Further Refinement of Plexiglass Ground Glass

[Ari]

Hi,
In the past, I would buy a ground glass and have it ground further to get a very fine grind, which I prefer to have for better focusing.
My new camera came with a plexiglass GG; it's very good for aerial viewing, but the grain is quite coarse for my taste.
Can I apply the same treatment and techniques to the plexiglass in order to achieve a finer grain?
Or do I have to be extra careful with plexi?
Thanks in advance.

[Kevin Crisp]

I've had good luck with valve grinding paste, you can get it at auto parts stores, I think Permatex brand in a tube. A 2000 grit boat polish like Aqua Buff might be too fine, but they make 1000 too.

[jnantz]

hi ari
i use an asymetrical sander to sand my plexi, it works but might be not to your taste.

[Peter De Smidt]

The valve grinding paste I tried must've been coarser than Kevin's. I didn't get good results, but I don't remember the brand.

I recently made a couple of plexiglass screens, and it was much more prone to scratching than glass. It's possible that my plexi just wasn't flat enough.... I used 3 micron aluminum oxide. The center ground ok, but the edges did not. Kirk has the screen, I don't know if he tried focusing with it.

http://www.lortone.com/abrasives_polish.html
Aluminum Oxide Pre-polish
#591-021 14 oz. $5.75

[Ari]

Thanks, John, the sander is probably more expedient, but I'd rather use the traditional method.
Kevin, thanks for the suggestion, I'll look into valve grinding paste.

[Bob Salomon]

A lot of guys use to just use tooth paste.

[Ari]

A lot of guys use to just use tooth paste.

I prefer hanging a pine-scented car freshener from my camera. :)

[unixrevolution]

I prefer hanging a pine-scented car freshener from my camera. :)

I should try that with my 8x10, it still has pretty bad halitosis inside the bellows.

[el french]

What technique are you using to grind the plexiglass? I recently watched a video about lapping, which unfortunately can't locate now. It showed the difference between lapping where the abrasive was embedded in the lap which was softer than the material being lapped and where the abrasive stayed on the surface of the lap. The later produces a rolling motion in the abrasive grains and each grain produces short cuts. The former made longer cuts. I wonder which would work better for glass and plexiglass.

[richardman]

I see we have the same issue ;-)

[Peter De Smidt]

For a small glass, I put some compound on a big and thick piece of glass. Add some water. Mix to a paste. I double-sided tape the screen to a flat block. Place the screen down on the base and move the screen in a swirly pattern across the base, regularly changing directions and rotating the screen in small increments every now and then.

[Drew Wiley]

Oh gosh. Don't get me started, of all people. I'm sitting smack in the middle of the polishing candy store. But nobody seems to get past me without spending at least a thousand bucks, whether its your yacht or your groundglass.

[maxotics]

Let us not forget the under $1,000 solution 153414

[Ari]

Reviving this thread as I now need a replacement plexiglass GG for my Wehman 8x10.
On the last plexi/gg I used an orbital sander with 1000 wet/dry sandpaper. It worked nicely, the grain is much finer, but it did leave a mild hotspot.
Now I'd like to find someone who can make a fine-grained plexiglass GG for my 8x10.
Anyone out there?

[cowanw]

Fine grain = hotspot; coarse grain = less sharp image.
Choose one.
Or I have often wondered if a coarse grain could also have a smaller fine grain centre circle.

[Bob Salomon]

Reviving this thread as I now need a replacement plexiglass GG for my Wehman 8x10.
On the last plexi/gg I used an orbital sander with 1000 wet/dry sandpaper. It worked nicely, the grain is much finer, but it did leave a mild hotspot.
Now I'd like to find someone who can make a fine-grained plexiglass GG for my 8x10.
Anyone out there?

That is why a fresnel is necessary

[Ari]

Fine grain = hotspot; coarse grain = less sharp image.
Choose one.
Or I have often wondered if a coarse grain could also have a smaller fine grain centre circle.

