Min Thickness for Countersunk Bolts
 

In attempting to improve last years drive train, I've run into a slight snag with multiple possible solutions. One is to use countersunk bolts in 4 locations to alleviate the issue. It's the most elegant solution so far, yet brings up a question.

The two pieces of metal to be attached are 1x1x1/16" box tubing and 1x2x1/8" box tubing. The countersink will happen in the 1x2x1/8" box tubing. Is this too thin? Is there a table of bolt diameters and/or minimum thicknesses for angles of countersunk bolts? There will be some sheer stresses on the bolts/boxes as well. Google hasn't been terribly helpful.

CAD to come on the overall drive train, soon. Thanks!

Re: Min Thickness for Countersunk Bolts
 

We have used countersunk #10s in .125 without issue many times.

Re: Min Thickness for Countersunk Bolts
 

If it gives you a reference:
I have used #6 in .0625 with lots of issues.

Re: Min Thickness for Countersunk Bolts
 

Do you really need countersunk? can you bore a large clearance and just bolt through one wall instead?

Could you use a larger diameter (like .5") insert the length of the tubing, and countersink that? That'd give a stronger connection anyway.

The had on a #6 is way thicker than .0625, I'd expect issues as well (unless you're not countersinking flush).

It looks like you have to go all the way down to a #2-56 to get close, and that's a .064" tall head still.

Re: Min Thickness for Countersunk Bolts
 

Are you going to run a screw through a piece of tubing, in a such a way that it could collapse the tubing? or will there be a spacer in it? or are you going to drill a larger hole through one side for access to the screw head, and have the screw only go thru one side of the tube?

I'd need more info before being able to answer the question, or suggest other ways of building it.

Re: Min Thickness for Countersunk Bolts
 

http://www.smithfast.com/msflathead.html. For both 82º and 100º screws it lists all the relevant dimensions of the countersunk head. It lists 82º #10 screws as between .098 and .116. 100º is even smaller, so you should be good (although just barely).

I really like finding websites like that that give every imaginable chain, fastener, or whatever dimensions you would ever need.

Re: Min Thickness for Countersunk Bolts
 

Thanks for the responses! The CAD will clear some things up, but the idea is to attach the 2 box tubings along their 1" faces. The bolt will not go through the entire thing; instead only the two touching faces will be bolted -- like what Jim described in his last sentence. The bolt itself wouldn't be longer than 1/2".

The countersink is for chain clearance so the chain (or belt) doesn't rub against the bolt head if it jerks while slightly loose. The sprocket & chain will be housed within the 2x1. Other solutions are low-head machine bolts and/or simply putting teflon over the bolt head. Or I could put the chain elsewhere, but the new location is part of the potential improvement.

Re: Min Thickness for Countersunk Bolts
 

Is this a west coast style drivetrain? What are the advantages of moving the chain inside?

We (and many others) have run nearly identical chain placement tucked inside (as in adjacent, inside the driveframe but not the tubing) the 2x1 for seasons without any failure. If you look at 60, 254, 968, 1538 (and now 1323) and us, the number of drivetrains using that configuration including practice bots is probably above 60 among those teams. Thousands of hours of runtime as well.

I'm not saying it's a bad design without even looking at it, I just don't see what potential advantage internal chain has aside from being "protected". The cost of putting chain internal seem huge to me as well in terms of assembly and risk.

Re: Min Thickness for Countersunk Bolts
 

You might just want to use button head screws, they don't protrude much and are rounded, and not likely to cause problems for chains or belts, and no countersinking necessary.

Or just weld the two tubes together....or rivet them....or something like that. It's fun figuring these things out with just a few clues :D

Re: Min Thickness for Countersunk Bolts
 

It's not like WCD, though I have thoroughly studied WCD and tried to deduce many of the design decisions along the way. Mainly the wheels aren't cantilevered and there's no need to add additional supports for bumpers. I know our welder would balk at a 973/254 side bumper rail, yet a couple of our mentors want more support than what 2793 and a few others had this year. The axles are live axles, which is why the 2x1 is needed. The big thing we lack is knowledge about automated CNC'ing*. While I understand we don't *need* CNC to do WCD-esque drives, I do have other mentors to convince (and they're a stubborn bunch). There are some other things that are 'wanted', like direct drive & a multi-level frame, yet we also have to live within our limitations. I think I've come up with a concept that can do those few things without increasing complexity of the drive train.

