Core Elements — Guide to Selecting a Stylus for Coordinate Measuring Machines
When it comes to determining the best method for measuring a workpiece on a coordinate measuring machine (CMM), many choices are defaulted to because they have been carefully considered beforehand. The CMM's accuracy specifications, the optimal type of sensor used (trigger or scan), and the optimal measurement method are often taken for granted and no longer questioned. However, this foundation of accurate measurement can be undermined by selecting an unsuitable stylus, resulting in compromised measurement accuracy.
When evaluating the required precision of coordinate measuring machine (CMM) measurements, the common practice is to use a CMM uncertainty to characteristic tolerance ratio of at least 1:5 (1:10 is the ideal ratio, but too expensive and impractical in many cases). This ratio provides a safety margin, ensuring that the measurement uncertainty is relatively small compared to the expected range of workpiece error. As long as a 1:5 ratio can be maintained at extremely tight tolerance levels, the debate regarding precision will cease.
Unfortunately, seemingly insignificant operations such as changing the stylus on the probe can have a significant impact on the achievable actual accuracy, leading to substantial changes in measurement results. Relying on the annual calibration of the coordinate measuring machine to check this accuracy is insufficient, as this only confirms the measurement results of the stylus used in the test (which is usually very short). This may only represent the best-case scenario accuracy. To gain a more comprehensive understanding of the potential accuracy of various measurements, we need to evaluate how the stylus affects measurement uncertainty.
The size and configuration of the probe can affect the accuracy of the measurement results.
This white paper will elaborate on four main aspects of stylus selection that affect the overall accuracy of coordinate measuring machines:
1. Measuring the sphericity (roundness) of a sphere
2. Stylus deformation
3. Thermal stability
4. Tip Material Selection (Scanning Applications)
1. Measuring the sphericity (roundness) of a sphere
Most probes have a spherical tip, most commonly made of synthetic ruby. Any error in the sphericity (roundness) of such a probe tip can affect the measurement uncertainty of a coordinate measuring machine (CMM), potentially reducing its accuracy by as much as 10%.
spherical ruby tip
Ruby spherical gauges have various accuracy grades defined as "grades," referring to the maximum deviation of the gauge from the ideal sphere. Two of the most commonly used gauge grades are 5 and 10 (the lower the grade number, the better the gauge). "Downgrading" the gauge grade from 5 to 10 might save a slight cost on the probe, but it could very likely affect the theoretical 1:5 ratio. The problem is that the gauge grade is not visually identifiable and its impact on the measurement results is not significant, making it difficult to assess its importance. One approach is to specify a 5-grade gauge as the standard: this might be slightly more expensive, but compared to the high risk of a qualified part becoming scrap due to gauge defects, or a defective part being mistakenly measured as qualified, this cost is negligible. Paradoxically, the higher the accuracy of the coordinate measuring machine, the greater the impact of the gauge grade. On the highest-specification coordinate measuring machines, this effect can reduce accuracy by up to 10%. See the following example…
For applications with extremely demanding requirements, Renishaw offers a range of styluses with a sphericity of only 0.08 μm and a 3-stage sphere.
2. Stylus deformation
When using trigger-type probes (such as the industry-standard TP20), it's common practice to interchange probe modules, using optimized probes to perform different measurement tasks. The reason long probes aren't used in all feature measurements is that longer probes result in greater accuracy loss. A good approach is to choose short and rigid probes whenever possible—but why?
Although the stylus is not the direct cause of this particular error, the error does increase with stylus length. The error stems from the varying measuring forces required to trigger the probe in different directions. Most probes are not triggered the instant the stylus contacts the workpiece; they require a continuously increasing measuring force to exceed the spring load within the sensor mechanism. This spring force forces the stylus to deform. This deformation allows the probe to move a short distance relative to the workpiece after physical contact but before triggering. This movement is known as the pre-trigger stroke.
Most probes' three-point mechanical positioning mechanism can provide different measuring forces to generate triggering as required. In the direction of greater rigidity, the probe will resist triggering until a larger amount of stylus deformation occurs. This also means that the coordinate measuring machine will move further, so the pre-trigger travel varies with the advance angle (see figure below). When using compound advance angles (X, Y, and Z axes), this pre-trigger travel variation is more complex.
Comparison of advance angle and trigger pre-stroke of TP6 trigger probe.
