Hard to find: Custom-made wheels

[Deniw]

Hi, I'm a college student in university and I'm doing a lunar robot project. For that I need to find wheels with a precise custom diameter, that do not involve air, it would be good if it had tires, but no air usage, probably Full body polymer rubber tires.

I've been looking and looking, but I can't find much, does anyone know of a good company ( the wheels need to be one 344 mm diameter and 554 mm diameter)??

[Soeren]

Hi,

I need to find wheels with a precise custom diameter, that do not involve air,  [...] ( the wheels need to be one 344 mm diameter and 554 mm diameter)??

Why does it need to be "precise" and what does precise mean in your book?
In engineering we don't deal with adjectives like "Very precise", "Somewhat precise" and so on, but something like eg. : Measure, 554 mm, Tolerance 0.5 mm.
You indicate a needed precision of better than 0.2% on the largest wheel (and 0.3 on the smaller), is that really a must?
Will it drive anywhere except on a polished floor without even the slightest bump?

In case you need the precision for dead reckoning, forget it, no matter how precise, it won't make dead reckoning precise.

If you get wheels that are slightly oversize, a lathe will get them into shape, if you can find one that can handle the size of the workpieces.

[Deniw]

Thanks, good point,
the tolerance would be of 0.5 mm, but we can work up to 1 mm tolerance if we need to

[vinito]

coupla more points...

1/2mm tolerance for a rubber tire? really? A difference in downward pressure on a wheel/tire essentially reduces its working diameter. As a tire is used, it wears which also reduces its diameter. So is this more accurately called a "roller" which controls motion of a rod or the like, or is the tire actually in contact with the ground? Honestly I can't think of a reason a wheel's diameter can't vary widely as long as too large doesn't cause interference and too small cause chassis clearance problems. You might discuss this with your group and see if they can determine what they actually need. It would help us to advise you if you could describe the function of the wheel in more detail too.

I am a machinist by trade, and I can tell you what you are already finding out, which is that tight tolerance translates to higher cost (eg. you are having to put effort into finding a specific diameter rather than buying something off the shelf). If you can work with 1mm, then specify that because it's easier to "hit" the target and costs less.

My first thought for you is to find something close and see if you can adjust it to fit. You can spin a tire against a belt sander and remove material to reduce diameter. Just use a little careful hand work and it should work fine (measure often) and this will also true up an out-of-round wheel to be more round and smooth running.
My first thought for myself would be to simply make one from scratch, but I have a machine shop at my disposal so that's easy to say. However, if you're at a school, you likely have a machine shop and some skilled students somewhere at your disposal to accomplish the same.

One word of advice, if you take the time to make a dimensioned drawing of what you need rather than just describing it to somebody, you should have better luck getting back what you actually wanted. Memory has a way of morphing between sleeping sessions. Also, a drawing is a document which you can use to compare to a part once it's made so if it's wrong, it's black and white instead of he said/she said. But really, the machinist will appreciate having a drawing. It doesn't need to be CAD and in color, just a sketch which describes the details and tolerances is all you need. It does need to have all dimensions noted, and every dimension needs to have a tolerance noted. Google "tolerance block" to see how it's done. Essentially you just decide what the "generic" tolerances for most of the features on your part and make a not on the drawing like "all tolerance +/- .5mm unless noted", then note the few exceptions. Theres a bit more to it than that so look it up, but it's pretty simple.

Rubber isn't difficult to machine, but the material requires grinding (or sanding) rather than standard cutting like you'd do with metal or plastic. It would be good practice for a "new" machinist to work with the material, so an instructor might welcome the chance for a practical application to drift into the shop. If you have no machine shop at your own school, there is probably some school in the area which could do it for you.

On tolerances: when you say .5mm, that can mean a few things. Do you mean +.5mm/-.5mm, +/-.25mm (totaling .5mm), +0/-.5mm or +.5mm/-0
Also, if your tolerance is .5mm but you can work with 1mm, then your tolerance is not .5mm but actually 1mm. ".5mm but can work with 1mm" is two different tolerances. Pick what you need and leave the other one out of the conversation and off the drawing when talking to the machinist. Every part on the HST, including the mirrors, had a tolerance that allowed the machine to be assembled and work properly. If those rocket surgeons can do it with a million parts, you should be able to take care of the few you need for the wheels.

One additional tolerance you need to specify is time! Tell the shop the date you need to have it back. This is very important. Any shop, whether commercial or in a school, has a lot going on. It's not uncommon to ask to have something made and wait six months to find out they haven't even started on it. You just need to tell them you HAVE to have it eg. "no later than three weeks from today" when you actually don't need it for four or five. Count on it being a little late and pad your due date because late delivery is pretty common. And if you need it tomorrow, triple what you expect to pay (if you have to pay anything). If it's a school and the machining is free, please tip the shop a pizza or something at least. It's just the courteous thing to do.

So in summary, first talk with your group and decide what the actual parameters of your wheels really need to be. Make a sketch of it as you go. That's the hard part. From there it's a simple matter of either altering something you buy to fit, or having something made from scratch.

Welcome to the machine.

[Admin]

Just design your own wheel in CAD and have it laser/water cut. Shouldn't cost more than $100 for two wheels including shipping/materials.

I needed custom wheels too, this is what I did:
http://www.societyofrobots.com/robot_ERP.shtml#changes
http://www.societyofrobots.com/mechanics_robot_suspension_system.shtml