Plastic screws

[Phil Hobbs]

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Gerhard Hoffmann]

Am 17.07.19 um 15:25 schrieb Phil Hobbs:

> Anybody got a source of plastic screws other than nylon?

M1 stainless steel + 2* TO-3 / TO-220 fiberglass-reinforced insulation
washers?

cheers, Gerhard

 

[Joseph Gwinn]

On Jul 17, 2019, Phil Hobbs wrote
(in article<RKudnVUn6NouvLLAnZ2dnUU7-VfNnZ2d@supernews.com&gt:

For that SEM cathodoluminescence gizmo I mentioned in the distributed

attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

I think you will have to machine the non-nylon hardware. Delrin is a good
choice, if it´s OK in vacuum. Note that unlike most plastics, one can
press-fit delrin, so threads may not be needed.

For vacuum, I´d go to ceramic spacers with stainless steel hardware keeping
them in compression.

Or, if thermal insulation is the only rest, a stack of shiny steel plates
with staggered metal separators (which can be spot-welded), so the leakage
paths are very long, and radiation coupling between plates dominates (there
been no air to support convection).

Joe Gwinn

 

[Phil Hobbs]

On 7/17/19 9:37 AM, Gerhard Hoffmann wrote:

Am 17.07.19 um 15:25 schrieb Phil Hobbs:

Anybody got a source of plastic screws other than nylon?

M1 stainless steel + 2* TO-3 / TO-220 fiberglass-reinforced insulation
washers?

cheers, Gerhard

Too big a heat leak. A larger plastic part with steel screws could
work, but this gizmo has to fit in a really tight space.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Wed, 17 Jul 2019 09:25:04 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Got a sketch of the geometry?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Phil Hobbs]

On 7/17/19 9:41 AM, Joseph Gwinn wrote:

On Jul 17, 2019, Phil Hobbs wrote
(in article<RKudnVUn6NouvLLAnZ2dnUU7-VfNnZ2d@supernews.com&gt:

For that SEM cathodoluminescence gizmo I mentioned in the distributed

attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.
I think you will have to machine the non-nylon hardware. Delrin is a good
choice, if it´s OK in vacuum. Note that unlike most plastics, one can
press-fit delrin, so threads may not be needed.

For vacuum, I´d go to ceramic spacers with stainless steel hardware keeping
them in compression.

Or, if thermal insulation is the only rest, a stack of shiny steel plates
with staggered metal separators (which can be spot-welded), so the leakage
paths are very long, and radiation coupling between plates dominates (there
been no air to support convection).

Joe Gwinn

Ceramic is a surprisingly decent thermal conductor,
unfortunately--better than glass, and much better than plastic. SEMs
don't run in UHV, so FR-4 is okay. At lower delta-T you can just have
the board hang ten over the TEC and use the FR-4 for insulation.

Interesting idea about the delrin--maybe I could use unthreaded plastic
studs with those speed-nut-like grip gizmos on the other end. (What are
those things called, anyway?)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Wed, 17 Jul 2019 14:53:19 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Wed, 17 Jul 2019 09:25:04 -0400) it happened Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote in
RKudnVUn6NouvLLAnZ2dnUU7-VfNnZ2d@supernews.com>:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Dunno,
magnetic levitation?

Stainless steel spring? That wouldn't conduct much heat.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Wed, 17 Jul 2019 07:47:40 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Wednesday, July 17, 2019 at 9:25:17 AM UTC-4, Phil Hobbs wrote:
For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.
Well a bunch of silly ideas...
The Delrin sounds OK, maybe a machine shop can cut a thread on the end
of a rod for you. (There are a few places on the web listing delrin
screws, but no prices.)

If the sizes work out, you can just scrunch a ss nut onto a Delrin
rod.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Martin Brown]

On 17/07/2019 14:25, Phil Hobbs wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C.  It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design.  Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws.  Switching from #2
stainless to #6 nylon reduces the heat leak by 20x.  However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart.  (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

PEEK is a pretty good engineering plastic that will tolerate hard vacuum
fairly well provided that you don't want to bake it too hard.
I don't think you are going to like their prices though. eg.

http://www.fastenercomponents.com/peek-screws-ezp-20.html

Or for a one off buy some PEEK rod and thread it yourself.

https://www.directplastics.co.uk/peek-rod

--
Regards,
Martin Brown

 

[Carl]

"Phil Hobbs" wrote in message
news:94ednTJ5EJEHtbLAnZ2dnUU7-QnNnZ2d@supernews.com...

Interesting idea about the delrin--maybe I could use unthreaded plastic
studs with those speed-nut-like grip gizmos on the other end. (What are
those things called, anyway?)

Push nuts or self threading nuts. Also, mcmaster.com has fiberglass
threaded rod but the smallest is 3/8"-16, for $15/foot so not completely
hateful. They also have plain FRP fiberglass rod down to 1/8" diameter you
could use with the push nuts. I think that would be stiffer, stronger, and
less prone to creep than delrin.

--
Regards,
Carl Ijames

 

[Jan Panteltje]

On a sunny day (Wed, 17 Jul 2019 09:25:04 -0400) it happened Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote in
<RKudnVUn6NouvLLAnZ2dnUU7-VfNnZ2d@supernews.com>:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Dunno,
magnetic levitation?

 

[George Herold]

On Wednesday, July 17, 2019 at 9:25:17 AM UTC-4, Phil Hobbs wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

Well a bunch of silly ideas...
The Delrin sounds OK, maybe a machine shop can cut a thread on the end
of a rod for you. (There are a few places on the web listing delrin
screws, but no prices.)

