The tolerance stack problem
Most machined components have multiple features that must relate to each other within tight tolerances. The challenge is that each machining operation adds its own variation, and those variations stack up.
A part that requires a bore position to within ±0.05 mm relative to a datum face needs a machining process capable of holding that tolerance consistently — not just occasionally. And it needs an inspection process capable of detecting when it doesn't.
What CNC machines can actually hold
Modern CNC machining centres are capable of impressive accuracy, but there's a significant difference between what a machine can achieve under ideal conditions and what it will hold in production.
Positioning accuracy (the ability to move to a commanded position) is typically ±0.005–0.010 mm for a quality VMC. But this is measured under controlled conditions with a laser interferometer. In production, thermal growth, tool wear, fixturing variation and cutting forces all degrade this.
Repeatability (the ability to return to the same position) is usually better than positioning accuracy — typically ±0.002–0.005 mm. This is what matters most for production consistency.
Process capability (Cpk) is the real measure. A Cpk of 1.33 means your process is producing parts with the tolerance band at ±4σ — acceptable for most applications. Cpk of 1.67 (±5σ) is required for safety-critical aerospace and automotive applications.
Aligning your inspection strategy with your process
The purpose of inspection is not to sort good parts from bad parts — it's to control the process so that bad parts aren't made in the first place.
This means:
- Measuring the right features at the right frequency
- Using the measurement data to adjust the process before it goes out of control
- Having clear action limits that trigger intervention before the tolerance limit is reached
First article inspection (FAI)
Before production starts, a full dimensional inspection of the first article confirms that the process is capable of producing the part to drawing. For AS9100 applications, this is a formal requirement (AS9102).
A first article inspection should measure every dimension on the drawing, not just the critical ones. It's your baseline — if the process drifts, you need to know where it started.
In-process gauging
For high-volume production, in-process gauging (measuring on the machine before the part is removed) is the most effective way to close the loop. Post-process gauging (measuring after the part comes off the machine) is better than nothing but introduces a lag.
Automatic in-process gauging systems can feed correction data directly back to the CNC control, compensating for tool wear and thermal drift in real time.
Statistical process control (SPC)
SPC uses control charts to distinguish between common cause variation (inherent to the process) and special cause variation (something has changed). When a control chart shows a special cause signal — a point outside the control limits, a run of points on one side of the mean — it triggers investigation and corrective action.
For effective SPC, you need:
- A measurement system with adequate resolution and repeatability (Gauge R&R < 10% of tolerance)
- Sufficient sample size and frequency to detect process shifts quickly
- Clear response plans for each type of signal
GD&T and CMM inspection
Geometric dimensioning and tolerancing (GD&T) is the language of precision engineering. It defines not just the size of features but their form, orientation and position relative to datums.
Inspecting GD&T callouts correctly requires a CMM with appropriate software. A surface plate and height gauge can measure linear dimensions but cannot correctly evaluate composite tolerances, true position or profile of a surface.
Common mistakes in GD&T inspection:
- Measuring position from the wrong datum
- Ignoring the material condition modifier (MMC/LMC)
- Measuring perpendicularity with a square rather than a CMM
- Confusing runout with concentricity
The cost of getting it wrong
Scrap and rework are the obvious costs of poor process control. But the hidden costs are often larger: the engineering time to investigate non-conformances, the disruption to production scheduling, the risk of non-conforming product reaching a customer, and the damage to your quality system audit record.
Investing in the right measurement equipment and using it correctly is not a cost — it's insurance against much larger costs downstream.
Summary
Align your inspection strategy with your machining capability. Use first article inspection to establish your baseline. Use in-process gauging and SPC to control the process in real time. Make sure your CMM software can correctly evaluate your GD&T callouts. And measure the right things at the right frequency — not everything, all the time.