Invar Machining Tolerances: What a Shop Can Hold and What It Costs

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

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. A shop that cuts Invar 36 regularly holds ±0.01 to ±0.025 mm on a stable part as a matter of routine, and tighter on a few features with extra work. Knowing which features deserve that work, and how to state it on the drawing, 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 and Kovar, which are often used in thin sections and sheet, are usually toleranced more conservatively, around ±0.025 mm, because fixturing and flatness dominate the achievable accuracy more than the cutting process does.

The table below gives typical achievable values by feature type on Invar 36, and what the shop has to add to reach them. These are typical figures, not a spec. The shop’s quote governs for your geometry.

Feature type	Typical achievable tolerance	What it takes	Typical cost effect
Overall length or width, general	±0.05 to ±0.1 mm	One setup, normal practice	Baseline
Bore diameter, reamed or bored	±0.01 to ±0.02 mm	Finishing pass after stress relief	Small increase
Bore position to a datum	±0.01 to ±0.025 mm	Finishing in one setup, CMM check	Moderate increase
Flatness on a mounting face under 100 mm	0.005 to 0.01 mm	Finish skim after relief, soak, CMM at 20 °C	Moderate increase
Flatness on a face over 300 mm	0.01 to 0.03 mm	Full sequence, low-stress fixturing, per-part CMM	Large increase
Parallelism between two faces	0.01 to 0.02 mm	Both faces finished after relief, same setup where possible	Moderate increase
Thin wall thickness under 2 mm	±0.025 to ±0.05 mm	Vacuum or low-force fixturing, alternating passes	Large increase, scrap risk
Small feature, extra passes	±0.005 mm	Extra pass, soak, per-part CMM, scrap allowance	Two to three times the ±0.025 mm line

A thin walled part, a deep pocket or a feature far from a fixturing point holds a looser tolerance than a short, well supported boss, regardless of the alloy. The cost side of the same table is in why Invar 36 parts are costly.

Inspection at 20 °C and ISO 1

Invar’s whole purpose is to change size very little with temperature, but “very little” is not “not at all.” ISO 1 fixes 20 °C as the standard reference temperature for geometrical product specifications, which means every dimension on your drawing is, by default, a dimension at 20 °C. A part measured on the shop floor at 24 °C has grown by a small but real amount. On the Invar part itself that growth is tiny. On the CMM scales, on a steel fixture and on a mating part in aluminum, it is not, and on a feature toleranced to a few microns the difference can be the whole tolerance band.

Three things follow for the buyer:

• Ask for the soak. A precision Invar inspection report should state the measurement temperature and how long the part sat in the room before measurement. Several hours is typical for a part of a few kilograms.
• Ask where the CMM lives. A CMM in a temperature-controlled room held near 20 °C gives numbers you can compare with your own incoming inspection. A CMM on the shop floor does not, on tight features.
• Say if your own inspection happens elsewhere. If the part ships to a different climate and is measured there, the report needs the temperature so the two sets of numbers can be reconciled.

What a good report looks like, and what to ask for, is in Invar CMM inspection reports. Definitions of soak, reference temperature and related terms are in the glossary.

GD&T callouts that matter on Invar

Geometric tolerances, written to ASME Y14.5 on most drawings, are where Invar parts most often need extra thought. Three callouts interact directly with the stress relief sequence described in stress relief for Invar machined parts:

• Flatness on a mounting or reference face. The part’s shape shifts slightly during the furnace cycle, so a flatness tight enough to matter should be produced by a finishing skim after stress relief and verified after the part has soaked. A flatness measured after roughing tells you nothing about the delivered part.
• Position of locating bores and dowel holes. Position is held by finishing all the bores in one setup after relief, with the datum faces already finished. Ask the shop to confirm the datum scheme on the drawing matches how it will fixture the part.
• Parallelism and perpendicularity between reference faces. These are cheapest when both faces can be finished in the same setup. A drawing that forces two datum faces into different setups will cost more and hold less.

Two things to avoid. Do not put a tight profile tolerance on an outer surface that touches nothing. Do not use a general note like “all surfaces flat within 0.005” in place of callouts, because the shop will either price every face at that number or ignore it, and both outcomes are bad. Call out the feature, the datum and the number.

