Guides / Materials

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

September 20, 2026

Invar machining is ordinary CNC work on an unusual alloy. The machines are the same mills and lathes that cut stainless. What changes is how the shop sequences the work, because the property you are paying for, very low thermal expansion, can be damaged by the machining itself. If you understand that one point, you can read an Invar quote properly and write an RFQ that gets you a part that stays in tolerance.

What Invar is and why it costs more

Invar 36 is an iron alloy with about 36 percent nickel. Near room temperature it expands roughly 1.2 to 1.6 millionths of its length per degree Celsius. Carbon steel expands about 12 and aluminum about 23. A 1 m aluminum bar grows around 0.23 mm when it warms by 10 °C. The same bar in Invar 36 grows around 0.013 mm.

That is the whole reason people buy it. Optical mounts that must hold alignment, layup tools for carbon fiber parts, metrology frames and space 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 more than stainless, and thick plate can take weeks to arrive. Then the alloy cuts more slowly than steel and needs an extra heat treatment step. None of that is a shop padding the quote. It is the material.

For related alloys, see Invar 36 vs Kovar vs Super Invar.

How Invar behaves on the machine

Machinists compare Invar to austenitic stainless like 304. It is soft and gummy, it work hardens if the tool rubs instead of cutting and it produces long, stringy chips. It also has low thermal conductivity, so heat stays at the cutting edge.

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

• Sharp carbide tools with positive rake, replaced before they dull
• Steady feed so the tool always takes a real chip
• 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. It needs a shop that has cut the alloy before and knows not to treat it like mild steel.

Milling and turning

Most Invar parts are milled from plate: mounts, frames, base plates and tooling faces. Round parts such as spacers, lens cells and shafts are turned from bar, often on a lathe with live tooling so cross holes and flats are done in the same setup. Turning brings its own trap. The long chips wrap around the part and the chuck unless the shop uses chip breaking inserts and controls feed carefully.

Thin parts

Thin Invar plates and rings are where most problems start. Clamping stress distorts them, and when the clamps come off, the part springs. Shops handle this with vacuum or low-force fixtures, by machining both sides in alternating passes and by leaving the final skim until after stress relief. If your design has walls under about 2 mm, expect the shop to ask questions or suggest a small change.

Burrs and chips

Because the alloy is ductile, it burrs. Deburring small cross holes by hand is slow, and on optical parts a loose burr can end up on a lens. Ask how the shop deburrs and cleans parts if your assembly is sensitive to particles. Invar is also magnetic, so fine chips cling to the part and to the fixture. Good shops clean with that in mind.

Why good shops machine Invar in stages

This is the part most buyers do not see on the quote. Cutting leaves 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 can raise the expansion of the material near the surface above the value on your mill cert.

So a typical sequence for a precision Invar part looks like this:

1. Rough the part and leave an even allowance on every critical face.
2. Stress relieve in a furnace. The cycle comes from the alloy spec, the drawing or the shop’s experience.
3. Finish with light cuts and sharp tools so the surface is not worked hard again.
4. Stabilize when the drawing asks for it. Some optics and metrology drawings call for a stabilization cycle that includes a low temperature hold.
5. Inspect after the part has soaked to 20 °C.

Each step adds cost and lead time. If your part is not thermally critical and the tolerances are loose, you can 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.

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. Tighter numbers are possible on small features, but they add passes, inspection time and scrap risk.

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. Parts measured at 20 °C after a soak give you numbers you can trust.

Surface finish, plating and cleaning

Invar 36 rusts slowly in humid air, more like a low alloy steel than a stainless. Many parts are used bare in controlled rooms. When corrosion is a concern, electroless nickel is a common coating, but it adds a layer with its own expansion, so it is usually kept thin and specified on the drawing. For vacuum or cleanroom use, say so on the RFQ. The shop will plan cleaning and packing, and may avoid some cutting fluids.

A machined surface of Ra 0.8 to 1.6 µm is normal on finished faces. Lapped or ground surfaces for gauge or reference parts are possible but add a separate step and often a separate supplier.

Lead time

Expect Invar parts to take longer than the same part in stainless. Material may need to be ordered, stress relief adds a furnace run and a second setup, and inspection waits for the part to reach room temperature. For a small batch of precision parts, several weeks is common. If you have a hard date, put it on the RFQ so shops can say plainly whether they can meet it.

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 or Super Invar. “Invar” alone leaves the shop guessing.
• 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.
• Certs you need. A mill cert is normal. Measured CTE data for your heat lot costs extra and takes time.
• Quantity and timing. Including whether it will repeat.

If you want a sense of how each of these moves the price, read why Invar 36 parts cost more and how shops price them.

Choosing a shop

Plenty of shops will say yes to Invar. Fewer have cut it recently and can arrange stress relief without sending the part across the country. When you talk to a shop, ask which low-expansion alloys it has machined in the past year, where the heat treatment happens and how it measures flatness and position.

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

Frequently asked questions

Keep reading

• Why Invar 36 Parts Are Costly and How Shops Price Them