Re: White Paper Discuss: Nothing But Dewalts
 

I just ran an FEA w/ the plate being 3/8 thick. At just two times the stall torque of a CIM, the keyway already had a lot of deformation. I really don't think it could take the repeated shock loads of being in the drivetrainn.

I have a question though, If you can cut the keyway into delrin, why can't you cut it into Aluminum? You have a laser cutter, right?

Re: White Paper Discuss: Nothing But Dewalts
 

Any chance you could post screenshots of your FEA results? What was the total deformation?

Our laser cutter can only cut plastic, for the most part. It has a hard time with polycarbonate, as it scores the edges, and melts polyethylenes, for example. It cannot cut reflective materials at all and just isn't powerful enough to get through metal of any useful thickness.

On the other end of the spectrum, our plasma cutter can fight through 1" thick chunks of steel, but its kerf is pretty bad and it does a bad job of cutting fine details in parts. For example, holes almost never end up being round by the time it's done :)

Re: White Paper Discuss: Nothing But Dewalts
 

It's coming to the gallery shortly.

Just to make sure my numbers make sense, I took the stall torque of the small CIM (343.4 oz-in) doubled it, and converted it to lbs (42.9250 lb-in). Then I divided it by 4mm converted to inches to get the force at 4mm (272.57375 lbs). I put a pin constraint on each of the 4, 4-40 holes (at .455 radius... taken from the whitepaper) and applied the calculated load to the keyway.

Does that sound right? Since I'm right out of high school I'd like it if someone verified this.... The sketch is in the NBD whitepaper, and took me 10 minutes from sketch to analysis.

EDIT: It's uploaded.

Re: White Paper Discuss: Nothing But Dewalts
 

Adam,

I ran more or less the same analysis just now. The true deformation isn't near as bad as your picture, you're looking at the scaled deformation. At any rate, the stresses are marginal at the stall torque of the CIM, but keys don't apply load on the keyway exactly like that in practice, and the worst stresses are at the corners of the keyway, and might be relieved by localized yielding in that area. Plus, the CIM is very rarely going to be stalled during actual use. So, again, I think the best test would be a mock-up to test the design in practice.

Re: White Paper Discuss: Nothing But Dewalts
 

If you're playing Defense it's feasible to stall the motors often. Also, everyone time you go from standing still to full voltage. And going from full forward to full reverse would put a good amount of load into it. I know the deformation isn't as bad as pictured, but that just seems like it would fail with the shock loads it will see in a match.

But then again, let's see a test; can't argue with the facts.

-Adam

Re: White Paper Discuss: Nothing But Dewalts
 

Risking embarrassment, I'll note that due to a bug in our PID code from shifting signed variables or something, the PID loop for the aforementioned shooter wheel had a fun quirk. When it overspun by a significant amount, it would throw the CIM into full reverse, or near to. Despite that crazy amount of shock loading, our bootlegged version of a keyway still held up. So I think things might work out better than you'd expect. If you still aim for some kind of press between the gear and shaft, that will also help relieve some of the load on the keyway.

Re: White Paper Discuss: Nothing But Dewalts
 

On a similar note, I talked to a team at Nationals who used a metal bonding agent (I don't know what the industry term is.... that's just what they call it where I work at Northrop) in addition to the press fit for the gear. How much of a difference does that make to the integrity of the press fit?

Re: White Paper Discuss: Nothing But Dewalts
 

JB Weld, maybe?

Re: White Paper Discuss: Nothing But Dewalts
 

So I was wondering if the Dewalts transmission worked good for all of you how have it made and running it on the robot? My team started making our first one, we arn't done yet and I was wondering if they help out or did you all thing it was more of a waste of time.

Re: White Paper Discuss: Nothing But Dewalts
 

My team used the Dewalts last year and they worked very well for us, we had some problems with shifting at our first event,but after we fixed the problem they worked wonders. It was nice to be able to go fast, to score and get away from defensive robots, and at the same time shift to be able to push back if we had to. We also used the low gear to climb up a ramp the one time we had to. I would say they are are not a waste of time, but it also depends on the game that is being played, if 3 speeds are not needed in the game then it may possibly be a waste of time, but they are a nice transmission to use, especially over the bane bots from last year.

Re: White Paper Discuss: Nothing But Dewalts
 

We have used the dewalts from the very moment Dr. Joe invented them.

Some comments:

To get the press fit correct you MUST wire EDM to exacting size.
We use a fixture to press on to the shaft of the motor for correct alignment and distance.
We created a fixture to control the sizes of the housings (we do no sand as per instructions) using a mill.

We do run 4 cims with 4 shifters that are in coordination....no issues

I believe the NBD System is still one of the easiest, lowest cost, reliable drive transmissions that all teams can make.

We have yet to have a transmission failure.

Re: White Paper Discuss: Nothing But Dewalts
 

I'd like to echo everything Dr. Joe said, especially about the EDM, and the fixtures.

Carbide will not work well to machine the sun gears. They are some ridiculously hard powdered metal. If you want to use these transmissions, you really must find someone to EDM them for you. It's quick, pretty simple, and most shops will be happy to do it for you.

There are two distinct variables that affect the CIM mate to the gearbox. One is the final length on the plastic housing, and the other is the press fit of the sun onto the CIM. Both of these will need to be quite accurate, or else you'll need to modify one or the other. We ended up doing this, but it's really not a good idea.

If you do choose to shift (not at all necessary, these gearboxes are amazing just locked into second gear!), you'll need to make absolutely certain that your shift ring slips well, and there are no binds. You'll also need a longer horn to get adequate torque- I recommend buying a new, longer one, and not relying on the adjustable ones that come with the servos. They slip.

Ultimately, the Dewalts, as Dr. Joe has said, really are the best imaginable gearbox that can be adopted for FIRST. They mate to everything, they're super light, easy to mount, have awesome three speed capability built in, and are tough as nails, even if you abuse them. However, the challenge is in making them. If you are willing to invest the planning time and schedule dedication to get everything fabricated and ordered for them, they will give you few hassles, and really make the design of the rest of the robot easier.

However, if you hack your way through the process, you'll wind up with a hacked piece of junk. Imagine that... If you put the proper effort into the Dewalts, they will return it a hundred-fold.

Re: White Paper Discuss: Nothing But Dewalts
 

id like to beg the differ on the modification of the sun gears. my team (1625) had 4 dewalts on the drive of this years robot. and all of the sun gears were modified on a manual lathe with a combination of HSS end mill and 1 carbide ream. and they never had any problems the whole season.

Re: White Paper Discuss: Nothing But Dewalts
 

Could we get drawings of these fixtures? that would be an awesome addition to the whitepaper.

Re: White Paper Discuss: Nothing But Dewalts
 

We have no drawings, but I will have students take some digital pictures of all the fixtures we use and upload them to the cd media section. I will post here when the students have done them.

The fixtures really help.