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Camera lens filter as sight glass?
My 2kg drum roaster currently doesn't have a sight glass, and I've read posts about using borosilicate glass discs with two different sized hole saws to create a sight glass. While I could take that approach, i can envision that cleaning the glass would be an issue (since it would be held in place with hi-temp silicone).

I had a thought that would instead use a camera lens "step down ring". For instance, a 55mm to 52mm step down ring. I would cut a 54mm hole in the front plate of my roaster, then silicone this threaded ring into that hole. Once that ring is affixed, then 52mm camera lens filters (which are super cheap) could be screwed into place. That would make it easy to unscrew/clean, as well as replace if one ever broke.

Has anyone tried this? At the filter too thin to withstand the heat of roasting? Would the fact that it's not as "flush mounted" have any negative impacts? I'm not afraid of spending the money on this even if it fails. I AM afraid of drilling a hole in the front of my roaster if it fails. :) Any experiences/thoughts would be appreciated.

Step-down ring:

UV filter:

NOTE: There are many other filter sizes I could choose from... 33.5mm, 37mm, 39mm, 40.5mm... Smaller filters might be even more durable, but the premise of my question would remain the same.
You can replace the 52mm UV filter glass with 2" borosilicate disc:
Thanks for that link! I was having trouble finding sources for a 2" borosilicate disc. Are you just suggesting that I use the glass in this link INSTEAD of a lens filter? Or are you suggesting that I knock out the super-thin lens filter glass and then affix the borosilicate to the aluminum housing w/ silicone adhesive?

To me, the allure of a lens-filter approach is two-fold... but using the step-down ring, you only cut ONE hole in the front of the roater... the inner threads of the step-down ring would go inside that hole, but then there's a nice wide, flat flange that has good surface area to adhere to the face of the roaster.
Next, and this is the biggie, the glass would then be THREADED in place... super easy to remove for cleaning!
Actually, I think I've just convinced myself to order that 2" disc from your link above, along w/ a few Amazon lens filter parts. I'll be the guinea pig on this one and post my updates. :)
I meant replacement of a filter glass, keeping the metal mount.
All photo filters I own have removable glass by design, not glued or other fixed mount.
Usually a photo filter metal mount is made as a tube with two threads of same size, in order to allow filters stacking, or placing a cap on the filter.
The glass is fixed with a retaining ring inside the tube, and can be removed.
Edited by renatoa on 03/07/2020 1:44 AM
I had NO idea the glass was held in by a threaded ring! I was even looking at a couple of the filter rings that I have on my Canon camera last night. Thanks for that tidbit. I'm 100% taking the plunge on this. I'll drill a 1-7/8" (47.6mm) hole in the front metal with a bi-metal hole saw. Then I'll get a 46mm x 52mm step up ring. The small 46mm end will fit inside the 47.6mm hold, then the flange will be adhered to the metal with silicone. Then I'll pop the glass out of the 52mm filter and replace it with the 2" glass linked above... that's only 50.8mm, so hopefully the retaining ring in a 52mm filter is at least 1.2mm to make up the difference. If not, a dab more silicone should do the trick. I'll post pictures and a report when it's done. But this will be a few weeks before I'm able to get everything together.
Thanks again!
Just FYI, at the Chemglass site above, the cheapest shipping for the $20 glass disc was $15. I wasn't about to pay $35 for a 2" glass disc. I found one for half that here:
Hmmm... a serious hole in the front plate... some beans will remain stuck in the ring probably...
No other setup possible, to have glass at front plate level ?
What about using a step-down ring in the front plate, so the glass comes flush with metal, more or less... 1-2 mm will not catch beans, but 5 mm will do for sure...
That's a good point... You now have me second guessing the whole lens filter concept. My glass is already ordered. Perhaps I'll just do the "cut halfway through with a hole saw, then all the way through with a smaller hole saw" and silicone it in place. Perhaps I need to just have an aluminum ring machined that has a 1/8" "routered" lip that the class would fit in, then a couple of bolt holes to secure it to the face of the roaster.
I THOUGHT the lens filter held promise, but your thoughtful input makes me think otherwise.
(on a side note, I have 2 different UV filters for my camera lens, and neither has removable glass... it's just a tolerance fit that keeps the glass in place in the AL housing.)
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