Plastic screws

[Joseph Gwinn]

On Jul 18, 2019, Phil Hobbs wrote
(in article<tNCdnecJusHGiqzAnZ2dnUU7-Y-dnZ2d@supernews.com&gt:

On 7/17/19 10:08 PM, Joseph Gwinn wrote:
On Jul 17, 2019, Phil Hobbs wrote
(in article<94ednTJ5EJEHtbLAnZ2dnUU7-QnNnZ2d@supernews.com&gt:

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

K/W thermal resistance, so I need to keep the heat leak to a minimum o

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, b
t
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 thereabout
..)

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 forge

Vespel or any of the other plastics you really want.
I think you will have to machine the non-nylon hardware. Delrin is a goo

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 reqt, a stack of shiny steel plate

with staggered metal separators (which can be spot-welded), so the leaka
e
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.

How much vacuum does a SEM need? I was thinking bake-out being needed.

Nah, SEMs work in the low microtorr range. Most don't even use a
turbopump, because diffusion pumps are good enough and don't cause
vibration.

Ahh.

.
UHV (<~ 1 nanotorr) is another beast altogether.

Yes, 1000 times harder.

.
If FR-4 is good enough for that vacuum, then use FR-4 stepped washers, steel
hardware, and belleville spring washers to maintain tension despite thermal
variation.

Washers don't provide enough insulation, because they're short and fat.

This being the reason to use skinny rods then. OK.

.
Or use steel washers, and stagger them to make the thermal path long and
windy.

Hard to get a factor of 100 that way.

I´m not visualizing this, With the classic spaced parallel thin plates, the
heat is forced to zig-zag from side to side, and a 100:1 path ratio can be
arranged.

There is a tradeoff to be made. Stainless steel has low thermal conductivity
compared to aluminum, but aluminum is a better reflector when polished. I
suppose the NASA approach would be gold-plated stainless steel. Or just wrap
the article to be insulated with layers of crumpled gold-plated mylar film -
we see this on satellites all the time.

.

Note that delrin is very slippery, and so delrin nut and bolt will tend to
unscrew itself. A SS nut forced to thread onto a slightly oversize delrin
rod should stay put. As will a press fit into an undersize hole.

The speed nuts are called Tinnerman nuts, as someone else also mentioned.
Tinnerman invented this kind of nut.

Thanks!
Welcome.

We learn from each others problem projects.

Joe Gwinn

 

[George Herold]

On Friday, July 19, 2019 at 6:55:08 PM UTC-4, Joseph Gwinn wrote:

On Jul 18, 2019, Phil Hobbs wrote
(in article<tNCdnecJusHGiqzAnZ2dnUU7-Y-dnZ2d@supernews.com&gt:

On 7/17/19 10:08 PM, Joseph Gwinn wrote:
On Jul 17, 2019, Phil Hobbs wrote
(in article<94ednTJ5EJEHtbLAnZ2dnUU7-QnNnZ2d@supernews.com&gt:

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

K/W thermal resistance, so I need to keep the heat leak to a minimum o

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, b
t
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 thereabout
.)

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 forge

Vespel or any of the other plastics you really want.
I think you will have to machine the non-nylon hardware. Delrin is a goo

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 reqt, a stack of shiny steel plate

with staggered metal separators (which can be spot-welded), so the leaka
e
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.

How much vacuum does a SEM need? I was thinking bake-out being needed..

Nah, SEMs work in the low microtorr range. Most don't even use a
turbopump, because diffusion pumps are good enough and don't cause
vibration.

Ahh.

Yeah me too. I didn't know diff pumps were still in
commercial use. My only memory of diff pumps,
(besides them working just fine.) Is cleaning/
repairing ones that some previous grad student had burned
out... never switched to the backing pump or
something.

George H.

.
UHV (<~ 1 nanotorr) is another beast altogether.

Yes, 1000 times harder.

.
If FR-4 is good enough for that vacuum, then use FR-4 stepped washers, steel
hardware, and belleville spring washers to maintain tension despite thermal
variation.

Washers don't provide enough insulation, because they're short and fat.

This being the reason to use skinny rods then. OK.

.
Or use steel washers, and stagger them to make the thermal path long and
windy.

Hard to get a factor of 100 that way.

I´m not visualizing this, With the classic spaced parallel thin plates, the
heat is forced to zig-zag from side to side, and a 100:1 path ratio can be
arranged.

There is a tradeoff to be made. Stainless steel has low thermal conductivity
compared to aluminum, but aluminum is a better reflector when polished. I
suppose the NASA approach would be gold-plated stainless steel. Or just wrap
the article to be insulated with layers of crumpled gold-plated mylar film -
we see this on satellites all the time.

.

