Inspection & Metrology
GD&T, true position, surface finish Ra/Rz, CMM programming, gauge capability studies, and hand gauge selection — the precision measurement reference for machinists and quality engineers.
Measurement: The Last Step That Cannot Fail
A part machined to perfection is only as good as the inspection that confirms it. The three pillars of effective shop metrology are: selecting the right measurement method for the tolerance (CMM vs surface plate vs hand gauge), understanding what each GD&T symbol actually controls, and maintaining gauge capability — typically a Gauge R&R study showing measurement system variation is <10% of tolerance.
GD&T (per ASME Y14.5) assigns cylindrical, planar, and other 3D tolerance zones that give machinists more usable tolerance than plus/minus coordinates — especially at maximum material condition (MMC), where a bonus tolerance equal to the departure from MMC is available.
Surface finish (Ra, Rz, Rmax) affects function — sealing surfaces, bearing fits, friction, fatigue life — and must be measured with a profilometer, not eyeballed. The guides below explain what each parameter means and when to specify which one.
Metrology Guides
GD&T
True Position GD&T: Tolerance Zones, MMC & Inspection
What the position symbol controls, cylindrical vs square tolerance zones, MMC bonus tolerance, and step-by-step CMM inspection procedure.
Read guideSurface Finish
Surface Finish Ra Chart: Values, Symbols & Process Capabilities
Ra, Rz, and Rmax explained with a process-capability chart showing achievable Ra for turning, milling, grinding, lapping, and EDM.
Read guideSurface Finish Process Capability
| Process | Ra Typical (µm) | Ra Min Achievable (µm) | Notes |
|---|---|---|---|
| Rough Turning | 3.2–12.5 | 1.6 | Feed-rate dependent |
| Finish Turning | 0.8–3.2 | 0.4 | Sharp insert, low feed |
| Rough Milling | 3.2–12.5 | 1.6 | Shoulder mill scallops |
| Finish Milling (ball) | 0.8–3.2 | 0.2 | Stepover dependent |
| Surface Grinding | 0.2–0.8 | 0.05 | Wheel and dress dependent |
| Honing | 0.1–0.4 | 0.025 | Bore finishing |
| Lapping | 0.025–0.1 | 0.010 | Mating flat surfaces |
| EDM (die-sinking) | 0.4–3.2 | 0.1 | Polish setting dependent |
Featured Metrology Instruments
Representative instruments across metrology categories. Specifications are indicative; verify with manufacturers before purchasing.
Crysta-Apex S 574
CNC Coordinate Measuring Machine
- Measuring range: 500 × 700 × 400 mm
- Volumetric accuracy: (1.7+3L/1000) µm
- Probe: Renishaw TP20 compatible
- Control: MCOSMOS software
Equator 300
Comparative Gauging System
- Working volume: 300 mm dia × 150 mm
- Repeatability: 0.001 mm 2σ
- Shopfloor-rated (no climate control)
- Runs beside the CNC machine
IM-8030
Image Dimension Measurement System
- Stage: 300 × 200 mm
- Accuracy: ±1 µm
- Instant multi-point measurement
- No fixturing required
SJ-210
Surface Roughness Tester
- Parameters: Ra, Rz, Rmax, Ry, Rq
- Measuring range: 17.5 mm (X-axis)
- Resolution: 0.001 µm
- Portable with internal printer
OMP600
Machine Tool Spindle Probe
- Triggering repeatability: 0.25 µm 2σ
- Transmission: optical (radio option)
- For: part setting, tool length, broken tool
- Compatible: all major CNC controllers
TESA MICRO-HITE 600
Digital Height Gauge
- Measuring range: 600 mm
- Resolution: 0.001 mm
- Air bearing column for smooth traverse
- USB data output