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The importance of measuring for precision measurement

In recent years, quality assurance standards have been continuously improved and improved. Only with superior process stability and excellent quality, so called “as fast as possible” companies can remain competitive. Quality assurance and coordinate measurement techniques play a key role in these processes. In order to remain competitive, manufacturers are constantly upgrading CMMs. Coordinate Measuring Machines (CMMs) are now directly integrated into the manufacturing industry as part of the process, while high-speed, high performance scanning systems ensure extremely accurate measurements in very short deadlines. The stylus is the first connection between the CMM and the workpiece, so it is important that the stylus provides the highest possible accuracy at the point of contact. When measuring with a touch probe, the machine uses styli to collect data points on the surface of the workpiece. The points generated by each trigger are defined by X,Y, and Z coordinate values. The features, dimensions, shapes and locations are then calculated from these points.

Select and use styli

Be very careful when selecting styli to ensure the most suitable measurement for your application. Selecting a stylus always involves connecting threads on the CMM sensor, typically M2, M3, M4 and M5 threads. The geometry of the workpiece determines the choice of the stylus assembly. The stylus must be easily accessible to all measuring points of the workpiece, enabling it to achieve the required measurement standards and measurement capabilities for each measured feature. A comprehensive measurement of the workpiece on a CMM with a fixed sensor usually requires a lot of styli, which are mounted in different directions and require differently shaped stylus assemblies, extensions and joints. The combination of all these components is called the stylus configuration and is mounted on the adapter block. The stylus is very flexible to use with a threaded adapter. For example, you can use the M2/M3/M4 stylus on the M5 sensor connection thread.

To maintain the accuracy of the touch point, we recommend you:
  • Try to use short stylus, The smaller the stylus bend or deformation, the higher the accuracy. Using the shortest possible stylus is the best choice for you.
  • Minimize joints, Each additional stylus is connected to the extension stem to add a potential bend and deformation point. Minimize the number of stylus assemblies in your application.
  • The diameter of the selected ball should be as large as possible, This increases the distance between the ball/stem and reduces false triggers caused by the impact bar. The larger the diameter of the ball, the less affected by the surface finish of the workpiece being tested.

Ball material

Ruby

For most measurement applications, ruby is one of the hardest materials found in ball gauges with industry standard and best characteristics. The artificial ruby is an alumina crystal (corundum) having a purity of 99% sintered at 2000 °C by the Verneuil process.
The corundum is then cut and gradually processed into a high precision sphere. The surface of the ruby ball is extremely smooth, with extremely high compressive strength and high mechanical wear resistance.

Tungsten carbide

Tungsten carbide is a compound composed of tungsten and carbon. It has a metallic luster and a hardness similar to that of diamond. It is a good conductor for electricity and heat. Tungsten carbide styli is ideal for applications where the contact points of the workpiece being tested have extremely high wear requirements.

Stainless steel

Stems made of stainless steel are widely used for measuring styli with a ball/tip diameter of 2 mm or more and a maximum length of 30 mm. Within this range, the integral steel bar provides the optimum stiffness to weight ratio, with a joint between the stem and the threaded body that provides sufficient ball/stem clearance without reducing stiffness.

Stem material

Stainless steel

Stems made of stainless steel are widely used for measuring styli with a ball/tip diameter of 2 mm or more and a maximum length of 30 mm. Within this range, the integral steel bar provides the optimum stiffness to weight ratio, with a joint between the stem and the threaded body that provides sufficient ball/stem clearance without reducing stiffness.

Tungsten carbide

For small diameter stems or short stems (lengths up to 50 mm) that require greater rigidity, tungsten carbide is the best choice. Small diameter stems can be used to measure styli with a ball diameter of 1 mm or less. In addition, the weight of the pole will have a negative impact, and the rigidity may also be reduced due to the possible deflection of the joint between the stem and the body.

Ceramics

For stems with a ball diameter bigger than 3 mm and a length more than 30mm, the ceramic stem has a stiffness which similar to that of a steel stem, but is much lighter than a tungsten carbide stem. When the stylus with the ceramic stem is broken due to the collision, the test has additional collision protection for the probe.

Carbon fiber

Carbon fiber materials come in a variety of grades. The materials used by Unimetro provide optimum stiffness characteristics under tensile and twisting conditions and are extremely low in weight. Carbon fiber is an inert material that combines with a special resin matrix to provide excellent protection in most extremely harsh machine environments.

For stems longer than 50 mm, Unimetro is the ideal material for the highest rigidity and extremely light weight. It is the best stem material for high-precision strain gauge probes with excellent vibration damping and a negligible coefficient of thermal expansion.