Cutting Corners...of a fresnel

[Dave Gesell]

When I ordered my Chamonix 045F-1, I added the fresnel lens to the package. The GG has clipped corners, but the fresnel has 90º corners, and when I check the corners for vignetting, I often don't see much but diffraction (more with the 90mm than with longer lenses). Any tips for cutting the corners off the fresnel? I have clamps, straightedges (including cork backed ones for drafting work), and self healing cuttings matts. For cutting tools I have a fine toothed razor saw and utility knives with lots of new sharp blades, but I have no problem buying something more suitable.

Thanks.

[Jim Jones]

People with more experience with Plexiglas suggest scoring and snapping it. It seems intuitive to use the sharp edge of a utility knife blade for scoring. However, the sharp edge tries to wedge material to the side, and might make a deep score only with difficulty. I'd use the back of the blade tip, and scrape the groove fairly deeply on both sides before snapping the corners off..

[Jon Shiu]

I think it might be difficult to snap off a small piece cleanly, so I would use the saw.

Jon

[BarryS]

I just cut a fresnel for my Primar Reflex and tried scoring/snapping vs. fine tooth sawing. The saw worked much better.

[Cletus]

I made a "fresnel" a while back - a big 9x12 one too, for a Sinar 8x10 - out of a book magnifier. It was a pretty thick and pretty brittle piece of plexiglass, or more properly, polycarbonate.

I scored the edges deeply (and carefully) with a sharp utility knife and was able to snap pieces off a little at a time using a duck-bill type plier. Even the edges, where I was only removing about a quarter inch or less broke without cracking the rest of the fresnel. The rough edges were then sanded with a emory cloth until smooth and clean.

If all you're doing is the corners, I'd think you'd have no problem scoring then snapping. Take ya five minutes.

[Jerry Bodine]

I recently cut some rectangular pieces of 1/8" polycarbonate diffusion material to replace those in my enlarger that had yellowed with age. I used a saber saw with a blade that had 40 teeth per inch. I also used a straight aluminum piece to guide the saw. The cuts were so fine they required no additional sanding, and the pieces were firmly clamped. Had I been concerned about a rough cut, I would've clamped the diffuser between two pieces of thin plywood.

[Dave Gesell]

Thanks all. I was leaning toward the razor saw, and in light of the responses I think I'll go ahead with that plan. If I also have to cut back the GG protector (a much thinner piece of plastic), I should be able to get through it with a fresh blade in a utility knife pretty easily.

[Jerry Bodine]

Dave, if your GG protector is made of polycarbonate (a nearly indestructible material), I expect you will find out what I learned trying to scribe it with a knife designed for scribing plastics in general: I was not able to scribe deep enough to break it off, which is why I chose the saber saw after testing on scrap pieces. The saw cut quickly like going through butter. I was fortunate to already have the saber saw available, as I've used it on many items before for making scrolling cuts (scrolling feature was locked for straight cuts, of course). Here is a video on cutting plastic sheets you may find helpful. And here is a bonus on how to repair and polish scratched plastic. BTW I have no connection to these sites; I just use the business to cut plastic parts to my dimensional specifications while I wait, since they're local to me.

[Drew Wiley]

Most fresnels are made of butyl acetate, which is relatively brittle. It doesn't cut like polycarbonate. Score and snap might work but it's risky, and applying clamping
pressure on the fresnel to do it could damage the fine lines. You could sand it a bit. A very fine hobby saw, japanese saw, or fretsaw would work. I would personally use a true Fein fine-toothed fiberglass blade on my Multimaster (never one of those worthless Chinese knockoff blades, which are likely to shatter the plastic!).

[Jerry Bodine]

Dave, don't be confused. My post #8 was for the GG protector (as stated). For the fresnel, I'd pay close attention to Drew's comments, as fresnels are usually very expensive to replace. Also, I'm wondering why you'd want to cut the corners of the GG protector, since the protector would probably be removed anyway when checking for vignetting.