The choice of materials for the stylus tip and stem is critical.
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 exists where ruby is not the preferred ball material, however there are two 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.
The second is 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.
Silicon nitride possesses many similar properties to ruby. It is a very hard and wear-resistant ceramic which can be machined into high precision spheres. It can also be polished to an extremely smooth surface finish.
Silicon nitride does not have an attraction to aluminium and so does not exhibit the adhesive wear seen with ruby in similar applications. However, silicon nitride does show significant abrasive wear characteristics when scanning on steel surfaces and so its applications are best confined to aluminium.
Zirconia is a particularly tough ceramic material with hardness and wear characteristics approaching those of ruby. Its surface properties make it an ideal material for aggressive scanning applications on cast iron components.
Stylus 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 lengths up to 50 mm. Beyond this, weight can become a problem and 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 but are significantly lighter 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 styli weigh around 20% less than tungsten carbide styli, making it a suitable material for long styli. Its thermal stability is also advantageous, particularly with very long styli, which makes it suitable for use in a production environment.
This is a very light material so is ideally suited for extensions, but only in a stable air-conditioned environment due to thermal growth.
Titanium is thermally stable compared to aluminium, has good flexural rigidity and is also very light. These characteristics make it very suitable for long extensions.