I thing that among all the pre-kickoff game hint madness, it’s about time for a good old technical thread.
My team does what would probably be considered a very “West Coast” frame setup (2x1 aluminium box tubing, etc) but tends to prefer using 3/16" rivets over welding. We use a lot of rivets, probably on the order of a few hundred in a serious weekend of building, so I thought it would be good to get the CD perspective on a few small problems with rivets that have bothered us in the past.
First, we sometimes see the mandrel break about halfway down (instead of inside the rivet body like it’s supposed to) when we’re installing a rivet, which gets to be a real pain. They stick out, look ugly, and make it very hard to take out the rivet. Is there a root cause to this type of problem? Is there some sort of technique that can keep this from happening? If the mandrel does break halfway, what’s the best way to uninstall the rivet? For normally installed rivets, we usually just take a #10 drill and drill through the center of the rivet. However, with the nub of the mandrel still sticking out in the center of the rivet, it becomes very difficult to start the drill on the center. Is there another way to remove this type of rivets?
Second, what drill size is the “right” size for 3/16" rivets? On our team we’ve used everywhere from 3/16" (fractional) drill bits to #6 number bits. The #8-6 bits are nice because they’re also clearance for 10-32 screws, but they also produce loose fits for the rivets, which can sometimes make out frames wobbly. Is there a correct size (or at least best size) for 3/16" rivets in FRC?
Finally, when we drill out rivets, the lazier among us (including yours truly) like to let the heads of the rivets stack up along the drill bit. If we’re drilling out ten rivets, it can become a pain to clear the drill of each head individually, so we just let them stack up. However, some of the mentors have said that this is bad for the drill bit. Is this baloney or the wisdom of an experience machinist?
If other people have cool/interesting tips about rivets in FRC, feel free to share them on this thread too.
We’ve had rivet pull studs break in the wrong place too. It’s really a pain, and it’s ugly. Usually, there’s enough sticking out for us to grab with pliers and wiggle back and forth to break off. If not, then we’ll file it down. I think it may stem from using a rivet outside of its specified grip range, but I’m not certain. I too would like to know how to avoid this.
We always drill a 3/16" hole for a 3/16" rivet. I don’t see any problem in letting the rivet heads stack up on a drill bit. I just grab them with pliers afterward and reverse the drill bit out of them.
We have a Harbor Freight pneumatic rivet puller that works great. I don’t think I’ll manually pull a rivet ever again.
That said, I am interested in what manual rivet pullers teams are using. All of our Arrow brand ones are total garbage and no longer grip the pull stud well at all.
Could you take a flat-tipped punch and knock the remnant through the hole? For closed-end or tight rivets, or parts you can’t hammer very hard, I suppose you could grind it flat with a die grinder or similar tool, then centrepunch it before drilling. I’ve never had occasion to try either one.
It varies: check the manufacturer’s specifications. If those aren’t handy, look at McMaster-Carr’s blind rivet section, and try to find something similar, and use the drill recommended. It’s probably either 0.192 in–0.196 in (11) or 0.201 in–0.204 in (7).
Presumably you’re not exposing the heads to any unusual force that would cause them to do anything but spin on the drill bit. So I don’t see any problem with that. The worst case scenario is that they are held tight, and the drill drills through them, enlarging the hole. I can’t imagine why that would be especially bad either.
We’re pretty wild on hole size, as it varies with manufacture method. I try to keep the holes small in our gussets, as often we have at least a .196" hole in the tube it’s mating with. We generally square up our frames with clamps or squares before riveting. Also, pieces like bellypans do a great job of squaring assemblies even with large holes in them. Larger gussets that cover more area and parts will get better alignment for the same size hole.
We use a pneumatic riveter whenever we can, it makes it easier for the operator to pop a bunch of rivets without getting tired and using poor technique.
As for drilling out rivets, we have a few designated bits we use for it. Drill bits are dirt cheap, don’t be afraid to mess them up. That being said, I’m not sure how the heads damage the bit any more appreciably than just drilling (especially if that bit is never drilled into deep material where the upper flutes do anything).
We use a 9 Drill (.196) for anything 3/16 or 10 hardware related. It’s recommended on many drill charts for a 10 clearance hole, including the Solidworks “Close” fits. Gives a nice close fit, but still just loose enough to allow fasteners to easily go through. Going much bigger just seems to make the assembly less precise, especially if you rely on the rivets for location.
This is the practice that I’ve found works best, too, both with riveting and bolting. It’s far easier to allow some slop in your hole placement/sizing and clamp it in the right position before tightening than it is to try to machine everything to a perfect fit - in fact, I’ve found the latter to be effectively impossible if you’re stuck with just a drill press, and still surprisingly difficult even with a good mill.
Edit: Also, if you decide that you want tighter holes to prevent the frame from getting out of square after taking some abuse, match-drilling through your gussets once you’ve clamped the whole assembly is pretty foolproof.
The danger here is for teams that may not be placing ALL their holes across all parts precisely. Some teams may need the larger hole size for the frame to even have a chance of lining up.
I don’t at all disagree with what you’re saying if a team is capable of it, but no shame in oversizing holes if it’s what you have to do it get the frame to happen.
For the mandrels that are sticking up and you want to remove the rivet just use a punch to drive it through.
For drilling out 3/16" rivets use a 5/16" or larger drill bit and stop the second that the head of the rivet starts to spin, then use a punch to knock the remains out. That prevents the rivet heads from stacking up on the drill bit and from wallowing out the hole so that the rivet you put back in there doesn’t grip correctly.
No matter how careful you are you cannot consistently drill right down the center of the existing hole so you create ovals, do it a couple of times and the rivet will not hold correctly anymore.
It does take skill to not drill into the material that the rivet is holding but it is easier to achieve that drilling down the center of the existing hole.
I agree that a pneumatic riveter is a great tool. I was one of the people who helped instal some of the 10,000+ rivets while building the field cases for the PNW division and w/o pneumatic riveters I don’t know how we would have done it.