The sprockets are housed inside the box tubing blocks more for the fact that we won't have to purchase a hex broach and learn its process than anything. The 2x1 tubing will be in 4" segments, so the whole chain isn't inside the tubing. I still wonder about how much of a hassle it will be during assembly -- particularly with belts (I want to leave the option open for next year). We want live hex axles so there's a naturally-sturdy structure between the wheel blocks if we lathe the ends down for round bearings. We want live axles because we don't want slight imperfections in a bearing-wheel combination to throw off the chain on impact like it's done every year (e.g. Plaction wheel + pressed bearing was discovered this year). Hex axles will also allow us to use 4" AM performance wheels without an additional hub, which is a decent money saver when we're talking about 6 wheels. I may even try to find a sponsor with a 3D printer so we can get a custom wheel diameter and naturally-purple coloring (maybe).

We're going down to minimalistic mechanical mentor support for the Fall from what I can tell (work/school issues) so mechanical knowledge will be scarce. Additionally, we only have 1 mechanical FRC student returning this year. Sticking with what we know, at least for this year, will be key for us to expand upon our 2011 successes.

Jim, I'll look into the button head screws to see what's available. If we use 16T sprockets, there will be about 0.15" of clearance for a bolt head before a chain touches it.

*The year we tried auto-CNC at our knowledge level was disastrous (2009 ... record of 6-18-2 because we could barely move). We do have a no-slop, ~0.01" precision manual mill now, so I'm stepping up the game somewhat with direct drive.

Re: Min Thickness for Countersunk Bolts
 

You might try a .125-187 Dia. stainless steel pop rivet. They are also available in counter sunk heads. A shoulder bolt could work if you don't mine the chain rubbing across the head.

High bond double back tape works wonders.

Re: Min Thickness for Countersunk Bolts
 

Would they balk at our drivetrain with no welds (at all, anywhere)? or our .125 plate bumper supports that could be made by hand in 30 minutes? :D (seen pretty well here)

It's easier than you think, and sounds like it's easier/less parts than what you're planning.

Hex broaches are pretty cheap too for what they do.

Re: Min Thickness for Countersunk Bolts
 

Running chain in tube can be done, but I would recommend a bigger tube (3x1 at the minimum) so that you have room to do maintenance if it does fail. 2x1 would have been incredibly cramped for us, but 3x1 gave us some breathing room.

Re: Min Thickness for Countersunk Bolts
 

Not trying to offend, but same question as before (and to Jesse as well). Why? What is the advantage?

Re: Min Thickness for Countersunk Bolts
 

We used belts in ours, so I think some advantages would be lost on chain, but the main reason is to protect the belts from damage and to keep them out of the way of the electronics. Since timing belts don't need any real adjustment once you get them "just right" (properly tensioned and aligned), ease of maintenance is not much of an issue. Even so, we can get a belt out in 7 minutes, enough to do a belt change between matches.

25 chain tends to stretch, and I would be concerned about that and sprocket wear. Especially because you cannot see the chain and thus have little indication how badly it needs to be retensioned.

Re: Min Thickness for Countersunk Bolts
 

Sorry Adam, I meant 254/1868. They had a box tube that ran down the side, above the wheel. We like that reinforcement, but don't want to risk warping of that box tubing.

Attached is what I could come up with in an hour. In hindsight, I could still put the sprockets wherever I wanted -- either in the box tubing or next to the wheel -- and maintain the constraints above.

The middle wheel isn't shown because it's dependent upon the attached gearbox. The gearbox would attach using a slightly modified wheel block (though still 2x1). I also didn't add in the 3/16" spacer to drop the center wheel.

Maybe more to come after the dog park.

Re: Min Thickness for Countersunk Bolts
 

That looks like it could be riveted relatively easily

Re: Min Thickness for Countersunk Bolts
 

I know I probably sound like a desperate salesman now, but that frame looks like substantially more work than a proper west coast drive. A wcd doesn't NEED CNCing or anything fancy, 973 ran a very, very, very simple and reliable wcd for four consecutive seasons before my time.

I'm not tooting my own horn either, we've made some tweaks and optimizations, but it's really NOT our design. We just love it so much, and despite our best efforts, haven't really found a way to improve it!

Re: Min Thickness for Countersunk Bolts
 

Ok, your raw crazy loyalty has me interested. Can you tell me what the pro's Con's of this are versus six wheels with dead axles supported on both sides (http://www.chiefdelphi.com/media/photos/36993)?

