PDA

View Full Version : Grind your own ground glass



Tom Persinger
03-06-2006, 10:04 PM
To make your own ground glass, I use Sears Valve grinding compound (small white vial), some water (in the yogurt container, (if you haven't had the Stonyfield Farm Whole Milk French Vanilla go out and get some it's delicious) the piece of glass you want to be your ground glass -in this case the large one on the foam (to keep it from slipping once it gets wet) and a piece of glass to grind with -i put a duct tape "handle" on this piece to aid in gripping.
Attached files http://f295.f295.org/uploads/im002243_3124.jpg (http://f295.f295.org/uploads/im002243_3124.jpg)

Tom Persinger
03-06-2006, 10:05 PM
Here's the glass with a dollop of grinding compound and some water to help lubricate it, the 2nd shot is about 15 minutes into the grinding process. You'll want to work the top piece of glass in small circles all over the future ground glass. It's the same sort of motion that one would use waxing a car.

Attached files http://f295.f295.org/uploads/im002246_9006.jpg (http://f295.f295.org/uploads/im002246_9006.jpg) http://f295.f295.org/uploads/im002250_6689.jpg (http://f295.f295.org/uploads/im002250_6689.jpg)

Tom Persinger
03-06-2006, 10:09 PM
Time for a 1st rinse in the laundry tray, and then an inspection. It's hard to see on these images but at this point it was quite uneven, so back for more grinding.

Attached files http://f295.f295.org/uploads/im002251_9193.jpg (http://f295.f295.org/uploads/im002251_9193.jpg) http://f295.f295.org/uploads/im002252_1775.jpg (http://f295.f295.org/uploads/im002252_1775.jpg) http://f295.f295.org/uploads/im002253_7938.jpg (http://f295.f295.org/uploads/im002253_7938.jpg)

Tom Persinger
03-06-2006, 10:10 PM
After about 25 more minutes of grinding you can see that there are still some minor areas of uneven grinding -in this image near the middle top is most obvious... so yet more grinding!

Attached files http://f295.f295.org/uploads/im002255_5439.jpg (http://f295.f295.org/uploads/im002255_5439.jpg) http://f295.f295.org/uploads/im002258_3695.jpg (http://f295.f295.org/uploads/im002258_3695.jpg)

Tom Persinger
03-06-2006, 10:16 PM
It ended up taking about an hour to grind this to a mostly even state. There's still a minor imperfection in the top that probably another 15 minutes of grinding would fix, but I don't plan to use the entire piece, will most likely cut it down to use as several smaller pieces.

Couple of things to keep in mind:
* Maintain even distribution of grinding over the entire sheet of glass as you work.
* You'll have to periodically add grinding compound and some water as you work. When it feels like there is too much friction add some water, when you don't hear any "grinding noise" add more compound.
* Make sure that the entire piece of glass your grinding is evenly supported - I forgot this part and added the second piece of foam after the first rinse
* Don't be afraid to rinse periodically and inspect your progress. It's an easy water cleanup and rinsing to check on things aids the process

Hope this helps.
tp Attached files http://f295.f295.org/uploads/im002260_9090.jpg (http://f295.f295.org/uploads/im002260_9090.jpg)

moot
03-07-2006, 12:11 AM
Thanks for the tutorial, Tom. I need to try this.

theothersimon
03-07-2006, 06:36 AM
I recently had to make a new piece of ground glass for a project. I tried two methods, one pretty much followed the same process as yours.

The other down 'n' dirty approach I tried was to get an old CD Jewel case and some fine grade wire wool. The results from this were surprisingly good. And very quick.

staft
03-07-2006, 08:00 AM
wax on, wax off. wax on, wax off. you are ready, grasshopper.

thanks, tom. if i ever gird myself to try this, i hope the results are as good. are you bearing down at all, or just moving the grinder without pressure?

Tom Persinger
03-07-2006, 12:03 PM
just a little pressure.... you don't need to apply too much. the grinding compound is fairly abrasive just on it's own.

murrayatuptowngallery
03-07-2006, 02:41 PM
I've never broken a piece of glass (with this process), but someone somewhere usually says avoid too much pressure. Too much finger pressure can cause uneven pressure on the glass.

