinability, and coefficient of friction

Look at www.mcmaster.com and type in "plastics" in the search area. The resulting page will provide general descriptions of common plastics.

I am guessing that Acetal (commonly known as Delrin), Nylon, and UHMW would be your best materials to use. Each of these are fairly hard and will be slick wheels, unless you put some sort of treads on them. Polycarbonate will be a bit stronger, but also more expensive.

If I had to pick one plastic to work with for machined robot parts, it would be Acetal (Delrin). From McMaster, you can get a 1 foot bar of 1"x1" stock for $11 and change.

<edit... crap, I misread mech-brain's notes, and did not realize his needs were for a 1 ft x 1 ft piece... see other suggestions below for cheaper materials>

Good luck,
Andy B.

Re: I need help with plastics
 

Andy's comments are spot on.

Delrin is a great material, but for what you're looking for, it's way over your budget - at least from McMaster.

One alternative is eBay. I bought some UHMW* at under a dollar a pound, 1" thick, includes shipping cost. You may want to approach a local plastics supplier - in person - and explain about FIRST - they might offer a discount, or give you some 'scrap' for free. Don't forget about places that use plastic (as opposed to selling it). I once visited a plastic moulding factory, and they were happy to give me piles and piles of the coolest scrap you have ever seen - big gobs of plastic, many pounds each, that came from melting pots that needed to be emptied.

(*UHMW - Ultra High Molecular Weight Polyethelene)

The only plastics you want to stay away from are those that will be, as you put it, brittle. Solid polystyrene, for example.

As an alternative, you can save a bunch of money by assembling your wheels - instead of machining from a solid block, machine from a block that is put together, so as to reduce waste and material costs.

An alternative would be wood. Not pine, but something strong, flexible and machinable, like mahogany, poplar (strong if layered), maple or basswood. A specialty woodworking store can help with selection, and in many cases wood can be much stronger and durable than plastic. Can look a lot cooler, too.

Some resources are a free plastics properties handbook, a place to learn about the properties of plastic AND wood is here, these guys sell plastics and have some good info, and more detailed properties of many woods can be found here.

Don

Re: I need help with plastics
 

A lot of plastic supplies have samples of different plastic types. As stated before, they may be more than willing to donate some scrap pieces. You may even be able to get samples in several different types and do a comparison. Also, they should be able to give more good advice on which may be good choices.

Have fun!

Re: I need help with plastics
 

I'll second Andy's rec for Delrin. This stuff is very strong, stable, easy to machine and gives a nice surface finish. Polyethylene (HDPE, UHMW, etc.) is good for high wear surfaces and for chemical resistance; I use it quite a bit in the lab. HDPE has pretty good stiffness, but is difficult to get really good dimensional stability and finish in machined parts. Tensile strength is much higher and more consistent with Delrin (70 MPa) than HDPE (15-40 MPa), too.

What that means for you is Delrin is about twice as strong, so you need half the thickness for a given load (saving weight and space) and you can tap threads in it with good load capability.

One thing to watch for with buying any material in stock size: the size tolerance is usually pretty loose. For the 1 x 1 bar stock Delrin, McMaster shows +.025" tolerance, meaning it may be anywhere from 1.000 to 1.025 thick. It is not uncommon to find wider tolerances than this in stock sizes.

Re: I need help with plastics
 

Delrin is an awesome material, and as an added feature it is self lubricating.

Basically, this means if you are going to be using it as a axel bushing for example, it may not take as much load as a brass bushing true, but it won't need oiling to keep it rolling on smoothly, unless you are in a exteremly high heat application where it dries out.

What kind of "thing" are you going to be making out of this plastic material?

Re: I need help with plastics
 

Well as I said before I'm looking at making some special wheels. Unfortunately assembling the wheel out of pieces would be less then ideal but I might go with the idea of wood. I am looking at giving whatever material I use a coating of rubber or a high friction plastic (I'm spraying the coating on) so it's personal friction is not too important but I'm thinking wood would actually take a sprayed coating better. Again my major concern is that because these are wheels and rather thin (well they will be at some points), they need to take some stress. Thanks for the all the help so far. This is the hardest part of the drive train I'm designing so I really can use the help.

Re: I need help with plastics
 

There is nothing wrong with wood. I "wood" suggest birch plywood. The wheel can be built up from several layers. To go a step further and build some really tough structures, laminate the plywood with 4 oz s2 fiberglass cloth. Use a good laminating epoxy. Our team has used wood, epoxy and fiberglass for several years now. Easy, tough, strong and cheap. Our shooter was made out of 3/4 birch plywood and was built in one night. If you want to try the fiberglass thing, let me know and I'll post some sources and info links. the RC airplane forums are a good place for composite construction info.

Re: I need help with plastics
 

Also look into a pvc building material called Azek. It is available at 'lumber yards', comes in 4x8 sheets and 1"x 4,6,8 etc. It is a replacement for pine for exterior use, is easier to machine & cut than wood, and can be glued with pvc glues, and accepts wood screws.

We have used this for wheels for the last two years, covered with conveyer belting.

Re: I need help with plastics
 

Thanks that would work fine. I kind of prefer plastic because I could see my wheels splintering at the edges where it's going to be very thin. I can just cut the board to size. I'll still keep an eye out for other plastics. But I'm happy to find something thats big enough.

Re: I need help with plastics
 

Consider composite structures of plywood, epoxy and fiberglass, too. Like Gdeaver sez, you can accomplish a lot more than you might think buy building up composites rather than carving down blocks of solid material. I know most student robot builders are obsessed with aluminum and Lexan, but there a lot more materials around that can be superior in the right use.

As soon as you can, share exactly what you want to do and let us make more detailed suggestions. I've done a lot of woodworking and composite boat building using glass and epoxy (death to polyester resin...). I'd be glad to share what I've learned, but I need more to go on than you've described so far.

In any event, good luck on your project.

Re: I need help with plastics
 

Will do. I'll consider looking at other materials but for the prototype I think the board will work for now. I don't want to share anything about it until my friend and I start the patent papers (assuming it even works).

Re: I need help with plastics
 

Perhaps you could build something up using ABS pipe and sheet for little money. Or just buy some of the larger Skyway wheels. I think some of the 8-9 inch diameter wheels were about $18. each.

Team 975's first year robot had wooden wheels laminated from pine 1 x 4s (or whatever lumber size we were restricted to back then).

Re: I need help with plastics
 

Something I had thought of while trying to make my own wheels: what about making a mold, and then making the wheels out of some sort of resin? I don't really have much experience with this (save for building a tape dispenser in my sophomore ME design class), but perhaps someone could enlighten us on the best way to do this?