Prepurchased mecanum wheels

[teammelbourne]

Hi there,
the team was curious as to whether it was in the spirit of the competition to purchase mecanum wheels or if it was a requirement to build them like has been done in the past.
Thanks for your help,
Team Melbourne

[Mk.32]

You can defininty buy them... AndyMark has just about everything you would need. And a LOT of teams do buy them.

[techedguy]

My points on this matter are these:

1) Designing, manufacturing, and running your own wheels is a great experience in the processes, design, and skills needed to fabricate just about anything. But if you custom make something, you can't reuse it later on.

2) COTS wheels, while not as good, are COTS so you can use them again. In a competition relying on fundraising for a budget, COTS parts can be recycled and repurposed...huge advantage.

[Billfred]

2) COTS wheels, while not as good...
Define "good". If a team has the resources (financial, manufacturing, design, manpower), then it might be worth it for them to build a custom set (and there are teams that do). But if all a team has available to them is a bandsaw and a hand drill, they're probably foolish not to look at COTS wheels.

The build-versus-buy debate is almost a decade old now (ever since "buy" became a plausible option)...and the answer is "Whatever is legal and works for your team."

OP: You may want to reread the earlier parts of the Robot section of the manual, starting around section 4.1.3 (http://frc-manual.usfirst.org/viewItem/57#4.1.3), to determine what's legal. As I said, what's right for your team within those limits is up to you.

[EricH]

Building versus buying... An age-old debate.

If a COTS mecanum wheel will fit your needs, you can buy it. However, if for some reason the COTS just won't work, build it. This is a classic engineering tradeoff. To say that COTS isn't as good--what you really mean is "Not as X as you'd like", and usually the reason is that the wheel is a compromise of multiple factors, and they all got shortchanged somewhat.

It's never been a requirement to build your own mecanums. However, some teams do it because they can. Back when 357 first applied them to a competition robot (2005), there were no COTS mecanum wheels that would work for the competition. So, they invested in the infrastructure to build their own. The result was Jester Drive--molded parts of various materials, assembled into one wheel. See the 2006 Behind the Design book. Then AndyMark realized that there was a market for these wheels.

With engineering in general, the build/buy debate is critical. Usually, if you can buy a solution that can be modified to work, that is the way you go. However, sometimes that isn't possible, or the potential solution will not work. Then you have to engineer your solution and build it, which tends to be more expensive because you need to test it--and if you did it wrong, it doesn't work right, so you have to redesign and rebuild.

[Andrew Schreiber]

Allow me to share two stories, one from work and the other from robots.

At work part of my job involved finding a way to provide support to users of a product. Being competent programmers we could have written a custom application to handle this for us. Being smart programmers we realized that searching for COTS solution that met most of our needs was a much better use of our time and resources.

In robotics I often have the need to reduce the speed of a motor. In 2011 I needed to reduce the FP motor to a reasonable speed. To do this I could have built a custom transmission but I determined that using the COTS transmission designed for the motor by people much smarter than I was a better idea. Instead we took some time to figure out a good way of interfacing with it. Was this an 80% solution? Absolutely!

Point is, in engineering, if you can find a COTS product that meets most of your needs you can save yourself a lot of work. Am I capable of producing custom transmissions? Yup, but I'd much rather spend a little extra money and buy one than dedicate the hours to developing one.

Another data point for you, if you decide to order mecanum wheels (or any COTS part) you can order them before season and have them on the shelf as long as you don't assemble them. For international teams this might not be a terrible idea as shipping times tend to be long. If you decide to make your own you cannot produce the ones you run on your competition robot until the season starts.

[philso]

Quite a few posters have covered the build vs. buy trade-off quite adequately.

I would like to suggest that you research the implications of using mechanum wheels thoroughly before commiting to using them. You should have solid justification for using any component. You should also be willing to accept any potential negative consequences or find a way to negate them.

It may be beneficial to ask someone who has used that type of component for their advice. We wish we did, abandoning mecanums after Alamo. Our chassis was too stiff for them to work correctly and it was too easy to damage them trying to get onto the bridge.


Phil

[techedguy]

In defining 'good' in relation to our use, this was done in "Breakaway" We didn't want the first contact point with the angled transistion to be the stamped sheet metal plate of the COTS mechanum wheels (and since one of our sponsors was willing to machine on their 5 axis that year) we built wheels similar to the Jester Drive system we'd read about.

We had also seen used COTS mechanums with chewed up rubber on the rollers from wear and believed we could make a more durable roller through casting those ourselves.