Cartesian Bot options

While I was looking through my parts bin last night, I came across two really big steppers that I've had kicking around for a while. They about a 3/8" shaft, come with toothed pulleys and weigh something like 5 lb each. I'll update with specs tonight when I have them in front of me.

I'm trying to work out what kind of cartesian bot to build.

I have a lot of torque to move things really quickly, but those motors are *heavy*. I want to be able to do milling at some point, so bracing is important.

What sort of cartesian bot should I be looking at?

You probably don't want to move either of those motors if you can avoid it.. if you want to keep your X and Y motors fixed you're talking about a positioning stage gantry like the one I've built but heavier duty (there's a topic in "Reprappers" with photos), I think. You move a stage as one axis, and the Z axis is mounted to the other. It limits the weight of your Z stage, though. If you have a bunch of good linear bearings you could rig a positioning system using the motors that moves either the toolhead or stage in X and Y without moving the motors as well.

Ok, so the steppers I have are unipolar NEMA 34 steppers. They draw 4.8A/phase. I'm looking for the best option for using these to build the basis of my cartesian bot.

4.8 amps! you have a direct line to Hoover dam?

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

The rated holding voltage is 1.8V, so it's really not that bad.

Yes, however, when you're not "holding", there is induction to conquer, so it's not uncommon to use driving voltages 10-20 times the rated holding voltage. The idea is that even with 1.8V, the motor will be able to consume 4.8 amps and achieve its holding power. When you drive it with 12 V, you'll need to control current. Seems this is some beefy stepper, which will require some expensive electronics to drive. I guess it would be cheaper to buy a smaller stepper *and* simpler electronics rather than buy electronics that can drive your motor reasonably well.

-Geert

GeertB Wrote:
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> Seems this is some
> beefy stepper,

That's the idea

> which will require some expensive
> electronics to drive.

I've spent time doing power supply design. I'll design a controller for it.

After spending some time looking at cartesian bot options, I've decided a couple of things.
I want to keep a high maximum acceleration in the bot, which means that I'll need to keep the mass it moves down. To that end, the only thing I want to move is the toolhead. This rules out gantry robots.

For toolhead-only cartesian robots, there are two options that I can see:
1) The reprap style which uses 4 linear bearings, 3 leadscrews and moves one stepper. The additional disadvantage with this architecture is that it gives asymmetrical response: the head will accelerate faster in x than in y.

2) a beefed up version of the reprap style which uses 8 linear bearings and 4 leadscrews, but keeps both steppers static. This architecture is totally symmetrical. Both x & y will be moving the same mass given a square frame.

I like the second version a bit better in terms of symmetrical operation. But both bring up the question: what do I use for linear bearings?

So far, my best idea is to use aluminum channel with bearing blocks made from three skate bearings on a mounting, one bearing in each side of the channel and one holding the block up on top. But that will mean that I use a total of 24 skate bearings.

Edited 1 time(s). Last edit at 01/28/2009 10:57AM by Annirak.

Annirak Wrote:
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>
>
> So far, my best idea is to use aluminum channel
> with bearing blocks made from three skate bearings
> on a mounting, one bearing in each side of the
> channel and one holding the block up on top. But
> that will mean that I use a total of 24 skate
> bearings.
>
Use the aluminum channel for the guide and then cut a groove in HDPE for the moving guides. For x and y that works great. Loads aren't big so wear isn't really a factor. I do use skateboard bearings on the z-axis, though.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison