Greetings ALl,

I'm building a vacuum chamber for coating larger telescope mirrors (up
to 24" in diameter) (Amateur adventure, not really for financial
gains!)

I'm currently perplexed on the best method to mount the 36 evaporative
heating elements supports on the back wall of the chamber.

Currently I have a plan on how to complete the mounts, but first I
must support the 36 x 1/2" long ceramic stand offs.

The wall that these 36 points will be mounted to is 1/2" thick sheet
steel, and the end results has to be a leak free semi-strong support
(there's no real weight being supported, just the occasional need to
change the tungsten filaments.)

My Current ideas a

#1 Drill 36 holes thru the back wall, insert 5/32" bolt with an
O-ring on the outside, and a nut on the inside. However this becomes
36 new points to search for vacuum leaks!

#2 Drill 36 holes, but only 1/4" deep into the backside of the 1/2"
thick back wall. This leads to three possible mounting methods

A) Big Hammer, and just "Nail" the threaded rod into the
under-sized hole, and hope it holds together (Yea, uh huh g)

B) Heat the hole and braze the threaded rod into place, but
this might cause un-wanted heat-shrinkage in the back wall...

C) Use an adhesive that won't out-gass to hold the threaded
rods in place. (Like some type of epoxy, but these might see somewhat
high heat loads, and I worry the epoxy will break down over time)

#3 Drill the 36 holes, 1/4" deep, and Blind Thread the holes (sounds
like I'll be breaking a BUNCH of taps!!)

Any other thoughts??








Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

Can you double wall this thing? mount the rods to an 1/8" plate or so and
then affix this thinner plate to the 1/2" structural plate? Or mount the
standoffs on strips of metal say 6 strips of 6 and affix the strip to the
back plate?
lg
no neat sig line
"James Lerch" wrote in message
...
Greetings ALl,

I'm building a vacuum chamber for coating larger telescope mirrors (up
to 24" in diameter) (Amateur adventure, not really for financial
gains!)

I'm currently perplexed on the best method to mount the 36 evaporative
heating elements supports on the back wall of the chamber.

Currently I have a plan on how to complete the mounts, but first I
must support the 36 x 1/2" long ceramic stand offs.

The wall that these 36 points will be mounted to is 1/2" thick sheet
steel, and the end results has to be a leak free semi-strong support
(there's no real weight being supported, just the occasional need to
change the tungsten filaments.)

My Current ideas a

#1 Drill 36 holes thru the back wall, insert 5/32" bolt with an
O-ring on the outside, and a nut on the inside. However this becomes
36 new points to search for vacuum leaks!

#2 Drill 36 holes, but only 1/4" deep into the backside of the 1/2"
thick back wall. This leads to three possible mounting methods

A) Big Hammer, and just "Nail" the threaded rod into the
under-sized hole, and hope it holds together (Yea, uh huh g)

B) Heat the hole and braze the threaded rod into place, but
this might cause un-wanted heat-shrinkage in the back wall...

C) Use an adhesive that won't out-gass to hold the threaded
rods in place. (Like some type of epoxy, but these might see somewhat
high heat loads, and I worry the epoxy will break down over time)

#3 Drill the 36 holes, 1/4" deep, and Blind Thread the holes (sounds
like I'll be breaking a BUNCH of taps!!)

Any other thoughts??








Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and
Coating site)

In article ,
says...
Greetings ALl,

I'm building a vacuum chamber for coating larger telescope mirrors (up
to 24" in diameter) (Amateur adventure, not really for financial
gains!)

I'm currently perplexed on the best method to mount the 36 evaporative
heating elements supports on the back wall of the chamber.

Currently I have a plan on how to complete the mounts, but first I
must support the 36 x 1/2" long ceramic stand offs.

Given it's a vacuum chamber that is also going to operate at quite a high
temperature of around 676 deg C with a 10^-6 atmosphere vaccum (based on
30 deg C external ambient temperature) you'd be best off tapping the
holes in to the plate steel chamber. If you take your time tapping the
holes by hand you shouldn't break a single tap.

The other alternative that is more elegant is to have a ceramic ring and
combined standoffs made by a specialist ceramic caster. Such a ring can
be mounted on half a dozen bolts around its circumference. Again the use
of blind mounting holes is a very good idea.

Ian

James Lerch wrote:
....
I'm building a vacuum chamber for coating larger telescope mirrors
....
I'm currently perplexed on the best method to mount the 36 evaporative
heating elements supports on the back wall of the chamber.

Currently I have a plan on how to complete the mounts, but first I
must support the 36 x 1/2" long ceramic stand offs.

The wall that these 36 points will be mounted to is 1/2" thick sheet
steel, and the end results has to be a leak free semi-strong support
(there's no real weight being supported, just the occasional need to
change the tungsten filaments.)

My Current ideas a

#1 Drill 36 holes thru the back wall, insert 5/32" bolt with an
O-ring on the outside [...] #2 [...] #3 [...]

You mentioned a spot welder in your later post; I think
spot welding the supports to the wall would be a reasonable
approach. You would need flat-head bolts and a spot welder
pressure foot with a 5/32" hole in it to press the flat head
against the wall without munging the threads during the weld.
You probably should run a few tests with some bolts, your
welder, and scrap 1/2" plate to decide if this will work.
-jiw

A variant of someone else's suggestion is to use a thin plate of sheet metal
(16 ga or so) and press in PEM-style studs. Yes you have to drill or punch
the holes first but no welding and thus no warping. Put in lots of
ventilation holes between the studs to pump out the space between sheet
metal and wall (and get the surface area down for faster pumpdown), and
attach the sheet to the wall with a hole tapped into the 1/2" wall at each
corner. If you do drill and tap 1/4" into your 1/2" wall, make sure to vent
the threads. Either drill a hole down the center of each stud or grind a
flat on one side to make the threaded end somewhat D shaped. That lets the
gas (air at first, then water forever) escape much faster. Also, if you
have fasteners threaded into both ends of your ceramic spacers you need a
way to vent the middle. Either a hole drilled into the side or again, drill
through the screw or D shaped threads (McMaster Carr sells a small selection
of vented screws with the holes already drilled). If you do braze or weld
the studs into place, don't do it into a blind hole without a vent path or
you can get an impossible to find virtual leak if the braze doesn't make a
good seal and the air behind it leaks into the chamber (it's impossible to
find because you can't get to the back side of the leak).

--
Regards,
Carl Ijames