Cutting GG/fresnel combo (plastic,Wista/Tachi-type)

[darkroom_rookie]

Currently assembling an 8x10. Got a Cambo SC spring back and a ground glass - but with a few caveats. It's slightly larger and needs to be cut to size. There are no markings on it at all, but I gather it's a Japanese product for Wista or Tachihara camera - tough plastic, ground on one side, extremely fine fresnel grooves on the other (except for a small circle in the center). How to cut it properly?

 

[John Koehrer]

Depending on how large it is, it may be easier to sand it down,
cover the field, both sides, with microfiber cloth on the surface and cover the cloth with
a substantsial paper, like grocery bag.
YOU NEED TO PROTECT THE SURFACES FROM THE DUST.
Tape the layered stuff over the entire surface and using masking tape or painters tape folded over the edges
to hold it in place.
You're going to take off about half of the excess from each dimension.
I use a wet or dry paper taped to the bench, using it wet and around 280 grit. Holding the glass vertically, rub it back & forth
until you reduce the edge a bit. Flip it over & keep on going for the other sides.
I finish up with a 400 or 600 grit just to polish it up.

 

[darkroom_rookie]

Well that's a pretty good idea - but it is a large piece. I.e., larger than the needed size. Sanding would take a while. I guess I should probably get one of those special Japanese knives for plastic scoring and learn how to use it.

 

[gone]

This is how I cut plexi for frames and such, and it should work for your application assuming you have enough to cut off. If you're only cutting off a very small amount you'll have to try a different method.

Take the fresnel and lay it on a table covered w/ a soft cloth, grid side up. Mark how much you want to cut off w/ a felt tip pen and ruler, then take a box cutter w/ a new blade and find a 1x1, 1x2, etc. Lay another soft cloth on top of the fresnel where the 1x1 will be laid down to give you a straight cut, and either gently clamp it down on each end, or have someone on the other side of the table reach over and hold the board down firmly right at the edge of the fresnel on both sides. Clamps are preferred. Then slowly cut the line on the fresnel. Try to do it in one cut because if you do it twice or more the blade will usually walk outside the first line. You really need a straight, somewhat deep score

Once you have that done, scoot the screen to the very edge of the table (you need a straight edged table for this), lay some extra cloth on top of the fresnel, and either go w/ the clamped 1x2 or have your helper hold it down. Position the fresnel exactly on the edge of the table, take two pair of big pliers, grip the fresnel on the piece you wish to break off, and snab it off w/ a quick and firm downward movement. If it breaks in places and doesn't make a clean snap all the way across, no worries. Grab the pieces w/ the pliers and snap or cut them off. Needless to say you need a spotlessly clean table for this, and lay the fresnel on top of a thin cloth on the table if you have any doubts. The other option is to use a large picture book. Scribe your fresnel, open the book to the middle, put the screen in there, close the book, have someone hold it firmly closed on top of your table, and try the snapping off process.

If you only have a small edge to trim, John's way will work better. I would use a file, but sandpaper will work. Whatever you do, just make sure to have the fresnel firmly clamped down somehow, and protected from sanding/filing dust.

 

[shutterfinger]

Score at least 1/2 way through using a plastic scoring tool http://www.amazon.com/Hyde-Tools-45...247942&sr=8-3-spell&keywords=pexiglass+cutter and a straight edge.
Pull the notched end toward you with slight pressure on the tool.

 

[darkroom_rookie]

Gentlemen, thank you for your suggestions, most helpful. I have obtained the needed tool, and I was told this one is among the better models. Wish me luck and a steady hand.