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Topic: Polishing the sound drum

Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 07-26-2001 04:15 PM

Harry,
Take it to a small job shop and have them grind the drum section only between centers on a Center Grinding Machine. I do this on all my sound head rebuilds where applicable. This makes em like new. Having it hard chromed is also a great idea but you still need to have it ground for best results. Of course you could always get another soundhead for 50 bucks and use that sound drum too. I DO NOT reccommend polishing it at all. Just does not work out well.
MArk @ GTS

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Harry Robinson
Expert Film Handler

Posts: 155
From: Franklin Tennessee
Registered: Dec 1999

posted 07-26-2001 04:28 PM

Well, as far as I can tell, the votes are in. As luck would have it, I talked to a friend who is a commercial pilot today. There is a precision machine shop at the general aviation airport here in Nashville. He says the guy who runs it is sort of cantankerous, but a total perfectionist. I'll print out all of your comments and take them over to him. When I get my results I'll post them on this thread.

Incidentally, I am sending a few frames of some virgin trailers I ran through the system five times. The scratching is almost invisible, but I want to be absolutely sure my system is running as clean as humanly possible. Take my word that in no other facet of my life am I this anal.

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Greg Mueller
Phenomenal Film Handler

Posts: 1687
From: Port Gamble, WA
Registered: Jun 99

posted 07-26-2001 07:12 PM

Mark's right about the center type grinding machine. That's what those holes are for in the center of both ends. And, make sure the shop owner has a few years under his belt

------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/

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Harry Robinson
Expert Film Handler

Posts: 155
From: Franklin Tennessee
Registered: Dec 1999

posted 07-26-2001 10:34 PM

To paraphrase the original Mash, this guy is supposed to be the "pro from Dover."

We'll find out a week from tomorrow. Incidentally, I printed this entire thread out, and I'll take it along for him to read. That way he'll get the gist of what's expected of him.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 07-26-2001 11:42 PM

Harry,

Are your scratches contained to just the middle of the film or are they kinda random?

If they are just in the middle, often the culprit is not the drum but the pad roller for the sprocket just past the drum. If the isolation loop between the upper and lower sprocket is too large, the film will pucker into the middle of the pad roller (this is true on the simplex gear-box soundheads too). The solution is to reduce the size of the isolation loop until the film clears the pad roller (typically it will look like a ")"

Steve

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"Old projectionists never die, they just changeover!"

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Harry Robinson
Expert Film Handler

Posts: 155
From: Franklin Tennessee
Registered: Dec 1999

posted 08-10-2001 01:41 PM

First of all, I'd like to thank everybody for your advise. Brad took the time to look at some of this film and send me some great input.

Most of all I'd like to thank John Ulery for his help in regrinding the sound drum. He told me that the surface had some crowning.

It looks like new. I replaced the bearings as well. The difference with the new bearings and drum is dramatic.

I ran a virgin trailer through the system today, and it looks like we're scratch-free. I looked at it through a magnifying glass while reflecting the surface off of a light. Time will tell, but I think we licked it.

My screening room is approaching a place where I will send pix. I think ya'll will be pleased. All you guys deserve some of the credit.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-10-2001 01:54 PM

Cautionary note some sounddrums had a slight crown to keep the film from climbing off them

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Will Kutler
Phenomenal Film Handler

Posts: 1506
From: Tucson, AZ, USA
Registered: Feb 2001

posted 08-10-2001 04:27 PM

Thanks for that info, Gordon!

So now, the best thing to do would be to put the soundrum on V-blocks and a surface plate and measure the "crown" with a base and dial indicator. That way the O.D. Grinder can be properly adjusted for a taper, or the grinding wheel can also be "dressed" for the proper profile.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-10-2001 06:14 PM

The DP-70 uses a novel feature in that the lateral guide roller pivot shaft is offset by a few thousandths from being perfectly straight with the sound drum. This causes the film to wander to one side and eliminates the film walking back and forth in the scanning beam. If the rollers are in good shape this works very well. I have seen some that do wander all over though. This is very bad for stereo reproduction!
MArk @ GTS

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Josh Jones
Redhat

Posts: 1207
From: Plano, TX
Registered: Apr 2000

posted 08-14-2001 12:10 AM

since no one seemed to point this out....
DO NOT overpolish the drum. the film will slip on startup and cause scratching. you want a surface smooth enough to prevent scratching, but not so slick that the film slips.