Note that delrin is very slippery, and so delrin nut and bolt will tend to
unscrew itself. A SS nut forced to thread onto a slightly oversize delrin
rod should stay put. As will a press fit into an undersize hole.

The speed nuts are called Tinnerman nuts, as someone else also mentioned.
Tinnerman invented this kind of nut.

Thanks!
Welcome.

We learn from each others problem projects.

Joe Gwinn

 

[Joerg]

On 2019-07-18 18:28, Phil Hobbs wrote:

On 7/17/19 6:05 PM, Joerg wrote:
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.

Interesting idea, I don't know.

There is also this sort:

https://images-na.ssl-images-amazon.com/images/I/51siOyY3%2BDL._SL1000_.jpg

https://www.youtube.com/watch?v=LUa169kRuz4

--
Regards, Joerg

http://www.analogconsultants.com/

 

[whit3rd]

On Wednesday, July 17, 2019 at 7:08:11 PM UTC-7, Joseph Gwinn wrote:

Note that delrin is very slippery, and so delrin nut and bolt will tend to
unscrew itself. A SS nut forced to thread onto a slightly oversize delrin rod
should stay put. As will a press fit into an undersize hole.

i'm uncertain about this approach; if you start with elastic material under
stress, and heat-cycle it (it's a heat pump, after all), the thermal expansion and
the strain will interact. A thermal cycle can stretch the member, it'll just become
loose. Something that's stiffer than nominally required for the target force would
be preferable to a thermoplastic.

Prestressed music wires? Thinned SS bicycle spokes?

 

whit3rd <whit3rd@gmail.com> wrote in
news:12041aa9-ee6a-41d1-b9b7-05726a359433@googlegroups.com:

On Wednesday, July 17, 2019 at 7:08:11 PM UTC-7, Joseph Gwinn
wrote:

Note that delrin is very slippery, and so delrin nut and bolt
will tend to unscrew itself. A SS nut forced to thread onto a
slightly oversize delrin rod should stay put. As will a press fit
into an undersize hole.

i'm uncertain about this approach; if you start with elastic
material under stress, and heat-cycle it (it's a heat pump, after
all), the thermal expansion and the strain will interact. A
thermal cycle can stretch the member, it'll just become loose.
Something that's stiffer than nominally required for the target
force would be preferable to a thermoplastic.

Prestressed music wires? Thinned SS bicycle spokes?

I think Delrin 'cold flows' just like Teflon does, so his 'swaged'
thread impingement method would likely not work under stress as well
as properly formed threads and additional engagement policies (more
threads captivation).

The rod end would split easier and if it does cold flow in a
similar way as Teflon, they would also 'pull out' (shear) easier as
well.

 

[Chris Jones]

On 19/07/2019 11:29, Phil Hobbs wrote:

On 7/18/19 8:05 AM, Chris Jones wrote:
On 17/07/2019 23:25, 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?

Cheers

Phil Hobbs


Crazy idea:
sandwich the TEC between big NdFeB magnets. Lots of compressive force
and no heat leak.

Right, a 1T magnet inside an SEM chamber.  That'd go over real big.

hehe, yes, even more crazy than I thought - I forgot the SEM context and
ws thinking of some optical instrument in a vacuum, for some reason.

 

[Joseph Gwinn]

On Jul 20, 2019, DecadentLinuxUserNumeroUno@decadence.org wrote
(in article <qgv2f9$1ack$1@gioia.aioe.org&gt:

whit3rd <whit3rd@gmail.com> wrote in
news:12041aa9-ee6a-41d1-b9b7-05726a359433@googlegroups.com:

On Wednesday, July 17, 2019 at 7:08:11 PM UTC-7, Joseph Gwinn
wrote:

Note that delrin is very slippery, and so delrin nut and bolt
will tend to unscrew itself. A SS nut forced to thread onto a
slightly oversize delrin rod should stay put. As will a press fit
into an undersize hole.

i'm uncertain about this approach; if you start with elastic
material under stress, and heat-cycle it (it's a heat pump, after
all), the thermal expansion and the strain will interact. A
thermal cycle can stretch the member, it'll just become loose.
Something that's stiffer than nominally required for the target
force would be preferable to a thermoplastic.

Prestressed music wires? Thinned SS bicycle spokes?

I think Delrin 'cold flows' just like Teflon does, so his 'swaged'
thread impingement method would likely not work under stress as well
as properly formed threads and additional engagement policies (more
threads captivation).

The rod end would split easier and if it does cold flow in a
similar way as Teflon, they would also 'pull out' (shear) easier as
well.

Delrin does not cold flow like teflon.

