pic: 4587-Sheet Metal Chassis V.2

[tim-tim]

Three thou is way too much interference for a press fit.
Around half a thou (.0005") is pretty standard in FRC. I suggest getting a 1.1245 (or 1.124) and .8745 (or .874) reamers to do the final finish.

Here is an old thread to reference: http://www.chiefdelphi.com/forums/ar...p/t-98825.html

[AllenGregoryIV]

I'm trying to think of other little annoyances from our drive train this year.

Are you going to spring up the modules or rely on the pneumatics to bring them up too? We really liked the springs because if we lost air or wanted to run our practice robot with out pneumatics it didn't really affect anything. If you go with springs make sure to fully CAD them we didn't and luckily had enough room to the side of the omni wheels to slide them over because in the intended configuration when the traction wheel was down the springs hit the omnis.

Think about the interface between the modules and the cylinders. We made small aluminum discs that just threaded on to the end of the piston. These pushed down on to VEXpro tube shaft. Over time we were able to dig pretty big groves into the tube shaft and would have to rotate them to get our ride height correct.

Also bumper mounts as always.

[Greg Woelki]

Originally Posted by AustinSchuh

Big squares save even more weight for less machining cost.

This frame is quite beefy but keep in mind that the sharper the corners the higher the stress concentrations.

[Cash4587]

Originally Posted by AllenGregoryIV

I'm trying to think of other little annoyances from our drive train this year.

Are you going to spring up the modules or rely on the pneumatics to bring them up too? We really liked the springs because if we lost air or wanted to run our practice robot with out pneumatics it didn't really affect anything. If you go with springs make sure to fully CAD them we didn't and luckily had enough room to the side of the omni wheels to slide them over because in the intended configuration when the traction wheel was down the springs hit the omnis.

Think about the interface between the modules and the cylinders. We made small aluminum discs that just threaded on to the end of the piston. These pushed down on to VEXpro tube shaft. Over time we were able to dig pretty big groves into the tube shaft and would have to rotate them to get our ride height correct.

Also bumper mounts as always.

I've actually thought about the sprung up. I think the way 148 does it is actually with surgical tubing that spans from a standoff on their modules then they ziptie surgical tube between the modules to pull them up. I am not completely sure of this but it seems like a good idea. If not, I plan to put extension springs around the tube axle standoff that the piston actually pushes down on and attach them to the sheet metal above them.

As for the wear, I think if I were to put a delrin or nylon tube over the 1/2" tube axle that the piston pushes down on, then it should be fine.

Also for bumper mounts. I haven't really looked into ways to doing them, But I have a pretty good idea of how they will need to be set up. Tips and pointers however are always useful.

[Cash4587]

Originally Posted by Greg Woelki

This frame is quite beefy but keep in mind that the sharper the corners the higher the stress concentrations.

I've actually changed majority of the lightening holes to parallelograms. They look nicer to me, and I have used a pretty big fillet on all the corners (3/16 radius)I have not however put any Swiss cheese effect on the drive rails yet. They are not finalized, neither is the belly pan.
Seen here:
http://prntscr.com/40gv2q

[Greg Woelki]

Originally Posted by Cash4587

I've actually changed majority of the lightening holes to parallelograms. They look nicer to me, and I have used a pretty big fillet on all the corners (3/16 radius)I have not however put any Swiss cheese effect on the drive rails yet. They are not finalized, neither is the belly pan.
Seen here:
http://prntscr.com/40gv2q

It is probably still strong enough, but taking out the same amount of material that way instead of with circles makes it significantly weaker.

[Cash4587]

Originally Posted by Greg Woelki

It is probably still strong enough, but taking out the same amount of material that way instead of with circles makes it significantly weaker.

I think It will be okay.. But then again IDK because really, I am just a student. I have only put in about 15 months of solid works use on and off. However, 148 uses a similar pattern on their drive train:

https://www.flickr.com/photos/robowr...7642758521834/

[AdamHeard]

Originally Posted by Greg Woelki

It is probably still strong enough, but taking out the same amount of material that way instead of with circles makes it significantly weaker.

Eh? I don't agree with this statement.

Triangular pocketing is far more optimal (unless too much is taken out) than circular. If an equal amount of weight is removed the triangular pattern is stronger unless it's laid out awful.

[Greg Woelki]

Originally Posted by AdamHeard

Eh? I don't agree with this statement.

Triangular pocketing is far more optimal (unless too much is taken out) than circular. If an equal amount of weight is removed the triangular pattern is stronger unless it's laid out awful.

I was under the impression that the smaller the radius, the higher the stress concentrations, like so: https://drive.google.com/file/d/0B4u...it?usp=sharing

I'll try doing the same thing with triangular cut outs though. Perhaps it also depends on how it is loaded.