Which features need the tightest tolerance

The features that justify a tight Invar tolerance are 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, bearings or kinematic elements
• Flatness on a reference surface used for downstream assembly
• Spacing between two mounting features when that spacing is the reason the part is Invar

Features that do not affect fit or function, such as an outer profile, a non-mating edge or a relief cut, can run at a standard machining tolerance like ±0.1 mm without affecting the part’s performance. Tolerancing those tightly just because the material is Invar adds inspection time for no functional benefit.

Worked example: turning a temperature band into a CTE

Tolerance and material choice are the same decision seen from two sides. Take a hypothetical 250 mm metrology fixture that locates two probes. Their spacing must stay within 0.004 mm in total, and the lab is held at 20 ± 5 °C. The designer splits the budget: 0.002 mm for thermal growth, 0.002 mm for machining and assembly.

The thermal part sets the CTE the material must have:

CTE needed = allowed growth / (length × temperature swing) = 0.002 mm / (250 mm × 5 °C) = 1.6 ppm/°C

The table shows how typical materials measure up. CTE values are typical handbook figures at room temperature. The mill cert governs.

Material	Typical CTE, ppm/°C	Growth over 250 mm and 5 °C	Fits the 0.002 mm budget
6061 aluminum	23	0.029 mm	No
316 stainless	16	0.020 mm	No
Titanium	8.6	0.011 mm	No
Kovar	5.2	0.0065 mm	No
Invar 36, annealed	1.2 to 1.6	0.0015 to 0.0020 mm	Yes, at the edge
Super Invar	0.3 to 0.6	0.0004 to 0.0008 mm	Yes, with margin

Annealed Invar 36 fits the band with nothing to spare at the top of its range. That has two consequences for the drawing. First, the 0.002 mm machining budget on the spacing is real and should be called out as a position tolerance on the two probe bores, finished after stress relief and measured at 20 °C. Second, the stress relief is not optional, because a cold-worked surface with a higher effective CTE would push the thermal growth past the budget. If the lab band widened to 20 ± 10 °C, the needed CTE would fall to 0.8 ppm/°C and the fixture would have to move to Super Invar, or the budget would have to change. That trade is covered in when Invar 36 is not enough and the temperature dependence itself in Invar 36 CTE vs temperature.

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 aluminum part expands at 23 ppm/°C. Over a 100 mm bolt pattern and a 10 °C swing the aluminum moves 0.023 mm while the Invar moves about 0.0014 mm, so the joint has to allow for 0.02 mm of relative motion or the bolts will load the Invar part and bend it. 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 and whether slotted holes or flexures are the better answer. The mismatch also matters for long-term dimensional stability, because a joint under load creeps.

A practical approach to tolerancing your drawing

1. Identify the two or three features that control fit, alignment or optical performance, and tolerance those tightly.
2. Leave everything else at a general machining tolerance such as ±0.1 mm.
3. Write the tight requirements as GD&T callouts with datums, not as a general note.
4. State the measurement temperature, 20 °C after soak, on the drawing or the RFQ if any tolerance is under about ±0.02 mm.
5. Specify stress relief between rough and finish so the tight features are cut after the part has stopped moving.
6. Ask the shop what it can 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.

What to put on the RFQ

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. On the RFQ, include:

• The alloy and spec, for example Invar 36 to ASTM F1684
• The critical features, their tolerances and the datum scheme
• The general tolerance for everything else
• The heat treatment: stress relief between rough and finish, or full stabilization
• The inspection temperature and whether you need a per-part report on the critical features
• The operating temperature band, so the shop can see whether the tolerance and the alloy agree

Send your drawing through the RFQ form with those points called out. We match you with shops that can tell you, before you commit, whether your tolerance is realistic on that geometry and what it will add to the price.

Which features need the full treatment for stability is covered in Invar dimensional stability.

Frequently asked questions

Reviewed against

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

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
• Reading an Invar CMM Inspection Report: What Each Number Means · Process
• Why Invar 36 Parts Are Costly and How Shops Price Them · Cost
• How the CTE of Invar 36 Changes With Temperature and Condition · Materials

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