Invar Machining: What Buyers Need to Know Before Sending a Drawing

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

Invar machining is ordinary CNC work on an unusual alloy. The mills and lathes are the same ones that cut stainless. What changes is the sequence, because the property you are paying for, very low thermal expansion, can be damaged by the cutting itself and restored by a furnace. Understand that one point and you can read an Invar quote line by line, write an RFQ that gets comparable prices and receive a part that is still in tolerance six months after delivery.

What Invar is and why it costs more

Invar 36 is an iron alloy with about 36 % nickel, carbon held under 0.1 %, manganese under 0.6 % and silicon under 0.35 %. At that nickel content the alloy’s magnetic ordering cancels most of its normal thermal expansion near room temperature. The result is a coefficient of thermal expansion (CTE) of roughly 1.2 to 1.6 ppm/°C between 20 and 100 °C, against 11 to 12 for carbon steel and 23 to 24 for 6061 aluminum. Above the Curie temperature, about 230 °C for Invar 36, the effect disappears and the alloy expands like an ordinary nickel steel. The Wikipedia article on Invar covers the physics.

For a 1 m bar that warms by 10 °C, the numbers mean this. Values are typical handbook figures at room temperature. The mill cert for your heat lot governs.

Material	Typical CTE, ppm/°C	Growth of a 1 m bar over +10 °C
6061 aluminum	23 to 24	0.23 mm
316 stainless	16 to 17	0.16 mm
Carbon steel	11 to 12	0.12 mm
Titanium	8.6	0.09 mm
Invar 36, annealed	1.2 to 1.6	0.012 mm

That last row is why people buy the alloy. Optical mounts, layup tools for carbon fibre parts, wafer stage components, metrology frames and spacecraft structures all use Invar because the part keeps its size when the room, the autoclave or the orbit changes temperature.

The price reflects the nickel. Invar bar and plate cost several times what 304 stainless costs and thick plate can take weeks to arrive. Then the alloy cuts more slowly than steel and needs at least one extra furnace step. None of that is a shop padding the quote. It is the material.

Which alloy to ask for

“Invar” on a drawing is not a specification. Four alloys share the family name in practice and differ in CTE, temperature range and price. Values below are typical room temperature figures. The mill cert governs.

Alloy	Nominal composition	Typical CTE, ppm/°C	Spec to cite	Usual reason to choose it
Invar 36	36 % Ni, balance Fe	1.2 to 1.6 (20 to 100 °C)	ASTM F1684, UNS K93600	Default for optics, tooling and metrology
Super Invar	32 % Ni, 5 % Co	0.3 to 0.6 (about 20 to 90 °C)	Supplier data sheet	When Invar 36 is not stable enough over a narrow band
Kovar	29 % Ni, 17 % Co	5.0 to 5.5 (30 to 200 °C)	ASTM F15, UNS K94610	Matched to borosilicate glass for seals and windows
Alloy 42	42 % Ni	4.0 to 6.0	ASTM F30, UNS K94100	Lead frames and glass sealing at lower cost

Invar 36 is the right answer for most machined parts. Super Invar buys a lower CTE, but its expansion rises sharply above about 90 °C and it costs more. Kovar is chosen because its expansion tracks glass, not because it is low. The full comparison is in Invar 36 vs Kovar vs Super Invar. The pages for Invar 36, Super Invar and Kovar carry machining notes, and the spec numbers are on the Invar specs reference.

How Invar behaves on the machine

Machinists compare Invar 36 to austenitic stainless like 304. Annealed hardness is about 70 to 80 HRB and machinability sits around 40 to 50 % of B1112, the same band as 304. It is gummy, it work hardens if the tool rubs instead of cutting and it produces long stringy chips. Thermal conductivity is low, so heat stays at the cutting edge.

Shops that cut it well tend to do the same things:

• Sharp carbide or HSS tools with positive rake, replaced before they dull
• Steady feed so the tool always takes a real chip, at roughly 30 to 60 m/min for carbide milling and 15 to 25 m/min for HSS
• Flood coolant to carry heat and chips away
• Rigid, low-stress fixturing, because the part moves when clamping stress is released
• Climb milling and no dwelling in corners

None of this needs special equipment. Cutting data is in Invar 36 machining parameters.

Milling, turning and thin parts

Most Invar parts are milled from plate: mounts, frames, base plates and tooling faces. Round parts such as spacers and lens cells are turned from bar. In turning, the long chips wrap the part and the chuck unless the shop uses chip breaking inserts and controls feed.

Thin plates and rings are where most problems start. Clamping stress distorts them, and when the clamps come off the part springs. Shops use vacuum or low-force fixtures, alternating passes on both sides and a final skim after stress relief. If your design has walls under about 2 mm, expect questions. See thin wall Invar machining.

The alloy burrs, and on optical parts a loose burr can end up on a lens. It is also ferromagnetic, so fine chips cling to the part. Ask how the shop deburrs and cleans if your assembly is sensitive to particles.

Why good shops machine Invar in stages

Cutting leaves residual stress and a thin cold-worked layer at the surface. In most alloys that only matters for flatness. In Invar it matters twice:

1. The part can relax and move after it leaves the machine, sometimes days later.
2. Cold work raises the CTE of the material near the surface above the annealed value on your mill cert.