Joe Gwinn

 

[bitrex]

On 7/17/19 11:28 AM, Tim Williams wrote:

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

Always keep the screws you need in stock for now, you can always get
plenty more to screw it up later.

 

[Jamie M]

On 2019-07-19 10:00 p.m., whit3rd wrote:

On Wednesday, July 17, 2019 at 7:08:11 PM UTC-7, Joseph Gwinn wrote:

Note that delrin is very slippery, and so delrin nut and bolt will tend to
unscrew itself. A SS nut forced to thread onto a slightly oversize delrin rod
should stay put. As will a press fit into an undersize hole.

i'm uncertain about this approach; if you start with elastic material under
stress, and heat-cycle it (it's a heat pump, after all), the thermal expansion and
the strain will interact. A thermal cycle can stretch the member, it'll just become
loose. Something that's stiffer than nominally required for the target force would
be preferable to a thermoplastic.

Prestressed music wires? Thinned SS bicycle spokes?

Hi,

I was also thinking thin low conductivity metal wire, pre-stressed by
coiling the ends together, but a better solution is to use a spring of
similar material, and fasten it to each end, this creates a longer
thermal conduction path than a straight wire, and also can be more
accurately pre-stressed, and is would have less variance in tension
with thermal cycling than a straight wire too.

The only tricky part is fastening the ends of the springs so they dont
go through the holes but actually if the spring is sized right it could
actually act as a screw itself in the holes and allow for adjusting the
tension that way.

cheers,
Jamie

 

Joseph Gwinn <joegwinn@comcast.net> wrote in
news:0001HW.22E3834C011AAF33700003BCB2EF@news.giganews.com:

On Jul 20, 2019, DecadentLinuxUserNumeroUno@decadence.org wrote
(in article <qgv2f9$1ack$1@gioia.aioe.org&gt:

whit3rd <whit3rd@gmail.com> wrote in
news:12041aa9-ee6a-41d1-b9b7-05726a359433@googlegroups.com:

On Wednesday, July 17, 2019 at 7:08:11 PM UTC-7, Joseph Gwinn
wrote:

Note that delrin is very slippery, and so delrin nut and bolt
will tend to unscrew itself. A SS nut forced to thread onto a
slightly oversize delrin rod should stay put. As will a press
fit into an undersize hole.

i'm uncertain about this approach; if you start with elastic
material under stress, and heat-cycle it (it's a heat pump,
after all), the thermal expansion and the strain will interact.
A thermal cycle can stretch the member, it'll just become
loose. Something that's stiffer than nominally required for the
target force would be preferable to a thermoplastic.

Prestressed music wires? Thinned SS bicycle spokes?

I think Delrin 'cold flows' just like Teflon does, so his
'swaged' thread impingement method would likely not work under
stress as well as properly formed threads and additional
engagement policies (more threads captivation).

The rod end would split easier and if it does cold flow in a
similar way as Teflon, they would also 'pull out' (shear) easier
as well.

Delrin does not cold flow like teflon.

Joe Gwinn

It would still follow logically that cut threads would have better
shear characteristics than force swaging them in.

 

[John S]

On 7/21/2019 4:58 AM, DecadentLinuxUserNumeroUno@decadence.org wrote:

Joseph Gwinn <joegwinn@comcast.net> wrote in
news:0001HW.22E3834C011AAF33700003BCB2EF@news.giganews.com:

On Jul 20, 2019, DecadentLinuxUserNumeroUno@decadence.org wrote
(in article <qgv2f9$1ack$1@gioia.aioe.org&gt:

whit3rd <whit3rd@gmail.com> wrote in
news:12041aa9-ee6a-41d1-b9b7-05726a359433@googlegroups.com:

On Wednesday, July 17, 2019 at 7:08:11 PM UTC-7, Joseph Gwinn
wrote:

Note that delrin is very slippery, and so delrin nut and bolt
will tend to unscrew itself. A SS nut forced to thread onto a
slightly oversize delrin rod should stay put. As will a press
fit into an undersize hole.

i'm uncertain about this approach; if you start with elastic
material under stress, and heat-cycle it (it's a heat pump,
after all), the thermal expansion and the strain will interact.
A thermal cycle can stretch the member, it'll just become
loose. Something that's stiffer than nominally required for the
target force would be preferable to a thermoplastic.

Prestressed music wires? Thinned SS bicycle spokes?

I think Delrin 'cold flows' just like Teflon does, so his
'swaged' thread impingement method would likely not work under
stress as well as properly formed threads and additional
engagement policies (more threads captivation).

The rod end would split easier and if it does cold flow in a
similar way as Teflon, they would also 'pull out' (shear) easier
as well.

Delrin does not cold flow like teflon.

Joe Gwinn



It would still follow logically that cut threads would have better
shear characteristics than force swaging them in.

