Guides / Process

Invar Machining Tolerances: What Is Realistic on Your Drawing

September 20, 2026

A drawing that tolerances every feature to ±0.005 mm because the part is Invar usually costs more than it needs to, and a drawing that leaves every feature loose risks a part that does not do what Invar is for. Knowing what a shop can realistically hold, and where that precision actually matters, is what makes the difference.

What tolerance is realistic

For a well machined Invar 36 or Super Invar part that has gone through rough, stress relieve and finish, shops commonly quote critical features in the ±0.01 to ±0.025 mm range. Tighter tolerances are achievable on specific, well supported features, particularly after a stabilization cycle, but they add cost and inspection time. Alloy 42, which is often thinner and used in sheet or stamped forms, is usually toleranced more conservatively, around ±0.025 mm, because fixturing and part flatness dominate the achievable accuracy more than the cutting process does.

These numbers describe typical shop capability, not a spec you can copy onto a drawing and expect any shop to hit on any geometry. A thin walled part, a deep pocket or a feature far from a fixturing point will hold a looser tolerance than a short, well supported boss, regardless of the alloy.

Why measurement temperature matters

Invar’s whole purpose is to change size very little with temperature, but “very little” is not “not at all.” A part measured on the shop floor at 24 degrees C instead of the 20 degrees C the drawing assumes has grown by a small but real amount, and on a feature toleranced to a few microns, that difference can be the whole tolerance band. This is why precision Invar inspection reports usually note the soak time and measurement temperature, not just the dimension.

For buyers, the practical takeaway is to ask whether the shop’s CMM report states the measurement temperature on tight features, especially if your own downstream inspection will happen at a different temperature or after shipping to a different climate.

Which features actually need the tightest tolerance

The features that justify a tight Invar tolerance are usually the ones that control alignment, fit or optical path length:

• Mounting faces that set the position of an optic, sensor or fixture
• Bore diameters and positions that locate dowels or bearings
• Flatness on a reference surface used for downstream assembly

Features that do not directly affect fit or function, such as an outer profile, a non-mating edge or a relief cut, can usually run at a standard machining tolerance like ±0.1 mm without affecting the part’s performance. Tolerancing those features tightly just because the material is Invar adds inspection time for no functional benefit, a point also covered from the cost side in why Invar 36 parts are costly.

Flatness, position and form tolerances

Geometric tolerances are where Invar parts most often need extra thought, because flatness and position callouts interact directly with the stress relief sequence described in stress relief for Invar machined parts. A flatness spec that is tight enough to matter should be verified after stress relief and finishing, not just after roughing, since the part’s shape can shift during the heat cycle. A shop that understands this sequences its finishing cuts and fixturing to correct for that expected movement rather than fighting it.

Tolerance stack-up on assemblies

When an Invar part mates with a housing or bracket in a different material, the assembly’s overall tolerance stack has to account for the CTE mismatch, not just the individual part tolerances. A tight position tolerance on the Invar part alone does not guarantee alignment across a temperature range if the mating part expands and contracts at a very different rate. This is a design level decision, but it is worth raising with your shop early, since it can change which features on the Invar part need the tightest control.

A practical approach to tolerancing your drawing

1. Identify the two or three features that actually control fit, alignment or optical performance, and tolerance those tightly.
2. Leave everything else at a standard machining tolerance.
3. State the required measurement temperature and soak time if your application is sensitive to it.
4. Call out flatness or position requirements as GD&T rather than a general note, so the shop knows exactly which feature and datum structure to hold.
5. Ask the shop what it can realistically hold on your specific geometry before finalizing the drawing. A shop with Invar experience will often flag a feature that is harder to hold than it looks on paper.

Getting a quote that matches your tolerance

Two shops quoting the same drawing can reach very different numbers if one assumes standard tolerance on every feature and the other assumes every dimension is critical. Be explicit about which features matter, and mention any measurement temperature requirement on the RFQ itself.

Send your drawing through our RFQ form with your critical features called out, and we match you with shops that can tell you, before you commit, whether your tolerance is realistic on that geometry.

Frequently asked questions

Keep reading

• Invar 36 Machining Parameters: Speeds, Feeds and Tool Wear
• Invar Drilling and Tapping: Hole Quality and Thread Practices