The stylus is the point of physical contact between the CMM and the component being measured and just as components come in different shapes, sizes and materials, so do styli. Made up of an adapter, a stem and a stylus tip, the stylus takes a measurement when triggered by the operator or by an automated programme. The stylus’ shape, the stiffness and length of its stem, its thermal stability and the material it is made from all determine performance.
Which Stylus is Right for the Job?
When it comes to shape, the most popular choice is the straight stylus which is used to perform most measurements, including of holes and cylinders. Manufacturers that need to measure recesses and grooves typically opt for a disc stylus whereas a star configuration combines several styli to efficiently measure rotational symmetrical toothed parts such as gears. Cylindrical styli, meanwhile, have a relatively large surface area, which is ideal for measuring thin-walled parts. And when it comes to deep holes and rough surfaces, a hollow ball stylus is recommended.
Once the optimal shape has been determined, it’s important to consider which materials make up the tip and the shaft of the stylus. One of the challenges when creating a stylus is to prevent it flexing, which can result in measurement variations. To reduce the risk of flexing the stem, the shaft needs to be as short as possible, have minimal joints and be made of a very hard material that is light enough to comply with the permissible total weight for the sensor. Several factors dictate the choice of styli, including the weight the sensor can bear.
Carbon fibre is best adapted to some of the most challenging measuring environments. Not only does it combine maximum torsional and longitudinal stiffness, it is also the most lightweight styli material, inert and insensitive to temperature, making it ideal, for example, for CMM styli operating within large temperature gradients. Ceramic is lighter than either tungsten carbide and steel, while tungsten carbide’s stiffness makes it useful for small stem diameters of 1 mm or less and lengths of up to 130 mm, which are more susceptible to flexing.
Tipping the Balance Towards Accuracy
The stylus tip is the final point of contact with the component being measured. Both the material used and small deviations in shape and size can have an impact on the measurement result. For example, a smaller sphere is ideal for measuring holes and bores, whereas larger spheres are better adapted to surface measurements and larger characteristics.
The most commonly deployed material for the sphere is ruby because of its extreme hardness and relative affordability, although it is not ideal for use with aluminium, which tends to accumulate as a residue on ruby. Instead, when it comes to measuring aluminium, we recommend silicon nitride. Its interaction with the part’s surface is similar to that of ruby, but it is more resistant to wear and less susceptible to aluminium build-up. A ceramic tip is used for longer, heavy styli and disc styli, as well as for scanning rough surfaces, such as cast iron because of its resistance to wear. Diamond’s hardness makes it the most resistant to wear and in addition it isn’t susceptible to residue build up.
To Get the Most from Your Styli and Ensure Accuracy:
- Check the permissible total weight of your sensor and make sure your sensor complies
- Replace worn or mechanically deformed or damaged styli immediately
- Regularly check the contact surface of the individual accessories for damage
- Clean spheres frequently
- Align the stylus against the part feature to be measured or configure it with either hand tools or a stylus pre-setter
Manuel Müller is a Product Marketing Manager at Hexagon’s Manufacturing Intelligence division. He is responsible for the product marketing strategy for the sensors and components product line, located in Waldburg, Germany. While studying for his bachelor’s degree in Business Management and Marketing, he gained wide experience working for global brands in Germany and North America. Manuel has been with Hexagon since 2014.