Rotary tank - how to build?

I'm doing some casual experimentation at home and would like suggestions on building a rotary tank to study fluid motion. I have many questions and probably could have
gotten a few of them answered on my own but what the heck. I have no experience in metalwork, though I have experimented with brushed DC motors and PWM controllers.
Kalliroscope is a type of rheoscopic fluid used to study fluid motion. For example: http://www.kalliroscope.com/matisse/index.htm click on Kalliroscopes / Revolving Kalliroscopes
A circular tank about 6-8" diameter by 3-4" deep, is mounted to a shaft supported by a suitable bearing block assembly. It will be filled with water and kalliroscope or other particulate matter, driven by a PC controlled reversible DC motor. The tank could be a cylinder, but prefer a hemiphere. The problem is finding a hemiphere that can be mounted accurately to a shaft. Perhaps a metal spinning co. might have some leftover I could work with, however haven't yet asked around. If I had to have one built, what would be the ~ cost of a hemiphere, aluminum or stainless steel, 6" dia x 3" height with an additional 1" height vertical wall section, with a boss or mating surface at the bottom that would allow accurate placement on a flange or shaft? I imagine the less TIR, the more expensive. Wall thickness can be whatever is necessary given it only has to contain water. If it's stainless steel then it can be similar to a kitchen mixing bowl. Max speed is around 200 rpm or whatever that doesn't cause the water to be thrown out of the tank.
Coupling the motor to the tank. Should it be belt driven, or direct using a flexible coupler? Thinking of low speed DC servo motors in the ~300-550 rpm range at www.surpluscenter.com and perhaps they can be direct coupled? Or should I consider a higher rpm motor and reduce the speed thru a belt drive? Since tank rotation will be reversed frequently, I'm concerned with backlash of a coupler. I've seen zero backlash couplers but would then have to adapt shaft sizes, unless there are some available with different sizes on each end. But I guess it depends on what the motor shaft and tank shaft diameters happen to be. Now if I had one of those 7x12's...
Another thought (though seems insane) was using something like this: http://www.etech4sale.com/commerce/catalog/product.jsp?product_id 08 but the max rpm was I believe 63 rpm for this model. Other models go to about 220 rpm or so but were not on sale. However I'm not at all familiar with servo actuators, and if it would be worth it/too much trouble to adapt. The controller is expensive. Seems like overkill at 25 lbs. and they refer to it as compact, which kinda strikes me funny. Of course what am I thinking - I'm talking about a 6" diameter tank on my desktop. Will a servo actuator of that type allow for programming a smooth change in velocity without much trouble? What other smaller servos with bearings suitable for direct mounting would be reasonable for this application? A surplus servo and controller would be ideal but I need help in finding the right one that won't cost an arm. Having one that can take the tank directly would save a lot of work.
All this just to have the ability to repeat the rotary motions in case a particular fluid pattern interests me.
I appreciate your help.
Ben
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Oops - I mean hemisphere. Where did the "s" go...
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Sounds like another application for the ubiquitous DC treadmill motor and controller. With the one I've got, I'd definitely gear it down or use a jackshaft to reduce the speed, top speed is around 6000 rpm. See www.surpluscenter.com(now online!). Loads of goodies that you may or may not need.
Stan
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How would a metal wok do for this? They are relatively cheap, readily available, and the right shape. Tack weld a shaft to the bottom, and spin it to your heart's content.
Michael

on building a

could have gotten a

in metalwork,

supported
kalliroscope or

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about 220 rpm or

actuators, and if it

expensive. Seems like

me funny. Of

desktop. Will a

velocity without much

mounting would be

ideal but I need

can take the tank

particular fluid

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Must be a hemisphere with height = dia/2. A stainless steel mixing bowl has the correct shape (except small flat on the bottom), but trying to spin it by hand while floating in water makes it difficult to fine tune. If the spin is off center, especially when trying to reverse rotation, the bowl wobbles and there's no control. Only one attempt was successful in producing an especially interesting pattern in the bowl consisting of water and a measured amount of baking soda . So I'm trying to figure a way to gain some control in the process, and it needs to be precise. If it were possible to obtain a direct drive DC servo with a bearing for direct mounting of the tank, capable of fine resolution over a 0-200 rpm range, that would ease the project considerably. Of course the one I mentioned before shown at etech4sale was overkill and happened to be too slow. Something smaller would do. Otherwise there's bearing blocks, pulleys, whatever. Positional accuracy isn't needed. Only change in velocity.

