I’ve seen a lot of teams building their own chassis instead of using the basic andy mark chassis that comes with the kit of parts. What are some of the benefits of doing this and should our team consider building over buying? And if we do build a chassis should we weld it or bolt it together?
We do it to save on our scraps/bare bones budget. We also have the resources to machine our own drive rails. Those were the two prerequisites for our decision. Last year we made gussets and riveted it together but for this year are thinking about welding it together (not that riveting it caused us any trouble).
The AM14U family of chassis is quite tough and flexible. Essentially, teams should probably be working from a kit chassis unless they find some advantage in making their own. If you find it to be an advantage, then your second question (how to assemble) is likely to be obvious, based on the advantage you found and the resources you have available to you. Here are paraphrases of many of what I would consider reasonable justifications in recent years:
- We’ve been developing a [name your poison] drive during off seasons for the past several years; we know what we’re doing here and are ready to do it
- We have a great machine/sheet metal/welding/whatever shop and talent available to us (whether as sponsors, mentors, or students) and are ready to take advantage of it
- We find that it is more inspirational or educational to build our own chassis than to build a kit
In the interest of full disclosure, 3946 built kit robots in 2012, 2015, 2016, and 2017, and did our own in 2013 and 2014.
Our reasons in 2013 included access to an aluminum welder (mentor’s garage), but were largely based on dissatisfaction with the 2012 C channel chassis after we modified it by cutting structural parts out which we did not suitably replace. Our 2013 robot design depended on welding, because we had that as a given. Unfortunately, we overextended ourselves to get a 30 point climb that between the frame spending so much time off-site and some poorly constructed reversible bumpers, we got no climb points at all.
In 2014, we rolled our own based purely on dissatisfaction with our ill-advised mods in 2012. We actually did pretty well, apart from our strategic mistake of selecting a mecanum drive for what turned out to be the heaviest defensive game in our team history, and a misunderstanding in converting from the prototype (2x4s) to the aluminum competition chassis (1.5" square x 1/8" c-channel).
In 2015, we flubbed our way into what was the right decision. We forgot/missed the KoP opt-out, and wound up with an AM14U2 that we did not really want. When we came up with our robot design (including an H-drive), we realized that we could build this using the kit chassis we already had.
In preparation for the 2016, 2017, and 2018 build seasons, we have purchased two of the previous year’s kit chassis “frame only” kits from AndyMark when they are the Deal of the Day, and opted out of the kit for that year. By leveraging our stock of TB-mini parts, this has proved to be to our advantage for 2016 and 2017, and we will likely continue this in future years. OBTW, we purchase all-new bearings for at least four TB-minis each year; this keeps the gearboxes up to scratch, and also helps us comply with the fabrication schedule rules.
The Andymark Chassis family is a very good system. I feel like something many teams miss, is how incredibly upgradable/modifiable it is. Andymark has most chassis configurations that I could think of for sale on their website. Here is a link to just the upgrades :http://www.andymark.com/AM14U-s/514.htm
I also highly reccomended checking out the different varieties of wheels and gearboxes are available from Andymark, as many of them will work on the kit chassis. So… If you are looking for some chassis that is more complex than a 6-wheel tank, the kit chassis still may meet your needs.
That being said, we built a custom chassis last year, and plan to do it again this season. It is still a 6-wheel tank, but has some small advantages we like. Here is our reasoning for doing so.
-it makes a great educational project durring the off-season. The key here, is that we design it, build it and test it in the off season, where we aren’t bound by a 6-week timeline. We knew the chassis design worked well before we used it on the robot.
-We make the chassis out of welded 2"x1" tubing (1/8" wall). This makes the chassis more rigid, and matches our team’s building style on the rest of the bot. (Down side is that it weights more).
As far as the question of bolts vs Welds. This is based on what your team has the best access to and experience with. If it’s done well, both of these methods will work well.
For example, we have access to a TIG welder, so welding the chassis makes a lot of sense for us. I think it looks clean, is reasonably quick, is light, and is strong. That being said, if you don’t have a welder, or access to a sponsor who is willing to weld things quickly… don’t weld the chassis.
Bolts are a great option as well. Most teams have a drill, and can use bolts as fasteners very easily. They can weigh more, and may require some additional parts such as gussets plates sometimes, but they hold part together well. Another advantage to bolts, is you can disassemble things, and modify them if needed.
A third option here is rivits. I personally have never had a ton of luck with them, but do not have a lot love experience with them either. I do know that many teams like them a lot. They may be worth looking into depending on what you have access to.
Remember that your chassis is one of the most important parts of your robot. If you can’t drive, you usually can’t score many points. A design flaw in the chassis can easily ruin your season, so if you design something custom, test it thoroughly.
On 2386 we build a custom chassis every year (including our rookie year back in 08). Ours are always bolted, or in recent years rivites. Although we have the capability to weld our chassis, we are most knowledgeable with sheet metal, which lends itself more to fasteners than welding IMO. I also don’t fully trust the quality of welds we can produce, so try and avoid them whenever possible (most events we attend don’t have access to welding facilities, so if a weld breaks and we don’t have a spare, we’re out of luck). As for why we build our own chassis, these are our three major reasons.
- We have a very well equipped shop capable of producing said chassis’.
- We feel it provides the students a more in depth and engaging learning experience.
- It gives us more flexibility/freedom in our design.
The first 2 kind of go hand in hand. Because we have a well equipped shop, we want to teach our students how to use the tools in a hands on and engaging experience. The last point is also a major one for us (but not nearly as important as the others). Being able to move around mounting positions, choose our shape/dimensions exactly and design in added features are all really nice bonuses to designing our own chassis.
