pic: 6WD Riveted Frame Development

[MrForbes]

Using fiberglass for the main frame members is really nice....no dents....

(although some of the team members are talking about using wood next year!)

[M. Mellott]

Everyone,

Thank you very much for the comments/questions--keep 'm coming!!

[Mike Nawrot]

Originally Posted by squirrel

Using fiberglass for the main frame members is really nice....no dents....

Fiberglass is a great material for this application. Just a few things to keep in mind while working with it:

1. Gloves. Fiberglass dust can itch and sting if it gets in your skin.
2. Goggles (a must no matter what you're doing)
3. Dust mask/vacuum. Fiberglass dust likes to float around in the air if you don't have a vacuum held to whatever you're machining. Inhaling the stuff is not recommended..
4. Pay attention to what kind of stresses you're putting on the fiber glass. We used fiberglass for our ball holding mechanism frame this year, and in the scheme of things, it worked great. The one issue we ran into with it (apart from the constant irritation of the fiberglass dust), was the fibrous nature of the material. Since fiberglass is a series of glass fibers held together by an epoxy, drilling holes in the material leaves exposed fiber ends, allowing the material to be pulled apart. This caused some failures in our design, resulting in tubing that was split down the middle. There were two fixes to the problem that we looked into. The first (and heavier solution) was to put reinforcing plastic inserts into the tubing to reduce the stress on the holes. The second (and less precise) solution was to epoxy any machined edges to reseal the fiber ends. Of course, this issue is only a problem under certain conditions. We had plenty of machined connections on the fiberglass that held up fine through the whole season. The key is predicting and observing the stresses that the parts will be under, and designing accordingly.

As for rivets..

The super structure for our ball gathering and shooting mechanism in 2006 was 1/8" aluminum with 1/8" rivets. Only after two regionals, championships, and two off seasons of constant abuse and the robot flipping over onto the super structure did the rivets start to come out. So riveting is definately a viable solution. As for maintenance, pop rivets are usually pretty easy to remove by simply drilling them out with a drill of the same diameter (i.e. 1/8" rivets removed with 1/8" drill bit) and using the hole from the shank as a center.

When using aluminum, I find that it's good practice to use aluminum rivets. Since the steel is harder and generally stronger than aluminum (except for the rare cases like 7068), using a steel rivet can mar the aluminum. 3/16" aluminum rivets should be sufficient for this design, especially with the number that you appear to be using. I could be wrong though, as I've never actually constructed a riveted drive train. If you decide to use steel rivets, use washers between the expanding part of the rivet and the aluminum to distribute the force over a greater area.

[s_forbes]

I might also want to mention that we also used rivets on our mechanisms this year, and we did have several shake loose. These were mostly in the high stress points in our launching mechanism and lifting mechanism (which took a severe beating). They were all easy to replace, but it took us a couple of attempts to realize that if the plates aren't clamped together very well when you rivet them, they will shake loose again in no time.

All the rivets in the frame stayed tight all season though!



(a mild escapade off topic)

Originally Posted by Mike Nawrot

The one issue we ran into with it (apart from the constant irritation of the fiberglass dust), was the fibrous nature of the material. Since fiberglass is a series of glass fibers held together by an epoxy, drilling holes in the material leaves exposed fiber ends, allowing the material to be pulled apart.

This is interesting, where did you get your fiberglass from? The stuff we use drills and cuts very cleanly, and I've never experienced any irritation from it (though I think some team members found it to be itchy). We get ours from creative pultrusions, http://www.creativepultrusions.com/ )

[Mike Nawrot]

Originally Posted by s_forbes

This is interesting, where did you get your fiberglass from? The stuff we use drills and cuts very cleanly, and I've never experienced any irritation from it (though I think some team members found it to be itchy). We get ours from creative pultrusions, http://www.creativepultrusions.com/ )

I've looked into creative pultrusions before, and I attempted to convince the superiors to order from them, but it was easier to order from mcmaster just because of the account we've had established there for years now. It's of notably lower quality, which might be the cause of the splitting effects we experienced. It still cut and machined quite cleanly and easily, but when we had too much radial force on some of the holes it basically destroyed itself. This was partially the material, and partially because of the location of the holes on the tube. That said, those problems would not have occurred with a metal of any sort. So really, the point I'm trying to make (in a rather roundabout way ) is to remember that fiberglass is just different from aluminum, and to design accordingly.

As for feeling the irritating effects of the fiberglass dust.. Everybody is different. People with thicker skin don't seem to have problems with it. I wish I could say the same for myself. Haha

[MrForbes]

I can easily imagine having problems with attaching hardware to fiberglass tube...we have only used channel, angle, and I beam shapes, which are much easier to deal with.

You do have to be careful with design, taking into account the properties of the material you are using.

For round tubing, clamping might be a safer way of attaching it.

[Mike Nawrot]

Originally Posted by squirrel

For round tubing, clamping might be a safer way of attaching it.

It was square tubing. Sorry for the ambiguity.

[Tom Line]

Originally Posted by IKE

We have been doing a folded sheet metal riveted frame for 3 years now. While in general I really like it, this years game was particularly rough on it. The high-speed nature of overdrive combined with the less than compliant barrier wall made for some interesting bends in the frame this year. Our material is also a lot thinner as we use mostly 0.060 and 0.050 AL to make our frame.
If you do riveted joints, make sure that you check them periodically after major impacts and/or the end of the day. After a lot of wear and tear, then tend to loosen. Not that I would stop using them, but they do become a maintenance item. Also don't use the largest rivet that will package because having an up size option at competition is handy should they become loose.

We got a particular chuckle out of you guys during the playoffs at MARC. Three of your students had pulled your robot over to the side of the staging area and were beating the poor thing. After a few minutes, someone went and got a hammer and you really started pounding on it.

Whatever you were beating on must have decided to cooperate, because your robot worked brilliantly after that!