Not always.
A friend of mine would regularly take in my GGs for refining, they could be stock or specialty GGs.
He worked it so fine that the grain was almost imperceptible. The images really snapped into focus and there were no hotspots, ever.
The screen was darker than normal, but that was ok with me.
He has some kind of voodoo with grinding glass, but plexiglass is a different animal.

[Ari]

That is why a fresnel is necessary

Not gonna happen on a Wehman.

[cowanw]

Not always.
A friend of mine would regularly take in my GGs for refining, they could be stock or specialty GGs.
He worked it so fine that the grain was almost imperceptible. The images really snapped into focus and there were no hotspots, ever.
The screen was darker than normal, but that was ok with me.
He has some kind of voodoo with grinding glass, but plexiglass is a different animal.

Please ask him his secret. Mother Nature wants to know too.

[Ari]

Please ask him his secret. Mother Nature wants to know too.

He's a mining engineer, so he probably has access to fairy dust.
He's never told me specifics, but it is a great screen to have, if a bit dark.

[Bob Salomon]

Ari, all lenses are hot, center to edge. Non wide angle lenses have about 1/3rd stop fall off from the center. So, unless someone ground a screen to encompass this fall off you can only reduce or eliminate it with a proper fresnel screen

[Eric Woodbury]

1600 grit wet/dry. Any finer and it is clear.

[Ari]

Oh, I realize that, Bob, but there's no provision on a Wehman to fit a Fresnel. I don't like using them much, anyway.
Eric, I went up to 1000, which I feared would clear the glass to its original state. I dared not go further.

[Drew Wiley]

There are just soooo many kinds of "wet/dry" paper, as well as options of "plexiglas" or acrylic, and even differing polishing solutions (water, glycerine, paraffin oil, salad oil - seen it all), that one needs to be more specific, unless you're trying to retain a personal trade secret. And then if you go to power polishing, add a whole other layer of significant variables. Even grit ratings differ between not only type of abrasive, but systems of measurement. In the finer grits, the European standard is different from the US; and there are even other standards. Kinda like metric versus Imperial systems, then throw in something wacky like Whitworth. That's just with sheets abrasives. Powder or lapidary grits, a whole other game. But for me, fresnels are much easier to classify - they all go into a single "don't like any of em" column in my book.

[nerologic]

For plexiglass, I really love the brightness and detail I get when lightly sandblasted. Takes about 45 seconds to do an 8x10 sheet nicely. The garnet grit in the blaster had been used a bit, and kept the pressure low.


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[Tin Can]

I made an 11X14 'Plexi' with DA and 1000 grit

For for a camera I never use, it is very useable

However I prefer my DIY glass

Maybe this year...

[John Layton]

Ha! I still have my tube of grit that was a gift to me from Will Whitaker all those years ago...

Problem most of the auto parts store variety of Permatex valve grinding paste (mentioned earlier) is that it is a "multi-grit" paste - based on the theory that as one lapps ones valves (as I have on several occasions) the coarser grits will get pushed out from the contact surfaces as the grind progresses, ending with the finest grit in the paste. Very difficult to get this to work properly (coarse grit gets sucked back in, leading to over-grinding) and for all of my future engine work I'll be using single-grit pastes...thank you very much!

I suspect that for the purposes of grinding plexiglass for a ground glass, a single grit paste should also be used. Valve paste also comes mixed as a water or oil based formula...and I'd suggest water based for the ground glass.

When I grind my own glass screens, I use my grit from Will W. (made for grinding telescope mirrors, which Will had actually done) and I believe it is 500 grit corundum (or carborundum?). At any rate...I could imagine that for grinding plexiglass, the suggestion of using valve grinding paste might actually be better than using the typical dry grit diluted with water scenario. Just a hunch!

Then again, in pinch as was pointed out...if you can lay rows of magic tape onto the plexiglass carefully enough, and do this horizontally - you will have a not-too-distracting series of faint lines with which to check your horizons!

[Tin Can]

I used DA discs dry, they did clog, but took only 3