pic: New to CADing for FRC. Critique? Would this work?



Obviously not the whole drivetrain, but how is this? Our team has access to a plasma cutter and we hope to cut those sidewalls out with it. We also are going to be using the new Vex Pro ball shifter which is attached.

Looks good. Are you using chain or belt? If you are using chain do you have a way to tension the chain? That lightening pattern looks inefficient. You probably can cut out more weight and spend less of your sponsors time by making larger lightening holes.

I would be wary of cutting those rather small holes and lightening patterns with a plasma cutter as they are very close together. You’ll want to take out larger pieces but leave enough material in between.

177 used their plasma cutter this past season to create their drivetrain plates.

http://www.chiefdelphi.com/forums/showthread.php?p=1173244&highlight=plasma#post1173244
http://www.chiefdelphi.com/forums/showthread.php?p=1173208&highlight=plasma#post1173208
http://www.chiefdelphi.com/forums/showthread.php?p=1173196&highlight=plasma#post1173196

Drop Peter a message on here and I’m sure he would be as helpful to you as he was for me. He has pictures of 177’s plasma cut parts on his facebook, not sure of any public albums though. His general suggestions: practice A LOT with the plasma cutter settings, types of patterns you can cut, thicknesses and cutting speeds, keep lightening shapes rather large, large radii, watch out for weakened areas due to the heat of the cut. Bearing bores will most likely need secondary machining to have a precise fit. Here’s a pic of one of their modules. Another great thing to do is discuss these things with your sponsor’s machinists. They know what’s best for your parts and can help you with many aspects of the design to make sure it will work with their machine.

Hope this helps! Good luck!

https://sphotos-a.xx.fbcdn.net/hphotos-ash4/423626_10150567376411915_369619069_n.jpg

Akash beat me to the punch.
He hit the highlights well, but that lightenging pattern will definitely not work.

It doesn’t look like there is any thought in the layout as to structure or function, it just appears to be there to remove weight. The pattern should have web stretching between all bolt/bearing/spacer holes to give strength where the loads are.

Plasma cutters are nice to have especially being able to cut out certain parts of the body to reduce weight. Looking really good!::safety::

Increase the diameter of the standoffs that hold the sides together. Doing so will distribute the shock load better and also provide more surface area to help stiffen the side plates when your assembly is tighten. If you added a bend along the leading edge that would help and you could add some brackets between the side plates to keep the plates parallel and square

In addition to the comments that have already been made. If I were reviewing the CAD with our guys - we would be looking to:

  1. Use the members that join the left wheel module with the right wheel module to provide structure/support for the wheel modules themselves- you can see the cuts and joining holes in the 177 images that Akash posted. With a couple bends, like Roy mentioned, or a piece of angled aluminum joined to your side plates, you can tie into the cross tubes, and everything will stay a lot more solid and square.
  2. Make the side plates as close together as possible while enabling chain maintenance.
  3. Increase the number of standoffs between the plates - especially around the wheels as the side loading on the frame will be highest close to the wheel that is on the ground.
  4. Either support the outboard end of axle out of the vexpro transmission or analyze its ability to withstand the cantilevered loading. I have not analyzed the vexpro transmission (so it may be fine in this application), but we generally avoid cantilevered drive members for our bot if we haven’t analyzed or prototyped the system with the cantilevered loading.

In general - it is looking really neat - looking forward to seeing some images of the KleinBOTS’ machine this year!

In addition to the many valuable recommendations already made, I’ll add these things to check for (I don’t know if you’ve already done some/all of these, just wanted to double-check):

  • Make a 2D sketch (or 3D model, if you feel like it) of the chains/belts to ensure none of them hit your standoffs.
  • It looks like a couple of your standoffs may be in the way, but it’s hard to tell, obviously
  • Design for chain tensioning and/or use a chain length calculator
  • Could be a floating wheel, tensioned by a screw or turnbuckle
  • Could be a larger sprocket placed in the chain run (like 118)
  • Could be a small sprocket (or piece of delrin) pushed against the chain until tight
  • I’m having a hard time judging the scale, but if you’re pressing bearings into your sidewalls, ensure your holes are toleranced to get a good press fit (depends a lot on machining method used… I’m unfamiliar with the plasma cutter)

  • Standoffs can be a lightweight way to make a more rigid structure between two sheet metal plates… I would recommend at least using some pretty large OD standoffs and potentially putting a couple plates across that have two holes in each sidewall. This should add significant rigidity

  • Also, you do want to ensure that your drive modules mount to the rest of your frame rigidly and without creating stress-concentrated areas…

What thickness of aluminum do you have modeled so far? With a better-optimized lightening pattern and good structural design I would say you don’t need anything thicker than .125; however, I don’t have any experience with flangeless sheet metal drivetrains.

Best of luck in your design!

Thank you everyone for the support and tips. We will take them into consideration.

To Nathan: I modeled for .125in sheet metal.

I have another question as well:
-What works better, bearings in the sidewalls or on the wheels themselves?
-Is there any mathematical method to determining where to cut out for lightening while providing support to the structure at the same time?