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Old August 11th, 2005, 10:04 AM   #16
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Or just do it in photoshop accuratly and print it out the right size.

Only problem is your only centring the outer diameter cut in relation to the holes in the centre of the disk. even with this perfect, there still maybe offset once mounted to the fan motor.

With my method, it marks the outer diameter perectly taking into account any inaccuracies made when drilling and any slight offset in relation to the centre of the fan motor. Challenge is to cut exactly on this line with the circular glass cutter. I guess both methods are open to potential inaccuracies during cutting/drilling, the question is which method is the most accurate? I guess the proof is in the puddling.

Im just wondering how much better real glass is going to be compared to the plastic CD in terms of light loss?

Wayne.
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Old August 11th, 2005, 10:18 AM   #17
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Quote:
Originally Posted by Wayne Kinney
Or just do it in photoshop accuratly and print it out the right size.

Only problem is your only centring the outer diameter cut in relation to the holes in the centre of the disk. even with this perfect, there still maybe offset once mounted to the fan motor.

With my method, it marks the outer diameter perectly taking into account any inaccuracies made when drilling and any slight offset in relation to the centre of the fan motor. Challenge is to cut exactly on this line with the circular glass cutter. I guess both methods are open to potential inaccuracies during cutting/drilling, the question is which method is the most accurate? I guess the proof is in the puddling.

Im just wondering how much better real glass is going to be compared to the plastic CD in terms of light loss?

Wayne.
I think the glass would maintain better optical properties than the plastic. Maybe not so much with light loss, but maintaining image clarity on the frosted surface. I am purely speculating here.

Totally agree that both methods should work in theory about centering the holes and that the end result depends alot on accuracy of machining. I was putting forward a method that should keep everything in close to perfect relation to the physical center of the glass. That may not get you perfect balance, but it's a good starting point.

-gb-
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Old August 11th, 2005, 10:26 AM   #18
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Greg,
You methed is very good and I think I will be trying both methods in the future. As you point out, getting it spinning without vibration is difficult without special tools, but i think its possible.

The cool thing about my method of using the pen to mark the outer diameter when its spinning, is that the mark will be perfectly central in relation to the axis of the motor, which is whats important.

Wayne.

Last edited by Wayne Kinney; August 11th, 2005 at 11:29 AM.
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Old August 11th, 2005, 11:07 PM   #19
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can you spin it and then mount it against sandpaper so it spins against the sandpaper until it's a perfect circle centered on the axis?
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Old August 12th, 2005, 06:33 AM   #20
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Ben,
I thought of that but think it would take too long. However, I was planning on using my method first to cut it, then do as you sujested to clean it up after. Thanks for your sujestion.

Wayne.
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Old August 12th, 2005, 07:49 AM   #21
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After speaking to Nick Bartleet on this design, he had tried it and failed. His main issues were balance of the glass and gyroscopic effect.

His glass was cut buy a company called Knight Optical. Nick had his glass crack twice. Due to the weight of the glass, the gyro effect has much more force.

So what do you guys think about this issue? Would thickness of the glass help? Would 1.5mm or 2mm be better thickness? 1.5mm would mean lighter but weaker, 2mm would mean heavier but stronger.

What do you all think?

Wayne.
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Old August 12th, 2005, 08:17 AM   #22
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Wayne.

Any of the .jpg files titled "ohara008.jpg" or similar at ww.dvinfo.net/media/hart are tests of a glass disk dressed to 5 micron and slightly backpolished for better light transmission at expense of the "filmlook"

The disks can be had in 1.2mm thickness from ohara in Japan but the downside is you have to dress and polish them both sides then groundglass one side yourself. It is a seriously difficult task and of the ten disks I got in, only two now survive. Four were lost in wax composite disk attempts, one (the best) was broken accidentally in service. Three are still in semi-complete state. If I go glass again, I will likely use the smaller CD-R size and spin them at 3000rpm than the 1500rpm I use with the full size CD-R glass disk. As AU$50 an item, the glass is not cheap to be breaking. I was pleased with the result I got before disaster struck.