A typical sequence for a precision part:

1. Rough the part and leave an even allowance on every critical face, often 0.5 to 1 mm.
2. Stress relieve in a furnace. A typical cycle for Invar 36 is 315 to 345 °C for 1 to 2 hours followed by a slow cool. The drawing or the spec governs.
3. Finish with light cuts and sharp tools so the surface is not worked hard again.
4. Stabilize when the drawing asks for it. Demanding optics and metrology drawings call for a fuller cycle: anneal, quench, a low temperature hold, then a long soak near 95 °C.
5. Inspect after the part has soaked to 20 °C.

Each step adds cost and lead time. If your part is not thermally critical, ask the shop whether a single stress relief is enough. If you are building a laser cavity or a reference frame, the full sequence is what you are buying. The cycles, and how to write them on a drawing, are in stress relief for Invar machined parts.

Tolerances you can expect

On a stable part, most shops that cut Invar regularly quote ±0.01 to ±0.025 mm on machined features. Flatness of 0.01 mm over a small mounting face is realistic after stress relief and finishing. ±0.005 mm is possible on small features with extra passes and 20 °C inspection, at the cost of CMM time and scrap risk. These are typical figures and the shop’s quote governs.

Two things help more than asking for a tighter number:

• Put tolerance only where it matters. A mirror mount needs the mounting face and the bores tight. The outside profile rarely does.
• State the measuring temperature. Invar moves very little, but the CMM and the fixture do. ISO 1 sets 20 °C as the reference temperature for dimensional specifications, so ask for inspection after a soak at 20 °C.

Which GD&T callouts matter on Invar, and what each costs, is in Invar machining tolerances.

Worked example: a hypothetical optical bench

Take a hypothetical 300 mm optical bench that mounts two mirrors and has to hold their spacing within 0.005 mm while the lab drifts between 18 and 24 °C. The growth of the bench over that 6 °C swing, with typical CTE values, is:

Material	Typical CTE, ppm/°C	Growth over 300 mm and 6 °C
6061 aluminum	23	0.041 mm
316 stainless	16	0.029 mm
Invar 36, annealed	1.4	0.0025 mm
Invar 36, cold-worked surface (illustrative)	2.5	0.0045 mm

Aluminum and stainless miss the 0.005 mm budget by a wide margin. Annealed Invar 36 uses half of it. The last row is the point of the sequence. A part left cold worked and unrelieved could show a higher effective CTE and use almost the whole budget before any machining error is counted. The 2.5 ppm/°C figure is illustrative, not a handbook value, but it shows why the buyer of that bench should specify stress relief and 20 °C inspection rather than tighter machining tolerances.

Price, finish and lead time

Most of an Invar quote sits in five lines: material, cutting time, heat treatment, inspection at 20 °C and certs. Programming and setup dominate a batch of one to ten. Material and cycle time take over above about a hundred. Stress relief is usually charged per furnace load, so ten parts in one load cost little more than one. How each line moves, and which drawing changes bring it down, is in why Invar 36 parts cost more.

Invar 36 rusts slowly in humid air, more like a low alloy steel than a stainless, so many parts are used bare in controlled rooms. When corrosion is a concern, electroless nickel is common, kept thin because it has its own expansion. A machined finish of Ra 0.8 to 1.6 µm is normal. For vacuum or cleanroom use, say so on the RFQ.

Expect Invar parts to take longer than stainless: material may need ordering, stress relief adds a furnace run and inspection waits for 20 °C. Several weeks is common for a small precision batch.

What to put on your RFQ

A clear RFQ gets a real quote instead of a padded one. Include:

• Alloy and spec. Invar 36 to ASTM F1684, Kovar to ASTM F15, Alloy 42 to ASTM F30 or Super Invar to a named data sheet.
• Heat treatment. Stress relief between roughing and finishing, a full stabilization cycle or “shop to recommend”.
• Operating temperature range. Where the part is assembled and where it works.
• Critical dimensions. Mark the features that must hold size. Loosen the rest.
• Inspection temperature. 20 °C after soak, if a tolerance is under about ±0.02 mm.
• Certs you need. A mill cert is normal. Measured CTE data for your heat lot costs extra.
• Quantity and timing. Including whether the part will repeat.

The RFQ drawing checklist expands each point. Send the drawing through the RFQ form with those notes attached and every shop that quotes will be pricing the same part.

How to vet a shop

Plenty of shops will say yes to Invar. Fewer have cut it recently and can arrange stress relief nearby. When you talk to a shop, ask:

• Which low-expansion alloys it has machined in the past year, and in what sizes
• Where the stress relief happens, in house or at a named heat treater, and how the cycle is recorded
• How much allowance it leaves before the furnace and how it fixtures the part for finishing
• How it measures flatness and position, and whether the CMM room is held at 20 °C

A shop that answers in specifics has done the work. One that answers in generalities is learning on your part. More questions are in how to find an Invar machine shop.

That is the check we run when we match an RFQ. You send one drawing and we pick up to three shops that cut the alloy and can run the sequence your part needs. Each one quotes you directly.

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
• Invar Machining Tolerances: What a Shop Can Hold and What It Costs · Process
• Why Invar 36 Parts Are Costly and How Shops Price Them · Cost
• How to Find a Machine Shop That Has Really Machined Invar · Choosing a Supplier

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