hard black plastic?

I need to make some new idler wheels for my wire EDM. The material is a very hard black plastic. Maybe the same stuff you see on a car electrical
distributor. It needs to be non-conductive and be able to have a light press fit for a ball bearing.
Can somebody give me a material name and source to order?
Karl
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Its not black, but maybe something like Tufnol. Maybe have a look at Radiospares in their engineering materials section
http://www.rs-components.com/index.html
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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 machines.
Micarta might also work, depending. I guess it's technically a form of phenolic.
--
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wrote:

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.
Wolfgang
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Bakelite is phenolic resin and a wood-flour filler. It's probably what the originals were made of. It can be colored black, or it can be dark brown, as I'm sure you know.

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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 ;-)
Thanks Ed.
--
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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 these laminates.
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.
-- Ed Huntress
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On Fri, 24 Oct 2008 16:00:11 GMT, Ecnerwal

You can add epoxy/glass, silicone/glass and melamine composites to the Micarta family, just to confound things a little more.
--
Ned Simmons

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wrote:

And Cycolac. Back when telephones had dials, some of them were made of Cycolac. Tough stuff. I think it's a form of ABS.
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    [ ... ]

    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. :-)
    Enjoy,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
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wrote:

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 available product.
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.
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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:

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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.
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wrote:

What voltage? Especially in a dusty environment, creepage may be as important as clearance. (Creepage is the distance of the path from a hot lead to another conductive surface, along the surface of the insulator.)
Dave
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wrote:

What voltage?
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.
-- Ed Huntress
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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.
--
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On Fri, 24 Oct 2008 11:47:37 -0400, "Ed Huntress"

Ed, this assertion doesn't nearly meet your usual high standard of editorial rigor.
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wrote:

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>
-- Ed Huntress
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On Sat, 25 Oct 2008 01:33:18 -0400, "Ed Huntress"

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 impedance.
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wrote:

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.
-- Ed Huntress
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