Re: multi segment single laye

To: tesla-at-pupman-dot-com
Subject: Re: multi segment single laye
From: "Daryl P. Dacko" <mycrump-at-concentric-dot-net>
Date: Thu, 21 Nov 1996 20:34:17 -0500 (EST)
At 03:32 PM 11/21/96 -0700, you wrote:

<snip snip, snip...>

>> >
>> > Does anyone have some specs for bulk resistivity that they can quote for
>> > tungsten carbide? I know there are dozens of grades, coatings etc. but
>> > an approximate figure will do.
>
>
>Tungsten carbide is a very effective insulator. It is used as an 
>abbrasive and protective coating.
>
>  I am assuming you meant elemental tungsten metal (as used in Spark 
>gaps).  Tungsten is a lousey conductor of electricity, but in the short 
>stub length we use in Rotary gaps, its resistance is of no concern 
>whatsoever.
>
>Richard Hull, TCBOR

I just got back from the lab, where I measured the resistance of three types
of 'carbide' tool bit inserts for my lathe, and found them to be about twice
as resistive as a tool steel bit... about .4 ohm from tip to tip of a one inch
diamond insert.

One of the bits had a hard black resistive coat, that once removed left it as
conductive as all the rest.

I seem to recall that these bits are really a 'cermit', a metal/ceramic
composite,
formed by sintering various combinations of metal and secret ingredients.

This might explain why they are so conductive, if the active ingredient is 
tungsten carbide - either excess carbon or excess metal in the mix.

(a quick trip back to the lab)

I just ran 15 amps DC through an arc between a carbon rod and one of my diamond
shaped inserts - at first a few sparks that looked a lot like the sparks formed
by high carbon tool steel when ground, and then a nice steady arc.

After about 10 min. I shut it down and looked at the tips, the carbon rod had 
worn down faster than the insert, and the insert had some build-up on it.

Sandpaper took off the build-up, and I see just a touch of erosion where the
arc had played...

I'd guess that these things would work fine as tips in a rotary, but tomorrow
I'll try to remember to bug a welder to give me a worn-out tungsten tip to 
try against my insert to see which is the winner ;'}

Worn-out carbide bits are pretty easy to come by around here (lots of machine 
shops), so this might be a worthwhile investigation.

Comments ?

Daryl