Obviously valve grinding compound does grind, or they wouldn't make it that way, but I wonder if the fluid in the compound works against the efficiency of hand work.

I use plain dry SiC grit (or experimental mixes of SiC and Al2O3 (if I have the numbers right) and water. I don't think I even spent an hour on my 20"x24" piece!

I do 4x5 in probably under 10 minutes. Maybe I get more of a workout.

I use a hard flat surface lined with damp kraft paper to keep the bottom piece from sliding. I wonder how much the foam underneath disperses the pressure.

Maybe Tom's glass is more relaxed than mine and passes that on to the photographer.

moot
03-07-2006, 04:20 PM
Murray, what grit do you use? Have you tried different grits?

brainwagon
03-07-2006, 05:07 PM
In grinding telescope mirrors, we generally use either silicon carbide or aluminum oxide as grinding abrasives. If you have two pieces which are very nearly flat (more on this in a second), then I'd recommend using either #400 or #600 aluminum oxide and just enough water to make it flow. I might think about getting some double stick tape and attaching a piece of wood to the glass as a handle, to keep the piece from warping under your fingertips. Use fairly short grinding strokes, with probably pretty mild pressure, and rotate the both pieces semi-regularly to help ensure even grinding.

If the glass is reasonably flat, you can tell by cleaning two pieces of polished glass and placing them face to face against each other, and looking for Newton's Rings (interference patterns) using a flourescent light against a dark background (certain bug lights work even better). If you can see a countable number of dark and light bands, then the surfaces are probably flat enough to grind against each other with #400 grit. It might actually be faster to grind with #400 first, and then #600 than to try to do all the work with #600 grit.

(incidently, these grits are numbered in grains per inch).

staft
03-07-2006, 10:35 PM
brainwagon, welcome! and remember, there is much knowledge in useless pleasure, too...
i'm not sure what is involved, but more information on telescope optics/lens and mirror making would be very interesting on this forum. if it is a homespun possibility to make lenses, you could start a stampede.

ImageMaker
03-07-2006, 11:20 PM
Really, for the tolerances needed with ground glass, warping from finger pressure or heat isn't a big problem (though warping the work piece on that soft foam bed is likely why your example piece had that slow-grinding area, Tom). You most definitely wouldn't worry about Newton rings to test for flatness when you're just trying to convert a piece of float glass made for a window into a ground glass for your next camera project. Even grinding is good, though, so the rotating part is still worth doing (and it's no extra effort).

However, having ground a couple telescope mirrors myself (one 6" mirror ground through 220 grit in 1977 as a high school project, and an 8" fully ground, polished, figured, and aluminized and in my main telescope since 1999), I can say that you MUST have the water for the process to work correctly, and to protect youself. First, the water keeps the grit from sliding off the glass almost immediately; second, it helps the grit particles roll instead of sliding (which makes them chip out chunks of glass instead of just scratching). Third, the water traps the glass particles chipped off by the grit, so you don't wind up breathing them (which would be very bad). And fourth, the water lets the grit make a gentle grinding sound instead of something that strongly resembles fingernails on a chalkboard... :o

staft
03-07-2006, 11:45 PM
back in my printmaking days, we ground large slabs of bavarian limestone flat for lithographic purposes using a levigater- essentially a grinding disk with offset pivoting handle. the job went fast and seemingly accurate. does anyone know of something like this for glass? would it be too coarse to use a smooth disc and grit, just for the mechanical advantage and momentum?

Tom Persinger
03-08-2006, 09:20 AM
There are many ways to cut tofu, this is just my method. Thanks for everyones input and suggestion. Next time i'll not use the foam (it was handy so i grabbed it, i'm sure you can tellthat i shoved everything off that corner of my workbench to get the space for this ;) )

ok, who's next with a detailed tutorial (thanks for the tripod piece marv)
tp

murrayatuptowngallery
03-08-2006, 07:05 PM
I got two tubs of dry SiC in an eBay barter from a rock polishing guy.

One was a blend of something like 220/300. It was fast but I was sure that there was something to be gained by going to the finer stuff. I don't have it in front of me; I seem to recall it's something like 400/500 blend.

