Archived posting to the Leica Users Group, 1999/04/11

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Subject: Re: [Leica] Re: Problem with M6 scratching film
From: DonjR43198@aol.com
Date: Sun, 11 Apr 1999 10:05:49 EDT

Just a few thoughts about the film scratches this Sunday morning.

In the early days of the M-3's, the pressure plate was ceramic, if my memory 
serves me correctly, but that was changed when static electricity discharges 
on the film were apparently traced back to the film rubbing across the 
ceramic plate thus generating static electricity.  At least that was the 
story in the 1950's.

Polishing the current pressure plate.  It is likely the current pressure 
plate is an aluminum alloy that has been plated or anodized.  Thus, if it is 
polished with fine abrasives such as silicon carbide paper or crocus cloth, 
the surface may be removed.  If the pressure plate is aluminum and the 
anodized surface is removed, the very soft aluminum will likely pick up specs 
of dirt and grit that will really cause a scratching problem.  If the surface 
is anodized, the surface will be harder than tungsten carbide and should not 
be removed in my view.

Another problem is getting the pressure plate too smooth.  If the plate is 
too smooth, the film may "wring" to the pressure plate just like Jo-blocks 
ring together.  Then you would really expect to see scratches on the film 
since the film would stick to the pressure plate and resist moving across the 
pressure plate.

There is a fine line between getting a surface too smooth and not smooth 
enough and each application requires its own specification.

Further, if the black finish is removed, light will travel through the film, 
reflect off the pressure plate and begin a return journey through the film 
from back to front this time thus degrading the film image.

I am looking at the pressure plate on my M-6 that was purchased about 6 
months ago and the finishing marks are NOT in a straight line but somewhat 
circular which leads me to believe the pressure plate was finished on a flat 
lapping machine that employees a lapping wheel to provide the final finish.  
I have just checked my old M-3 pressure plate and the finishing marks appear 
identical to the M-6 so the manufacturing methods must not have changed much 
over the years.

One of the LUG members suggested taping protective covering over the rest of 
the camera while sanding the pressure plate to keep out the grit generated 
from that operation.  If an M camera owner is going to attack the pressure 
plate, I would suggest he or she remove from the camera body the back cover 
on which the pressure plate is mounted.  The left hinge moves laterally by 
pressing the stop pin to the right which should allow the back cover and 
pressure plate to be totally removed from the camera body.  It is likely ALL 
of the abrasive used in the effort to further smooth the pressure cannot be 
cleaned out of the back cover and it will eventually get into the works of 
the camera and really make a mess of things.

If there is a spec of grit on the pressure plate, you will probably be able 
to see it by using a strong light and positioning the pressure plate in 
relation to the light to get a terrific view.  You will likely be able to see 
specs so small that they would not put visible scratches on the film backing.

Why not return the camera to Leica with the scratched film so the matter may 
be properly examined and resolved.  May be that the factory let out pressure 
plates that are either too smooth or not smooth enough.  Bringing it to the 
attention of the company would benefit everyone involved.

It might even be worth a call to see if Leica would send you a back cover 
while yours is in the shop since the back cover is removable.

Good luck with the project!

DonjR43198@aol.com