Choosing a measurement story before the RFQ goes out
Machined-part buyers often write “CMM report required” on every drawing, or they leave inspection method blank and hope the shop’s default matches the tolerance stack. Coordinate measuring machines (touch-probe CMMs and related contact systems) and optical / vision / structured-light systems answer different questions. Picking the wrong family—or demanding both without a plan—adds cost, delays first article inspection and still misses the feature that actually fails in assembly.
This quality-inspection guide compares CMM-style contact measurement with optical and vision approaches for CNC and sheet-metal RFQs. It uses typical industry roles only: no invented accuracy claims, lab accreditations, brands or prices. Use it with first article inspection reports, GD&T basics for buyers, flatness parallelism perpendicularity for machined parts, and surface roughness Ra for machined parts (Ra usually needs a profilometer, not a CMM alone).
What each family is good at
Contact CMM and related probing
A probe touches the part; software fits points to planes, cylinders, circles and GD&T features. Typical strengths:
- Internal diameters, deep bores and features optical lines of sight cannot reach
- Datum schemes that need physical establishment of planes and cylinders
- True position of hole patterns relative to datums
- Many shop-floor and metrology-lab workflows already built around CMM programs
Limits buyers should respect: stylus tip compensation, accessibility, time per point and soft materials that mark. Very large thin sheets may need fixturing that itself influences flatness readings.
Optical, vision and non-contact scanning
Cameras, vision systems, laser line probes and structured light capture edges and surfaces without touch (or with light touch for hybrid systems). Typical strengths:
- Fast capture of external profiles, cutouts and large flat patterns
- Burr and edge-break visualization when magnification is part of the method
- Thin or delicate parts that probe marks would spoil
- Sheet-metal blanks and laser/waterjet external profiles
Limits: recessed internals, shiny or transparent surfaces without preparation, and GD&T that truly needs a physical datum simulator. Optical “pretty color maps” are not automatically a substitute for a position tolerance report unless the method and software are qualified for that characteristic.
Illustration 1 places common feature types against a preferred method family.
Map features to methods on the drawing notes
Hole patterns and true position
Contact CMM is the default language for many machined hole patterns. Optical systems can measure hole centers on thin plates when edges are crisp and the system is calibrated for that task—state the method if you require optical only. Mixed stacks (thick housing + sheet cover) may need CMM on the housing and optical on the cover.
Flatness and large faces
Both families appear in industry. Surface plate + indicator remains valid. Optical flatness maps help large plates; CMM plane fits help localized regions and datums. Call the span and the datum—not only “flat.”
Profiles of complex exteriors
Optical scanning often wins for freeform exteriors and castings after CNC clean-up. For sealed CNC pockets with undercuts, contact or a combination may be required.
Threads, undercuts and O-ring grooves
Threads need thread gauges or specialized methods more than a general optical photo. O-ring groove width/depth often use pins, optical comparators or CMM with small styli—see O-ring groove design for CNC housings. Undercuts need accessibility notes—see CNC pockets, thin walls and undercuts.
Edges, chamfers and burrs
Optical magnification and vision systems excel at edge-break bands and burr presence when acceptance criteria are visual or size-banded. Pair with edge break, chamfer and deburr callouts so “no burr” is measurable.
How to write RFQ inspection language without over-buying
Prefer characteristic-based notes
Good: “True position of 6× Ø6.6 holes to datums A-B-C per drawing; CMM or equivalent contact method acceptable.” Good: “External profile of sheet blank within …; optical or laser scan acceptable with documented correlation.” Weak: “Full CMM of all features” on a 200-dimension plate when only eight characteristics are critical. Weak: “Vision inspection” with no list of which edges or holes.
First article vs in-process
FAI may use a fuller CMM program; production may sample optically or with gauges. Say whether the RFQ method applies to FAIR only, to every lot or to a sampling plan. See first article inspection reports for ballooning and report structure.
Correlation when methods differ
If design engineering used optical scan data and production will use CMM (or the reverse), ask for a short correlation on the critical characteristics during FA. Do not assume identical numbers from different physics.
