View Full Version : Magnifier for optical center punch (pics)

03-18-2007, 04:41 AM
Here's a followup to this thread: How to approximate a lens shape
I figured I'd post some pictures as a thanks for the help I have received, since its usually neat to see pictures of what other people are working on.

I have succeeded in free-handing a lens shape in acrylic for an optical centerpunch. The view is a little distorted on the edges, but the center is fairly clear and does a decent job of magnifying, so it's good enough for me.

First, I fired up a CAD program and drew some circles to get a visual template to work with. Then I filed down the acrylic using the calibrated eye-ball technique by comparing to the paper template. Polishing was really easy, just hit it with some 220 (the only grit I have handy,) which seems to cut, then clog, then pre-polish. Then I hit it with some hardware store polish I have, and it was good. I also held a lighter under it for a few seconds to flame polish it, and that made it a tiny bit better.

Here's a shot of it lining up with the circle, you can kind of see I messed up near the edge and should have smoothed it more:


Here's a closeup where you can kind of see how the shape follows the curve. I had the camera in macro mode with full zoom to reduce the near/far difference of the acrylic and paper beneath it.
BTW, this was the first time I found the digital zoom handy, it made it much easier to see if the curves were lined up on the tiny camera screen.


The circle in the center used for aiming is about 0.020" in diameter, and I added some crosshairs leading to the circle just for fun. After cutting the shallow marks, I hit it with the paint pen and wiped off the excess.

For the markings, I used the corner of the cutoff tool (what I had handy,) but I think I needed a sharper point to get finer scribe marks. The cutoff tool seemed to drag a bit below the centerline and you can see cloudy areas in each quadrant near the center. I think this dragging only showed up (after painting!) on the center because of less SFM there in the sanding phase which left the bottom face ever so slightly concave.

Here's what it looks like zeroing-in on the decimal point of "1.5" on the printout: (FYI, printout is 600dpi laserjet, you can kind of see the dots on the "5".)


I have been considering making a radius form tool by using and endmill to cut a radius in some drill rod, to quickly get the shape to the right ballpark.

Your Old Dog
03-18-2007, 07:37 AM
I have to admit, I thought you were nuts but you persisted and got it to work! What kind of plastic am I looking for?

03-18-2007, 12:04 PM
That's real nice, but I have to say, you've taken the time to do an even more beautiful job photographing it!

I love the way you lined up the curve with the drawings and shot exactly what it looks like to use this tool.



03-18-2007, 02:28 PM
I have to admit, I thought you were nuts but you persisted and got it to work! What kind of plastic am I looking for?

Trust your instincts, I AM nuts! ;)

This plastic is just plain acrylic obtained from the local supplier, Tap plastics. It was a whopping $1.36 for 2 feet of 3/4".
Now that I see that this kind of works, I'll have to try making one out of polycarbonate.


03-18-2007, 04:18 PM
I just made one for fun. Not having any acrylic rod I sacrificed a very old screwdriver with a (somewhat) clear lucite handle. Lucite is softer than acrylic but takes an optical quality finish and polish with ease. The trick is to not overheat it and use water or alcohol with the polishing compound at low speed.

The lucite in this screwdriver isn't water clear but has a slight purple cast to it. Still, it was easy to make. I just free handed the lens with a fine file to what looked about right and polished it up wet with ethanol using 400, 600 and 1500 papers followed by fine alumina and then cerium oxide.



03-18-2007, 04:37 PM
Lucite is an acrylic.

Acrylite, Lucite, Perspex, and Plexiglas are common brand names for polymethyl methacrylate or PMMA, developed in the 1930s by Rohm & Haas.

Excuse me, marketed by R&H in the '30s. I don't know who developed it originally.

03-18-2007, 04:48 PM
It's acrylic but doesn't have the same properties as plexiglas. Lucite won't shatter no matter what you do to it and is noticeably softer than the Plexiglas formulation.

BTW, Lucite was invented by DuPont.

03-20-2007, 05:01 PM
It's acrylic but doesn't have the same properties as plexiglas. Lucite won't shatter no matter what you do to it and is noticeably softer than the Plexiglas formulation.

BTW, Lucite was invented by DuPont.

Sounds like you're thinking of Lexan, a GE brand name for a polycarbonate resin. Softer surface than the acrylics, relatively shatterproof (even, in some cases, bulletproof). Working it sometimes reveals some strange delamination properties I've not encountered when working with acrylics of any brand.

03-20-2007, 06:30 PM
Definitely not thinking of Lexan. I have sheets of that downstairs. The old Lucite screwdriver handles are virtually indestructible. The one I used for the magnifier was probably made in the 60's. You can hit that stuff with a sledge hammer and it deforms but won't shatter. It's softer than polycarbonate and based on the smoke sniff test is definitely not polycarbonate but an acrylic. It has very different properties in that regard from garden variety acrylics. Not sure if they make it anymore as DuPont went out of the acrylic sheet/shapes business AFAIK.

When I say soft I mean you could take a small piece and twist it into a pretzel with pliers.

Lew Hartswick
03-20-2007, 09:36 PM
Lucite: Acrylic resin E.I.duPont de Nemours & Co
Plexiglas Acrylics Rohm & Hass Co.
Crystalex Acrylics Rohm & Hass Co.
A-100 SM Acrylics Resolite Corp.
Methacrol Acrylic emulsions E.I.duPont de Nemours & Co.

The only ones listed as ONLY Acrylics. There are quite a few that list a name
and then an asortment of materials as well as acrylic.
For example:

Polypenco Acrylics, chlorinated polyethers, fluorocarbons, nylons, polycarbonates, (by) Polymer Corp.

Acording to the "bible" Handbook of Chemistry and Physics , CRC Press

When all else fails the "Ultimate Source" for me. :-)

03-20-2007, 09:46 PM
Many screwdriver handles are made of butyrate.

Not to heave this thread off the tracks, but speaking of butyrate tool handles, has anyone else encountered the handles on Excellite brand nut drivers that even brand new, smell alarmingly like vomit? I was given a set AGES ago, maybe 25 years, and the blow molded clamshell case was really, genuinely awful smelling when you opened it. They finally moved to the garage.

Anyway, butyrate is pretty clear, usually about Rockwell R75 hardness, and it machines very easily. When I was in biotech, the machine shop I sent a lot of tiny, tricky parts would sometimes inquire if they could use it instead of acrylic, but generally it was biologically incompatible on the basis of proteins binding to it or some such...

Todd Tolhurst
03-20-2007, 10:19 PM
Ever had a whiff of butyric acid? That's what you're smelling.

03-20-2007, 10:31 PM
The screwdriver is definitely acrylic. The sniff test is a very accurate way to identify plastics. Each family has a very distinctive odor and I have a very sensitive nose.

Butyrate plastics are derived from butyric acid which smells amazingly similar to fresh puke.

(Todd beat me to it.)

03-21-2007, 12:19 AM
So that's the reason those Craftsman nut drivers smell. Every time I open the drawer in the tool chest I wonder if the smell will ever go away. It's been 25 years, so far, and I guess the answer is "NO".


03-21-2007, 01:00 AM
I have some xcelite tools like that. Good tools, bad smell. Thanks for giving me a name to put to it. What was it- pukeric acid :)

Ryobiguy, can you show a pic of your center punch? I'd like to see the final configuration.