Invar Parts for Diagnostic and Lab Equipment Manufacturers

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

A diagnostic analyzer, an imaging system or a lab automation platform rarely needs Invar 36 throughout. It needs Invar 36 in the one place where a small dimensional shift moves a calibration: an optical reference bench inside a spectrometer, an alignment structure in an imaging gantry or a stage component in an automated liquid handler. This guide covers where that requirement shows up in diagnostic and lab equipment, how the parts are machined and stress relieved and what tolerance is realistic. None of the applications here involve implantation or patient contact; the material is chosen for calibration stability, not for any biological property, and this site makes no claim about the regulatory status of the finished instrument, which is a question for its manufacturer.

Where dimensional stability matters in diagnostic equipment

A calibration is a mapping between a physical position and a measured value. If the physical position moves with room temperature, the calibration drifts, even though nothing electronically has changed. That is the problem Invar 36 solves, and it shows up in a handful of recurring places:

• Imaging system alignment structures. Frames and brackets that hold a source and a detector in fixed relative position, where thermal drift shows up as a shift in image geometry rather than a hard failure.
• Optical benches inside analyzers. Spectrometers, flow cytometers and similar bench instruments that hold mirrors, gratings or filters at a fixed spacing, where a drift of a few micrometers moves a wavelength calibration.
• Lab automation stage components. Reference rails, gauge blocks and positioning fixtures in liquid handlers and automated sample processors, where repeatability across a room’s daily temperature swing matters more than absolute accuracy.
• Calibration standards and fixtures. Reference artifacts used to check an instrument’s own calibration, which need to hold their size better than the instrument they check.

The common thread is that the part never touches a sample or a patient. It sits behind the scenes, holding a geometry that the instrument’s measurement depends on. A useful way to spot a candidate part on your own design is to ask what happens to the calibration if that structure grows or shrinks by a few micrometers. If the answer is a shifted wavelength, a misaligned beam path or a stage that no longer repeats to its taught position, the part is a candidate for a low expansion alloy. If the answer is nothing measurable, a conventional material is almost always the better choice on cost and machinability alone.

Why the CTE difference matters here

The table lists typical mean CTE values used for planning; a real design should use measured data for the specific structure and temperature range involved.

Material	Typical CTE, ppm/°C	Interval
Invar 36, ASTM F1684	1.2 to 1.6	20 to 100 °C
Borosilicate glass (optics)	about 3.3	20 to 300 °C
304 stainless steel	16 to 17	20 to 100 °C
6061 aluminum	about 23.5	20 to 100 °C

Invar 36, the iron-nickel alloy described in the Wikipedia entry on Invar, moves roughly a tenth as much as aluminum over the same temperature swing. In a lab that is not tightly climate controlled, a few degrees of daily drift is normal, which is enough to shift an aluminum optical bench by several micrometers per 100 mm of length, well within the range that moves a sensitive calibration. The physics of how CTE changes with temperature and why the mean value depends on the interval chosen is covered in Invar 36 CTE vs temperature, and the reference page of Invar specifications lists the standard and the typical property table for the alloy.

Machining and stress relief

Invar 36 parts for diagnostic equipment are machined the same way as any other Invar 36 part: sharp positive rake tooling, steady feed, flood coolant, because the alloy is soft, gummy and work hardens readily. The full recipe is in Invar 36 machining, for the grade defined by ASTM F1684. What decides whether the finished part actually keeps its low expansion advantage is what happens between roughing and finishing. Machining cold works the surface, which raises its local CTE and stores stress that relaxes slowly after the part leaves the machine, sometimes over days. A stress relief cycle, typically 315 to 345 °C for one to two hours with a slow cool, run between roughing and finishing, removes that stress before the final light cuts are taken. Skipping it is the most common reason a supposedly stable optical bench keeps drifting after it is installed. Long term behavior of a finished part, including how much drift to expect after the initial settling period, is covered in Invar long term dimensional stability.

