6 Aug 2006 tbenedict   » (Master)

No new work on the ROV, but some thoughts in case anyone is reading this and planning to duplicate what I'm doing:

First, I'm beginning to think the idea of using 30kRPM motors is a mistake. I still don't have the props, so I can't test what they do under load, but I'm beginning to suspect it'll overload the motors. 10:1 gearboxes to bring them down to 3kRPM and 10x torque (minus geartrain losses) would be good. But if I'm going to get wrapped around the axle about this, I'd rather start over with gearmotors from Lynxmotion and build pressure compensated enclosures. Which defeats the whole purpose of making it low-cost and easy to build. So I'll run with what I have and replace them with motors wound for torque when they finally burn out.

On the framework, I wound up scrapping the cable clips. Oddly enough, lining up the holes in the plexi to mate up with the cable clips was problematic. I made a new motor mount plate from 1/8" plexi and just drilled some holes so I could zip-tie it to the frame. This worked out better and made for a cleaner arrangement, believe it or not.

But that got me thinking... Why not just build the entire ROV frame out of 1/8" plexi? At the scale I'm working at (3" on a side, give or take) it's plenty rigid. And since I'm already looking at making yet another motor mount plate with a 90 degree bend in it to support the vertical motor, I'm seriously considering starting over on the entire framework.

Here's the argument in favor of re-doing the whole thing in plexi: Plexiglass is cheap. An 8"x8" sheet at the hardware store is much less than half of what I paid for all the drip irrigation fittings and poly hose. And I can make several ROV frames out of a single 8"x8" sheet. Cost is low.

Next, you can cut plexi with a jeweler's saw, a coping saw, or a scroll saw. (I've got my scroll saw set up to take jeweler's saw blades, so it can cut anything from 3/4" plywood down to thin sheet silver.) Almost anyone can work the stuff.

Next, hot-forming plexi is entirely doable in the home shop. So the whole thing can be laid out in a single sheet and bent to shape over a form. And since all the bends are 90 degree bends, the form can be a simple block.

Finally, thanks to modern printers and print drivers, making sawcut parts from CAD drawings is dead-easy. Print 1:1, spray some Super 77 adhesive on the back of the printout (LIGHTLY!) let it cure for a couple of minutes until tacky, and stick it on the plexi sheet (with the protective film left on!) Drill all the drill holes, sawcut to the lines, file the edges clean, and peel the protective film off the plexi. The drawing comes off with the film and voila, you have a made-to-order plexiglass part for your robot or ROV.

And before leaving this topic, most laser cutting services will cut plexi. So if the cost of a scroll saw or the thought of using a jeweler's saw frame for hours is too daunting, consider having it cut by places like Pololu, who charges by the inch.

Since this is a demonstration ROV, I'll use my scroll saw. If it looks like the local club wants to adopt it for a club build, we can mass-produce the plexi frames at a laser cutting service and package them with the parts OR use it as an opportunity to teach people to use a scroll saw and a drill press.

No props = no new fabrication tonight. But I'll probably spend the evening playing with layouts for a plexi frame and try to come up with a new design based around that idea.

Wish I could post pictures. I need to start a new area on my web page.

Tom

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