It works nicely. More water seems better than too little. It tends to freeze up (friction, almost glue) as the grit turns to mud. I have seen no problem in erring on the side of more water. I suppose when the grit washes off you know that might have been too much water.

Then I read a reprint of an article from some photographer in Ann Arbor MI who finishes with fine ALuminum Oxide, claiming it doesn't chip the glass and you get brighter glass.

I talked some grit company into sending me a sample of 5 micron (roughly 1000-1200 grit) Aluminum oxide. I went straight to that and it seemed more like polish. I spent a LONG time and my arm was getting sore, and of course I didn't have the sense to try something small - I tried a roughly 7x10" piece.

It's nearly transparent and I can'tget the 'ripples' out of the glass. I read there can be 'waves' in flat glass (it's not 100 year old farmhouse window glass that physics teachers say 'flows' due to gravity as a supercooled fluid). I use picture framing glass which is 'float glass', made by floating molten glass on molten tin. Once in a while I see a bad piece with a spot of tin on it.

So the 5 micron Al Ox alone was too far in the wrong direction.

Someone told me glass doesn't sand like wood and you can't increase grit number to get rid of larger grit scratches. I'm nto sure that makes sense because that's exactly what people do when making telescope optics, right?

Last piece I made I just guessed at a blend of the 400/500 SiC plus a couple dashes of the 5 micron Aluminum Oxide. I argue that mixed grit will give me an average smoother grind but I haven't done any measurements. I was trying toget between the two extremes of what I had.

Next time I'll keep track of what I mix.

I mainly tried making piecs to fit each folder camera I had to check focus but I must have a problem combining befocals and a loupe because I couldn't get anything consistent.

As far as LF glass experiments, I have so many cameras started & not finished I had to be content with the image on glass...haven't proven out one grind over another with photo comparisons.

Murray

moot
03-08-2006, 07:59 PM
Brainwagon & Murray: thanks for the info. I have some 400 SiC so I'll give that a try.

ImageMaker
03-10-2006, 12:27 PM
You're right, Murray, changing to progressively finer grits is *exactly* what one does when grinding an optical surface. On my telescope mirror, I started with 60/80 SiC grit blend, then went to 120, 320, 15 micron aluminum oxide, 5 micron, 3 micron, and then I was ready to put sticky-back polishing pads on the tool and use cerium oxide rouge to polish back to an optically smooth surface before pouring a pitch tool for figuring.

Starting from float glass, where you don't need to make a big change in the shape (I ground away almost 1/4" depth in the center of my 8" diameter mirror), you could probably start with 320 or even 500 SiC, then once the glass is evenly frosted change to 15 or 10 micron and finish at 5 micron for the brightest glass (actually, from 500 you could jump directly to 5 micron). The SiC will cut a lot faster than the aluminum oxide, but the alox will leave a smoother surface for a given grit size (so even though 500 and 10 micron aren't much different, that's a good jump). Best result in least time, I'd suggest 320, 15 micron, and 5 micron. By the time you have an even frost in 320, you'll have all the waves out of the glass, then it's just a matter of reducing the average pit size so the glass doesn't look too grainy (and it'll get brighter as it gets finer and smoother, too). An 8x loupe makes a nice inspection magnifier to check that you don't have any big pits hiding among the smaller ones -- when you don't, you're ready for the next grit, or you're done, depending where you are.

staft
03-10-2006, 05:05 PM
pardon my ignorance, but how do you get the initial telescope mirror or lens shape?

earlj
03-11-2006, 07:51 AM
staft:

You start with two flat glass disks. The grinding is done with the disk offset - the natural consequence is that the top disk is ground down more in the center, and the bottom disk is ground down more around the edges. After many hours, you are able to make the mirror into a very accurate paraboloid that will focus parrallel rays to a point.

