A new chemically engineered GG

[Stephen Pipe]

I think we have to give Tony the benfit of the doubt here. I've shot with the P+S and the mini 35 and had a quick look at Waynes adapter at it's early stages 6 months ago. I also am going to get a sample of the glass so hope I can shed some light onto this. I'm keeping my fingers crossed it's the real deal.

[Tony Relph]

If I wanted to deceive people I would make sure that I did it properly. That is why I have insisted that people test the GG to ensure a fair test and see for themselves.

May I thank everyone for their help, input and interest.

[Stephen Pipe]

Tony I hope you don't think I was implying you were trying to deceive anyone. I was trying to say people should stop getting on your case here and let your product do the talking when it's been tested. I'm right behind you buddy. You could potentially blow this 35mm adaptor thing wide apart. Good luck man and I'll email you later about the size of glass.

[Bob Hart]

A fair call Stephen.

I have been looking for a holygrail which resolves like a wax GG without the problems. It costs me $AU50 to land a raw glass disk here in Australia and then I have to do the figuring and polishing, then the one surface of five micron finish.

The whole thing comes to about 15 - 20 hours assuming things go right and don't have to be repeated. Factor in the value of the hours and dressing/polishing materials and somebody else's work starts to become attractive especially if it is of superior performance.

Keep up the good work Tony. R and D is not an easy business.

[Alex Chong]

Hi Tony,

Can your relative glass factory cut out a 90mm diameter glass (3 or 4mm thick) with a center hole same size as a CDR and the glass then coated with your special wax? if can, I would like to order a couple of waxed GG and couple of unwaxed GG. I currently have a DIY mini35 using ground glass which I would like to replace. I don't mind buying some from you if you can do for me in the next couple of week.

Kind regards,

Alex

Email me at ccm@brunet.bn

[Tony Relph]

Thank you for your support Stephen and Bob.

That is exactly what I want to do - let the product do the talking. Hopefully it will say what I hope.

Alex - With regards to your glass the diameter and thickness is fine. You have a choice out of several materials:

- BK7
- Sapphire
- Fused Silica
- Fused Quartz

The first 2 choices are the best for us but the glass is only as good as the spec. Wrong thickness, material, coating = Bad GG.

Email me the exact measurements, material and quantity then I can get a price for you.

[Tony Relph]

NOTE FOR ALL GG REQUESTS:

For all of those who have emailed requesting a GG sample will have to wait until the GG goes into commercial production. Only 10 samples are being sent out for testing. All DiGi35 products both Adapter and GG should be ready for order September 2006 time.

Before that, footage and stills from testers and myself will be available for viewing at digi35.co.uk shortly. A brochure will also be available via post or email which will include product images including accessories, final pricing and other relavent information.

The option to pre order will be given before the release date This will be limited to about 20 units. Take a note of email address: preorder@eldavinci.com

Just out of interest, what would you expect to pay for a high quality GG? At the same time we want to be competitive but we must allow for the cost of process, seals and design which make the glass what it is. The GG will be guaranteed for probably 2 years. This will be finalised.

[Alex Chong]

Hi Tony,

I would like to try the BK7. The size is 95mm diameter with centre hole exactly the same size as the centre hole of a CDR. I think its exactly 15mm. But can't be sure. Can you check if that is correct. I need it to be roughly 4mm or 5mm being max as any thinner it may break. If the price including shipping is good, I would like to order 2 of each, i.e. 2 disk with the wax and 2 black disks. Please get back to me with the price including shipping. Do give me the best offer you can as I would at the same time do a test run for you with my mini35.

[Bob Hart]

Alex.

My disks from Ohara in Japan have been BK7 so I can say the material works.

Your choice of 90mm? That may leave you a little tight on two corners of your movie frame if you are using a CD motor and hub to run the disk. I looked at the smaller than CD-R sized disks and ruled them out for that reason

Your choice for disks may also be a little too thick. I use disks which start out at 1.3mm thickness but come down to 0.9mm by the time they are figured, polished and then one side groundglassed.

Thicker may make them less likely to break but too thick and some nasty optical things may start to happen.

[Wayne Kinney]

I have found I can fit a 36x24mm image onto a disk of 87mm dia. The SGpro uses a slightly bigger frame then this with a disk diameter of 93mm.

[Alex Chong]

Hi Bob,

Thanks for the info. How do you get your disk? You order online? 90mm was what I started with and with my MX500, it was ok. Upping it by 5mm should give me extra room for filming.

