[volt-nuts] plastic caps on 3458A reference board

[Charles Steinmetz]

Dave wrote:

>If you want thermal insulation, then PTFE ...

John wrote:

>I[t] needs to insulate electrical and thermal and metal would not do well.

Two machined pieces of glass-filled PTFE with holes to pass screws or 
plastic rivets through should work pretty well.  You want some 
thermal insulation, but not too much (there has to be a heat flow 
path to ambient to allow the heater to work at a reasonable fraction 
of its capacity), so the thickness of the top part would need to be 
right and some experimentation might be required.  I've made a number 
of machined parts out of plain PTFE and glass-filled PTFE -- either 
one is definitely do-able.

Straight PTFE is subject to cold-flowing, so if you used that some 
thought would need to be given to the through-bolts so everything 
would stay tight.  Belleville washers are one common solution.  But 
glass-filled PTFE is harder and more stable, and is the better choice 
for this job.

I've never machined polysulfone, but it is routinely done, so that is 
another choice.  It is also available with glass filling.

Once a design was set and programmed, a small CNC mill could zip out 
enough parts to fill the next 100 years' volt-nut needs in ten minutes.

For those who haven't seen one, here is a top view of a 3458A 
reference board showing the LTZ1000 with its hat 
on: 
<http://cache.amobbs.com/bbs_upload782111/files_17/ourdev_466753.jpg>. 
As Frank noted, the bottom piece (not shown) is very important 
because it insulates the thermocouple joints where the part is 
soldered to the board.

Best regards,

Charles