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shooting s16 on unmodified 16mm mags with Arri SRII


Luke Randall

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Inverse is a tricky word there.

Proposition...Unmodified mags => No scratches
Inverse proposition...Modified mags => Scratches.

I think you just mean a simple negation...Unmodified mags => Scratches.

 

Anyway, hard to prove that something never happens. And if it happens rarely, how often does it have to happen before you should worry?

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Just me having fun. It would be nice to have all the real data, so you knew how often it actually happened (scratches from unmodified mags).

 

Have you carefully scratch tested the unmodified mags you want to use?

 

No. I have two s16 mag's, I want to get a third so I can have a backup mag/low light (500t) mag on set. There are a lot of unmodified mags going cheap on ebay.

 

Plenty of people say they shoot s16 on regular mags without issue, but there also must be reasonable motivation for modding the mags in the first place.

Edited by Luke Randall
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I recall being told that when the first S16 SR conversions were done the mags were not altered, and in most cases there was no scratching, but with the amount of 16mm being shot professionally even a small percentage of footage getting damaged was a big deal, so mags started to get converted too. Certainly if unconverted mags are not being regularly serviced the risk of scratching will increase. It's mainly a case of turning down one shoulder of the film rollers in a lathe, although when I was at Panavision Sydney in the 90s we were modifying parts of the film guide too.

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Scratching does occur, and you need to turn down the rollers as well as cut back the film guides. Will every un-converted mag scratch every roll of film? Definitely not. But do you want to take a chance that the one spectacular take you got on that one shot, ends up being scratched when you process and scan the film? Probably not.

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Now you know why.

 

Ha, maybe. I'm not sure even s16 SR mags have much market value at the moment. I picked up a SRII s16 with ccd tap, 2 mags, 3 batteries for $1500.

 

Hopefully the prices will encourage more people to use film. You can barely get a couple days rental on an alexa for that price.

 

 

Scratching does occur, and you need to turn down the rollers as well as cut back the film guides. Will every un-converted mag scratch every roll of film? Definitely not. But do you want to take a chance that the one spectacular take you got on that one shot, ends up being scratched when you process and scan the film? Probably not.

 

Indeed, I do not. I'll either have to get one modded or rent an extra mag. Cheers

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I recall being told that when the first S16 SR conversions were done the mags were not altered, and in most cases there was no scratching, but with the amount of 16mm being shot professionally even a small percentage of footage getting damaged was a big deal, so mags started to get converted too. Certainly if unconverted mags are not being regularly serviced the risk of scratching will increase. It's mainly a case of turning down one shoulder of the film rollers in a lathe, although when I was at Panavision Sydney in the 90s we were modifying parts of the film guide too.

 

Are the roller steel or Aluminum? Is the part of the roller being turned down in contact with the film? Or does it need to be reduced in size so it doesn't contact the film at all? If the former and they are steel, then it's likely that the roller would need to be ground (on a cylindrical grinder) to size, not turned on a lathe to achieve an acceptable finish. If they are Aluminum, you should be able to achieve an acceptable finish on a lathe.

Edited by JD Hartman
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You will lathe the soundtrack area of the roller's contact surface away but leave just a little narrow bit of it to support the film edge, i think it's half or quarter of a mm or something like that

 

Machinists don't "lathe" anything, we turn down material to a desired diameter. Not knowing the type of plastic, might be difficult to get a smooth finish. Most plastics are thermoplastic, acrylics as cast.

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Machinists don't "lathe" anything, we turn down material to a desired diameter.

Sure, just like cinematographers don't "lens" a project, they capture images with the desired characteristics.. ;)

 

Language is a fluid thing, sometimes technical accuracy is important, other times not so much. It's also worth remembering that this is an international forum, and English is not everyone's first language.

 

I don't know exactly what type of thermoplastic Arri used for the rollers, but they turn down just fine.

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Sure, just like cinematographers don't "lens" a project, they capture images with the desired characteristics.. ;)

 

Language is a fluid thing, sometimes technical accuracy is important, other times not so much. It's also worth remembering that this is an international forum, and English is not everyone's first language.

 

I don't know exactly what type of thermoplastic Arri used for the rollers, but they turn down just fine.

 

Well as a member of a professional machining forum with many international members, I can say that none of the machinists from abroad "lathe" anything either. A lathe is a specific machine, not an operation.

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Should I use the term machining for that, in Finnish it is normal to specify also the work process if it is known (eg do you use lathe or milling machine for removing the metal or in this case plastic, and if you use multiple tools for the job it is usually said that you are machining the material. So, in Finnish you would definitely "lathe the soundtrack area away". I have used metal lathe and milling machine so I know the process quite well but don't know all the pro terminology for all the processes)

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That said, I think it is very important in this case to specify which kind of tools one would need to modify the rollers so one would understand the amount of work needed and could then find out which kind of place could do the actual mod and order the service more easily

Edited by aapo lettinen
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What tools would depend on how the roller was to be held in the lathe, if both end were to be turned in one operation or turned down one end at a time with then the roller being flipped. Then there are tools and tooling.....will it be held in a four jaw chuck or collet and live center? Will they need right and left hand turning tools? These details are superfluous to the customer, all you need to do is bring the rollers to a machine shop with an accurately dimensioned drawing of the modified roller. How they accomplish the operation is their business. Most machine shops would not want you telling them what to do anymore than a heart surgeon would want you telling them how do perform your transplant.