Re: Min Thickness for Countersunk Bolts
 

WCD requires only 4 pieces of very rigid aluminum tubing for its frame -- which is the key to its success. So long as bumper rules are complied with, it's still the simplest most versatile drive train frame for teams who have these capabilities:

1.) Welder who can keep things square
2.) Low tolerance, low slop drilling capability
3.) Some sort of milling capability is probably needed regardless of variant
4.) It doesn't hurt to be able to do your own gearboxes so you can customize the gearing while also driving chains to each end.
5.) It probably also doesn't hurt to be able to make custom wheels of any diameter/hub configuration

With WCD, I would have to figure out gearbox configurations for the wheel size such that the gearbox doesn't bottom out on the field when put into direct drive configuration. For 4" wheels, this is somewhat difficult without doing a custom gearbox (with plates). For 6" wheels, it's a bit better since a large variety of COTS gearboxes exist with decent spacing between the output and bottom of the gearbox.

Sheet metal is only simpler if the mentors in charge are comfortable with it and have th resources for it. It's like apples and oranges, imo.

Adam, show me a team with sloppy drill presses, hand drills, a basic lathe (and 3 tools), and a surplused mill who has done WCD ... then you'll probably be able to convince my other mentors.Until then, they want to do what we know with the drive train and focus learning on other parts of the robot. It's not me you have to convince, heh.

Re: Min Thickness for Countersunk Bolts
 

1) No need to weld, robot delivered and took some beatings this year and it survived.
2/3) a poor quality manual mill is adequate, kind of like our poor quality manual mill.... ;)
4) This is certainly nice, but Super Shifters (we ran them stock in 08, and "stock" in 09) work just fine.
5) This is no longer as big as advantage as it used to be. The AM performance line is fantastic.

The super shifter is a great choice here as you can change gears out in that final stage for easy ratio adjustment.

Uhhhh... us right here! While we've recently (last 3 weeks) expanded on our in house capabilities, you described the capabilities of us in house. With some know how and practice our kids make very high quality parts (that often get mistaken for CNC'd parts). We do have a large amount of industry sponsors, but that only started for the teams 6th year of doing west coast drive. We get a lot of drive components CNCd and outsourced more out of convenience than necessity (the parts are ready to go really early in season, when sponsors are generally idle).

Also, most of the complaints you listed seem applicable to any drivetrain style. Our CAD models are out there, and I'm here to answer any questions (along with all the other teams that run it I bet). I've helped quite a few teams make easier to make WCD's that are tailored to the resources they have available to them.

Back in the day, I was also against WCD out of some preconcieved notion it was harder, and would make these massive plate sandwich drivetrains (294 07 and 08) that just can't compare at all to WCD in terms of weight or efficiency of manufacture. Once I saw and understood how simple, clean, and easy they are to make, I never looked back.

Re: Min Thickness for Countersunk Bolts
 

A west coast style drive can be done in a simple manner too. We don't even use bearing blocks (the trickiest part IMO), we just made some bearing holes on a manual mill and use a simpler tensioning system. We have an in house setup that's nothing to write home about and we did a west coast like drive quite easily. We even finished our frame earlier this year than any other.

I like the posted frame design for things like obstacle climbing, etc though. Pretty neat stuff.

Re: Min Thickness for Countersunk Bolts
 

We also have all the sloppy tools you mention (manual mill and lathe, some ok drill presses, chop saw, etc.), as well as basic welding that we can do square (which could be foregone with some more holes, gusset plates, and riveting), and a WCD is something I think is very much within our capabilities.

It seems to me that the biggest thing is easier maintenance of the wheels, but after just replacing one last night at the shop due to a failed bearing on our drivetrain that's supported on both sides, it was really a snap, so those benefits are really the last few percent. Also, frame simplicity and weight are improved.

We could even probably do the bearing blocks in house on our manual lathe and mill if we felt like spending a lot of time on it, but since we know some people with CNC capabilities we probably wouldn't bother. I actually think I would prefer dead axles because that gives you more wheel options (namely, plaction, omni, etc.) and you could just slot the axle hole on a manual mill for chain tensioning.

Re: Min Thickness for Countersunk Bolts
 

You could also try using a Pem Stud.

http://www.pemnet.com/fastening_products/pdf/fhdata.pdf

Roy