Sheet Metal Frame Feedback Requested

[John_Keyzer]

We are very fortunate to have new labs this year, along with the new spaces we are getting the following machines for summer:
• Haas VF2SS with probing, high speed machining, minimum qty lubricant and thru spindle air
• Haas TRT210 5 axis rotary with 5th axis dovetail vise/ riser
• Boss Accucut 1kw fiber laser
• Baleigh 33 ton 60” cnc press brake
• Omax Protomax water jet
• 50 ton hydraulic press
• Tormach Slantpro 15L lathe with turret

The fiber laser has the ability to fly cut which makes cutting multiple holes fast and easy. Below is a link to a demo of a similar machine (same optics, power and servos but different travels and ours is enclosed). https://youtu.be/WFtPYJ7Cv0E

Limitations on our capabilities
• Due to assist gas requirements (not allowing compressed O2 in a school) we can only use nitrogen which means we can laser aluminum and stainless steels no carbon steel
• Laser will cut up to 48” square material, we can also fixture to precisely cut holes in square tubing on multiple sides
• Top punches in the press brake are two 30” sections, we will purchase shorter punches but for now we can only bend metal along two sides (no pans or four sided bends)

We are looking for feedback on our first in house sheet metal frame. We used sheet metal in 2019 and it worked great until we got to worlds. Frame was 3000 series aluminum and had an opening for cargo. The flanges got metal fatigue due to damage from impacts and repairs. It ended up snapping off right where a swerve module mounted. We were paying a lot of heavy defense and the damage occurred on the front side which had the opening for cargo. The entire side was covered in bumper.

In looking at this frame design and materials we started to research sheet metal design on line and found that while Gauge thickness is important strategically placed reinforcements can be equally important.

We used Inventor sheet metal to design this frame, it is parametric and linked to a master parameter part that sets material, material thickness, and overall frame dimensions across multiple parts. Keeping in mind our design limitation of only having flanges on two sides of the face we used flanges and secondary sheet metal parts as well as aluminum U channels to provide reinforcements.

CAD Files can be found here: https://grabcad.com/library/sheet-metal-first-robotics-chassis-1

Drive system is going to be Swerve Drive Specialties MK4i with dual falcons. We do have a reputation for playing high quality aggressive defense and are planning to be able to take and give hits as needed. This was our frames failure point in 2019.

We are looking to use either 5052 Aluminum 0.0625 thick or stainless steel 0.0325 thick. We can cut and bend both of these materials pretty easily with the laser and brake.
The battery will be contained by milled out ABS or HDPE ring with a 2” battery strap across the top. We’ve used these straps for years and have a very high degree of confidence using them.

Electronics will be in the belly pan mounted to a piece to polycarbonate milled in the Haas. This will allow electrical team to assemble and write the plate independent of the robot frame. The radio will likely be mounted up high on hand specific super structure.

Connections will be either 1/8” large head stainless-steel rivets or ¼-20 nuts and bolts. All of the extra holes in the outside allow for various bumper heights and the inside and top flange holes allow for upper superstructure to be added (collector shooter climber etc).

Our plan is to make one frame out of each material and test them for strength, rigidity and impact resistance and pick a winner.

Please feel free to provide any insight into improvements that we can make, it would be greatly appreciated.

[memtech3]

Have y’all considered using aluminum tubing for the perimeter instead of sheet metal? Or to brace the bends around the perimeter?

[NustinJewton]

Another way would be to do something like 148 and 118 where they use 2 U channels to make a tube like structure which would be much more rigid.

[cad321]

I started on a team that runs similar sheet metal chassis each year (they run 0.03" mild steel, but the design concepts are extremely similar). The biggest thing I notice with your design is that the large triangular pockets seem extremely aggressive. While we would add a little pocketing or holes for weight savings, it was much more conservative than this.

With a sheet metal chassis, one of the benefits is being able to use lighter gauges, and then adding bends for a huge increase in strength. But by adding those extremely large triangular holes, I’d say you’re drastically reducing the effective strength of your entire chassis.

That’s my $0.02 based on personal experience working with sheet metal chassis.

[Knufire]

@John_Keyzer

Drive system is going to be Swerve Drive Specialties MK4i with dual falcons. We do have a reputation for playing high quality aggressive defense and are planning to be able to take and give hits as needed. This was our frames failure point in 2019.

Honestly, if you plan on continuing to use the SDS modules, I would just do a simple box tubing chassis. Integrating them into a sheet metal chassis is likely way more time and effort than it’s worth, given the modules themselves were designed around using a 2x1 tube frame. It should be no problem to manufacture with the VF2 you acquired. Sheet metal construction could still be used for your superstructure and mechanisms on top.

At least going off prior art, 5052 aluminum is likely to be your best bet. 1/16" may be a little thin, but adequate with enough internal bracing. 0.090" has been popular in the past.

971 and 148 are successful teams that have extensively used sheet metal in the past, and have public CAD files available. I would strongly suggest looking at their designs:

https://frc971.org/content/team-documents#RobotDesign

Resources

Both have moved to more tube + gusset construction in recent times, so you may need to go back a few years.

[troy_dietz]

If you’d like a related fun fact, a decent number of low tech vehicle frames are/were constructed this way as well. Makes for easy and cheap large frame members that can curve in various directions, and incredibly easy to spot weld.

