How to Find a Machine Shop That Has Really Machined Invar

By Invar Machining editorial team· Updated September 26, 2026 · How we write these guides

An Invar machine shop is any CNC shop with a rigid mill or lathe, sharp carbide tooling and a program written for a gummy, work-hardening alloy. That describes thousands of shops. The one you want also runs stress relief between roughing and finishing, measures at 20 °C after the part has soaked and can point to Invar lots it cut in the last year. This guide gives you the questions, a qualification table, a hypothetical quote comparison and the red flags that separate the two.

What “Invar experience” actually means

“We have machined Invar” covers everything from one bracket five years ago to a monthly run of metrology frames. Three concrete things tell you which one you are talking to.

Why “we can cut anything” is not enough

A shop can machine Invar to the drawing’s dimensions on day one and still hand you a part that drifts out of tolerance weeks later. The alloy’s whole value, very low thermal expansion, depends on the machining process not reintroducing instability. That instability comes from residual stress and work hardening the cutting itself creates, covered in why Invar work hardens and stress relief for Invar machined parts. A shop unfamiliar with this either skips the sequence or applies a generic cycle that was not chosen for your part. The drawing tolerance alone will not tell you which one you are getting.

Recent lots, by alloy

Ask which low-expansion alloys the shop cut in the last 12 months and roughly how many lots. Invar 36, Super Invar and Kovar cut similarly on the machine but are specified, heat treated and inspected differently, so an answer by alloy matters. A shop that names the alloy, the rough part size and what the parts were for has the experience in recent memory, where the programmer and the setup person can still use it. A shop that answers with its material list is describing purchasing, not machining. The alloy pages for Invar 36 and Kovar summarize what changes between them.

Furnace access and a cycle chosen for the part

A precision Invar part goes through rough, stress relieve, finish and, on the tightest parts, a final stabilize. That sequence needs a furnace. It does not have to be in house. A contracted heat treater is normal and often better controlled. What matters is that the shop can say where the furnace is, what a typical cycle looks like and how the cycle for your part is chosen: from the drawing, from the spec or from the shop’s own record on similar parts. For Invar 36 after roughing, a typical cycle is a hold at 315 to 345 °C for 1 to 2 hours followed by a slow cool, with the drawing or the governing spec deciding the final numbers. A shop that cannot describe any cycle has not run one recently.

Inspection at 20 °C

Geometrical product specifications are referenced to 20 °C under ISO 1. On 316 stainless, a 3 °C room swing moves a 300 mm dimension by about 0.015 mm, which is why the reference temperature exists. On Invar 36 the same swing moves it by about 0.001 mm, so the part is not the problem. The CMM scales, the fixture and the probe are steel, aluminum or ceramic, and they move. A shop that soaks parts and lets the machine settle before a critical measurement understands this. A shop that measures straight off the mill is reporting a number that includes its own thermal error. Invar machining tolerances covers what ±0.01 mm actually needs, and reading an Invar CMM inspection report shows where the temperature should appear on the paperwork.

Shop qualification checklist

The table below is a typical screen for a first conversation, not a pass or fail test. A shop can be a good fit and still miss one row. A shop that misses most of them is not the shop for a thermally critical part.

Question	Good answer	Red flag
Which low-expansion alloys did you cut in the last 12 months?	Names alloys, rough lot count and part types	“We can source any material”
Where does stress relief run?	In-house furnace or a named contracted heat treater	“We can add that if you need it”
How is the cycle chosen for my part?	From the drawing, the spec or a record on similar parts	One cycle for everything
How do you fixture thin sections?	Low-force or vacuum fixtures, alternating sides, final skim after stress relief	“Carefully”
At what temperature do you inspect?	20 °C after a soak, stated on the report	No stated temperature
How do you run the cutter?	Climb milling, tool changes on a schedule set for Invar	Steel parameters, tools run to failure
What paperwork ships with the part?	Mill cert by default, CTE data and full reports on request	Cert “if required”, no process for it
What would you change on my drawing?	Flags a wall, radius or tolerance that is hard to hold	Nothing to say

Red flags in a quote

• No heat treatment line item on a part with tight tolerances or a thermally sensitive application. Its absence usually means the shop either forgot it or does not plan to run it, not that your part does not need it.
• A single-setup plan on a tight, thin-walled optical or metrology part, with no mention of roughing, stress relief and a separate finishing pass.
• No stated measurement temperature on a CMM report for a part toleranced tighter than about ±0.02 mm.
• Vague answers about alloy experience, especially “we can source any material,” which describes buying power, not machining experience.
• A lead time shorter than the furnace allows. Roughing, a furnace run, cooling and a second setup rarely fit inside a week on a real part.

