三坐标测量机如何选配测杆及测球 How to Select Stylus Shanks and Probe Tips for Coordinate Measuring Machines

三坐标测头的测尖种类丰富，有球体、圆柱体等等，大多数测针的测尖是一个球头，而一个测球的等级取决于其相对于理想球体的较大偏差。测尖球度（圆度）的任何误差都可能成为三坐标测量机测量不确定度的一个影响因素，这很可能造成坐标测量机精度降低百分之十之多，总之，坐标测量机精度越高，测球等级的重要性就越大。

The types of stylus tips for CMM probes are diverse, including spheres, cylinders, and others. Most styli have a spherical tip, and the grade of a stylus ball depends on its maximum deviation from an ideal sphere. Any error in tip sphericity (roundness) can become a contributing factor to the measurement uncertainty of the CMM, potentially reducing the machine's accuracy by as much as 10%. In short, the higher the accuracy of the CMM, the more important the grade of the stylus ball becomes.

材料

Materials

红宝石的“硬度”有目共睹，所以红宝石是三坐标常用的测球材料。人造红宝石的主要成分是氧化铝。从相对较软的铝件上转移到测球上的材料可能生成一层“涂层”，从而降低测球的圆度精度。在这种情况下，氮化硅测球是更好的选择，因为这种材料具有良好的耐磨性，而且对铝没有亲合力。 除此之外，氧化锆测球硬度更高的也是一种不错的选择。要求刚性好，又要求重量轻的测量任务时，可以选择陶瓷测杆。还有可塑性强的碳纤维测杆不可忽略。

The hardness of ruby is well known, which is why ruby is a commonly used stylus ball material for CMMs. Synthetic ruby is mainly composed of aluminium oxide. Material transferred from relatively soft aluminium workpieces onto the stylus ball can form a coating that reduces the roundness accuracy of the ball. In such cases, silicon nitride stylus balls are a better choice, as this material offers good wear resistance and has no affinity for aluminium. In addition, zirconia stylus balls, which have higher hardness, are also a good option. For measurement tasks that require both good rigidity and light weight, ceramic stylus shanks can be selected. Also, carbon fibre shanks, which are highly adaptable, should not be overlooked.

挠曲

Deflection

并不是测杆越长测量精度就越高，因为，长测杆并不适合测量所有的工件特征，所以，测量精度随着测杆长度的增加而降低。那么如何选择呢？采用具有较大刚度、尽可能短的测杆才是正确的做法。另外，测杆的挠性会放大预行程的变化。虽然钢也适合制造一些短测杆，但硬质合金是刚性较好的测杆材料。然而，由于硬质合金密度大，因此不适合用于长测杆。陶瓷测杆通常可用于既需要刚性好，又要求重量轻的测量任务。同样，碳纤维通常也可用于制造很长的测杆。 

A longer stylus shank does not necessarily lead to higher measurement accuracy, because long shanks are not suitable for measuring all workpiece features. In fact, measurement accuracy decreases as shank length increases. So how should one choose? The correct approach is to use a shank with high stiffness and the shortest possible length. In addition, the flexibility of the shank amplifies variations in pre‑travel. Although steel is suitable for making some short shanks, tungsten carbide is a shank material with better rigidity. However, due to its high density, tungsten carbide is not suitable for long shanks. Ceramic shanks are often used for measurement tasks that require both good rigidity and light weight. Similarly, carbon fibre is also commonly used for manufacturing very long shanks.

热稳定性

thermal stability

热稳定性是选择测杆及测球的一个因素，也是影响程度较大的一个因素。用户可以选择具有低热胀系数的材料如碳纤维、钛合金。

Thermal stability is a factor in selecting stylus shanks and balls, and it is also a factor with a significant degree of influence. Users can choose materials with low coefficients of thermal expansion, such as carbon fibre and titanium alloy.