Could it be that it was never filed properly to begin with? Skillful filing (by someone who knows how to file plates) will compensate nicely for the off-angle projection.

Again, before any filing is done, a target film should be used, to determine exactly what the situation is.

Since the projector is at an angle to the screen, I suspect you are seeing a form of geometric distortion known as "keystoning". In effect, one side of the screen is farther from the projector than the other, so it has a slightly larger image. Minor keystone distortion requires bring the masking in to cover the uneven edge. Significant distortion will require moving the projector to be more centered relative to the screen.

A useful tool for calculating lens focal lengths, screen curvature and distortion factors is available from Schneider Optics. It actually simulates the design of a theatre, and graphically shows any distortion due to the design. Check out their "Theatre Design Pro" software at:
http://www.schneideroptics.com/theatre/desprodn.html

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As to Tom Kroening's problem:
Tom, I would guess by what you said that plates have been switched around from one projector to another. You said that you even switched plates with another projector.

My advice is that all aperture plates should be attached to a chain that is bolted to the wall and will only reach the projector that it is filed for. Maybe a deadbolt so that they can't be switched???

Please remember there are techs who do not file plates out the way they should be for fear of too much spillage. Also, there are plenty of projectionists who are trying to learn. I think the guys here made it pretty clear that this was not something to be taken lightly and he should check with the tech. Still, if he felt it had to be done (many times a manager will give the instruction to do so), at least he's got a few tips and he doesn't go out with a power drill or something crazy.

When our theatre was first opened, we didn't get all the projectors running right until right before opening night. (As usual) Some of the plates were filed rather hastily and there were some 'burrs' on the edges of a couple of them. That's why I recommended using an emery board. It's good enough to get the burrs off but not so aggressive that it'll open up the hole unless you really "have at it". But, again, I assume that the other person has the intelligence to understand things like that.

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1) Check your mirror alignment.
It is possible when aligning the mirror to bring it in so far that you get shadows on the side.
2) Check your apperture alignment. Can you alter the alignment settings to fit the screen evenly? E.g it's auto-stop position.
3) Has your projector moved? It will over time particulary if it is a new installation.

These few points should always be checked before you bust out the files.
If you go ahead and file the appeture and it is one of these things you will waste a lot of time and probably end up with a badly over cut apperture which is very anoying.

If it's none of these, good luck!

Some of the best files on the market for doing this type of work are Grobet tm. They are not cheap, and should be a standard item in any good lapidary store. This company does produce an assortment of non-tapered safe files. So what does this mean? Non-tapered simply means just that, the file body does not have a taper, and all surfaces are perpendicular. Safe means that one or more surfaces of the file body do not have any cutting teeth. This is perfect when filing 90 degree corners! Another feature of Grobets is that they also produce files that have square-non-radiused corners. Another asset when cutting 90 degree surfaces!

A little about file care and safety:

One of the biggest mistakes that people tend to make when storing files is to throw them together into a drawer unprotected. This is the quickest way to dull and damage files. Always store them in their individual pouches so that they do not rub against each other.

Aperture plates are made of a soft leaded brass that produces fine chips that will easily clog a file. One of the oldest machinist tricks on the books is to pre-load the file with common chalk-board chalk prior to filing. This will prevent the file from becomming clogged. A soft tap on a block of wood, and a few swipes with a good file card will remove the debris. Yes, the file needs to be reloaded with chalk every time it is cleaned. Please note that I did mention to use a file card!

A note about file safety:

Never use a file without some type of protective handle, many serious accidents have happened due to people not having protective handles on files!

Always use protective evewear. Never touch your eyes without washing your hands first! This type of brass produces fine chips that can seriously injure your eyes in a heart-beat!

Remember the motto: Take care of your quality tools, and they will take care of you!

Happy filing!

"I came accross this old thread and wanted to add my two cents."

#1 Never press the "submit" button before you finish typing. Your first message said "ravgrgiua". Then you next message was my quote above. This means you had to have pressed the "submit" button at least three times to achieve your post above. The simple answer...just don't press "submit" until you are done. This is not a race, and as such there is no reason to get a post up before anything is typed.

#2 Compose the post in a text editor and then paste it into the reply box via EDIT/PASTE commands.