DIY Borosilicate Focusing Screens

This is not so much of a step-by-step ‘how to’ as a recount of information and material sources I used to allow me to grind focusing screens for my newly purchased 4”x5” Chamonix 45H-1 and the optional 5”x7” back. The 4x5 back had a combined GG with Fresnel and the 5x7 came with just GG.

The reason that I started down this route was that I had purchased a borosilicate GG screen for my Ebony 4x5 from Steve Hopf years ago and found that it really was an improvement over the original Ebony screen (GG + Fresnel). I found it just as bright and, without the Fresnel, easier for me to focus. (Please, no replies about Fresnel. I know that they work very well for some - just not for me).

Anyway, I tried to contact Steve again after I got my new Chamonix this year but no luck. But I found several sources of instructions online on how to do it that seemed reasonable for me (most useful for me was from a LF guy named Dokas – sorry, asked him if it was OK to share a link here but have not heard from him so you’ll have to find it yourself). Decided I’d try. Can’t scare me, I ground my own 6” telescope mirror when I was 13. I figured this had to be easier – and it was – once I got the right materials.

My first lesson was NOT to purchase borosilicate glass made for 3d printers. This is plate glass made to handle the high temperatures those machines create. Bought 2, broke 1 and only then found out why. (Will be happy to sell you the other 200mm x 200mm x3mm piece in original wrapping for shipping costs if you can use it – PM me.)

Anyway, I then found this outfit called Mountain Glass that sells uncoated sheets of the right size. Cuts like regular ‘window’ glass and, at 1/16” thickness, was perfect for the task. https://www.mountainglass.com/Borofl...eet-1-16-Thick Used regular ‘hardware store’ glass cutter and it was fairly easy, even given my lack of experience.

So I had the blanks, now needed the grinding grit. Some instructions I found warned against silicon carbide grits as several authors found that the hardness of the grit caused the glass to micro-chip and require much more end stage grinding to remove them.

The consensus seemed to be that Aluminum Oxide grits were better as they did not seem to cause this problem. Based on this, and the grit sizes suggested as best, I purchased small bottles of F800 and F1200 from Amazon sold by an outfit called Gritmatic. Since the larger 5x7 screen used maybe 3/4 tsp. of grit, these 7oz and 5.5oz. bottles are a 30x lifetime supply. Probably overpaid but I’m an Amazon-oholic.

Anyway, after 1/2 hr. of grinding w/ F800 and a little less than 3/4 hr. with the F1200 for each screen, I was done. I’m very happy with the results. Compared with the original Chamonix screens, they definitely seemed brighter and were easier for me to focus.

PS. If you want to just use regular glass instead of borosilicate, and cut the cost way down, this post should work for you as well.

Hope this helps you if you are thinking of doing this project. I found it definitely doable.
Best, Jim

Good write-up, Jim. I've been using aluminum oxide to grind my screens for awhile, and the results have been good.

In case there are a few who don’t know of the advantages of borosilicate:
1. It makes for a brighter screen as it transmits more light than standard glass
2. It is harder so will not break as easily (but can if impacted hard enough).

Best, Jim

What did you use as your grinder-another piece of glass?

Yes. As you could see, the glass from the vendor was not rectangular. In cutting out the screens, I had a couple odd size pieces left over and used one.

As boro is harder than regular glass, I felt it was better to use like to like for the grinding tool. Not sure that was required but...
Best, Jim