As usual, Alwayswrong, you were wrong. So who would trust your logic,
Alwayswrong? Do you have any links to back you up?... I thought not.

 

John S <Sophi.2@invalid.org> wrote in
news:qh2gh0$3h7$1@dont-email.me:

As usual, Alwayswrong, you were wrong. So who would trust your
logic, Alwayswrong? Do you have any links to back you up?... I
thought not.

Do you always spout retarded insulting horseshit?

Thread a hole on the end of a (delrin) plastic rod. Which rod will
split first? The one with precision cut threads, and the screw
nicely captivated fully, or the one with threads swaged in by force,
with the entire rod/screw assembly still under (outward) stress?

I am betting that scenario 2 with the ram it, cram it, jam it, and
slam it, damn it, Janet version will split first.

You? I don't give a fat flying fuck what an insulting, retarded
bastard like you feels is correct. You can ram it, cram it, jam it,
and slam it up your ass for all I care. FOAD, troll punk.

 

[John S]

On 7/21/2019 6:34 PM, DecadentLinuxUserNumeroUno@decadence.org wrote:

John S <Sophi.2@invalid.org> wrote in
news:qh2gh0$3h7$1@dont-email.me:

As usual, Alwayswrong, you were wrong. So who would trust your
logic, Alwayswrong? Do you have any links to back you up?... I
thought not.

Do you always spout retarded insulting horseshit?

Only when it is deserved.

 

John S <Sophi.2@invalid.org> wrote in news:qh6qq7$mb8$2@dont-
email.me:

On 7/21/2019 6:34 PM, DecadentLinuxUserNumeroUno@decadence.org
wrote:
John S <Sophi.2@invalid.org> wrote in
news:qh2gh0$3h7$1@dont-email.me:

As usual, Alwayswrong, you were wrong. So who would trust your
logic, Alwayswrong? Do you have any links to back you up?... I
thought not.

Do you always spout retarded insulting horseshit?


Only when it is deserved.

Right... said the jackass whom conveniently snipped the segment of
the post proving that you are full of shit.

 

[david eather]

On 19/07/2019 1:07 pm, david eather wrote:

On 17/07/2019 11:25 pm, 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


Are M3 polycarbonite screws of any use?

is there anything wrong with polycarbonite for use in a vacuum.

 

[Rick C]

On Tuesday, July 23, 2019 at 11:45:04 PM UTC-4, david eather wrote:

On 19/07/2019 1:07 pm, david eather wrote:


On 17/07/2019 11:25 pm, 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


Are M3 polycarbonite screws of any use?

is there anything wrong with polycarbonite for use in a vacuum.

I already suggested that. When I looked it up the properties were better than nylon in the important respects for this application. All plastic will absorb some moisture which it will out gas. But some are worse than others and it can impact the material properties. Polycarbonate (which is what you probably meant rather than "polycarbonite") is a lot better in this regard than many plastics. Mostly it should be a lot better price than some of the plastics with standard size hardware available off the shelf.

--

Rick C.

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

 

[david eather]

On 24/07/2019 2:48 pm, Rick C wrote:

On Tuesday, July 23, 2019 at 11:45:04 PM UTC-4, david eather wrote:
On 19/07/2019 1:07 pm, david eather wrote:


On 17/07/2019 11:25 pm, 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


Are M3 polycarbonite screws of any use?

is there anything wrong with polycarbonite for use in a vacuum.

I already suggested that. When I looked it up the properties were better than nylon in the important respects for this application. All plastic will absorb some moisture which it will out gas. But some are worse than others and it can impact the material properties. Polycarbonate (which is what you probably meant rather than "polycarbonite")

Yep. Mia Culpa.

M3 and M4 $AUD30 for 100, $AUD250 per 1000. Plus postage.

is a lot better in this regard than many plastics. Mostly it should be
a lot better price than some of the plastics with standard size hardware
available off the shelf.
>

 

[John S]

On 7/23/2019 7:58 AM, DecadentLinuxUserNumeroUno@decadence.org wrote:

John S <Sophi.2@invalid.org> wrote in news:qh6qq7$mb8$2@dont-
email.me:

On 7/21/2019 6:34 PM, DecadentLinuxUserNumeroUno@decadence.org
wrote:
John S <Sophi.2@invalid.org> wrote in
news:qh2gh0$3h7$1@dont-email.me:

As usual, Alwayswrong, you were wrong. So who would trust your
logic, Alwayswrong? Do you have any links to back you up?... I
thought not.

Do you always spout retarded insulting horseshit?


Only when it is deserved.

Right... said the jackass whom conveniently snipped the segment of
the post proving that you are full of shit.

And YOU didn't snip, Dip?