Subject: Re: INVAR

From: [email-masked] (Mark Fraser)
Date: 10 Jan 1995 04:48:27 GMT

Microwave waveguides are sometimes [frequently?] made of Invar.
Check your local junkshop, electrojunk, that is, or maybe the
folks in your department [grinnnn]

Cheers / Mark

[email-masked] wrote:
: Derek Abbott ([email-masked]) wrote:
: : Where can I buy a 2 cm block of the metal alloy called invar (commonly used
: : in watch and clock manufacture) ?

: : Invar has a zero coefficient of thermal expansion -- i'm interested in any
: : metal alloy that can do that.

: I'm almost sure you realize that is not true.  It has a low thermal expansion
: coefficient but it is not zero.

: P.S.  It also has a low machinability rating.

: Dave Parsons

---

From: [email-masked] (Derek Abbott)
Date: 9 Jan 1995 17:04:30 +1030

Where can I buy a 2 cm block of the metal alloy called invar (commonly used
in watch and clock manufacture) ?

Invar has a zero coefficient of thermal expansion -- i'm interested in any
metal alloy that can do that.

---

From: [email-masked] (James Kirkpatrick)
Date: 10 Jan 95 08:46:01 MST

In article <[email-masked]>, [email-masked] () writes:
|>Derek Abbott ([email-masked]) wrote:
|>: Where can I buy a 2 cm block of the metal alloy called invar (commonly used
|>: in watch and clock manufacture) ?
|>
|>: Invar has a zero coefficient of thermal expansion -- i'm interested in any
|>: metal alloy that can do that.
|>
|>I'm almost sure you realize that is not true.  It has a low thermal expansion
|>coefficient but it is not zero.
|>
|>P.S.  It also has a low machinability rating.

I too am looking for a small piece of Invar.  I need a sleeve roughly
1/2 inch outside diameter, 5/16 inside, and about 2 inches long.  Or
any piece in that size range.

As for Dave's comments, he's generally correct,  but:

  1.  There have been specific examples of Invar that exhibited zero
      or even negative thermal expansion.  Trouble is, you can't reliably
      make it that way, it's more of an accident when it happens.
  2.  There are different Invar alloys.  The original, Invar 36 I believe,
      is roughly as tough to machine as stainless.  Another alloy has
      about the same thermal characteristics but has added Cobalt to make
      it "free cutting"; can't remember the alloy name and of course my
      references are at home.
  3.  There is a newer alloy called "Super Invar" that is tough, but better
      than Invar 36.  I can't remember if it has better thermal characteristics
      or long-term dimensional stability.

An alternative that was suggested to me just yesterday was a machinable
ceramic from Coors.  You machine it (carefully) then bake it to harden it.
I haven't had time to research this yet.  Maybe I can get an engineering
sample :-).

There was an article in the Horological Sciences Newsletter about Invar and
how to obtain it, some time last year I think.  There are suppliers but the
minimum order is the catch for folks like me.  If others are interested
perhaps we can put together a group order!

Jim

---

From: [email-masked] (Richard Ballew)
Date: 10 Jan 95 18:32:39 GMT

>any piece in that size range.

(deletions for brevity)

>Jim

A source I have used for invar rod for laser cavity length stabilization
is Burleigh Instruments, Inc.  Their address is Burleigh Park, P.O.Box E,
Fishers, NY 14453-0755.  They make and sell scientific instrumentation
but also sell materials such as invar.  They probably also have super
invar.  I do not have a price list or any other information apart from
their phone number which is (716) 924-9355.  Just call them and tell them
what you want.  I do not know if they have a minumum order.  

Good luck,
Richard Ballew
University of Illinois, School of Chemical Sciences
[email-masked]

---

From: [email-masked] (Dan Caster)
Date: 11 Jan 1995 01:23:30 GMT

Nasa came up with a graphite glass composite which had a zero coeficient
of expansion.  Glass is plus and graphite is minus.  i forget the ratios.
-- 
 					Cheers,
					Dan
Greetings from Bainbridge Island, Washington

---

From: David McQuiston <[email-masked]>
Date: Wed, 11 Jan 1995 00:39:43 -0800

> perhaps we can put together a group order!
For machinable ceramic you could use Macor from Corning Glass Works,
CTE = 9.4*10^-6/K.