None of these alone disqualifies a shop, but two or more together, in the table or in the quote, are worth a direct follow-up question before you send a purchase order.

Worked example: two quotes for a hypothetical optical bench

Take a hypothetical 300 mm by 200 mm by 25 mm Invar 36 optical bench, milled from plate, with a mounting face flat to 0.01 mm and six bores at ±0.01 mm true position. Two shops quote it. The lines below are illustrative and typical, not a price list, and the shop’s own quote governs.

Quote line	Shop A	Shop B
Material	Invar 36 plate to ASTM F1684, mill cert	Invar 36 plate, mill cert
Roughing	Listed, 0.5 mm stock left	Listed
Stress relief	Furnace line item, cycle stated	Not listed
Finishing	Second setup, low-force fixture	Same setup as roughing
Inspection	CMM at 20 °C after 4 h soak, report per part	Final inspection, no temperature
Lead time	5 weeks	3 weeks
Price	About 25 % higher	Lower

Shop B is not cheaper. It is quoting a different part. Without the furnace step the bench carries its roughing stress out of the door and can move as that stress relaxes over the following weeks. Without a 20 °C measurement, the 0.01 mm flatness number was never really checked. If the bench holds lenses, the re-machine or the re-order costs more than the 25 % gap. If it is a lab breadboard at ±0.1 mm, Shop B’s plan may be fine, and you should say so on the RFQ so Shop A can drop the lines you do not need.

Regional notes for US, Japan, Korea and EU buyers

Where the buyer sits changes the paperwork more than the machining. Typical points to settle before the quote:

Buyer region	What tends to differ	Put on the RFQ
United States	Material called out to ASTM F1684 or a UNS number; aerospace and space programs may ask for AMS 2454 style heat treatment records and export control handling	Spec, export control status, whether domestic melt matters
Japan	JIS style drawings with tight surface finish callouts, strong expectation of first article data and a dimensional report per part	Finish parameter (Ra or Rz), report format, inspection temperature
South Korea	Semiconductor and display equipment parts, cleanroom packaging, short recurring lots	Cleaning level, packaging, expected reorder cadence
European Union	ISO GPS drawings, EN or DIN material designations alongside UNS, declarations on assemblies	ISO 1 reference temperature, material designation cross-reference, declarations required

None of these change what the shop has to do to keep the part stable. They change whether the quote and the paperwork match your program’s expectations the first time. Across regions, put the designation cross-reference on the drawing so the shop is not guessing at an equivalent. The Invar specs reference lists the common designations side by side.

Sample parts and first article inspection

For a new supplier on a thermally critical part, a small sample order or a first article run ahead of the full quantity confirms that the shop’s stated process holds up on your geometry. This matters more for Invar than for most materials, since the difference between a shop that understands the sequence and one that does not often shows up only after the part has had time to move. Ask for the first article to be measured twice, once after finishing and once a week or two later, so you see the drift and not just the delivery state. Planning for this stage is covered in prototype vs production Invar machining, and the same sample logic applies when you add a backup supplier, covered in second source qualification for Invar parts.

Directories, marketplaces and a matching service

There are three common ways to find a shop. A directory lists shops by capability and location, which tells you who owns a five-axis mill but nothing about whether they run stress relief. A marketplace takes your drawing and returns bids, usually fast, usually from shops competing on price, and leaves the qualification above to you. A matching service asks the questions in the checklist before a shop sees your drawing, then passes the RFQ to a small number of shops that answered them specifically.

Each fits a different job. For a bracket at ±0.1 mm, a marketplace bid is fine. For a mount that has to hold alignment across a 20 °C swing, the qualification is the work, and it is the part a directory or a bid list does not do for you. When we match a request, we ask these questions on your behalf and send your drawing to up to three shops that have already answered them, under the terms in Invar NDA and IP protection.

What to put on the RFQ

Give the shop what it needs to answer the checklist in its quote instead of in a follow-up call:

• Alloy and spec, for example Invar 36 to ASTM F1684
• Heat treatment: stress relief, full stabilization or “shop to recommend”
• The two or three tolerances that actually control function
• Inspection temperature and report format
• Certs and traceability your program requires
• Quantity now and a realistic reorder estimate

The full list, with sample drawing notes, is in what to include on an Invar RFQ. Send your drawing through the RFQ form with those items covered, and we match you with up to three shops that have already passed this check, so your search starts further along than a cold search would.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Invar

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Stress Relief for Invar Machined Parts: When and How Shops Do It · Process
• What to Include on an Invar RFQ: A Drawing Checklist · Choosing a Supplier
• Second-Source Qualification: Requalifying an Invar Part at a New Shop · Choosing a Supplier
• Invar Machining: What Buyers Need to Know Before Sending a Drawing · Materials

All choosing a supplier guides