Just my $.02

Josh

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-14-2001 09:58 AM

Josh,
The pressure roller(rubber impedance roller) is what provides the friction or keeps things from slipping on startup. There are not many machines that do not have one of some sort. Simplex/RCA uses a rubber roller while others (Norelco, Cinemeccannicca) only provide the pressure to the outer edges of the film, and this is the preferred method. Century and Christie use the Davis tight loop system but also use full contact sound drums. Christie used to use only an edge contact drum but changed(for the worse) to a whole contact drum. All in all over years of use sound drums usually become quite highly polished. This is normal from slipping on startup and slipping again on run-down(if film is still threaded).
MArk @ GTS

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Will Kutler
Phenomenal Film Handler

Posts: 1506
From: Tucson, AZ, USA
Registered: Feb 2001

posted 08-14-2001 02:26 PM

Mark,

I agree with you about the Davis Tight Loop seeming to be the prefered method. However, when I started to rebuild my RCA 1040 I was concerned about scratching prints with the full drum and roller assembly. I was also concerned about possible print scratches caused by dirt being trapped in the indentations in the pressure roller. So, I asked several parties for their opinions about the Davis Loop vs the full drum/roller assembly. One of the parties I asked was Dolby Labs. Everyone has said that as long as the componets are kept clean and properly maintained, then you should not have damaged prints. Everyone has said that the two methods are accomplishing the same task in a different manner. Notice that the Simplex 5 Star is a hybred between the two methods. However, having the film be in minimal contact with a surface seems to logically be better. One thing to note on the full drum/roller assemblies is that the pressure roller should be left open when not in use-to prevent flat spots from forming. On older soundheads like the 1040, this will prevent you from closing the compartment door. But there is a little pin pressed into the compartment door that I removed--so that I can close the door and keep the pressure pad in the open position when not in use. But this may be a bad idea because that pin in the door can act as a safety to prevent the pressure roller from accidently opening while in operation. Also remember that there were several types of pressure rollers used over the years. Metal, felt, and now the rubber with indentations. The rubber with indentations is the last one introduced and is the only new style available and is the preferend style. Wolk makes them and they are around $30.00

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Harry Robinson
Expert Film Handler

Posts: 155
From: Franklin Tennessee
Registered: Dec 1999

posted 08-14-2001 10:39 PM

I thought I might throw in my two cents worth, as long as we're still discussing this situation. The sound drum I received from John Ulery after he had reground it is like a new component. By changing the bearings as well, I'm finally totally scratch-free. The print doesn't look like it's ever been through a projector, and that's not at all the way it was before these changes. So I guess this is one more problem solved.

I know you guys have sometimes between 12 and 25 projectors going at once in your theaters. And you techs...good lord, you have to service hundreds of them. But this huge thing that I refer to as "Black Betty," sits right next to my desk in my home. I nursed her back to health after she sat around in a barn for who knows how long. She was just a rusty hulk. Thanks to a lot of help from everybody, she shows a bright, steady picture again, this time in my screening room. All my friends think she's really cool. Part of me is allowed to be a kid again. Life is good.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-14-2001 11:09 PM

Harry i am glad filmtech helped
On the issue of which damper system is bestwell it opens a can of worms.
The five star is a hybrid that seems to have taken the worst of both designs the Western Electric dynamic rotary stabalizer (the soft loop of the RCA/SH1000) and the hydraulic damper of the Davis tight loop.
When it worked the best was the version in the R2 soundhead that in effect Cineemcanica copied. I have never heard any measurable wow or flutter on any cinemecanica product that had the combined dashpot option and the rotory stabalizer

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 08-14-2001 11:35 PM

Mark, even with the impedance roller, the film will still slip on the scanner drum on start-up or run-down (if the film is still in the machine during rundown) on the SH-100Xs' and RCA's. Weber Synchro-film was another.

When I was with Josh in July, we "cleaned up" (very gingerly) one of his scanner drums.

I will agree with Josh - If they are too glossy, they'll slip more. I heard somewhere that if they are too glossy, they will cause problems and should be replaced or refurbished. I think it was MTS that told me that. I also heard something to that effect in MPO school in 1967.

Paul

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