How Micro Parts Are Measured: CMM, Optical and Vision Inspection

By the Micropartz editorial team · Published October 27, 2026

On a large part, measurement is a check at the end of the process. On a micro part, it is often the hardest step. A ±0.005 mm tolerance on a 0.4 mm bore is a band of 10 μm on a feature you cannot see clearly without magnification. This guide explains the methods used to inspect micro machined parts, what limits them and what to agree with a shop before production. It builds on the guide to feature sizes and tolerances in micro milling and turning.

Why inspection is harder on micro parts

Four things make measuring small parts different.

The tolerance is close to the uncertainty of the method. A good workshop micrometer is accurate to about 1 to 2 μm. That is enough for ±0.02 mm, marginal for ±0.005 mm and useless for ±0.002 mm.

Contact force deflects the part. A CMM probe pressing on a 0.15 mm wall or a PEEK feature can move it by more than the tolerance. Measured size then depends on how hard you push.

Features are hard to reach. A 0.3 mm hole cannot take a 1 mm stylus. A slot deeper than it is wide hides its bottom edge from a camera.

Dirt and burrs are the same size as the tolerance. A single chip or a burr of a few microns changes the reading. Micro parts must be cleaned and deburred before they are measured, not after.

Measurement methods and where they fit

Micro machining shops usually combine several methods. The table shows which kind of feature each one suits.

Method	Typical features	Contact	Typical resolution	Limits
CMM with small stylus	Bores, positions, flatness, datums	Yes	0.1 to 1 μm	Stylus size, contact force
Micro-CMM	Very small bores and 3D features	Very low force	Below 0.1 μm	Cost, speed, few shops have one
Vision / optical measuring system	Profiles, hole diameters, slot widths, positions in 2D	No	0.5 to 2 μm	Edge detection, depth
Measuring microscope	Edges, burrs, radii, thread form	No	About 1 μm	Mostly 2D, operator dependent
Pin and ring gauges	Hole and shaft size, go / no-go	Yes	Class dependent	Only size, not form
Air gauging	Bore size in production	No	Below 1 μm	Needs a master per size
Stylus or optical profilometer	Surface roughness Ra, Rz	Yes / no	Nanometre range	Access on small features

Values are typical for orientation, not equipment specifications.

CMMs with small styli are the workhorse for 3D features and datums. Ruby or ruby-like stylus tips go down to about 0.3 mm in normal use, smaller on special systems. Measurement strategy matters: how many points, which scanning speed and how the part is fixtured.

Vision systems measure silhouettes and edges with a camera and calibrated optics. They are fast, contact-free and well suited to flat parts, turned profiles and hole patterns seen from one direction. They struggle with depth and with edges that are burred or rounded, because the software has to decide where the edge is.

Measuring microscopes are the standard tool for edge condition. A drawing note like “burr-free at 10x” can only be checked this way.

Gauges remain the fastest method for size in production. A ⌀0.400 mm hole can be checked with go and no-go pins in seconds. Gauges say nothing about roundness or position, so they complement a CMM rather than replace it.

See also: the reference page on how micro parts are measured matches each feature type to an instrument.

Measurement uncertainty and the 10 to 1 rule

Every measurement has uncertainty. For a tolerance to be meaningful, the uncertainty must be a small part of the tolerance band.

Drawing tolerance	Tolerance band	Method should be good to (10:1)	Minimum often accepted (4:1)
±0.02 mm	40 μm	4 μm	10 μm
±0.01 mm	20 μm	2 μm	5 μm
±0.005 mm	10 μm	1 μm	2.5 μm
±0.002 mm	4 μm	0.4 μm	1 μm

Rule-of-thumb ratios, not a standard requirement.

The table explains why ±0.005 mm is a selected-feature tolerance. Checking it properly needs a method good to about 1 μm, which means a well-maintained CMM or vision system in a controlled room, not a hand tool on the shop floor.

Temperature and cleanliness

Dimensions are defined at the ISO 1 reference temperature of 20 °C. For micro parts, thermal expansion of the part itself is small: a 5 mm stainless part grows only about 0.1 μm per degree. The bigger problem is the measuring equipment and fixture, and the fact that a part carried straight from a warm machine can read differently from one that has settled.

Good practice in micro machining shops:

• Measure in a temperature-controlled room, usually 20 ± 1 °C.
• Let parts stabilize before measurement.
• Clean parts ultrasonically and dry them before inspection.
• Deburr before measuring, and inspect edge condition separately.
• Handle parts with tweezers or vacuum pickups, never fingers.