How about soldering/ or epoxy and then squeeze it some other way.
(Fishing line/ zip ties, something else?)

You need my beer can cooler on the hot plate :^)
Has anyone done a distributed two stage TEC, Some small TEC in
vacuum and then a bigger one outside the vacuum going to a
heatsink?

George H.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Tim Williams]

Don't base your pricing on McMaster. Their core business is the one-off,
priced accordingly.

Buy the parts you need now, and source them [cheaply] later. (They'll tell
you who made the part, on request.)

It really doesn't matter until you need actual boxes of, or pounds of,
fasteners.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message
news:RKudnVUn6NouvLLAnZ2dnUU7-VfNnZ2d@supernews.com...

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC) down
to about -10C. It's on the end of a piece of aluminum with about 5 K/W
thermal resistance, so I need to keep the heat leak to a minimum or I'll
run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a good
option, because it eliminates the heat leak through the screws, but I
really want to put a compressive preload on the TEC, because that improves
its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon isn't
good in a vacuum, because it outgasses, dries out, and falls apart. (At
least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Rick C]

On Wednesday, July 17, 2019 at 9:25:17 AM UTC-4, Phil Hobbs wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

I'm just curious about what you say of nylon in a vacuum. While plastics absorb moisture and that moisture will be released in a vacuum, the moisture should not strengthen the material, in fact moisture reduces the strength. I see they refer to moisture increasing the "toughness" of nylon which I believe means resistance to shock, so I guess pulling the moisture out makes it more brittle.

Polycarbonate is an affordable material. It seems to have better properties for your application than Nylon, although that's not a high bar. Here's one source.

https://www.solidspot.com/polycarbonate_socket_head_cap_screws.html

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Jon Elson]

On Wed, 17 Jul 2019 09:25:04 -0400, Phil Hobbs wrote:

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x.

How about stainless screws with nylon step washers as heat insulators.
Especially in vacuum, there is no heat conduction without compliant
thermal conductors or grease, only radiation. You'd be AMAZED at the
poor thermal conductivity of flat, clean surfaces clamped together, in
vacuum. Thermal conductivity drops by a factor of 100:1 when the air is
pumped out.

Jon

 

[Jamie M]

On 2019-07-17 6:25 a.m., Phil Hobbs wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C.  It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design.  Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws.  Switching from #2
stainless to #6 nylon reduces the heat leak by 20x.  However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart.  (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Hi,

If you can get the torque accurate, low conductivity sufficiently strong
wire through the same screw holes and twisted with pliers to tighten
might be simplest.

cheers,
Jamie

 

[John Larkin]

On Wed, 17 Jul 2019 09:25:04 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Amazon has acetal (delrin) screws for a few cents each.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Joerg]

On 2019-07-17 06:25, Phil Hobbs wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Can you use rubber rivets?

https://www.amazon.com/Mounting-Screws-Rivets-Silicone-Computer/dp/B01M4LVBY2

They could also provide tension.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[George Herold]

On Wednesday, July 17, 2019 at 4:02:14 PM UTC-4, John Larkin wrote:

On Wed, 17 Jul 2019 09:25:04 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Amazon has acetal (delrin) screws for a few cents each.

McMaster -Carr had threaded rod, but the smallest was
10-24 (or something)

Back in grad school I made threaded rod with the right size rod,
(maybe with help of a lathe) and a die.
I once just ran a die down a piece of rod,
(~1/8" Tellerium copper) by hand...
the thread got all wiggley-woggley after a while.
(I needed to hold the die perpendicular to the
rod axis... I was much younger then.)

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Wednesday, July 17, 2019 at 7:32:32 PM UTC-4, George Herold wrote:

On Wednesday, July 17, 2019 at 4:02:14 PM UTC-4, John Larkin wrote:
On Wed, 17 Jul 2019 09:25:04 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

For that SEM cathodoluminescence gizmo I mentioned in the distributed
attenuator thread, I need to cool a multi-pixel photon counter (MPPC)
down to about -10C. It's on the end of a piece of aluminum with about 5
K/W thermal resistance, so I need to keep the heat leak to a minimum or
I'll run out of Q-dot.

So I'm sort of breathing on the cold plate design. Silver epoxy is a
good option, because it eliminates the heat leak through the screws, but
I really want to put a compressive preload on the TEC, because that
improves its shear strength and shock resistance.

One traditional approach is to use nylon screws. Switching from #2
stainless to #6 nylon reduces the heat leak by 20x. However, nylon
isn't good in a vacuum, because it outgasses, dries out, and falls
apart. (At least it's reasonably cheap--a nickel a screw or thereabouts.)

I looked at other plastic screws at McMaster-Carr, and the price
difference is fairly shocking--even screws made of some totally
garden-variety plastic such as PVC cost dollars apiece, and just forget
Vespel or any of the other plastics you really want.

Anybody got a source of plastic screws other than nylon?

Cheers

Phil Hobbs

Amazon has acetal (delrin) screws for a few cents each.
McMaster -Carr had threaded rod, but the smallest was
10-24 (or something)

Back in grad school I made threaded rod with the right size rod,
(maybe with help of a lathe) and a die.
I once just ran a die down a piece of rod,
(~1/8" Tellerium copper) by hand...
the thread got all wiggley-woggley after a while.
(I needed to hold the die perpendicular to the
rod axis... I was much younger then.)

George H.

Oh Phil, I wanted to ask how many screws, you are
using to hold down the TEC. (I've used 2 and 4,
I can make arguments for 1 and 3.)
Curious minds want to know,
George :^)

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com