My team had a bad experience with 1/8" rivets. (“The front shooter fell off”.) Since then, we’ve used aluminum 3/16" rivets, and not repeated the experience. Our chassis have been west-coast-style, made of 2x1 1/8" aluminum extrusion, and held together entirely with rivets through 1/8" gussets. (Mechanisms and vertical members are usually bolted onto the chassis.) It’s been a good pattern for us – the chassis’s construction hasn’t been a limiting factor during build or on the field since we switched to larger rivets.
On a 1-inch-wide chassis rail, we have one row of rivet holes down the centerline, spaced 0.5 inches apart. Our gussets also have holes every half-inch. When assembling rails and gussets, we put lay out the rails, put gussets on them, insert rivets into all of the gusset holes, pull all of the rivets, then flip it over and gusset & rivet the backside.
We use a # 11 drill bit ( 0.1910") for rivet holes. This allows the rivets to slide in freely, and also makes it easier to fit # 10 bolts. A 3/16" drill works in a pinch, but has a tight fit, often requiring a small socket and a hammer to push 3/16" rivets into the holes. (Maybe reamed 3/16" holes would fit better?) We also use a 3/16" drill to remove rivets.
We prefer to have a sponsoring machine shop drill the long rows of rivet holes in our chassis rails. It’s repetitive, boring work that is better-suited for CNC mills. We can make the holes ourselves on our manual mill, and usually do that for smaller mechanism pieces. Really small pieces (5-ish or fewer holes in a row) can be reliably made with a ruler, a punch, and a hand drill. More than that, and errors can accumulate to make the holes not line up.
If we didn’t have machine shops to drill the chassis rails for us, we would probably drill them by hand with a jig like this.
We used to have a pneumatic rivet gun, but we don’t anymore. During the last half of the 2019 season, and all of the 2020 season, we used this rivet tool from Menards. It looks nearly identical to the Astro 23. For 3/16" rivets, this style is much, much easier to use than more basic rivet tools. It’s so much easier that we decided to not buy another pneumatic gun – the slight difference in force and speed is largely offset by the lower price, not requiring an air compressor, and allowing us to use the same tools in the shop as in the pit. If it breaks, somebody can drive to Menards, spend $30 and be back with another one in 30 minutes.
We only have one of these riveting tools. Riveting a chassis or mechanism together doesn’t take too long; we haven’t really needed to have two.
We still keep one of the basic-style of rivet tools lying around, in case we need to re-do a rivet in a tight spot. They mostly gather dust: they’re very difficult to use with 3/16" rivets, and we’ve broken a couple of them just through occasional use.
We have a sponsoring machine shop with a waterjet make us rivet gussets during the offseason. We exclusively use L- and T-shaped gussets, usually made of (IIRC) 0.125"-thick 6061 plate. Unlike the AndyMark gussets (which our otherwise quite similar to ours), we make ours with holes every 0.5 inch, and usually put rivets in all of them. This makes 8 rivets per corner (5 holes wide, 5 holes tall, with one hole in common at the corner, and one hole being over the butt joint between the 2 rails) on the top, and another 8 on the bottom. This is almost certainly overkill, but we really don’t want our robot to split in half on the field ever again. Lighter-duty mechanisms often only get 4 or 5 rivets per gusset.
We buy our rivets from Fastenal. All-aluminum, button-head, in grip ranges from 1/8" up to 3/4" inch. By far our most common grip range is 1/4", for riveting 1/8" gussets to 1/8" extrusion. We interchangeably use rivets with 1/4" and 3/8" max grip range for this purpose. We buy one or two boxes of 500 of those before every season, and replenish other lengths on a less-frequent “as-needed” basis. Shorter lengths are useful for riveting lexan sheets. (You can use longer rivets, but they will stick out more and weigh a tiny bit more.) Longer lengths can hold many pieces together, or be used in place of bolts when you can’t access the back side of an assembly.
With a west-coast-style chassis, two of the chassis rails are capped on both ends. When you drill out rivets in those holes, the back sides will remain trapped inside the tube. If the robot is modified a lot over the season, the weight of drilled-out rivets trapped inside the chassis may actually be measurable. The robot will also sound like a rain stick when tipped on its side. If this turns into a real weight problem, (unlikely but conceivable,) a 5/8" hole near the end of the chassis rails makes it easy to shake them out.
When drilling rivets, take extra care to protect eyes and electronics, just like you’re drilling new holes in your frame. A lot of small chips will be thrown out in the process! We keep old t-shirts around to cover the robot’s electronics. After drilling, carefully fold and remove the shirt so that it doesn’t spill into the robot, then shake it out in a trash bag.
Sometimes, a drilled-out a rivet still has enough meat left on it to hold two parts together. If all of the other rivets have been drilled out, these parts can usually be pried apart. If the remains of the rivets are still stuck inside the holes, they can usually be pushed out with a new rivet, a small socket, and a tap with a hammer.
Although we’ve designed the whole robot in CAD the last few seasons, we used to build riveted-together robots that were largely “designed” by just making parts, holding them together, drilling holes through them, and bolting or riveting through the parts. I say this to emphasize that, although rivets aren’t infinitely-adjustable like T-slot is, you can still heavily modify your design after you’ve fabricated and assembled it the first time. The only critical mistakes you can make are to drill a hole to big, or to put a heavy-load-bearing hole in the wrong place such that it intersects the spot it needs to be in.
Every time you pull a rivet, the stem gets pulled off the end. My team has a tradition of collecting these in “the rivet cup” just for fun.
That’s everything I can think of right now…
Edit: It is important to put the rivets in sorta-close to straight (normal to the surface). If you’re off by too much, the pulled rivet will only be clamping on one side. You’ll be able to see a gap under part of the rim, and possibly be able to fit a fingernail or piece of paper under it. This rivet is providing less clamping force than it should be, and is more easily loosed or broken. Drill it out and put another rivet in its place, taking a little more care to ensure it is straight.