"N_Cook" wrote in message
...
1.8mm slabs of porcelain under TO220 devices.

(This is continuation of Ampeg BA600-115, can of worms.
I've got mains back on it and is in working order , now
to try with high power.)

This is my educated guess as to what happened at manufacture as absent
but
distinctive 20mm long bolt not found around here. The 4 output devices
should be held to the heatsink via a cross-bar over all 4 of them and
2
bolts thru it into the heatsink. The insulators are ceramic and the 2
central ones, butted up together , do not clear the retaining bolts on
their
outer edges. After graunching the thread the assembler decided to leave
it
out and trust to luck one off-centre bolt would hold.
Coming to reassembly I found a 3mm screw for replacement but it would
not
hold, as tapped thread is stripped, so I tapped that one hole out to
4mm.

WARNING

Tried grinding a clearance notch on the side of one pad but got nowhere
as
ceramic.

WARNING

Not only that but I had no trouble holding the slab in fingers while
trying
to grind a slot , it was barely getting warm - good thermal insulator
or
small grinding wheel not generating heat as not cutting into the
material ?
So ,lateral thinking, decided to grind down the screw thread where it
interferes with the ceramic , on one bolt.
{snip}

Those ceramic insulators are made from beryllia (berylium oxide), a
ceramic that has a higher heat conductivity than aluminum. You should
be _extremely_ careful in handling them. Quoting from the Wikipedia
article: "Like all berylium compounds, BeO is carcinogenic and may
cause chronic beryllium disease. Once fired into solid form, it is
safe to handle as long as it is not subjected to any machining that
generates dust." Likewise the American Beryllia site says: "Exposure
to fine, airborne beryllium oxide powder or dust in sufficiently
large concentrations, may cause a lung disease in a small number
of hypersensitive people."

Grinding a beryllia insulator is considered exceptionally dangerous.
Please read the MSDS at
http://www.AmericanBeryllia.com/lit/...Oxide_MSDS.pdf.

I once needed some microwave windows made of beryllia. I was going
to order beryllia blanks, have the edges sputtered in gold, and braze
them into 316 stainless waveguide sections. I called the engineers
at National Beryllia (now American Beryllia) to discuss this application.
In short order, they convinced me that they should manufacture the
windows as there was a chance that the differential thermal expansion
between the beryllia and the stainless steel waveguide could potentially
shatter the ceramic and release beryllia powder. After learning the
cost, I went with sapphire windows instead.

Some people are hypersensitive to beryllium compounds. If you are
not in the hospital yet, consider yourself _very_ lucky. A small
percentage of the population is so hypersensitive that the small
amount of dust you created could be lethal.

Dr. Barry L. Ornitz WA4VZQ [my_ham_call at live.com]