I plan on using two junk CDROMs in my basic science class as a variable
capacitor for a crystal set -- shim brass cut to cover a little less than
half of each cdrom, glued to the surface, with a vinyl grommet in the middle
to hold the disks together and a disk of mylar cut from a sheet protector
between the brass plates as the dielectric.

Each student is going to have to prepare two disks each, so it isn't a
onesie-twosie job. I need a method that I can use fifty times a semester
every semester.

The problem is removing the metal "plates" that are there now. First
thought was to soak the disks in drano or aqueous lye and dissolve the
aluminum, but further reading and experimentation shows this isn't quite
enough. Apparently there is a lacquer or epoxy coating over the aluminum
and a nickel flash under the aluminum. This is only hearsay, but I know
from experimentation that a caustic solution by itself isn't enough.

Anybody KNOW what the various layers are on the disk and any suggestions on
how to remove them without destroying the plastic of the disk itself?

Jim


If you use CDRs, you can usually delaminate the coatings by flexing the

I plan on using two junk CDROMs in my basic science class as a variable
capacitor for a crystal set -- shim brass cut to cover a little less than
half of each cdrom, glued to the surface, with a vinyl grommet in the middle
to hold the disks together and a disk of mylar cut from a sheet protector
between the brass plates as the dielectric.

Each student is going to have to prepare two disks each, so it isn't a
onesie-twosie job. I need a method that I can use fifty times a semester
every semester.

The problem is removing the metal "plates" that are there now. First
thought was to soak the disks in drano or aqueous lye and dissolve the
aluminum, but further reading and experimentation shows this isn't quite
enough. Apparently there is a lacquer or epoxy coating over the aluminum
and a nickel flash under the aluminum. This is only hearsay, but I know
from experimentation that a caustic solution by itself isn't enough.

Anybody KNOW what the various layers are on the disk and any suggestions on
how to remove them without destroying the plastic of the disk itself?

Jim

I plan on using two junk CDROMs in my basic science class as a variable
capacitor for a crystal set --

If you use CDRs, you can usually delaminate the coatings by flexing the
disc a few times. Peeling with strong tape is another possibility.

Brainstorming ideas:
* Use orbital sander or try a scraper.

* Sandblasting

* Microwave the disk..this will create breaks in the coatings allowing
NaOH to attack the Al

* Drop the CD idea ...Buy poly carbonate sheets at the hardware
store... (lexan)...Get it laser cut to any shape..

I plan on using two junk CDROMs in my basic science class as a variable
capacitor for a crystal set -- shim brass cut to cover a little less than
half of each cdrom, glued to the surface, with a vinyl grommet in the middle
to hold the disks together and a disk of mylar cut from a sheet protector
between the brass plates as the dielectric.

Each student is going to have to prepare two disks each, so it isn't a
onesie-twosie job. I need a method that I can use fifty times a semester
every semester.

The problem is removing the metal "plates" that are there now. First
thought was to soak the disks in drano or aqueous lye and dissolve the
aluminum, but further reading and experimentation shows this isn't quite
enough. Apparently there is a lacquer or epoxy coating over the aluminum
and a nickel flash under the aluminum. This is only hearsay, but I know
from experimentation that a caustic solution by itself isn't enough.

Anybody KNOW what the various layers are on the disk and any suggestions on
how to remove them without destroying the plastic of the disk itself?

"D from BC" <myrealaddress@comic.com> wrote in message

Brainstorming ideas:
* Use orbital sander or try a scraper.

Labor intensive. Getting 50 kids to sand two disks apiece will either take
a lot of sanders or a lot of time.

* Sandblasting

School doesn't have a sandblaster.

* Microwave the disk..this will create breaks in the coatings allowing
NaOH to attack the Al

THAT was a hell of an idea ...fireworks city, but if you only do it for five
or ten seconds that just might work. It sure as hell crazed the surface.
I'll drop it in the lye bath and report back...

* Drop the CD idea ...Buy poly carbonate sheets at the hardware
store... (lexan)...Get it laser cut to any shape..

You wanna explain to the taxpayer why we have a $50 variable capacitor in
our $5 crystal set?

Jim

I just took an old Philips that I've been using for a coaster, and just
scraped some of the top layer off, and imagine my surprise when there
was nothing else there - the rest of the CD was transparent. I was
kinda surprised - I always assumed that the data stuff would be on
the LED side, but apparently the LED sees through the plastic, and the
data is on the underside of the label.

I plan on using two junk CDROMs in my basic science class as a variable
capacitor for a crystal set -- shim brass cut to cover a little less than
half of each cdrom, glued to the surface, with a vinyl grommet in the middle
to hold the disks together and a disk of mylar cut from a sheet protector
between the brass plates as the dielectric.

Each student is going to have to prepare two disks each, so it isn't a
onesie-twosie job. I need a method that I can use fifty times a semester
every semester.

The problem is removing the metal "plates" that are there now.
[...]
Anybody KNOW what the various layers are on the disk and any suggestions on
how to remove them without destroying the plastic of the disk itself?

Jim,

Can you tell us where you're teaching?

How'd you manage to convince the teachers' union to let an engineer teach
a class?

If the CDROMs are just there to support the brass plates--with the
mylar dielectric in the middle--why not just face the intact
metallized sides outward, whereby the large gap will minimize any
affect on total capacitance?

Namely, in cross-section: (view using Courier font)

. ||xxxxxxxxx #### mmm #### xxxxxxxxxx|| <--aluminized layer
. \_________/ \__________/ facing outward
. CDROM CDROM
. B M B
. r y r
. a l a
. s a s
. s r s

If the resulting minimum capacitance is acceptable it'll save a lot
of effort stripping.

Best,
James Arthur

"D from BC" <myrealaddress@comic.com> wrote in message

Brainstorming ideas:
* Use orbital sander or try a scraper.

Labor intensive. Getting 50 kids to sand two disks apiece will either take a
lot of sanders or a lot of time.

* Sandblasting

School doesn't have a sandblaster.

* Microwave the disk..this will create breaks in the coatings allowing
NaOH to attack the Al

THAT was a hell of an idea ...fireworks city, but if you only do it for five
or ten seconds that just might work. It sure as hell crazed the surface. I'll
drop it in the lye bath and report back...

* Drop the CD idea ...Buy poly carbonate sheets at the hardware
store... (lexan)...Get it laser cut to any shape..

You wanna explain to the taxpayer why we have a $50 variable capacitor in our
$5 crystal set?

Jim