Subjectively, I think the image looks slightly more solid than the plastic CD-R spacer result. I've since tried splitting DVD+Rs and I got a good result however they are also difficult to dress as the plastic seems to have some hardening enhancements which resist the grit and take longer to dress in my machine than the actual glass ones. The DVD+R when split is very thin and forgives misalignment if you spin it at 3000rpm (3vdc on a Mabuchi style CD spindle motor.) as it flexes enough to run true on the focal plane. When drewssing, you have to protect the clear surface with stick-on material you can peel off later. I found plastic book covering sheet to be adequate.

Due to some family issues the whole R&D is been on hold for about three months.
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Old August 12th, 2005, 08:36 AM   #23
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Wayne.

Furthur to above, "grabmon.jpg" on www.dvinfo.net/media/hart is an assembly of images shot in a low budget production environment, time pressures, a location, two man crew and powersaver flouro incandescent lamps of combined tungsten 340watts equivalent plus an overhead 18watt overhead flouro light. Lenses were Nikon f1.8 50mm and Nikon f1.8 85mm. Image path was Nikon Lens >> oharadisk >> 2 x 90 degree prism to erect image >> Century Optics 7+ achromatic dioptre >> PD150 P camcorder.
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Old August 12th, 2005, 09:01 AM   #24
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Quote:
Originally Posted by Bob Hart
glass disk dressed to 5 micron and slightly backpolished for better light transmission at expense of the "filmlook"
Bob,
Thanks very much for your reply. When you say at the expensive of the "filmlook", do you mean that backpolishing reduces overall diffussion and therefore letting more aerial image through and killing DOF?

The shots of your tests with the disk are very clean indeed. I think i'll try cutting my own glass, dont want to go to the trouble and expensive of getting hold of them disks from japan.

Was gyroscopic effect ever a problem for you?

Thanks,
Wayne.
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Old August 12th, 2005, 10:59 AM   #25
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Maybe the glass can be held on the outside so it doesn't have to be drilled or anything like that. Could build a gear on the outside of a short cylinder and let it rest on some gears that have ball bearings in them.

I've seen a p & s mount for an xl1 and that is how I think they might hold the glass and deal with the weight, but they spin it in the center so you have a spot in the center that doesn't move.
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Old August 12th, 2005, 11:47 AM   #26
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Keith,
Yes thats the design Nick Bartleet went with, but as you point out it spins slower towards the middle and in theory not at all in dead centre. Thats why i didn't want to go down that path.

Wayne.
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Old August 13th, 2005, 06:10 AM   #27
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You can still spin it off center, it just needs to be a larger ground glass, that way the groundglass looks like it is just a continous sheet moving up or down. The P & S I saw spun with the center of the spinning ground glass in the center of the screen, but using a larger gg the center would be off to the side.

You probably thought of that anyway and one side will still not spin as fast as the other but at least you don't see that its spinning in place.
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Old August 13th, 2005, 08:11 AM   #28
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Keith,
That would certainly make the centring of the glass a simple process. Nick Bartleet used a big bearing with a 50mm diameter hole with the ground glass fixed to the inner race.

Is this how you mean? Or do you mean the glass supported by a few smaller bearings? Could you elaborate?

Wayne.
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Old August 13th, 2005, 06:17 PM   #29
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I was thinking something maybe a little larger. Like I said a little earlier, just throwing out ideas, not to experienced with this and haven't followed all so I'm just thinking out loud a lot.

I was thinking a large metal gear with the ground glass placed in the center, possibly 10 - 12 cm in diameter, surrounded by a metal ring with gear teeth that rests surrounded by 3 smaller gears that mesh with the large one which holds the glass and one of the smaller gears is driven by a motor. The other two gears are only to hold it in place. The side of the ground glass would be used to reduce the spinning that you see.

This might not be the greatest setup for simplicity, I'm a little tainted by access to metal lathes and mills and cnc machines. So it might be better to replace the gears with a metal ring that rests on some wheels, with a rubber wheel for grip on the motor.

Again, just thinking out loud. I'm involved with designing some other stuff, but if I get the time I'll see if I can do up a simple drawing or something if needed
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Old August 13th, 2005, 07:29 PM   #30
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Keith,
Your idea certainly solves 3 of the problems of the other designs. I would just be concerned about the ammount of noise gears would make. Probably something you could develope?

Wayne.
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