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There are also salad bowls which are round bottomed. I would not have tried to do precise control with the wok by just spinning it by hand. Tack weld a rod to the center of the bottom. Put some bearings on the rod so it spins around the rod axis.
Michael

suggestions
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Your idea sounds like a good one except I'm not sure I'll make it with little runout. I made some spherical speaker enclosures using a rounter mounted on a turntable setup, but don't have that anymore. Though it may cost some, a metal spinner could make the tank. They'll make it more precisely in terms of TIR than anything I could do.
The stepping motor may work but I'm concerned with torque ripple transmitted to the tank contents, although less thru a gear reduction. It's unfortunate that servodisc motors are so expensive since they have no cogging and are brushed dc motors. Well, it's just like anything - it'll take some experimenting to see what effects things like cogging have on the contents. Perhaps it won't be a problem due to its mass, just like platters used for record turntables. I'm just a bit paranoid about building this contraption, because it's then a lot of trial and error for a certain pattern. The only other possibility is using say, three small motors with rubber tires around the circumfrence to turn a mixing bowl floating in a larger bowl of water. Just thinking out loud....

smooth
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You might be able to obtain an assembly with allowable run out by welding an over sized shaft as accuratey as possible to the bottom of your bowl. The end of the shaft to be welded to the bowl should be milled square, so the bowl will have best chance of being perpendicular to axis of the shaft. To eliminate any offset between shaft and bowl axes, place the weldment in a 4 jaw chuck on a lathe. Adjust 4 jaw chuck till you reduce runout at bowl edge to an acceptable level. You can now turn the shaft down to your bearing size and the axis of the bowl should coincide with the new shaft axis.
As an alternative you might design a coupling element to use between bowl and shaft that can adjust offset and tilt. It could be as simple as a long oversized collar with 8 set screws. Kind of like a Chrismas tree stand.
--
D. Miller
snipped-for-privacy@rochester.rr.com
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I'm hoping, that at a reasonable cost, a metal spinner could make the tank with a 3/8" shaft at the bottom. Then have an adapter made to fit the motor shaft. Since I already ordered a motor, the adapter will have to fit an 8mm dia. shaft. Just in case the motor doesn't work out, I'll just have to try another. Appreciate your suggestions. I'm trying to get this thing together as simply as possible. I was going to use an additional bearing and flexible coupler between the tank and motor in fear the motor bearings wouldn't support it, however I later learned that the particular motor I ordered will take up to a 7.5 lb axial load, and a 5" dia hemi filled with water will be well under that.
Ben
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"BR" writes:

[del]
I may try a stepper, but already ordered a small surplus Servodisc motor. It's shaft will take a 7.5 lb axial load, and the tank (decided on 5" dia) will weigh under 2 lbs, so it won't wear out any time soon. The Servodisc has zero cogging, and through the point of reversal the torque would be smooth.
[del]

That would be a problem.
Ben
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Perhaps you could use a servo motor from an old 51/4" floppy drive. These are build on an aluminium frame and have good bearing. There is also a hub to mount the bowl to. Some old turn tables used servo motors also.
Tom

on building a

could have gotten a

metalwork,
supported
kalliroscope or

The tank could be

that can be mounted

leftover I could work

would be the ~

an additional

bottom that would

more expensive.

water. If it's

is around 200

a flexible

at
consider a

rotation will be

zero backlash

available with

shaft and tank shaft

about 220 rpm or

actuators, and if it

Seems like

me funny. Of

desktop. Will a

velocity without much

mounting would be

ideal but I need

take the tank

particular fluid

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Now that's a thought. I do have a 5-1/4 floppy drive. Will it have enough torque to handle what I described? There's a lot of reversing going on.

These
a hub

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on building a

could have gotten a

metalwork,
Don't sweat the small stuff.. For example, you had mentioned: i) The small flat in the bottom of a mixing bowl. This would have very little impact on your results. Get on with your stuff and don't worry about perfection until everything else is working perfectly. ii) Back lash in the speed reducer. That will have no effect at all except to introduce a very tiny time delay in your direction reversals. Again, first make it work; then perfect it.
I did some very primitive experiments along these line using a phonographic turntable. It worked fine. Try borrowing an AC signal generator and driving a turntable with one of these. The variable frequency together with the 78/33/45 gearing would give a pretty good range. 16 RPM versions also exist. I would not recommend welding to the bottom of the bowl. I suspect you will waste a lot of time trying to adjust it and it will never be quite good enough.
I'd really love to see some of the pictures.
Walter.
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controllers.
You don't know how much time I've wasted thinking about results before getting them. :-) The motor won't need a speed reducer since it's a Kollmorgen Servodisc (found surplus recently), which has zero cogging and can turn as slow as 1 rpm, perhaps even slower. The speeds in the experiment would range between 0 to 150 rpm or a little faster, but certainly slower than centrifugated water all over the place.

it > will never be quite good enough.
No welding. A metal spinner should be able to provide a shaft at the bottom of the hemisphere for mounting. It will be direct coupled to the motor shaft with an adapter.

Well, if anything interesting happens there'll be movies.
Ben
--
>
> Walter.
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