Typical tolerances for calibration critical parts

The table lists what a shop can typically achieve on a stress relieved Invar 36 part, inspected after a soak at 20 °C, the reference temperature used across the industry for dimensional measurement.

Feature	Typical tolerance	Notes
Linear dimension	±0.01 to ±0.025 mm	Tighter with extra finishing passes
Flatness over 100 mm	0.02 to 0.05 mm	0.01 mm possible on smaller parts, added cost
Hole position	±0.01 to ±0.02 mm	Reamed or bored, not drilled only
Repeatability across a thermal cycle	Within the part’s own CTE band	Verify with a thermal cycling test if the spec is critical

General tolerance guidance across the alloy family, including how inspection temperature affects the reported number, is in Invar machining tolerances. A worked comparison of what a stable optical bench actually needs from its machining and mounting is in Invar optical bench worked example, which applies to a spectrometer bench inside an analyzer as directly as it applies to a standalone instrument.

Worked example: an optical bench in a benchtop analyzer

Take a hypothetical spectrometer bench, 250 mm long, that holds a grating and a detector at a fixed spacing inside a benchtop analyzer used in a lab without close temperature control, where the room swings roughly 5 °C between morning and afternoon. The question is how much the bench length changes over that swing, and how that compares between materials.

Growth of a 250 mm length over 5 K = 250 mm × CTE × 5.

Bench material	Typical CTE, ppm/°C	Length change over 5 K, mm	Length change, micrometers
6061 aluminum	23.5	0.029	29
304 stainless	16.5	0.021	21
Invar 36	1.4	0.0018	1.8

Values are typical and the calculation is illustrative; a real instrument’s calibration tolerance and its actual thermal environment determine whether any of these shifts matter. If the instrument’s wavelength calibration tolerates a few micrometers of bench movement, aluminum may be entirely adequate and Invar 36 unnecessary cost. If the calibration tolerance is closer to a micrometer, the aluminum bench moves 15 times past that limit on an ordinary room temperature swing, while the Invar 36 bench stays within it. This is a sizing illustration only; the actual decision depends on the instrument’s own calibration budget, which is the manufacturer’s engineering call, not a general rule.

Regulatory status is a separate question

Some diagnostic instruments are themselves subject to regulatory requirements depending on how they are marketed and used; others are general lab equipment with no such requirement. Either way, that status belongs to the finished instrument and its manufacturer, established through their own design controls, verification and, where applicable, regulatory submission. Choosing Invar 36 for one internal component because it holds a calibration does not create, and has no bearing on, the instrument’s regulatory status. This site is a matching service that connects buyers with independent machine shops; it does not manufacture instruments, and it makes no claim about any instrument’s or any shop’s regulatory or quality certification status. If your program requires supplier documentation for a regulated instrument, request it directly from the shop. See Supplier quality requirements for Invar and Kovar medical parts for what to ask, and the medical device manufacturing overview for related considerations across the vertical.

What to put on the RFQ

An accurate Invar 36 quote for a diagnostic equipment component depends on the shop knowing what the part is for. Include:

• Alloy and spec. Invar 36 to ASTM F1684, with the UNS grade and supply form.
• Flatness or straightness callouts, with the inspection method and temperature.
• Stress relief requirement. Between roughing and finishing, or “shop to recommend” with the operating range stated.
• Mating optical or mechanical components, so the shop understands which surfaces matter most.
• Quantity and expected repeat volume.

When you send an RFQ with the drawing, we match it with up to three shops that machine Invar 36 for stability critical parts. Each shop quotes you directly against your drawing and its own fixturing and stress relief process.

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

• How the CTE of Invar 36 Changes With Temperature and Condition · Materials
• Long-Term Dimensional Stability: Does Invar Age After Machining? · Materials
• A 300 by 200 mm optical bench in Invar, from alloy choice to quote · Choosing a Supplier
• Invar Machining Tolerances: What a Shop Can Hold and What It Costs · Process

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