ImageMaker
03-11-2006, 08:11 PM
Actually, in my case, I used a cast Pyrex blank (like a steep-sided pie plate only with the pie well filled in with glass) and a tool made from "field tile" (sold in mats to lay in a shower as a non-skid surface) and plaster of paris. In any case, generally, you buy the blank already cut round to the correct diameter, and cast or drawn to the correct thickness. The cast to size blanks like the one I used are hard to find now, it's much cheaper to use a water jet cutter to make circular plates out of a cast sheet of Pyrex.

constantine
03-27-2006, 04:02 AM
Hi,
i recently made a 8x10 gg using the glass out of a old picture frame and a spray can of glass frosting spray. took a few minutes, seems to work fine.
constantine

Steve Smith
05-09-2006, 11:05 AM
For quite a good alternative to ground glass (especially for experimenting) find a local screen printing company and see if they have any offcuts of fine textured polycarbonate or polyester. A common type is GE Lexan. This is available in various thicknesses. I usually use 0.2mm which on its own is not very stiff but mounted in a frame with all sides held (like a drum skin) is fine.
This material is not quite as good as a proper ground glass but is much better than tracing paper or greasproof paper when experimenting.

Steve.

derevaun
05-10-2006, 01:26 PM
I wonder if this stuff (http://www.armourproducts.com/item--3oz-Armour-Etch-Glass-Etching-Cream--15-0150.html) would work?

http://www.armourproducts.com/media/images/product_detail/15-0150_l.jpg

ImageMaker
05-10-2006, 03:37 PM
Eeek! Run the other way, if that's the stuff with hydrofluoric acid. If it's high strength sodium or potassium hydroxide, it's not much safer, but at least it won't wait hours for a drop on the skin to start causing pain -- not much comfort if you get a tiny bit in your eye and it makes a permanent opaque spot on your cornea before you can wash it out...

I'd stick with abrasives for grinding glass, thanks just the same...

murrayatuptowngallery
05-10-2006, 04:13 PM
Alot of glass etching artists use the cream stuff, but occupationally HF is scary stuff.

I always thought it was a weak acid, peculiar for liking glass for lunch, but was told me a photoetcher in response to a request for titanium parts, that they will not/ can not do titanium becasue it has to be etched with HF and they considered it too unsafe.

Again, if it's sold on the market, they must not have been sued yet :O) , and it probably comes with adequate instructions...

I suppose you need either adequate washing space or soem kind of neutralizer afterward...I'm guessing.

JoeVanCleave
05-10-2006, 07:17 PM
As I recall from working in Wet Etch in the semiconductor industry, contact with HF can cause flouresis of the bones. The flouride takes an affinity to minerals - like calcium.

At the company I work for, a one-time occupational exposure to HF is usually counteracted by a 20 minute whole-body rinse in the cold water safety shower, followed by another 20 minute shower at the nurse's office (just to make sure all your protruding body parts are mere freezing goose bumps), then treatment with calcium cream, to help draw the HF away from the bones. The cold water is supposed to close up the pores and reduce absorption of the chemical into the skin.

Luckily the company I work at is a stickler for safety, so I've never had to experience the joy.

derevaun
05-11-2006, 01:21 AM
Hmm, further research on the etching cream suggests that it uses ammonium bifluoride, which purportedly produces some hydrofluoric acid when in contact with water. And that's Etch-All. The MSDS for Armour Etch cream says it contains "inorganic fluoride compounds."

Seems pretty scary considering the stuff's on the shelf at the local Michael's craft store. Or at least more hassle to use it safely, compared with just using abrasives. Then again, check out this page of instructions (http://www.glassmart.com/etchalInst.htm), in which the user isn't even wearing gloves??? And I fancied myself a rebel for re-using the nitrile gloves when developing film :o

earlj
05-11-2006, 07:03 AM
In any case, it looks like it might be difficult to get an overall etch that is uniform and consisitent across the whole surface. I think that I will stick to the abrasives.

ImageMaker
05-11-2006, 05:58 PM
For my money, if you're etching designs on glass, the best way to do it is with a diamond wheel on an air-powered rotary tool, with the work contact point kept wet (so the glass dust doesn't become airborne). For making ground glass, i.e. all-over etching, loose abrasive still wins after 500 years.

jvh
05-27-2006, 08:38 AM
This is wonderful, like reading six different articles on one subject with a running critical dialog. I had no intention of ever doing my own ground glass until now. Thank you, I think...