By the way, I wrote to you before if you remember and you sent me some info on your Agus mini35 plan. I assume you are using spinning GG. Or have you moved towards vibrating GG instead.

Now I know this has been discussed before somewhere but may not be in details. But does anyone know how P+S vibrate their GG. I have done one before where I used a DIY vibrating motor to vibrate a focus screen on a bracket setup made out of plexiglass. Unfortunately the vibration was so loud the noise would just get recorded. I scraped the idea. You think by using a proper vibrating motor instead of a DIY one, it could reduce the sound considerably?

[Alex Chong]

Regarding my question about vibrating GG. Its ok, I search the site and tons of info.

[Tony Relph]

Sorry I haven't got to you, I've caught a chest infection off of my daughter so I'm having lots of sleep and Tramadol (wonder painkiller) and antibiotics which knock you out.

The thickness glass we use is 0.56mm (a tiny touch over half a mm) this is OK to use with the diameters that we use, so long as the diameter is within a certain ratio to the thickness you should have no problems. But bear in mind, and adaquate seal and O-rings are needed to add additional support to the glass and to reduce to risk of fracture.

NOTE TO GG DIY'ERS: When sandwiching 2 pieces of glass together make sure that you make some kind of seal to ensure no glass separation occurs otherwise you'll have more then just 2 pieces of glass.

The seal for the rotating adapter mount has been designed to accept a 36 x 24 image frame.

Alex - I take it that you are requesting glass to make a GG and not a specific GG. Email me at the address below with your requirements and I'll get a quotation put together for you. Please specify material, internal and external diameter, thickness and quantity.

I'm back off to bed cos the screen is making my head pound.

Many Thanks,

[Tony Relph]

Stephen - I will reply to your email possibly tomorrow. Haven't forgotten just lots on and not feeling to great.

[Bob Hart]

Originally Posted by Alex Chong

Hi Bob,

Thanks for the info. How do you get your disk? You order online? 90mm was what I started with and with my MX500, it was ok. Upping it by 5mm should give me extra room for filming.

REPLY:

I don't know if they still can be had. I ordered my blanks from Ohara in Japan. "hiro@ohara.co.jp" I think was my contact. I had to order a batch of ten blanks.

These were CD-R sized, complete with 15mm centre hole. They are 1.3mm slices off a round piece. They have a rough finish on them which has to be ground flat then the polish done then the 5 micron dressing done to one side.

It is not an easy job. I made a sort of a cement mixer from large diameter sewer pipe and caps. For the bearing, axle and hub for the mixer, I used a Ford Falcon water pump. I had to make a ring spacer to go between the pump hub and the pipe cap because the axle of the water pump sticks out. A power steer pump would also make a good axle and hub. With both, you already have the bolt holes in the main housing to fasten down onto some sort of frame.

For the first grind, I had large disks of thick window glass cut. This disk sits tightly in the bottom of the mixer barrel. I have a heavy brass disk about 15mm thick. I turned it in a lathe so it is perfectly round and has flat faces. To make it roll with constant face contact I have turned it so there is a little ridge higher at the front edge. This is closest to where the BK7 disk is glued on to the front of the brass disk. This little ridgeis there only to stop the brass disk from wobbling. As the mixer turns, the brass disk rolls around inside and rubs the BK7 glass disk against the larger window glass disk. The mixer barrel has to be at about the same angle as a real cement mixer. Too much back tilt and the brass disk tends to carry up and fall across. Not enough back tilt and the wear is very slow.I don't get the best results with this process because the path of contact between the two pieces of glass is a narrow ellipse not a true circle.

To glue the BK7 disk to the brass I use paraffin sealing wax. I shave some ieces onto the brass disk, t it on a eat source, rest the raw glass disk on top of the wax fragments then let them melt across under the glass and the turn off the heat and let it cool. don't use too much wax or it comes over onto the front of the glass and spoils the grind. To release the glass, I boil the brass disk in a large pot until the wax melts. I use a kitchen skewer or a bit of sharpened wire to lift the disk and hold it clear but don't take it out of the boiling water otherwise it will crack in the air cooling. Let the water cool down. There will be bits of wax all over. I use a fresh blade out of a disposeable shaver. Just watch your fingers when you break one open. I use this to peel the wax off the groundglass. Then, to shift the remaining wax, I use "preen" the great unstainer - a solvent based trigger spray for getting rid of collar stains from white shirts. I also use methylated spirits for a final cleanup, also kitchen liqid detergent and water. It is a messy business. It is important no wax get onto the groundglass finish otherwise it is almost impossible to get it off. Even if you have a good even 5 micron grind, there will be flicker because of the transparency of the wax.