Fixturing, datums and soft parts
Datum simulation
GD&T assumes datum features are contacted in a defined order. Optical systems must still establish an equivalent datum scheme in software. If your print is heavy on MMC modifiers and datum targets, confirm the lab’s method supports them—see GD&T basics for buyers.
Soft metals and plastics
Probe force can burnish aluminum cosmetic faces or dig into plastics. Call non-contact on cosmetic faces when needed, or accept witness marks outside cosmetic zones.
Temperature
Large aluminum plates measured warm will not match steel CMM scales the same way as stabilized parts. Typical practice is to stabilize; buyers can note “measure after temperature stabilization” without inventing a shop’s soak time.
Sheet metal and plate vs CNC housings
Laser-cut and waterjet blanks lean optical for external profile; CNC housings lean CMM for bores and patterns. Hybrid parts (machined boss on a cut plate) need a split plan. Laser cutting vs waterjet for metal parts and sheet metal enclosure design guide explain blank process differences that change which edges are sharp enough for vision systems.
Swiss-turned small diameters may use optical comparators and pneumatic gauges more than large bridge CMMs—see Swiss turning vs CNC lathe for small parts context.
Decision flowchart for buyers
1. List the characteristics that actually risk assembly or function (not every CAD dimension).
2. Internals, deep bores, classic true position → plan contact CMM (or dedicated bore gauges).
3. External thin profiles, burrs, large blank outlines → plan optical / vision.
4. Flatness → choose surface plate, CMM region or optical map based on size and tolerance; state the span.
5. Cosmetic faces → bar probe marks or zone them.
6. Write FA vs production expectations separately.
Illustration 2 shows that routing.
Side-by-side RFQ phrases
| Need | Prefer | Avoid |
|---|---|---|
| Housing hole pattern | CMM true position to datums | “Scan everything” |
| Sheet blank outline | Optical profile with tolerance | CMM-only on floppy sheet |
| Deep counterbore depth | Depth mic / CMM | Top-down photo only |
| Edge break band | Optical / comparator + size band | “Looks OK” |
| Ra on seal face | Profilometer | CMM point cloud alone |
| Plastic cover | Non-contact or light probe | Heavy stylus on cosmetic |
Package contents that make quotes fair
Include:
- Ballooned drawing or PMI with critical characteristics marked
- Material and finish (coatings change optical contrast and size)
- Whether pins, bushings and inserts are installed before inspection
- Any customer-mandated template or FAIR form
- Related service links when ready: CNC machining service and request a quote
Press-fit pins and bushings add another wrinkle: measure holes before press when the print sizes the open hole; measure after only if the requirement is post-assembly. Face milling vs peripheral milling flatness notes tell inspectors which faces were generated how—useful when tool lay confuses optical edge finders.
What not to put in the RFQ as fact
Do not claim a universal accuracy for “CMM” or “scanner” in marketing language inside the RFQ. Uncertainty depends on machine, probe, environment, part and program. Ask the supplier how they will report each critical characteristic and what the acceptance rule is (tolerance vs guard banding). That conversation prevents fighting over tenths that neither method was bought to argue.
Competitor lab names, fake ISO certificate numbers and invented micron guarantees do not belong in buyer guidance—or in this article. Stick to method family and characteristic.
Production sampling after a solid FA
Once FAIR passes on CMM for a housing pattern, production might check a subset with hard gauges or a shorter CMM routine, while optical spot-checks watch burrs on a break-edge note. Write the sampling intent if price sensitivity is high; silence often defaults to repeating the full FA method on every shippable lot.
Wire EDM openings and ground details may need specialized metrology beyond either family discussed here—see wire EDM for precision parts when those features dominate.
Summary
Contact CMM and optical methods are complementary tools, not rival slogans. Route deep, internal and datum-heavy GD&T toward contact measurement; route external profiles, delicate parts and many edge/burr checks toward optical or vision; keep Ra on a profilometer. Characteristic-based RFQ notes, clear FA vs production scope and honest correlation beat blanket “full CMM” stamps every time.