CNC Precision Machining: How Is Accuracy Inspected and Controlled?

Using probes for real-time, in-process measurement has become an important direction in CNC machine-tool accuracy inspection. During CNC setup, a probe can establish the work coordinate system quickly, reducing manual work and eliminating operator-related variation so that coordinate-system accuracy remains consistent. During machining, the CNC can automatically inspect the geometric accuracy of completed features, correct the coordinate system or tool offset, and improve the final part acceptance rate through in-process compensation.

How Touch-Trigger Probing Works

Probes used to inspect workpiece accuracy can be classified by measurement method, including touch-trigger and scanning probes. Touch-trigger probes are the type used most widely on machine tools. In a typical touch-trigger probe, contact between the stylus and the measured surface causes the stylus to deflect and rotate around an internal pivot. This changes the electrical resistance at the contact points between the stylus and its supporting structure. Once the resistance change reaches a defined threshold, the probe generates a trigger signal.

The trigger is normally a square-wave electrical signal with a very steep leading edge, typically measured in microseconds. Using this edge as the latch signal helps ensure that the captured machine coordinates accurately represent the contact position. High-precision probes can achieve measurement accuracy of 1 μm.

For on-machine measurement, the probing system usually consists of a probe and a receiver. The probe is commonly mounted in a toolholder that matches the machine spindle, while the receiver is installed at a fixed location on the machine. Before use, the probe radius, overall length, and other relevant parameters must be calibrated. When the stylus contacts the workpiece, the trigger signal is transmitted to the CNC by cable, infrared light, or radio. The CNC then processes the signal according to the inspection or compensation routine.

Machine-Vision Inspection

A machine-vision inspection system uses a charge-coupled device (CCD), lens, and light source to capture images of features on a machined workpiece. An image processor, such as a PC or the CNC itself, processes the acquired data. Software algorithms extract image features and combine them with camera-calibration parameters to calculate physical positions and dimensions for workpiece location and measurement.

The main hardware includes an industrial camera, lens, light source, protective enclosure, auxiliary brackets, and cables. The light source provides stable illumination, the lens focuses the image, and the camera captures it.

Unlike a touch-probe system, machine vision is non-contact and can inspect richer geometry, including arcs and curves, in a single acquisition. It can therefore improve inspection efficiency. Vision also has clear advantages for two-dimensional features that a probe cannot reach reliably, such as very small-diameter holes.

As intelligent CNC systems develop, image processing is becoming more tightly integrated with machine control. In newer intelligent controllers such as the i5, image-processing software is fully integrated into the CNC, improving synchronization and flexibility between the inspection system and CNC control.

Protecting Vision Hardware on the Machine

To obtain usable views of the workpiece, vision hardware on a CNC machine is normally installed near the spindle or tool turret. Chips, cutting fluid, and airborne mist can contaminate or damage the camera and lens. Effective protection for the camera, lens, and related components is therefore essential to prevent water, mist, and chips from degrading measurement accuracy or shortening camera life.