END OF REPLY THIS QUESTION


By the way, I wrote to you before if you remember and you sent me some info on your Agus mini35 plan. I assume you are using spinning GG. Or have you moved towards vibrating GG instead.

Now I know this has been discussed before somewhere but may not be in details. But does anyone know how P+S vibrate their GG. I have done one before where I used a DIY vibrating motor to vibrate a focus screen on a bracket setup made out of plexiglass. Unfortunately the vibration was so loud the noise would just get recorded. I scraped the idea. You think by using a proper vibrating motor instead of a DIY one, it could reduce the sound considerably?

REPLY

I did not progress to another method of moving the groundglass. Using something else other than a disk makes sense as a lot of groundglass arrea is inefficiently wasted with a spinning disk just to get a movie frame sized image.

The upside with a CD sized groundglass is that it is very simple. The rotating speed does not have to be fast because the linear surface speed of the disk even halfway in to the centre is quite fast for a given RPM.

This means very little vibration. Even with my single 1.5v battery 3/4 flat, the disk speed is fast enough to de-resolve the groundglass texture. On the other hand, an oscillating, orbiting or random vibrating groundglass movement can use a much smaller piece of glass.

However to get eough linear surface speed to de-resolve the groundglass texture, the cycle of movement must be much faster. Because the mechanical principle is not balanced in its nature, vibration will occur.

Three main methods of reducing the vibration are desirable,

---- make the groundglass area as small as possible to keep moving mass to a minimum, barely larger than the image frame, which can cause problems with picking up an edge in the image,

---- adding counter-mass to the mechanism which requires very precise engineering,

---- making the excursion of the groundglass as small as possible to reduce the actual motion of mass to its practical minimum and reducing the amount of countermass needed for balancing, which introduces a problem of needing the vibration of cycle of movement to be even faster for the linear surface speed to be maintained, another precision engineering process.

The upside is that all devices not using a spinning disk can be much smaller and lighter and the use of the groundglass material itself is more efficient.

Method of moving the groundlass in the P+S is according to their own literature as I comprehend it, several half cranks with small vee pulleys on them, driven by a common rubber belt by a electric motor.

This requires very precise engineering indeed. Counterweights can be added to the cranks but I don't know if anyone has bothered.

Keeping the movement very small and fast without adding counterweights and adding countermass in the form of a heavy housing is nearly as good a method and less complicated. But the appliance is then heavier than it should be and there is vibration still to be felt and heard.

Quyen's Letus35 as I understand it uses a different mechanical principle. It is elegently simple and requires fewer precision moving parts. Providing the flexible mounts on the pillars or sticks remain located as installed, there should not be movement of the groundglass off the focal plane due to wear. Vibration remains an issue as with the other methods and Quyen's passive method of moving the groundglass by onboard eccentric mass alone cannot be counterbalanced or the movement itself must fail. Counterbalance could be achieved with a second synchonous motor and contra-rotating counterweight mounted to the case however this would be an impracticable and diffucult system to engineer. Countermass is the only damping method that can be applied. However given there are only the motor bearings themselves which can wear, and there being a constant radial load being applied, there are about between nine and twelve fewer potential sources of future noise due to mechanical wear than in the other more complex methods using cranks.

The simple CD-R sized disk and CD player motor provides a very cheap, simple and reliable method of moving a groundglass. It is the groundglass itself which proves to be the most expensive item if real glass is used.

Making your own glass disk is a very hard task. If Tony Ralph can get his method up and running with an equivalent to an AO5 finish consistently around the groundglass, I would be inclined to buy that in rather than making my own. Getting my own groundglasses made and inventing the method cost me a 70% loss rate until I got the method sorted.

END OF REPLY THIS QUESTION


The tumbler method ("cement mixer") of dressing the glasses is a viable backyard method but not anywhere near the best. While a unifrom finish around the entire disk can be achieved with difficulty, getting a dead flat shape across the disk is another matter.

While my disks flicker or show grain effects little more if any than P+S products, they do move the image ever so slightly due to uneven shaping across the faces which cannot be controlled in a tumbler.