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Old December 26th, 2007, 06:12 PM   #1
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What you think? I've got a nice GG material, not a frosted CD, that'll be great for this project. I tried it with the Daniel's Vibrating holder, but the vibration wasn't able to rid all of the grain. So I'm going with a Spinner. What's the recommended RPM's? I've got one that's 9v at or around 18k RPM, but I plan on using a potentiometer to slow it down if I want to.

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-Rh

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Old December 26th, 2007, 07:12 PM   #2
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Rich.

You do not have to run the disk very fast at all, provided it has an even finish all over.

I run a full CD-R sized disk at anything between 1500rpm when the battery is new, to slowly moving when the battery is nearly flat. As long as the big disk is actually moving it is fine for 1/50th sec shutter.

With a smaller diameter disk, then motion may need to be faster to avoid slower surface speeds closer to the centre becoming visible, the so-called "vortex from hell" artifact.

If you have a controllable motor and can run the disk at exactly the period of or near to a multiple or fraction of the camera frame rate, your groundglass texture should not appear and any variable density artifact should be less apparent if at all.
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Old December 26th, 2007, 08:00 PM   #3
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The middle of the disk will have a hole 1.5mm in diameter. The size of the barrel from the motor. I didn't know for sure how fast the disk had to rotate so I went with the fastest. I'll be able to slow it down with a rheostat.
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Old December 27th, 2007, 09:06 AM   #4
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The centre diameter is 1.5mm. What is the shoulder diameter, is the disk positively retained onto the motor shaft, is the motor solid mounted or on compliant mounts?

High rpm is good for disappearing artifacts and dealing with axial runout but introduces some strong gyro forces which may cause the disk to flex off the focal plane in whip pans and tilts or cause it to chatter back and forth on the focal plane if there is endfloat in the motor shaft.

A compliant mount may permit such movements as well.

A good way of adding compliance to the motor mounting is to solid mount the motor to a panel which has at least three compliant mounting points of its own set wide apart, even wider thasn the disk diameter is good.

The compliant mounts help reduce noise which high rpm will possibly give you but being set widely apart gives them leverage over any eccentricy or runout of the disk or guro foces which want to move it off axis and off the focal plane.

Last edited by Bob Hart; December 27th, 2007 at 09:09 AM. Reason: error
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