How to Machine Invar 36 Without Dimensional Drift
Invar 36 has a reputation that is half legend, half warning: the alloy that measures perfect today and moves by the time it reaches your CMM. Shops that treat it like stainless steel learn the lesson the expensive way — a flat frame bows after pocketing, a bore goes oval, a datum face rocks on the granite table.
None of this is inevitable. Invar 36 is predictable once you understand the one property that makes it valuable — and the two consequences that property has for machining.
First, why Invar 36 is “difficult”
Invar 36 is roughly 36% nickel in iron, and its near-zero coefficient of thermal expansion (≈1.2 × 10⁻⁶/K between 20–100 °C) is exactly why buyers specify it: a metrology frame made of Invar holds its geometry when the room’s temperature changes. That’s the value.
But the same property creates two machining problems:
- Cutting heat stays as stress. In steel, some machining heat is accommodated by local expansion and relaxation. Invar barely expands, so the energy from cutting accumulates as residual stress around the machined zone instead of being released.
- Mill-supplied stock is pre-loaded. Bar and plate carry stresses from rolling, drawing and straightening. The part is in equilibrium until you remove material unevenly — then the equilibrium shifts and the part moves.
So dimensional drift in Invar 36 is almost never “bad machining.” It is unmanaged stress. The fix is a sequence, not a technique.
The goal is not to cut Invar carefully. The goal is to let every stress state out of the material before the final dimension is created.
The sequence that works
1 · Aggressive roughing, generous leave-on-stock
Counterintuitive but correct: rough Invar hard, with rigid setups and sharp carbide, but leave 0.5–1.0 mm on finished surfaces. Roughing deliberately disturbs the stock’s stress equilibrium while there is still stock to correct with. Light, timid roughing is the worst of both worlds — it disturbs stresses but leaves no allowance to recover.
2 · Stress-relief anneal between operations
After roughing, stress-relieve (typically 310–335 °C for Invar 36, time scaled to section thickness — your material supplier’s data sheet governs). This lets the structure settle at a dimension where correction is still possible. Skipping this step is the single most common cause of post-delivery drift.
3 · Re-datum, then finish with light passes
After annealing, the part is slightly different from what the roughing program assumed. Re-establish datums from the actual part, then finish-machine with light depths of cut, sharp positive-rake carbide, and flood coolant. Finishing passes exist to create geometry, not to remove errors left by roughing.
4 · Consider wire EDM for thin sections
Deep pockets, thin walls and window frames distort under milling forces no matter how careful the sequence. Slow-wire EDM cuts with almost no mechanical force — for thin Invar sections it is frequently the cheaper process once you count scrapped parts.
5 · Soak and inspect at 20 °C
Invar parts should reach thermal equilibrium before final measurement — a minimum soak on the granite plate, ideally overnight for large frames. Inspect on a temperature-controlled CMM. Measuring a warm part on a cold table guarantees a number that will be wrong by morning.
What this means for your supplier choice
When you evaluate an Invar 36 machining supplier, the questions that separate a specialist from a general shop are all about sequence:
- Do you stress-relieve between roughing and finishing, and at what temperature?
- Do you re-datum after heat treatment?
- At what temperature is your CMM operated, and how long do parts soak before inspection?
- Can you show a dimensional report where flatness was verified after a 72-hour rest, not straight off the machine?
A general shop will answer “we machine to the drawing.” A specialist will have numbers.
The honest caveat
No process eliminates movement entirely — Invar 36, like every metal, responds to how it was treated. What a controlled sequence does is make movement predictable and small, and verify the final geometry with data. That’s the difference between a frame that requalifies at your facility and one that starts an argument.
— GuKen Precision machines Invar 36, Kovar and other difficult alloys with exactly this sequence, documented per order. Send us your drawing and we’ll tell you what the process route looks like for your part — before you commit.
Related: Invar 36 machining services · Kovar machining services