Sunday, January 24, 2010

Drive System 2

I've had several false starts on the drive system upgrade.  At one point I thought I would be able to attach a gear to my old wheels and use my old drive frames and add the NPC motors in, but after I got the parts from The Surplus Center I could see that that wouldn't work.  So I reconsidered.

I've been really impressed with Mike Senna's drive system ever since he developed it.  So I've resolved to give it a go with some modifications.  I searched around Sacramento a bit for wheels and found a place that
sold me these:

The one's that Mike lists in his plan are also Colson caster wheels, but his are 5" x 1.5".  My distributor didn't have those, but he had these for about $10 a piece.  They are 5" x 2".  What that actually means is that the hubs are 2" across, but the treads are about 1.5".  And I think the tread width on the 5" x 1.5" is 1.25".  So mine are .25" wider.  I've had a picture in my head since I started this project of wide fat wheels with lots of torque--or even tank treads--that would conquer any terrain.  So I opted for the wide ones here.  We'll see if that's realistic.  The wide wheels may cause trouble in tight turns.  The wide wheels won't work on Senna's plans, however, because he's using a piece of rectangular tubular aluminum to fashion the frames, and I think the width on those is 2" on the outside, and .125 or .187 on the side walls.  So there's only about 1.75" clearance on the inside. But I have a solution for that in a minute.

I had some 1/8" aluminum plate and some 3" wide channel left over from before.  So I have cut a couple of pieces of the channel.  I think the lip on these is 1". Then I have drilled and counter sunk holes for bolts on both sides.

Next I have cut 4 trapezoidal side plates.  I'm following the Senna planes on outside dimensions.  (I've mentioned it before, but the Senna plans are part of the "Files" attachments to the R2 Builders Yahoo group.  Go down to Senna files, then look in the Senna Drivetrain folder.)  The side plates are going to bolt onto the channel top plates above to make the frame.

  Even after building my first set of drives, I had apprehension about cutting all of this aluminum.  I got tendonitis from the hacksaw before.  So I talked to a friend at a machine shop and he said to use a fine toothed jigsaw blade in my jigsaw that is intended for soft metals.  I was very skeptical.  But h was right; the jigsaw cuts this 1/8" plate just like wood.  Very easy to work with.

So I clamped these side plates into position on the U channel to mark and drill the holes for mounting in exactly the right spot, drilled them out, and then the bolts went through no problem.  My bolts were a bit long and protruding into the space inside, so I trimmed them off with the Dremel tool.

Here's the result.  I've put the plates on the inside but they can be easily be mounted on the outside to make a little more room.  So there's a bit more versatility with the interior width here than working with the rectangular tubing.  The drawback is that the bolts can work lose.  So I'm going to add some locktite to them on final assembly or superglue.  When the rest of my materials get shipped here next week, I'll start laying out the axle locations, add some bolts across the bottom for bracing, and so on.  More details then.  I'm excited to get those NPC motors in there.


Chris said...

Looking good.

Can't wait to see the next step.

Jamie McShan said...

Looks very interesting Matt! So you decided not to try out the cheaper Surplus motors?

Matt McCormick said...

Thanks guys. Jamie--Yeah, that was tempting, but that business with the short drive axle doesn't look right to me. I know that someone's managed to get a gear on there, but I suspect that the alignment will be off, or the attachment, because you can't get a key into a keyway slot, won't work out. I got NPC to give me the discount after some hassle, so they were a bit cheaper too.

I am going to buy some of those surplus motors, though, and put them in another robot. I think that there's a shaft connector out there what would be a sound way to lengthen it.


Jamie McShan said...

I will be following this build closely, I am still going forward with my scooter system, but I feel someday I will want to upgrade and I like where you are taking Mike's design. BTW, thanks for the tip on using a jig saw with aluminum. I picked up a blade last night and it worked like a charm. Better then a metal blade on my band saw.

Matt McCormick said...

Thanks Jamie. Yeah, I couldn't believe how easily the aluminum cut that way. I've had this idea in my head that I might try to cut my own aluminum frame pieces and this success with cutting it with the jigsaw made me think it might be plausible. But aluminum is expensive. I figure that just buying the 1/4" aluminum to cut the rings and plates for a JAG style frame will cost a couple hundred bucks.

I got the rest of the parts for the drive system yesterday, so I'm ready to continue. I'll try to document each step carefully. This is about the funnest part of the build for me.

I'd also advice not bothering with scooter motors, I think. Mine worked and I drove my droid around a lot and did several events with them. But they have never performed like I wanted them to. And once you buy the parts from The Surplus Center instead of McMaster and make some cost cutting choices, you can get the expense of the Senna or JAG style drive down. I'll try to post a spreadsheet on that too.