Please help with etching/grinding acrylic sheet

[Joe Young]

Hiya,

First post here but been following this board for a while. I wish to make an agus35 style adapter, but using a smaller disk then a cd. I wish to make my own spinning ground glass using clear acrylic sheet. Dont wont to try glass as im afraid it will break and cause injury.

However, im a little stuck on how to frost/grind/etch the surface of the acrylic in order to diffuse the light. Does anyone have any tips on how to do this? I have tried grinding with aluminium oxide, but its hard to get a nice uniform surface. I have heard about etching with acetone and sandblasting which seems expensive and messy.

Any help would be great.

Joe

[Oscar Spierenburg]

Put (frosted) window foil on it. It'll give a bright image, but you probably need a condenser lens to even out the light.

[Marcus Marchesseault]

I must ask the obvious. Have you used a wet-sanding method? Dry sanding is less uniform. I assume you have wet-sanded, but I had to ask.

[Bob Hart]

Joe.

If you want to make a small disk, have you thought about the small CD-R disks?

These are just, and only just big enough to yield a 24mm x 18mm image clear between the hub and outer rim. A smaller disk may have to be spun faster as the groundglass texture towards the center may be resolvable by the camcorder.

If you are cutting your own disk fromn sheet, you will likely have to make a dummy hub on a lathe, mount the rough disk and skim the outer edge to a perfect circle relative to the 15mm center hole or it will be almost impossible to balance.

Some CD-R disks seem to have had a surface hardening process and they are harder to dress with a groundglass finish than they used to be.

If you are going to dress acrylic material, how thick is it going to be? Thinner is better, as in CD disk thin.

Also, I am assuming you are going to dress the disk with loose 5 micron grit in a water solution. Fixed abrasives like sandpaper or wet and dry will give you a pattern of scratches, not pits. Scratches = inferior groundglass texture.

Also in this process, cleanliness is next to godliness. Do it in a place free of windbourne dust and grit or cats which might walk across your work.

[Joe Young]

Thanks for you replies guys!!!!

Bob,

The smaller CD-r disks are a good idea but are hard to find. The acrylic sheet i have is 2mm thick.

I am using 600 grit aluminium oxide and water, but as i say its not coming out uniform at all, like its ground more in some areas then others, very patchy. Any tips to grind it more uniform?

Thanks in advance

Joe.

[Marcus Marchesseault]

600 grit sounds a bit harsh. I would try 800 and 1500 since you can sand longer but take less material away each pass. Also, you should try using liquid dish soap as a lubricant while wet-sanding. Soapy water and 1500 grit should give a very even light haze. 800 grit might be necessary to get more diffusion.

[Joe Young]

Originally Posted by Marcus Marchesseault

Also, you should try using liquid dish soap as a lubricant while wet-sanding.

Thanks ill try that. You do mean with loose Aluminium Oxide powder/grit and not the sandpaper stuff, right?

[Mike Teutsch]

Joe,

Having done more sanding than I care to think about, owning a machine shop, I'll give you my take.

First, use silicone carbide sandpaper, not aluminum oxide. Silicone carbide is the black stuff used in automotive body work etc. It is waterproof and very sturdy. You will have to experiment with the grit to find one that gives you the diffusion you want.

Second, take a piece that is bigger than you need and use it to sand something else, like a piece of metal, not aluminum, or just sand your driveway for a bit. This serves two purposes, it makes the paper finer than when you started and it removes all of the pieces of grit that will come loose when you start sanding. It is the loose pieces of grit that cause the unwanted big scratches that don't match your sanding pattern. After your paper is smoothed some, trim off any parts that did not get worn, like the edges or corners. Make sure to use all of the paper, move your hand around to cover all areas.

Thirdly, as mentioned before is to wet sand. Since your part is small, just hold it under water, say in a sink and use some soap to keep the paper from clogging. Rinse constantly. Also, wet sand when you break-in the paper, the step above, then wash it well with soap and water before using it.

Then you just have to experiment with the grit size and sanding patterns. Probably a small circular pattern will look best, but you can experiment. One last note, start with the finest sandpaper your have and work up as necessary. If you start with a course paper you may not be able to sand out the big marks they make without a lot of work.

Good Luck---Mike

[Joe Young]

Thanks very much for the tips, i shell give them ago.

Joe

[Francois Poitras]

Interesting that you asked, Joe. I’m exactly at the same stage with my home made Micro35.

The plastic disc I got from Redrock was not plane and had an off-centre hole, which causes the disc to wobble when spinning. I get flickering.

I bought a 2mm acrylic sheet from Home Depot, drilled it and cut it to the right size and sanded it with 600 and then 800 grit paper (600 grit was too harsh). I tested it on my adapter and I get about the same results as the Redrock GG, minus the flickering. I think 2mm is too thick though.

I’m still experimenting (I like that). I just order a bunch of pre-cut and pre-drilled 1mm and 1.5mm acrylic discs (about $3 each) from a company that sells "optical" plastic (techplast.com).

So I will use the good advice given here to do my testing...