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

Reply to
Karl Townsend
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Its not black, but maybe something like Tufnol. Maybe have a look at Radiospares in their engineering materials section

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Reply to
K Ludger

Reply to
RoyJ

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.

Reply to
Don Foreman

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

Reply to
spamTHISbrp

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.

Reply to
Ecnerwal

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

Reply to
wfhabicher

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

Reply to
Ed Huntress

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.

Reply to
Ed Huntress

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.

Reply to
Ecnerwal

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.

Reply to
Ecnerwal

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. My pleasure.

-- Ed Huntress

Reply to
Ed Huntress

From your description, I suspect that it might be an Acetal. They are hard, hard wearing and have a "slipery" feel to them. They are also a good electrical insulator. Here in Australia, it is readily available from an industrial plastics supplier, in both shhet and rod form, but I would have no idea where you would go for it in the US

Reply to
Grumpy

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

Reply to
Ned Simmons

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

Reply to
Don Foreman

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

Reply to
Don Foreman

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?

-- Ed Huntress

Reply to
Ed Huntress

Acetal wins. But only because its one tenth the price of micarta in the grade I need.

Thanks for all the help ,everybody.

Karl

Reply to
Karl Townsend

I sell various forms of plastic offcuts on ebay.

From what you and the others say, I would agree that white delrin or phenolic is the way to go, but you could also have a look at unfilled Nylon in the natural form. It's quite a hard plastic and it wears very well. I think you'll find the black delrin or the black nylon both have carbon black in them - it's the usual way they manage to get a black colour.

If you want to have a look at some pieces, there are pictures at;

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If heat is a problem, delrin softens at a pretty low temperature ( say, around 300 F) - nylon's a bit more tolerant. Delrin is far more slippery, and is known for its insulating qualities in white as well as it's ability to be machined to extremely close tolerances.

Eric

Reply to
eric h

You might find something among the ceramic flanged pulleys here:

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They're surprisingly inexpensive and last forever, though probably not as cheap as a piece of Delrin.

Reply to
Ned Simmons

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