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Old April 24th, 2009, 04:56 AM   #1
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Ever heard of a ground glass exploding?

I just finished making my own ground glass to see if I could improve on the results I've been getting with the acrylic one in my DIY adaptor, and I'm looking forward to trying it out. It looks great. It's 84 mm in diameter and made from 1.5 mm thickness pane glass.

But someone has pointed something out to me. I've never heard of a GG exploding, but if it did, I could imagine the shards destroying both the achromat at one end, and one of my lovely Carl Zeiss lenses at the other.

Anyone ever heard of such a thing happening with a properly mounted GG?

Martin
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Old April 24th, 2009, 05:28 AM   #2
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Never heard of it but like every thing in life there is a first time for every thing and I cannot think what could make it explode apart from a stick of dynamite strapped to it. It will be interesting to see if any body has heard it of or even seen it.

Alan
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Old April 24th, 2009, 05:47 AM   #3
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I probably should have added two things:

1) My adaptor is the spinning, not vibrating kind.

2) This note of caution came from an engineer who says CDs explode inside drives frequently. Personally I've never heard of that happening, but as you say there's always a first time. (He made the remark in the context of some experimentation I'm doing to try and get my disc to spin even faster than it does now.)

Martin
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Old April 26th, 2009, 11:54 AM   #4
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Well it almost happened to me. I didn't properly tighten my spinning GG (made of glass) and this is what happened. It didn't explode, but it sure sounded awful!

DOF Adapter Oops!! By Marcel Van Someren On ExposureRoom
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Old April 26th, 2009, 08:39 PM   #5
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Ah, but that's understandable. Improper installation of the gg or any similar oversight in a comparable situation is asking for an accident.

I think what this person had in mind was an actual explosion of the material, which apparantly happens to CDs, but (I have learned since posting the above) at speeds much higher than what we are using to spin the discs in our adaptors.

After hearing from various people I no longer consider this an issue, though I will be a bit more careful in future now that I'm using real glass.

Martin
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Old April 26th, 2009, 09:40 PM   #6
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I have had two break up in a home made adaptor. The first was due to the adaptor hitting the ground from about three feet up with the groundglass motor running so the bits of disk took out the prism as well.

The next was when I was being a bit aggressive doing heldheld pans and tilts during a test.
In this case it was a groundglass I knew to be cracked and had patched with UV cure. I did end up with fine bits of glass in the back of the lens but fortunately no chips or gouges.

My groundglasses were CD-R sized glass disks, dressed down to 0.9mm thickness which is a wide span for such a thin piece. At that stage I was running the disks at close to 3000rpm. I subsequently found much slower speeds were just as effective.

With a smaller disk, if you have nounted it sensibly without excessive or uneven clamping pressure, you should have less risk from breakage due to centrifugal resistance to pans, tilts etc..
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Old April 26th, 2009, 11:07 PM   #7
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If there is any residual stress in the glass from when it cooled after being cast, there is a chance it might shatter. However, when you "grind" glass, you put a series of small chips all over the surface, so if it is going to break from internal stress, it most likely would have done so when ground.

Bob: 3000 RPM? What were you trying to do? Combine a DOF converter with a gyro stabilizer? I've heard of overkill, but that's REALLY pushing things.

Martin
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Old April 27th, 2009, 01:04 AM   #8
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I had used 3000rpm to get rid of an artifact from surface finishing on a plastic CD-R disk. The motor speed carried over onto the glass disk when I fitted that.

I very soon discovered that it vibrated very badly and the motion created a sort-of comb artifact on interlaced footage.

The glass disk resolves well enough even when the 1.5v battery is almost flat.

The flywheel business - the higher disk speed at the time came from my being small on the knowing of the downside of such arangements.
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