Inspecting burrs and edge condition

Burrs are the most common reason micro parts are rejected, and they are also the feature most often left undefined on the drawing. A burr of 20 μm is invisible to the eye but can block a 0.3 mm cross hole, scratch a mating bore or break off inside a medical device.

Edge condition is inspected optically, usually under a stereo or measuring microscope at a stated magnification. The drawing or purchase order should define three things:

• The requirement. For example “no burrs”, “edge break 0.01 to 0.03 mm” or “sharp edge, no burr exceeding 0.01 mm”.
• The magnification. 10x is common for general parts, 20x to 40x for medical and fluidic parts. What counts as a burr depends on how closely you look.
• Where it applies. All edges, or only functional edges such as cross hole exits, sealing faces and thread starts.

Edge requirement on the drawing	How it is usually inspected	Common deburring method
Burr-free at 10x	Stereo microscope, sample per lot	Brushing, vibratory finishing
Burr-free at 20x to 40x	Measuring microscope, often 100 % on critical edges	Hand deburring under microscope, electrochemical
Edge break 0.01 to 0.03 mm	Measuring microscope or vision system	Controlled chamfer in the program, abrasive
Sharp edge, no burr	Microscope, sample	Machining strategy, EDM for internal corners

Typical practice, not a standard.

Deburring methods also affect dimensions. Abrasive and electrochemical processes remove a few microns from surfaces and edges. If a feature is toleranced at ±0.005 mm, the deburring step must be part of the process that is measured, not an afterthought.

What to agree with the shop before production

Most inspection disputes on micro parts start with an assumption that was never written down. Agree these points in the RFQ or at order:

1. Method per critical feature. For example “bore ⌀0.40 H7 measured with pin gauges, position measured on CMM”.
2. Datums and fixturing. How the part is held for measurement, especially if it is thin or plastic.
3. Sampling plan. First article on every feature, then critical features on a sample per lot, or 100 percent on one feature only.
4. Edge condition and magnification. “No burrs visible at 10x” is inspectable. “Burr-free” alone is not.
5. Surface finish parameter and where it is measured. Ra on a 0.3 mm wide slot may not be measurable at all.
6. Report format. First article inspection report layout, whether measured values or pass / fail are recorded and whether raw data is needed.
7. Material and conformance documents. Mill certificates traceable to the lot, certificate of conformance.

How inspection shows up in the price

On tight-tolerance micro parts, inspection can take as long as machining. A first article report on a part with 40 dimensions can take several hours on a CMM. Sampling five critical features on every lot adds time to each delivery. That is not padding. It is the cost of knowing the parts are right. The guide on what micro machined parts cost explains how inspection interacts with setup and cycle time.

Hypothetical example: a medical buyer asks for 100 percent CMM inspection of eight features on 2,000 small housings. At about four minutes per part, that is over 130 hours of CMM time. Switching to 100 percent pin gauging of the two functional bores and a sampling plan for the rest cuts inspection to a fraction while still protecting the function.

Choosing a shop with the right measuring room

When you compare micro machining shops, the measuring room tells you as much as the machine list. Ask which CMM or vision system is used, which stylus sizes are available, whether measurement happens in a controlled room and to see a sample inspection report. The micro machining buyer’s guide lists the other questions worth asking.

When two shops measure the same part, see second source for micro parts for how to correlate them.

Micropartz matches each RFQ with up to 3 shops whose measurement fits your tolerances, and replies within 24 business hours. Typical tolerance and inspection values are on the tolerance and materials reference. When your drawing is ready, send an RFQ and state the tightest tolerance and how you expect it to be measured.

See also: measuring micro holes with pin gauges, air gauges and optical methods.

Frequently asked questions

Reviewed against

• ISO 10360 series, Acceptance and reverification tests for coordinate measuring systems
• ISO 21920-2:2021, Surface texture: profile, terms, definitions and parameters
• Wikipedia: Coordinate-measuring machine

Numbers in this guide are typical values for orientation. The drawing, the material certificate and the matched shop set the real ones.

Keep reading

• Minimum Feature Sizes, Tools and Tolerances in Micro Milling and Turning
• What Micro Machined Parts Cost and Why Inspection Is Half of It
• How to Choose a Micro Machining Supplier