Bakelite? That's a likely for the car distributor, and is a high temp
and hard black plastic.
Phenolic might work well, but it's not black. It is very hard and a good
insulator, based on several years I spent building plasma physics
Micarta might also work, depending. I guess it's technically a form of
Probably any of the THERMOSET plastics would work as long as they have
non-conductive fillers. Pick one with a high hardness,
Personally I like the idea of a metallic rim carrying the wire; it is
more precise and resists wear and tear better. An insulating bushing
containing the bearings would keep the magic smoke in.
I find that what I "know" is limited by what I've used, sometimes - all
the phenolic (technically, phenolic laminates) I worked with that were
generically referred to as phenolic when someone wanted that were the
dark-brown with brilliant yellow dust.
So my recommendations come down to "the phenolic referred to as bakelite"
The phenolic (probably paper-based) referred to as "phenolic" in the
labs I used to work for.
And "the phenolic trademared as Micarta, of which there are many more
varieties and colors than I ever knew".
Evidently, I recommend a phenolic ;-)
You got it right. The original Bakelite is phenolic resin
(phenol-formaldehyde, IIRC) and wood flour, as I mentioned. But the
laminates, generically known as "phenolic sheet," are similar except they
have stronger reinforcement. They're layers of paper or cloth bonded with
phenolic resin. Micarta is a brand name, I think, for a high-quality line of
Thinking about the uses for the sheets, I suspect you're right that those
wheels on the Andrew were more likely Micarta or similar, rather than
Bakelite. The laminates are much stronger and more wear-resistant than plain
Bakelite. They've been used for timing gears on V8 car engines, because they
wear pretty well and they run quiet.
I need to do something useful with this pile of trivia before my brain
collapses. <g> My pleasure.
Hmm ... that must have been Ma Bell's 500 series phones. I know
that the 300 series had aluminum (or brass) dials painted black. I
don't know if I ever *saw* a 400 series phone.
But I have some of these still around here -- even some
connected where I don't need to initiate calls. :-)
Email: < firstname.lastname@example.org> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
Phenolic plastics were the first, and still the commonest,
thermosetting plastics (they take heat well).
The earliest, inventor Baekeland used with lots of
asbestos as reinforcement and filler. That is not a currently
Bakelite (Union Carbide), and Formica (General Electric) and
Micarta (Westinghouse) are all tradenames for phenolic plastics,
and nowadays use either wood-fiber (paper) or for high
performance, linen reinforcement/filler.
Can't answer the question but my last project had to switch from black
delrin to white delrin after I noticed that the black is not considered
an insulator. Must have some carbon black in it.
Karl Townsend wrote:
On Fri, 24 Oct 2008 08:21:30 -0500, "Karl Townsend"
The most economical way to produce things like this is to mold them.
However, that isn't necessarily the only way, nor the best way if you
don't happen to have the molds.
I'd make an undersized aluminum wheel, press that into an insulating
bushing made of Delryn, Noryl, phenolic or whatever, and then possibly
press that assy into a metal "tire" that would resist wear. The
resulting wheel would be non-conductive from rim to center and would
be quite robust.
Especially in a dusty environment, creepage may be as important as
(Creepage is the distance of the path from a hot lead to another
conductive surface, along the surface of the insulator.)
On the order of several hundred volts, at very high impedance. If there's
any conductivity, the sparks won't initiate and the machine won't cut.
The actual cutting goes on at around the voltages you have with a welder --
quite low. The high initiation voltage is just to ionize the channel for the
actual current flow.
McMaster has micarta - they even have it in black, though I seem to
recall all the micarta I encoutered at the lab being white - but that
probably just means that the stuff we used happened to be.
I'd be wary of the metal rim that Wolfgang likes causing problems with
clearance to other parts, if the machine was designed with plastic
wheels. i.e., with a plastic wheel, the voltage is only present at the
part of the wheel with the wire on it. With a metal rim, you might get
sparks from the backside of the wheel-rim to some other part of the
machine, depending on the design.
I'm not following you. What I described is the way it works. You have a high
voltage at high impedance, and that voltage ionizes the channel. When it's
ionized and current starts to flow (the "spark"), the voltage drops to a
very low value, to reflect the low ohmic resistance of the ionized channel.
In an RC relaxation circuit, like most very old EDMs used, the open-circuit
voltage is high. In a somewhat newer electronic circuit, the low- and
high-impedance circuits actually are separate. A Sodick EDM of about 1980
vintage actually has three circuits.
If there is leakage in the high-impedance circuit, you won't have enough
voltage (it will drop in the high-impedance circuit because of the parallel
resistance of the leaky element of the circuit) to ionize the channel. Thus,
no spark will be able to initiate.
Is this editorially sufficient? <g>
Yes! I was questioning the assertion of high voltage and
particularly that of very high impedance.
You apparently know a lot more about EDM than I do. I thought they
were all RC circuits as described in Langlois' book. Those devices
were neither particularly high voltage nor particularly high
I wrote most of McGraw-Hill's old book on non-traditional machining; I was
the EDM editor at _American Machinist_; and I was the US Marketing and Sales
Manager at Sodick. I also gave Mitsubishi's East-coast lectures on wire EDM.
When we were hard-pressed at Sodick, I took off my tie and made board-level
repairs to power supplies.
<sigh> But, as with many other things, my knowledge is becoming something
that's mostly of historical interest. Hell, *I'm* becoming something that's
mostly of historical interest. Just ask my wife. d8-)
As for the high voltage, that was just one way to do it. If you have
patience, you can get sparks started with lower voltages, and the very
earliest EDMs had only one capacitor bank, which compromised several things,
including the time between sparks. But when Agie came up with sophisticated,
heavily researched power supplies during the '70s, the Japanese,
particularly, developed some work-arounds that kept them from violating
Agie's patents (more or less -- but that's another story <g>) while
competing with them on performance. Sodick's approach was common, although
they did it particularly well for the time.
Early wirecut machines tended to have pretty high initiation voltages. I
think the Andrew was in that category, but I'm not dead sure. It was a good
machine and an excellent buy at the time. It also was one of the few
American-made wirecut machines ever.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.