That said…there’s a reason FRC collectively has moved to box tubing and we don’t build robots like engineers design cars. Why make your own box tubing when you can just buy it…

[Knufire]

@John_Keyzer

Connections will be either 1/8” large head stainless-steel rivets or ¼-20 nuts and bolts. All of the extra holes in the outside allow for various bumper heights and the inside and top flange holes allow for upper superstructure to be added (collector shooter climber etc).

I would personally suggest some 3/16" aluminum body/steel mandrel rivets. The stainless steel rivets might be pretty difficult to drill out if rework is ever required (which it often is in FRC). Using 1/4-20 fasteners everywhere is likely to add a bit of unnecessary weight.

[Aidan-Mundy]

Or swap to 5/32 rivets to meet the Vex standard and take advantage of right-sized holes in their gussets

[Tim708]

I spent a couple designing complex sheet metal structures for flight simulators, we used 5052 aluminum all the time. Its easy to work with and saves a ton of weight, 1/16 should be sufficient if you use geometry to your advantage.

First thing I noticed was the front caps and spreader, these 3 parts should be 1 c-channel and you probably want 2 in of overlap between that c- channel and the c-tubes that make up the left and right side. It will be difficult to rivet together in this orientation and it creates shear planes that could fail during an impact. May want to consider clamping and match drilling for your riveted components, assembly will be a little longer but it will save a lot of frustration if your patterns are off a little. We did this on the simulators and I took the practice to 708 when we moved to mostly tube and gusset construction.

The second note I have is with the triangular pockets, they are taking away too much material and are saving you less than a pound. It’s not worth killing your stiffness for that little of a weight savings. A better option would be dimple holes, will get you the weight savings your looking for and increase the cross section of your tubes where your removing the material. I don’t expect a dimple dies set would be too big of an investment, can probably make them on your own with your set up.

A couple smaller things,

• Flipping the flanges for the ribs between the swerve modules and tying them into the top and bottom will make the tubes stiffer.
• You can throw some hat sections in high stress areas, may be a good options for mounting the swerve modules.
• It’s not clear to me how the modules are supposed to mount but I would put the bottom plate on the bottom side of your frame. Also they need to be accessible, that seems like it may be a bit of a problem in the current configuration.
• I don’t have an issue with 1/8 rivets, 708 uses them just about everywhere. But I would skip the 1/4-20 hardware, its overkill for most FRC applications. 10-32 should be sufficient, you’ll likely save as much weight going to 10-32s as you would with all the triangular cuts. Also rivet nuts are a great option for where you want bolted connections.

Would love to do something like this with my team if we had the machining capabilities in house.

[Mr.MetalSmith]

I’ll agree with the masses here in that box tube is the easiest solution here. Now for sheet metal let me offer some feedback on the design. It looks like your lightening pattern is the cause here. especially seeing the fatigue point being the "center of the beam like that. I would start off by shrinking the size of the lightening holes or switch to a more reliable method (see isogrid). C channel can work pretty well. but also consider running a support down the center of your bot to minimize the leverage affect.

[Weldingrod1]

You’d probably benefit from top and bottom sheets that tied your flanges into a box section. They can be open work and still be useful.
Another thought would be to make your C-channels with one VERY long leg and do them as sort-of mirror images. That way the tie sheet is actually part of the C-channel. IE, the two C-channels actually rivet to each other.
Closed sections (like tubes) are generally much stronger and more stable under varied loading than open sections (tape measures, U-channel, uni-strut, etc.).
I’d use a larger radius in your cutout corners too… If you really need to push the limits of weight you might end up with a round hole in the center of the cutout junctions to accommodate larger radii.

[MrMARVINMan]

After years and years of using box tubing for robot frames, 2614 made the switch to sheet metal in 2017. In 2019, we returned to the box tube to do swerve and had such a poor experience going back to that type of design (this isn’t true for all teams or even most, but our specific set of resources makes sheet metal a huge benefit during our construction process) that we basically decided to go back to sheet metal and 6-wheel drive rather than continue down that path. In 2021 we decided we wanted to use the offseason and 2022 season to completely switch to swerve, which meant developing a sheet metal swerve frame. What we ended up with was a bit of a hybrid, where 1x1 tube is placed inside the sheet metal to mount the swerves to.

We’ve used the system on an offseason robot (check out this thread to learn more about that) and our 2022 robot, and have been extremely happy with the results. It’s a little on the heavy side for now, and that’s mainly the improvement we’re targeting for 2023 as well as investigating swapping to the Mk.4i modules.

We basically have had no problems with this design so far, in fact we’ve been hugely impressed with its capabilities. The robot’s fallen from the high bar more than one time (thankfully in practice matches only) and we’ve not caught any damage to the frame whatsoever, including bending. It seems that the inherent flex in the sheet metal frame allows the robot to recover from impacts like falling or hitting the midfield cable guard really hard. Without the flex, that load is taken on by the Mk.4 mounting plates, and we’ve seen some pretty scary photos from other teams of those plates bending. We’ve also found that the flex of the frame pretty consistently keeps all 4 modules in contact with the ground, a problem we had in both 2015 and 2019 (admittedly, with different modules.)

You’ll also notice we swapped to placing our electronics on the underside between 2020 and 2022, we’ve found it really opens up everything above the drivetrain to be whatever we desire and provides us a really great organizer for all our electronics, keeping everything nice and clean. We use rivnuts bolts to keep the electronics cover on during matches.