[AdamHeard]

Originally Posted by Greg Woelki

I was under the impression that the smaller the radius, the higher the stress concentrations, like so: https://drive.google.com/file/d/0B4u...it?usp=sharing

I'll try doing the same thing with triangular cut outs though. Perhaps it also depends on how it is loaded.

Ah, yes. You make a good point but that isn't an argument against triangular pocketing, but rather an argument against too small of a radius.

Ideally triangular pocketing has webs sticking out in most directions of loading, so stress flows nicely and never has to take corners anyway. Isogrid is a really cool optimized pattern everyone should look up.

[Greg Woelki]

Originally Posted by AdamHeard

Ah, yes. You make a good point but that isn't an argument against triangular pocketing, but rather an argument against too small of a radius.

Ideally triangular pocketing has webs sticking out in most directions of loading, so stress flows nicely and never has to take corners anyway. Isogrid is a really cool optimized pattern everyone should look up.

Now that I think about it, lightening with triangles like you said so that it leaves a truss-like pattern should be stronger.

[Cash4587]

Originally Posted by AdamHeard

Ah, yes. You make a good point but that isn't an argument against triangular pocketing, but rather an argument against too small of a radius.

Ideally triangular pocketing has webs sticking out in most directions of loading, so stress flows nicely and never has to take corners anyway. Isogrid is a really cool optimized pattern everyone should look up.

Do you mean "118 Grid"?

[Greg Woelki]

Am I approaching this incorrectly? Each side is the same length, is under the same load, and is the same weight. I started the triangles with 0.1" round radius and I worked up to 0.25" (in picture), adjusting their dimensions to keep the volumes the same as I went. There is still higher stress in the piece lightened with triangles and if I increase the radius of the round much more then they might as well be circles.

https://drive.google.com/file/d/0B4u...it?usp=sharing

[Adrian Clark]

I really like this drivetrain, like a lot. Great job! It's been a lot of fun watching it get better and better. That being said, I think there's a few things left that could be optimized.

The first thing is space. I think electronic placement space is a very important aspect of a drivetrain. Without enough space you have to spend time coming up with ways to get everything to fit, and some of the solutions can make maintenance difficult. YMMV and I know a lot of teams are willing to make space sacrifices that make electronics placement difficult, and sometimes that trade off pays off. There's nothing wrong with finding sneaky ways to fit all your electronics on the bot, but I consider an ideal drivetrain one that keeps electronics placement simple. For that reason I suggest space as something to try to optimize.

The reason I think this drivetrains electronics space could be optimized is because of the large voids in between the modules and the voids created by the hexagonal frame perimeter. I think hex and octo frames are great, but I also think there's trade offs that should be considered. The first is space, is having such a wide hexagonal robot worth it if it limits space? And how will the frame shape effect superstructure and manipulation design? You may have accepted these trade offs, but what is brought into question is how hexagonal should you make your robot given those trade offs? Looking at your frame I notice that your sides are steeper than most octo and hex framed bots I've seen. Given that there's trade offs to having a non-rectangular frame, the key in designing this type of drivetrain is to balance those trade offs with the benefits of an octo or hex to get a shape that is effective in terms of space and interactions with other robots. Hex and octo bots are pretty new in frc, so not much is know as to how design one with the right shape. To determine how steep to make your corners I suggest building bumpers of different angles and testing it's effects on robot interaction to determine the best shape. Once you've done that you'll know how important frame shape is and then you can determine if having such a wide hexagon is worth the sacrifices when compared to a slightly smaller hexagon. There's teams that have done testing on this very subject, i'm hoping someone chimes in.

The first thing I thought of when I saw this design was that you could save quite a bit of space by putting the motors in the void between the modules. I read the previous thread and I noticed you had the same idea but didn't pursue it because it would be too complex. I don't know where you or your team draws the line for complexity, but I encourage you to not give up on that idea just yet. I think there's ways of getting a gearbox and motors in that space that are a little more complex than your current design but could be a great improvement and really take it to the next level. The most obvious reason to move the motors is so they don't take up valuable electronic space, but I think by moving the motors you can actually reduce the width of each side of your drivetrain pretty significantly. When I look at your power train I see two things that make each side wider than it needs to be: the gears for driving the wheels and the pulleys that connect the omni wheels together. My suggestion for narrowing your sides is to take the whole power train and put it in the gap between the outside of the hex frame and the inner yellow frame rail. In order to pull this off you would need to come up with a clever way to either replace each motor or remove the entire gearbox, which I think can be done. The advantage of this is that by putting a gearbox in between each module you eliminate the need for a belt to connect them, making the modules narrower. The main idea behind this is to take everything that's making the sides thick and put it in the void where there is space. There's a lot of ways you could do this without making it too complex, I've got some ideas but I'd like to see what you come up with

There's some other things, I ran out of time to post everything, I might post more later.

