SP80 and SP80H stylus selection
NOTE: For more detailed information about the range of Renishaw styli please refer to the styli and accessories technical specifications booklet (Renishaw part number H‑1000-3200).
Of particular interest to SP80 and SP80H users is M5 styli section. This details not only an extensive range of M5 styli that are compatible with the SP80 and SP80H, but also includes a complete range of carbon fibre extension bars of 11 mm or 20 mm diameter, and up to 500 mm long.
Accuracy at the point of contact
As industry has developed its requirement for increasingly diverse and complex manufactured parts, inspection systems have had to work hard to keep up. The use of CMMs with probing systems and in-process inspection on machine tools are two of the solutions offered by Renishaw to help you maximise your productivity and maintain the highest possible standards of quality.
Successful gauging depends very much on the ability of the probe's stylus to access a feature and then maintain accuracy at the point of contact. At Renishaw, we have used our expertise in probe and stylus design to develop a comprehensive range of CMM styli to offer you the greatest possible precision.
These notes explain the critical features of each stylus type, helping you to choose the right design for each inspection need.
What is a stylus?
A stylus is that part of the measuring system which makes contact with the component, causing the probe mechanism to displace. The generated signal enables a measurement to be taken. The feature to be inspected dictates the type and size of stylus used. In all cases, however, maximum rigidity of the stylus and perfect sphericity of the tip are vital.
The performance of your gauging can easily be degraded if you use a stylus with poor ball roundness, poor ball location, bad thread fit or a compromised design that allows excessive bending during measurement. To ensure the integrity of the data you gather, make certain that you specify and use a stylus from the comprehensive range of genuine Renishaw styli.
Best practice when using a stylusIn order to maintain accuracy at the point of contact we recommend that you:
- Keep styli short
- Minimise joints
- Maximise stem diameter and stylus ball size where possible
- Regularly inspect stylus tips for wear or damage
Ball materials available with Renishaw styli
The industry standard and the optimum stylus ball material for a vast majority of measurement applications, ruby is one of the hardest known materials. Synthetic ruby is 99% pure aluminium oxide, which is grown into crystals (or ‘boules') at 2000 °C using the Verneuil process.
The boules are then cut and gradually machined into a highly spherical form. Ruby balls are exceptionally smooth on the surface, have great compressive strength and a high resistance to mechanical corrosion.
Very few applications exist where ruby is not the best ball material, however there are two such applications where balls manufactured from other materials are recommended.
The first is for heavy duty scanning applications on aluminium. Because the materials attract, a phenomenon known as ‘adhesive wear' can occur which involves build up of aluminium from the surface onto the ball. A better ball material for such applications is silicon nitride.
Silicon nitride possesses many similar properties to ruby. It is a very hard and wear resistant ceramic which can be machined into very high precision spheres. It can also be polished to an extremely smooth surface finish. Silicon nitride does not have the attraction to aluminium and so does not exhibit the adhesive wear seen with ruby in similar applications. Silicon nitride does, however, show significant abrasive wear characteristics when scanning on steel surfaces so its applications are best confined to aluminium.
The second circumstance where ruby may be problematic is once again in heavy duty scanning applications on cast iron. Interaction between the two materials can result in ‘abrasive wear' of the ruby ball surface. For such applications, zirconia balls are recommended.
Zirconia is a particularly tough ceramic material with hardness and wear characteristics approaching those of ruby. Its surface properties, however, make it an ideal material for aggressive scanning applications on cast iron components.
Stem material available with Renishaw styli
Stems manufactured from non-magnetic stainless steel are used widely for styli with ball/tip diameters of 2 mm or greater and with lengths up to 30 mm. Within this range, one-piece steel stems offer the optimum stiffness to weight ratio, giving adequate ball/stem clearance without compromising stiffness with a joint between the stem and threaded body.
Tungsten carbide stems are best used for maximising stiffness with either small stem diameters, required for ball diameters of 1 mm and below, or over longer lengths up to 50 mm. Beyond this, weight can become a problem or stiffness is lost due to deflection at the stem-to-body joint.
For ball diameters greater than 3 mm, and lengths over 30 mm, ceramic stems offer stiffness comparable to steel, and are significantly lighter in weight than tungsten carbide. Ceramic stemmed styli can also offer additional crash protection to your probe as the stem will shatter in a collision.
Carbon fibre (Renishaw GF)
There are many grades of carbon fibre materials, however Renishaw GF combines optimum stiffness characteristics, both longitudinally and in torsion (important in star constructions) with extremely low weight. Carbon fibre is inert and this, combined with a special resin matrix, provides excellent protection in the most hostile machine tool environments.
Renishaw GF is ideal for maximising stiffness while giving very low mass for styli above 50 mm in length. It is the optimum stem material for high accuracy strain gauge technology probes with excellent vibration damping characteristics and negligible co-efficient of thermal expansion.