 

Conversely, telling them, "leave just a little narrow bit of it to support the film edge, i think it's half or quarter of a mm or something like that", would have them decline the job.

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Conversely, telling them, "leave just a little narrow bit of it to support the film edge, i think it's half or quarter of a mm or something like that", would have them decline the job.

I calculated it SHOULD be around 0.5 - 0.6mm but I hoped someone like Dom could tell the exact width of the support edge, it is very difficult to find it from the web.

I have some old footage shot with SR3 which I could use to measure the dimensions of the SR3adv gate and determine the width available but that would be unnecessary because someone already knows the exact dimensions and could just bring them up <_<

 

The point was that one will need a metal lathe for the work, whether if letting a pro machinist to do the job or by trying to do it by yourself (that is entirely possible though the end result may be not absolutely perfect) . first it is of course necessary to find out the exact width of the edge but after that it is very straightforward unless the guides have to also be modified

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I answered your post as best as I could, injecting some realities of the job into it. A person wanting to modify the magazine rollers would best examine the correct mag and take the dimensions directly from that. Any critical dimension needs to be specified with a tolerance, plus 0.00 or minus .05, never a range. So you can't measure a piece of film stock and guess this dimension. If I were doing the job, I probably would make new rollers from a suitable plastic and just ship them out. It eliminates the delay in waiting for the old part to arrive and the possibility that the old plastic has gone brittle with age.

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you can measure a S16 film piece and find out how wide the area between the film edge and the frame edge is, and then just decide how much support you will need on that side when knowing how much there is generally available. theoretically you could also made the support side a little wider than the gate edge < > film edge area because your ground glass markings always crop a little bit and thus you probably don't use the last 0.1mm anyway.

I would maybe first machine the edge to 0.6mm or 0.7mm and then add a 45° slant so that the actually support top would be 0.5 or 0.6mm wide. but difficult to say without seeing the actual rollers and I'm not a pro machinist or know how brittle the material is. there is also different ways to do this mod correctly so as long as you don't get pressure sensitisation marks or scratches it is "whatever works" based I believe :lol:

the film guides would be tougher to modify, I believe you would need a cnc milling machine for the work at least?

 

Luke said he already has two S16 mags so I suppose it would be possible to measure actual modified rollers to find out the dimensions needed :)

Edited by aapo lettinen
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I don't believe that you can't reverse engineer dimensions that way. You'd need a much larger sampling than two mags to determine minimum and maximum dimensions. What would make you think any modified mags are spot on in their dimensions? They may have just gotten lucky using an approach similar to what you are suggesting. It works....but it isn't right or perfect. Good luck what direction you choose to take.

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A calculation should be an OK start point. I think Aapo's calcs are a bit off.

I assumed 16mm stock was 16, measured some old stock, close enough to call 16.
Panavisions old rental catalogue gave me camera gate widths...
Regular 16 width = 10.26
S16 width = 12.35
S16-R16 width = 2.09mm
Sprocket side gate to film edge = (16-10.26)/2 = 2.87mm
Mod side gate to film edge = 2.87-2.09 = 0.78mm

 

There may be basic design issues that mean the remaining contact surface, say on a roller, for eample, is less than that. Like if the film has an edge clearance to end flanges on a roller, so the image area floats relative to the contact surface.

 

One can find out what some camera techs have used or specified for that dimension. On the Ecair community site 0.7mm was given, but it not accurately specified and I don't know who wrote that. I would follow the practice and specifications used or given by a camera tech in the past. Send Dom a beer voucher (gift card) and he might find it for you.

 

I think any machinist good with small parts, or even a skilled hobbyist could do the rollers if they know what they are trying to achieve. My guess is that one needs a collar penetrated by a little mandrill with a threaded end so one can clamp the roller to the collar and fit the collar to the chuck, but it's just a guess.

 

Measuring existing modded parts is a valuable and common step, I think, if one is trying to specify these mods. But the measurements need some interpretation.

 

Did anyone read the SR conversion notes on Cinematechnic's pages. They are pretty hard core about it.
http://cinematechnic.com/super_16mm/super_16_conversion_16SR.html

 

Reading that, buying a used S16 mag sounds like a better idea.

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Yes, no need to re-invent the wheel. Modifying these mags was common practice at places like cinematechnic. I believe Arri had some input too.

 

Whether anyone is still doing it is another question, but i'd say just picking one up already modded would be more cost effective. I've seen them go for as little as $150 recently....

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