For low CTE, Zerodur from Schott Glass Technologies, Duryea, PA
CTE = 0 +/- 0.10*10^-6/K   273K < T < 323K .

Dave

---

From: [email-masked]
Date: 11 Jan 1995 03:39:00 GMT

>folks in your department [grinnnn]

Are you sure about that? Most waveguide I've used has been a copper
alloy or (in 140 GHz-ish and up bands) silver alloy. If they really
make some waveguide from Invar, I would think it would have to be plated
for high surface conductivity for most uses. How do you identify Invar
waveguide? Pick it up with a magnet?

                      ...when I sit down to write
Don Holly             my intentions are always blameless however deplorable
                      the ultimate effect of the act may turn out to be.
                                           --Joseph Conrad

---

From: [email-masked] (James Kirkpatrick)
Date: 11 Jan 95 17:08:43 MST

In article <[email-masked]>,
 [email-masked] (Derek Abbott) writes:
|>Where can I buy a 2 cm block of the metal alloy called invar (commonly used
|>in watch and clock manufacture) ?
|>
|>Invar has a zero coefficient of thermal expansion -- i'm interested in any
|>metal alloy that can do that.

More followup than my previous:

There are three types of Invar.  Invar 36 is good stuff but a "first
class sumbitch" to machine; about like stringy stainless steel.
There is a variant called "FM" (Free Machining) that apparently is
a slightly different alloy but is nice to work with.  Then there is
Super Invar which has some Cobalt added, has about 3 times lower
thermal expansion, but once again is a sumbitch to work with.
There are some reports that Super Invar is not as dimensionally
stable as ordinary Invar, and will spontaneously change size (very
slightly, of course!) more often than Invar.  This problem can be
reduced by proper annealing, I believe.

Invar is used where a very stable dimension needs to be maintained
over temperature variations.  Precision clock pendulums are one example.
Other, newer materials are available such as ceramics and quartz but
are much more brittle and difficult to work with.

I did some checking around and there are three places I've found that
carry Invar here in the USA:

1.  Burleigh Instruments, (716) 924-9355.  They carry Super Invar in
    1/2 inch, 5/8 inch, 3/4 inch, and 1 inch rods (possibly other
    forms, not certain).  They charge per unit length, e.g. $2.25
    per cm as of today for the 1/2 inch.  Normally $125 minimum order
    unless paying by credit card.

2.  Scientific Alloys, (401) 596-4947.  Invar FM, $50 minimum.
    These folks are apparently a forging operation and they make
    the stuff.  I have more info on the way.

3.  Fry Steel Co., (310) 802-2721.  Invar FM.  Round stock, I only
    asked about 1/2 inch.  They usually sell 12-foot "sticks".
    For 1/2 inch, a 12-foot stick is 8 pounds, and it costs $12
    per pound.  If you want less, it is $12.95 per pound, plus
    an $8 cutting charge, $25 packaging charge, plus shipping.
    Note UPS won't take a 12-foot package!

Jim

---

From: [email-masked] (Derek Abbott)
Date: 12 Jan 1995 11:48:47 +1030

In article <[email-masked]> [email-masked] () writes:
>Derek Abbott ([email-masked]) wrote:
>: Where can I buy a 2 cm block of the metal alloy called invar (commonly used
>: in watch and clock manufacture) ?
>
>: Invar has a zero coefficient of thermal expansion -- i'm interested in any
>: metal alloy that can do that.
>
>I'm almost sure you realize that is not true.  It has a low thermal expansion
>coefficient but it is not zero.

Sure. I was making an engineering approximation!

>P.S.  It also has a low machinability rating.

You mean, it is hard to work with? What sort of cutting heads are needed?

---

From: [email-masked] (James Kirkpatrick)
Date: 12 Jan 95 10:48:48 MST

In article <[email-masked]>,
 [email-masked] (Derek Abbott) writes:
|>
|>You mean, it is hard to work with? What sort of cutting heads are needed?

