Roughing Out Small Plexiglass Disks?

It's been quite a few years (I did a lot of round plexiglas things back in my lab-rat days), but either a sharp holesaw or a flycutter, either one run slow enough and/or with enough air to keep it cool to limit chip-melting, might be worth a shot.

Then again, a coping saw or jewelers saw would let you control cut speed to a fair thee well and might get the job done as well or better than the bandsaw would. Generally backside roughness == melted and re-welded chips.

For thin stock shears or tinsnips might work, nipping the corners off - at some point (of thickness) they stop cutting and start shattering, though.

Score both sides with a utility knife and snap off with a pliers.

Hmm ... I think that 2-56 would not be strong enough for even a short stack. What I would do is get a replaceable tip tailstock live center, and make a tip with a center hole for it which would slide over the center rod and clamp the stack firmly in place. I've used a variation on this to turn new crystals for dial indicators. Once your workpieces are big enough in diameter, you can make a pusher plate with a center countersink drilled hole and a standard live center. I did this with Lexan instead of Plexiglass, because I wanted it to be flexible to take the dome for the crystal.

How much oversized do you plan to make the pieces? You know that Plexiglass breaks cleanly when bent at a score line. This could use a grid of lines to break into squares, or lines at 120 degree angles to break into triangles and get a cleaner edge before turning. (And yes, I have used a sheet metal brake to do the bending along the scribe when a piece was too small to grip easily.) You can also use a combination of a milling vise (smooth jaws) and a broad-jawed Vise Grip to do it with smaller pieces.

And avoid getting oil on the Plexiglass -- or many other solvents for that matter. They cause it to disintegrate into a bunch of cracked pieces.

Avoid the saw. Scribe and break. (Won't work on my Lexan, but great on Plexiglass.)

It would almost certainly.

Mine above.

Enjoy, DoN.

This came up once before, my advice is the same, make a punch and die, should work well with thin stock. Undoubtedly the way the commercial ones are made. Wouldn't take long to bore an oblong block of steel, then slot it, die block and punch guide all in one. If you need a concentric hole, a simple jig with a bushing would let you drill a centered pilot hole, then open it up with a clock reamer to whatever you want. A Sat. afternoon project. Most likely the originals were from acetate sheet which punches better.

Stan

You can buy a 12" long piece of 7/8" round cast acrylic for $4 and turn it to size before you part off the thickness you need:

Hi Doug,

What material?

How thick?

Tolerance on diameter?

How many?

I have a 35 Watt CO2 laser I built that could cut the disks perfectly. You would have to then machine the inner hole. You could also ask at a local engraving shop that has a laser.

Email me if you like.

Dave

"Dave, I can't do that" wrote in news:5d40b33a- snipped-for-privacy@l18g2000yql.googlegroups.com:

It's acrylic, rougly 0.10" thick. The tolerances on the final diameter are pretty tight, but slightly oversized blanks with a small center hole would save a lot of work. I assume the laser cuts cleanly enough that there wouldn't be optical distortion in the plastic after the fact?

Doug White

"David Courtney" wrote in news:inn7j6$bif$ snipped-for-privacy@dont-email.me:

These need to be pretty optically flat & clear. At least to the point where images are not distorted looking through the result. I'm not sure how I could achieve that easily. The raw material I have is mil-spec aircraft window grade cast acrylic.

Doug White

snipped-for-privacy@prolynx.com wrote in news:99100af4-0eac-4317-a37e-432a8b18f153 @f18g2000yqd.googlegroups.com:

They are made from 3mm thick aircraft window grade acrylic. Making a punch (assuming it didn't just shatter the things) is probably a lot more effort than making a half dozen or so by hand on the lathe.

Doug White

"DoN. Nichols" wrote in news: snipped-for-privacy@Katana.d-and-d.com:

Thanks! I'll try score & break. That should get me to squares, and I can just bide my time & turn those.

I agree, the 2-56 will flex if I stack more than a couple. I've been planning on supporting the other end with the tailstock & a center. For the first one or two, I will probably do them individually to work out feeds & speeds. I've got league matches starting in two weeks, and I'd really like to have a slightly smaller aperture than I have now.

My biggest problem at the moment is that the target "black" looks dark grey at 200 yards. I don't know if it's my prescription, dust in my rear sight or the beginnings of cataracts. I'm about to give my rear sight a thorough going over & hope I'm being tormented by a spec of lint in just the wrong place.

Doug White

These folks do custom laser cutting of acrylic and other materials:

I though about making plastic apertures. I decided it was going to be very difficult because of the precision required in the bevel of the hole, that is what you see when you look through it and it must be perfect.

I went to using an adjustable front aperture. I just looked at the current prices. Wow!

Champion shooters supply has the plastic apertures for $6.00.

What's the spec on the center hole? I once made a bunch of plastic washers in acrylic by sliding some blanks onto a bolt, clamping them together with a nut, chucking the bolt into a hand drill, and "grinding" them to size on the belt sander. But these were 1/4" thick, and the center hole was 1/4".

Good Luck! Rich

Hi Doug,

The laser can easily hold 0.005" The edge looks like flame finishing. There is no obvious distortion visible on stuff I have done as the laser dot size is only 0.006" diameter and at that thickness, little- to-no Kerf. Cast acrylic is good for clean cuts, extruded is not so good. There may be a small amount of distortion within about 1/32" of the cut edge but I am sure that would not matter.

I think I have some 1/16" here and if you want to Email a postal address I'll send you some samples. Is the 0.860" a max or min? Give me the tolerance. I think I have some in fluorescent orange and green too. Might be fun to try.

Dave

Can *you* part it to give a clear transparent finish? He needs to be able to see through this, except where the tapered hole is for contrast.

If you faced it with PCD (Poly Crystalline Diamond) you *might* be able to get a nice enough finish on one side -- but you then need to be able to flip it over and hold it parallel to face the other side. With a Jeweler's lathe, perhaps some step collets could be used for holding the disc.

Enjoy, DoN.

Indeed so.

O.K.

BTW -- in my dial crystals, I can't have a hole in the center at all. What I did to maximize the drive at about 2-1/4" diameter was to recess all but about the outer 1/8" or so on both the backing piece in the chuck and the pusher plate which the live center was engaging so the entire pressure was applied near the OD for maximum torque transmission. (If you clamp with a nut, the pressure is at too small a radius, and the workpieces are more likely to slip. With the near-OD clamping, I could handle something like eight or ten workpieces at once. But approach the maximum extension of corners carefully. :-)

Hmm ... perhaps diffraction from the smaller aperture? That may be why they are not made smaller?

Good Luck, DoN.

P.S. Wish that I had known about the target shooting at the "Toot" when I was there. :-)

Oooops, offer retracted, well at least temporarily anyway. Laser would not fire this morning. Not a good sign after going through all the procedures, it looks like a tickle pulse problem as I can get it to fire on full power, but not on adjustable pulse rate. Technology designed to keep one's retirement busy.

Dave

-snip nonsense-

Yes, *I* can but that doesn't necessarily mean that *you* could.

So -- if not for my enlightenment, perhaps for the OP's enlightenment, would you care to detail how he could do it. My snipped (by you) "nonsense" was all that I could think of for doing it, but I have not tried it. I do know that PCD on a rigide enough machine can produce an optical finish on aluminum, at least.

Enjoy, DoN.