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Rick Hudgell

Metrology

Selecting Styli for Optimum Accuracy

These accessories can be just as critical a choice as the machine to which they’re attached.

Published: Wednesday, April 13, 2005 - 21:00

As industry standards increase, so too does the demand for flexibility in specialized gaging procedures. Because there’s no room for error, choosing the right contact point or styli is paramount in conforming to the organization’s high quality standards. Accuracy can be jeopardized by any number of variables, but the contact point shouldn’t be one of them.Styli are manufactured in a number of different designs, shaft materials and tips. Each contact styli is designed for special applications. As the initial contact component of the measuring process, sphericity and rigidity are critical when choosing styli.

When choosing replacement styli, a few important qualities must be remembered:

  • Keep the styli as short as possible to prevent bending or deflecting.
  • Use as large a ball or tip as possible.
  • Minimize the use of joints or extensions. Custom-made styli are better choices.

The point of contact is the most critical aspect of accurate gaging. Therefore, careful consideration must be given to the probe tip and the ball material:

 Ruby-tipped probes

Ruby balls exhibit a very low coefficient factor and won’t scratch or mar delicate parts. High compressive strength and resistance to corrosion make them an industry standard for the majority of CMM applications. However, sphericity issues come into play when ruby contacts harsh metals and the probes life is dramatically shortened. Furthermore, a phenomenon known as adhesive wear, a film or buildup on the ball itself, can be an issue when scanning aluminum parts.

Tungsten carbide probes
Tungsten carbide is the most durable material used in contact points on the market today. Its compressive strength is higher than that of virtually all metals, and it’s two- to three-times more rigid than steel. With tungsten carbide styli exhibiting wear statistics of more than 100 times that of steel, the economical benefits are obvious. These characteristics make tungsten carbide the obvious choice for durability in large scale manufacturing while maintaining precise accuracy.

Delrin or Teflon
Occasionally applications arise in which the slightest contact will alter or mark a material’s surface. Delrin-tipped probes are often utilized in these applications. Delrin and its sister products are less durable than conventional styli tips. Sphericity issues are a concern and can be directly related to heat, compression, and/or environmental issues such as coolants and chemicals.

Environmental concerns must also be addressed when selecting styli for a company’s manufacturing needs. Melting points, magnetic properties and thermal expansion are among the factors that must be considered. Although ruby-tipped styli are considered the industry norm for most gaging needs, synthetic materials such as ruby and silicon nitride are more susceptible to environmental changes.

As the ball or point of the styli is the point of contact, the stem of the styli is equally important as the tip or contact point. Again, material selection is imperative to the entire probing process. Attention must be given to the overall length of the probe as well as the effective working length (EWL). The EWL is measured from the center of the ball (probe head) to the point of the stem that will contact the product, thereby causing an inaccurate reading. As joints or connections will create weak spots in the stem, they should be eliminated or kept to a minimum.

Ceramic stems are commonly used in the longer applications because of their lighter weight, compared to tungsten carbide, for example. Ceramic shafts also offer an insurance policy for expensive machinery. In case of an error in testing setup or any variable that could cause the styli to come in contact with the part to be measured, ceramic stems shatter at a fairly low tensile point. In comparison, tungsten carbide is the strongest stem material and maintains its strength and integrity when joined with extensions or knuckles. Heavier overall weight is the primary concern in using tungsten carbide stems. However, the benefits of longevity and adaptability may outweigh any weight concerns.

Carbon fiber stems are quickly gaining exposure in the manufacturing industry due to their extremely light weight and resistance to environmental issues. The tensile strength of carbon fiber stems is significantly higher than ceramic stems. Carbon fiber stems may splinter when recommended tensile strength is exceeded.

There isn’t one perfect gaging stylus for every industrial need. CMM’s, dial test indicators and height gages are only as accurate as their point of contact. Selecting the right styli for a particular application is essential to maintaining precision.

Situations exist in which custom-made contact points/styli are the only solution to precision gaging issues. Also, styli obtained from a replacement styli manufacturer provide significant savings over OEM. products. The standards and quality are generally higher than industry standards and resources are readily available for custom work.

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Rick Hudgell