The influence of measuring rod and measuring ball on the accuracy of coordinate measuring machine-EEWORLD

Collect

The stylus and ball are the key link between the sensor of the coordinate measuring machine (CMM) and the surface of the workpiece being measured. If the inappropriate stylus and ball are selected for a specific measurement task, it may lead to uncertainty and inconsistency in the measurement results. In order to optimize the measurement accuracy of the CMM, careful consideration must be given when selecting the stylus and ball. Renishaw, a British measurement product manufacturer, introduced the impact of the selection of stylus and ball on the overall measurement accuracy of the CMM from four aspects.

(1) Roundness of the stylus ball: The stylus ball at the top of the stylus of most CMM stylus bars is an artificial ruby ball. The roundness error of the stylus ball may cause the CMM to lose up to 10% of its potential measurement accuracy. The manufacturing accuracy level of ruby stylus balls is defined by grades, and the grade of a stylus ball is determined by its maximum deviation from an ideal sphere. The two most commonly used stylus ball grades are grade 5 and grade 10 (with sphericity errors of 0.13µm and 0.25µm respectively). Although some cost savings can be achieved by switching from a Grade 5 to a Grade 10 ball, the generally accepted 5:1 ratio of CMM measurement uncertainty to form tolerance may be jeopardized. Although a standard Grade 5 ball may cost slightly more, this cost is negligible compared to the possibility of scrapping a good part or allowing a bad part to pass.

(2) Material of the stylus ball: Although ruby is the most commonly used stylus ball material, other materials may be more suitable for contact scanning measurement. During scanning measurement, the stylus ball slides along the surface of the workpiece, causing friction and wear. In extreme cases, this long-term contact may cause the stylus ball material to be removed from the ball (abrasive wear) or the workpiece material to adhere to the stylus ball (adhesive wear). In either case, the roundness of the stylus ball will be affected. When a part of the stylus ball is in constant contact with the workpiece, this roundness error will continue to increase.

Abrasive wear may occur when scanning the surface of certain materials (such as cast iron). Tiny particles left behind can cause minor scratches on the ball and workpiece surfaces, creating a tiny "flat" on the ball. For this type of measurement, a harder zirconium oxide ball is a good choice. Adhesive wear usually occurs when the ball material has a chemical affinity with the workpiece material, which is common when scanning aluminum with a synthetic ruby ball, which is primarily composed of aluminum oxide. Material transferred from the relatively soft aluminum to the ball can form a "coating" that reduces the roundness accuracy of the ball. In this case, a silicon nitride ball is a better choice because it has good wear resistance and no affinity for aluminum.

(3) Stem deflection: Measurement accuracy decreases with increasing stem length, so long stems are not suitable for measuring all workpiece features (in fact, it is correct to use a stem that is as short as possible with maximum stiffness). Although the stem does not directly cause this specific error, the stem length can amplify the error. This error occurs when the probe contacts the workpiece from different directions, causing changes in pre-travel due to different measurement forces required to trigger. Calibrating the probe with a standard circle can reduce this pre-travel error, but it cannot completely eliminate it.

The flexibility of the probe stem will amplify changes in pre-travel. Although steel is also suitable for manufacturing some short probes, carbide is the most rigid probe material. However, due to its high density, carbide is not suitable for long probes. Ceramic probes are generally used for measurement tasks that require both good rigidity and light weight. Similarly, carbon fiber is also generally used to manufacture very long probes.

(3) Thermal stability: Temperature fluctuations can cause large CMM measurement errors. Selecting the appropriate extension rod material can achieve better stability and provide more consistent measurement results when environmental conditions change. Materials with low thermal expansion coefficients are the preferred probe rod materials (especially when using long probes) because the amount of thermal expansion depends on the length. Carbon fiber is the most commonly used material for long probes and extension rods because it is both rigid and light. Titanium alloys have good strength, stability and density, making them very suitable for manufacturing metal parts of probe rods (such as joints and movable joints). (end)

Reference address：The influence of measuring rod and measuring ball on the accuracy of coordinate measuring machine

Previous article：Testing indicators and methods for diamond circular saw blade substrate
Next article：Inspection of micro milling cutters can cut tool costs in half

Popular Resources
Popular amplifiers