Frame Material

Hello all,

Our team is having some trouble deciding on a material to use for our frame. Currently we’re debating between:

80/20 Aluminum frame, we’d need use 2x1 for the longer sides of our frame because of our wheel design.

A mix of 1x1 and 1x2 square aluminum tubing

The kit-of-part U shaped tubing with 80/20 bracings

Our main concerns are weight and structural stability. We’re not too sure how much force to expect from robot collisions and how much of it will be absorbed by the bumpers. If anybody knows of any tests or has any input we’d really appreciate it, considering we’re already behind schedule :frowning:

It’s going to come down to the corners, in terms of structural strength and stiffness. What methods of joining and securing the corners are available to you? (Components for joining corners include rivets, tubing inserts, bolts, welds, gussets, 80/20 hardware, etc.; tell us what you can use.)

Also, what’s your budget for the frame (roughly), and do you have any of the specialized tools needed for any of the above joinery techniques? Some methods will have higher costs, obviously.

What wall thicknesses of tubing did you have in mind? Conventionally, most teams that use it use either 0.050 in, 0.063 in or 0.125 in wall thicknesses. This decision will depend somewhat on what you want to do with the frame, and what needs to be mounted to it. This decision will also have a substantial impact on the robot weights.

The kit frame is a solid option, if you’re interested in making use of its predefined mounting positions. It’s somewhat less good if you have complicated machining in mind, or need something other than Ø0.25 in holes. The corner gussets are quite strong as well (but heavy).

As for bumpers, they’re 0.75 in of plywood—so take the opportunity to make them structural. When done right, the bumpers are often stronger than the frame, and are very difficult to break. Use hardwood plywood (birch, maple, etc.) with relatively smooth surfaces; it’s more expensive, but you can often get a good deal on a partial sheet in the offcut section of the lumberyard. You might as well get the lumberyard to cut it for you into 5.00 in tall strips (but insist that they guarantee the accuracy of the cut—don’t buy it until you’re satisfied that they’re correctly-sized).

Of course, the bumpers need to be quickly removable from the frame, so design a robust, yet simple mounting system. Many teams will use threaded inserts (e.g. T-nuts) with headless bolts drilled for strong cotter pins. If designed carefully, you can add substantial stiffness to the frame, and still be able to pull it apart in seconds.

As a guideline that will save you time when rushing to your next match, or when participating in inspection, your bumpers should each be removable by one person in ten seconds.

Test:

  1. Lift frame above eye level
  2. drop
  3. repeat

We do it every year, shows which welds are good, and which need to be ground and redone.

Not sure what you mean by tubing inserts, but we have access to everything else you listed.

We have access to a shop with welding materials, spare brackets and gussets, as well as some 80/20-specific parts, and enough room in our budget to order more.

We’ve used .125" in the past, so that’s probably what we’d stick to.

We certainly like the simplicity and how easy it would be to mount things, but our main concern is structural stability. In the past, we’ve used 80/20 and aluminum square tubing and have still had the occasional problem with warping/bending if we get hit hard enough. We’ve included supports in our frame designs in previous years and are worried that with a weaker material (the kit frame) our problems would be magnified.

If anyone’s used the kit frame successfully in the past, we’d appreciate tips or any data if it’s available. We’d love to test, but this far into the build season we really don’t have a lot of extra time. Are there any major flaws in using any of the three materials?

The kit frame is a solid option, if you’re interested in making use of its predefined mounting positions…The corner gussets are quite strong as well (but heavy).

I disagree, we are a second year team and we are trying to avoid the kit frame altogether from what we learned about it our rookie year. The C-Frame is made of sturdy materials yes, however, all of the holes make it structurally weak. our robot weiged close to nothing last year (all of our weight being in the kit frame, the battery, and a pneumatic compressor), and before a few matches were over, the frame was warped. the overall design isgood for starting with, or to use for prototyping, but avoid bringing it to compete with.

The kit bot is extremely reliable in my experience - I’m interested to see how you configured it to cause such failure. I’ve never seen anything like that happen.

We’ve never used it, assuming you’re still talking about the kit frame, but some arbitrary stress testing showed that it was kind of weak.

Our robot did okay at the competition, but when it underwent the stress of all of the banging and rough nature of the game Breakaway, the frame tended to warp. It was our rookie year and maybe i shouldn’t be so harsh on it, but I wouldn’t chance using it again for holding much weight.

In my experience, the kitbot generally works well enough.

Sure it will warp, but it seems completely worth the weight savings. If you notice that it starts to warp, it’s generally easy enough to add extra support.

One year we decided that kitbot was too warpy and we needed to use a stronger frame. We built a double-tiered frame out of aluminum stock that is roughly equivalent to 80/20. That was a bad life decision. It still warped because the corners were the weak spot. It was also incredibly heavy.

Kitbot is a strong option if you don’t have time to experiment with a lot of different things. It will warp, but the corner gussets are strong & its easy enough to add reinforcement. (80/20, although overkill, fits snugly inside the C-channel…) Most importantly, make sure the path from transmission to wheel is rigid: if that starts flexing, you’ll encounter chain issues.

We’ve been building off the kit frame for a few year now, and haven’t had any issues with reliability or warping. I think last year was a bit of an outlier. The standard config with the wheel brackets wasn’t rigid enough against the diagonal stresses from the chains, so thatb caused problems for teams. With the flat field and standard mounting this year, I think the kit frame will be plenty strong enough for most teams.

As for our usage of it, we’ve been building it mostly standard, and then putting uprights into the corners that bolt into the side of the corner bracket. It tightens up the corners nicely as well as giving us material to build a second deck on, etc. We build the rest of our frame out of various sizes of 0.065 square tubing, which gives us a nice light but stiff frame

One very easy way to pretty much fix all the warping problems is to mount a continuous piece of 3/8" baltic birch plywood to the bottom of the frame. Only cut out where you need to for the wheels and chain. This will greatly strengthen the frame plus give you a non-conductive mounting surface for all the electronics.