These are the primary reasons we do it the way we do, however all of this is still possible with a kit chassis (albeit, the last point becomes a little more challenging). The one thing I will recommend is not to attempt it for the first time during a build season. Design and build a custom chassis in the offseason to ensure you have experience come build.
What is your team’s objective? If it is to learn how to design and make things, designing and making your own chassis is a good means to that end. The learning Brian is writing about can occur if your team makes a custom scoring mechanism, which pretty much all teams do. If your objective is to win on the competition field, you will have to think about the big picture and consider what are all of the advantages, risks and costs associated with designing and manufacturing your own chassis. The teams that are able to do both, effectively, have spent many years learning what it takes to get to that point.
The AndyMark chassis have evolved over many years and can be a good foundation for a competitive robot. I will consume a lot of a team’s resources, during the build season, to design a chassis with superior features and performance. Naturally, it will consume less to design an inferior one ;-). This past season, I worked with a team that used a kit chassis. The other four teams working out of the same facility all used custom chassis. We were able to start practicing and refining our scoring mechanisms before the other teams We were also able to adapt a kit chassis to make a practice robot that was a very close copy with about 6-8 hours of effort from 4-5 students.
Having a design in CAD that looks good does not mean that it will give a team an advantage in competition. The downfall of many teams (including ones I have been on) is that their designs do not take into account manufacturability and serviceability.
If you have a manual mill and an experienced student machinist, you can get a West Coast Drive ready on Day 2. The main advantage I’ve found to doing custom is that you can easily add a robust, customized center cutout. Also, you get more area on the sides of your robot compared to doing a plate drive if you’re doing a WCD. Mounting things to the 2x1s of a WCD is also quite easy compared to dealing with KOP chassis side plates.
I have a few things to say on this topic.
I’ve been a pretty vocal proponent of teams using the kit chassis instead of rolling their own for a while now. For the vast majority of teams, I think one of the best things you can do for yourself during the build season is get a drive train that is or is relatively close to your competition driving as soon as possible. For a large amount of teams, the knowledge required to design and build a custom drive train simply isn’t there, even if for those who do have that knowledge it is dead easy.
Where the advantages really start showing is when you’re at a point where you can design and build your custom chassis in a negligible (relative to the iteration lengths for other parts of your robot) amount of time relative to the kit. A few people have drawn this line in the sub-2 weeks for a driving chassis ball park, and I tend to agree with that. If you’re going to spend longer than two weeks to do custom, I’m just not convinced it’s worth it competitively.
All that being said… it’s okay to prioritize the engineering experience for students over being competitive. Heck, you might find that for your team, having that emphasis makes you more competitive than a team more focused on using best practices to build a competitive robot. What works for teams varies considerably, and your team is doubtless as unique as each of the thousands of other FRC teams around the world.
And if we do build a chassis should we weld it or bolt it together?
I suggest rivits
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COTS parts make it super easy to put a a custom WCD together. If you use VEX versablocks, it removes the need to have precision machining. There are also plenty of WCD gearboxes available. Making you own chassis gives you more control over the design of your manipulators. As others have said, it is also easier to mount to.
I suggest that it depends on what resources are available.
If a team doesn’t have access to a welder, welding is obviously not an option.
If a team doesn’t have access to a riveter (air-powered, preferably), and can’t arrange that access, rivets aren’t an option.
Just about any team should have bolts, nuts, and wrenches.
My team’s been running WCD so long that we can have one ready for cutting inside two weeks, welded inside three, pretty easily. Last year we even had one ready in time to powdercoat.*
*Unfortunately, we got caught by the “bumpers are inside the volume” rule and had to re-build. That frame is now designated for demo-bot/defensive sparring partner.
I like rivets but the size of drivetrain gussets is starting to encourage me to use screws in the corners. Bellypan should be riveted to save weight, but for the corner gussets I’m looking at using screws instead; 2-3 3/16" rivets is much weaker than 2-3 10-32 screws + locknuts. It’s not hard to switch between the two anyway.
I think that building a “custom” chassis using Versa Frame and more or less* following the “Application Example” https://content.vexrobotics.com/vexpro/pdf/217-8000-6WD.pdf for a 6 wheel tank drive is not more difficult than putting together the kit chassis.
Biggest advantages of doing this: flexibility in size, flexibility in wheel choice and easy selection of robot speed.
More or less*
Use Single Speed gearbox and mount it to the side rail using half of VersBlock.
A range of gear options for the gearbox and drive train calculator on West Coast Products website http://www.wcproducts.net/how-to-drivetrain/ lets you select the proper gearing.
Use #35 chain, use the chainbreaker tool: https://cometkartsales.com/-35-Space-Chain-Tool.html
We bought this bolt-cutter-size hand riveter from Harbor Freight last year to do some 1/4" rivets someone purchased by accident, but we found that it greatly reduced the effort to set large numbers of smaller rivets as well. Planning to get another this year - the first one’s still going strong, but a second should simplify when we have two subteams riveting at the same time. At $20, most teams should be able to afford at least one.
The main benefit of a custom chassis is to adapt it to a certain strategy for a game. Second is to maximize performance in certain areas (more traction, less traction, more speed, less speed, more maneuverability, less maneuverability, etc…).
If your team has the capability to design a chassis within 3-4 days, and then machine/build/assemble the chassis in 7-10 days, you can consider going the custom chassis route. If it takes you longer than that, you’re better off spending your resources on improving the robot mechanisms to play the game IMO. If you’re determined to go the custom chassis route, I’d figure out how to get in that time frame with your resources.
I prefer rivets, lots of rivets, for assembly techniques.