To mitigate this effect, all probes are calibrated on a standard sphere of known size before use. Ideally, this process corrects for errors caused by the combined effect of the probe and the advance angle. In practice, to save time, only a few angles are usually sampled and averaged, so a small amount of error may still remain.
Without empirical testing, it is difficult to estimate the impact of these errors on measurement uncertainty. A key factor to note is that any remaining pre-trigger stroke variation error will be affected by different stylus choices. The importance of material selection in stylus design is emphasized here, considering factors such as flexural stiffness, weight, and cost. Steel is suitable for many shorter styluses, with a Young modulus E = 210 kN/mm². Tungsten carbide (E = 620 kN/mm²) is commonly used for its high stiffness, but its high density limits its use for longer styluses. In these cases, carbon fiber combines high stiffness (E ≥ 450 kN/mm²) with light weight. Meanwhile, ceramic styluses (E = 300–400 kN/mm²) are frequently used in machine tool measurement applications, offering advantages such as light weight and high thermal stability.
Long probes and extension rods are typically made of carbon fiber, which offers optimal rigidity and weight.
The rigidity of the stylus is also affected by the stylus assembly adapter. As a general guideline, it's best to avoid using adapters as much as possible, as they can introduce hysteresis; however, adapters may be unavoidable when measuring complex workpieces with fixed sensors. In these cases, a configuration consisting of a series of styluses, extension rods, connectors, and joints may be required. Again, careful consideration of the stylus material selection is crucial, as it affects the rigidity, weight, and robustness of the stylus configuration.
To maintain accuracy, complex probe configurations require careful selection of materials.
3. Thermal stability
Temperature variations can lead to significant measurement errors. Choosing the right material for the stylus extension ensures better stability under varying temperature conditions, resulting in more reliable measurement results. Materials with a low coefficient of thermal expansion are preferable, especially when using long styluses, as thermal expansion is related to stylus length.
Relevant coefficients of thermal expansion and density of the measuring rod material
As mentioned above, carbon fiber is the most commonly used material for long probes and extension rods because it is rigid, lightweight, and its length does not change with temperature. In applications requiring metallic materials—such as joints and connections—titanium offers a perfect combination of strength, stability, and density. Renishaw offers probes and stylus extension rods made from both of these materials.
4. Selection of probe material
For most applications, a ruby sphere is the default choice for the probe tip. However, in some cases, other materials offer better options.
In triggered measurements, the probe tip only contacts the workpiece surface briefly without relative movement. Scanning differs because the probe ball slides along the workpiece surface, causing frictional wear. Under harsh conditions, this continuous contact can cause workpiece material to detach or adhere to the probe ball, affecting its sphericity. These effects are amplified if a portion of the probe ball remains in continuous contact with the workpiece. Renishaw conducted extensive research on these effects, focusing on identifying two main types of wear:
• When scanning surfaces (such as cast iron surfaces), extremely small residual particles can cause fine scratches on the spherical probe and workpiece surface, resulting in small "shallow pits" on the probe tip. This type of wear is called frictional wear. Hard zirconia probe tips are the best choice for these applications.
Frictional wear (left) causes material to detach from the probe tip, while adhesive wear (right) causes surface material to adhere to the probe ball surface.
• Adhesive wear can occur when there is a chemical affinity between the probe and the workpiece. This phenomenon can occur when scanning aluminum workpieces with a ruby (alumina) probe. Material is transferred from the softer workpiece to the probe, resulting in an aluminum coating on the probe tip, which in turn affects its roundness. In this case, silicon nitride is the best choice because it has good wear resistance and does not adhere to aluminum.
5. Other factors
Other factors to consider when selecting a probe include:
• The probe thread size is adapted to the selected sensor.
• Stylus type — straight stylus, star stylus, rotatable stylus, or custom configuration
• Tip types—spherical, cylindrical, disc-shaped, hemispherical
• The probe tip size minimizes the impact of surface roughness on measurement accuracy.
in conclusion
The stylus is a critical factor in any measurement, providing a crucial connection point between the sensor and components. It measures features around the workpiece and must accurately transmit the surface position to the probe. For accurate measurements, the stylus must be assembled from precision components, each made of materials suited to the requirements of the measurement task. With careful selection, a suitable stylus will not significantly increase uncertainty but will instead provide consistently reliable results. In situations with tight workpiece tolerances and requiring longer styluses, the impact of these selections on accuracy must be carefully considered.