Oh, and just fyi: as for using loctite for holding cim screws in using the strongest loctite you have is not the way to go. Look up what strength thread locker you need for the fastener you're using and use that. If you put the wrong strength loctite on you might end up with an irremovable screw.

Originally Posted by Greg Woelki

Am I approaching this incorrectly? Each side is the same length, is under the same load, and is the same weight. I started the triangles with 0.1" round radius and I worked up to 0.25" (in picture), adjusting their dimensions to keep the volumes the same as I went. There is still higher stress in the piece lightened with triangles and if I increase the radius of the round much more then they might as well be circles.

https://drive.google.com/file/d/0B4u...it?usp=sharing

If you're seeing unusual stress risers in your results the first thing you should do is check your mesh. (assuming that's applicable to whatever program you're using). Also, for accurate results make sure you have at least three layers of mesh in the cross section.

-Adrian

[Cash4587]

Originally Posted by Adrian Clark

I really like this drivetrain, like a lot. Great job! It's been a lot of fun watching it get better and better. That being said, I think there's a few things left that could be optimized.

The first thing is space. I think electronic placement space is a very important aspect of a drivetrain. Without enough space you have to spend time coming up with ways to get everything to fit, and some of the solutions can make maintenance difficult. YMMV and I know a lot of teams are willing to make space sacrifices that make electronics placement difficult, and sometimes that trade off pays off. There's nothing wrong with finding sneaky ways to fit all your electronics on the bot, but I consider an ideal drivetrain one that keeps electronics placement simple. For that reason I suggest space as something to try to optimize.

The reason I think this drivetrains electronics space could be optimized is because of the large voids in between the modules and the voids created by the hexagonal frame perimeter. I think hex and octo frames are great, but I also think there's trade offs that should be considered. The first is space, is having such a wide hexagonal robot worth it if it limits space? And how will the frame shape effect superstructure and manipulation design? You may have accepted these trade offs, but what is brought into question is how hexagonal should you make your robot given those trade offs? Looking at your frame I notice that your sides are steeper than most octo and hex framed bots I've seen. Given that there's trade offs to having a non-rectangular frame, the key in designing this type of drivetrain is to balance those trade offs with the benefits of an octo or hex to get a shape that is effective in terms of space and interactions with other robots. Hex and octo bots are pretty new in frc, so not much is know as to how design one with the right shape. To determine how steep to make your corners I suggest building bumpers of different angles and testing it's effects on robot interaction to determine the best shape. Once you've done that you'll know how important frame shape is and then you can determine if having such a wide hexagon is worth the sacrifices when compared to a slightly smaller hexagon. There's teams that have done testing on this very subject, i'm hoping someone chimes in.

The first thing I thought of when I saw this design was that you could save quite a bit of space by putting the motors in the void between the modules. I read the previous thread and I noticed you had the same idea but didn't pursue it because it would be too complex. I don't know where you or your team draws the line for complexity, but I encourage you to not give up on that idea just yet. I think there's ways of getting a gearbox and motors in that space that are a little more complex than your current design but could be a great improvement and really take it to the next level. The most obvious reason to move the motors is so they don't take up valuable electronic space, but I think by moving the motors you can actually reduce the width of each side of your drivetrain pretty significantly. When I look at your power train I see two things that make each side wider than it needs to be: the gears for driving the wheels and the pulleys that connect the omni wheels together. My suggestion for narrowing your sides is to take the whole power train and put it in the gap between the outside of the hex frame and the inner yellow frame rail. In order to pull this off you would need to come up with a clever way to either replace each motor or remove the entire gearbox, which I think can be done. The advantage of this is that by putting a gearbox in between each module you eliminate the need for a belt to connect them, making the modules narrower. The main idea behind this is to take everything that's making the sides thick and put it in the void where there is space. There's a lot of ways you could do this without making it too complex, I've got some ideas but I'd like to see what you come up with

There's some other things, I ran out of time to post everything, I might post more later.

Oh, and just fyi: as for using loctite for holding cim screws in using the strongest loctite you have is not the way to go. Look up what strength thread locker you need for the fastener you're using and use that. If you put the wrong strength loctite on you might end up with an irremovable screw.




If you're seeing unusual stress risers in your results the first thing you should do is check your mesh. (assuming that's applicable to whatever program you're using). Also, for accurate results make sure you have at least three layers of mesh in the cross section.

-Adrian

How about this?
This isn't too bad for complication and it saves so much room. Although it does cost quite a bit for the bearing bore gears and for the bearings, It will leave plenty of room for electronics, at least I would say so.

http://prntscr.com/40ilna
http://prntscr.com/40ile3

Allen, Thanks for the idea. It looks very nice and makes everything SO MUCH more compact.