It is roughly like machining stainless steel (another engineering
approximation!).  It tends to be a bit "stringy", and will sort of
tear instead of cutting cleanly.  I have a 5/16 rod I threaded at
40 tpi for a pendulum rod, and purchased a real TiN coated carbide
outside threading tool to do it.  I practiced on a piece of random
stainless rod and got terrible results.  Out of frustration I stuck
the Invar rod in the lathe anyway, and got quite decent threads
after about an inch while the rod settled into the traveling rest.
I had intended to finish the threads with a die, but ended up just
using the lathe-chased thread instead.  The rating nut is currently
a piece of scrap brass rod that was threaded with a 5/16-40 tap, and
the Invar threads were cut until a snug but manageable fit was obtained
with the nut.  I may need to finish finishing the fit with a mild
abrasive but hope to avoid this because such abrasive would become
embedded in the nut (could use a temporary "waste" nut) and possibly
the rod threads as well.  I may also use the die, but I've had poor
luck in the past with dies trying to remove a small amount.  This is
the only work with Invar I've done.

So, the short answer is, use carbide tooling.  If at all possible
buy the "free machining" form of Invar 36; I think mine isn't but it
is second-hand so I'm not sure!  The "FM" form may be machinable with
tool steel.

Jim

---

From: [email-masked] ()
Date: 13 Jan 1995 02:16:13 GMT

James Kirkpatrick ([email-masked]) wrote:

: In article <[email-masked]>,
:  [email-masked] (Derek Abbott) writes:
: |>
: |>You mean, it is hard to work with? What sort of cutting heads are needed?

Yes, it is hard to work with.  It is In Var which I take to be a play on
Invariable.  It does not like to be changed.  However it is not the worst
stuff in the world to machine.

: So, the short answer is, use carbide tooling.

There are at least two schools of thinking on the type of cutter to use
Use carbide so it will hold it's edge longer or use tool steel that you
can sharpen.  I personaly preferr to use tool steel that I can sharpen.
One must say though that TiN is amazing stuff,  TiN Carbide that is.

When I said it has a low machinability rating I did not meen to say that
it can't be machined, but in ones design one might try to avoid small 
tapped holes, thin walls and other things that will piss off your machinist.

I think I might have a 2" diameter piece in my toolbox, I'll take a look for
you.

Dave
e

---

From: [email-masked] (James Phillips)
Date: Sat, 14 Jan 1995 05:23:07 GMT

David McQuiston ([email-masked]) wrote:
: On 10 Jan 1995, James Kirkpatrick wrote:
: > In article <[email-masked]>, [email-masked] () writes:
: > |>Derek Abbott ([email-masked]) wrote:
: > |>: Where can I buy a 2 cm block of the metal alloy called invar [...]

: > There was an article in the Horological Sciences Newsletter about Invar and
: > how to obtain it, some time last year I think.  There are suppliers but the
: > minimum order is the catch for folks like me.  If others are interested
: > perhaps we can put together a group order!

Burleigh Instruments, Pennsylvania somewhere, makers of the clever PZT
"Inchworm" drives, will sell you small quantities, at a high price per
lb, but you'd expect that for a small order.  Newport, southern CA,
deals in Invar and super-Invar, quite possibly small quantities, and
claims to know how to weld the latter while preserving the low CTE.
(BTW, super-Invar is more super than Invar because of lower CTE.)  My
group bought four ~12 ft long Invar rods, that turned out to have as
low a CTE as the super-Invar spec, from Carpenter Steel, somewhere in
the Eastern US.  (It was a decade ago.)

: For machinable ceramic you could use Macor from Corning Glass Works,
: CTE = 9.4*10^-6/K.  

(I don't think you need to bake Macor after machining and before use.)

: For low CTE, Zerodur from Schott Glass Technologies, Duryea, PA
: CTE = 0 +/- 0.10*10^-6/K   273K < T < 323K .

Also Graphite-epoxy and graphite-cyanate, not the ordinary stuff but
special fiber/resin ratios (try Composite Optics, Inc.).  WATCH THE
OUT OF PLANE CTE!  You can buy laminate with 0 CTE in both in-plane
directions, but out of plane it always has the CTE of the resin.  This
is so big that a single thickness can dominate the CTE of an assembly.
Careful joint design can eliminate any thickness in the thermal
expansion path.

I can supply addresses of any of the above upon request, on Tuesday
when I return to my office.  

--
James Phillips                Harvard-Smithsonian Center for Astrophysics
                           Opinions mine, not Harvard's or Smithsonian's.