TechArchive

This article was originally written in the period 1995-2000

Many difficult measuring problems can be solved by using balls of known size and accuracy. Alan Quinn looks at the latest from Spheric Engineering.

Think of where precision balls may be used and you generally come up with ball bearings. Which is quite reasonable, even though the price which manufacturers have been able to command for this application has fallen steadily over the last generation. Equally, a mind-boggling five billion balls for ball-point pens is manufactured every year!

But the latest trend for specialist ball manufacturer Spheric Engineering is to move more into added value systems and assemblies involving its high accuracy metal and ceramic products. Many difficult measuring problems can be solved by assemblies using balls of known size and accuracy.

Master balls are used for measuring the location of internal grooves, the depth of holes, internal tapers, flatness and parallelism of micrometer anvils and setting external gauges and comparators. Calibrated master balls are manufactured primarily in tungsten carbide. Calibrated balls have their uses mainly in metrology and standards laboratories, inspection departments and tool and gauge rooms. Because they are manufactured for use as reference standards, Spheric’s high precision ball sets are designated master balls.

A certificate of calibration is supplied with each master ball set, which provides the actual diameter and sphericity read in one plane. Also, a premium certificate of calibration is available, which provides the actual diameter and sphericity readings in three planes on each ball. The certificates are traceable to NAMAS (ISO 9000) and NIST.

A computer-controlled measuring machine (CMM) is a computer-controlled, three-axis system capable of repeating accurate inspection of high volume and complicated components and storing the data for quality control purposes.

The qualification sphere for a CMM is used to calibrate and set up the measuring stylus or probe and set the home/zero positions of the machine’s three measuring scales ready for use. The data from this initial set-up is stored in the computer to enable the program to control the table movements accurately and take measurements from the machine scales, the positions of which are determined when the measuring stylus or probe contacts the surface of the feature being measured.

Generally, the spheres are made from tungsten carbide, steel or zirconia, in a diameter range from 0.375in to 2in. This is mounted on a stainless steel stem of length between 50-100mm and having a diameter relevant to the size of the ball. The stem has a metric thread at the opposite end to where the ball is mounted, which is used to locate and secure the qualification sphere to a machine table of the CMM machine. The ball is mounted using a high strength adhesive and the joint is tested to a load of 20kgf. Variations include imperial and metric versions, cranked stems, and a reference ball with wire and id tag

Unusually, the joint strength is kept intentionally low to ensure that failure WILL occur at moderate loads. This serves as a means of limiting or preventing damage to expensive CMM equipment. Equally, in the event of a severe impact loading which may distort or damage the ball, failure of the joint acts as a positive indicator.

With a hardness of approximately 1500HV, high stiffness and very good dimensional stability, tungsten carbide is an ideal material for reference standards. Its high wear resistance ensures minimum deterioration in use and allows the time between recalibrations to be maximised. Its coefficient of expansion of 6 x 10^-6and low rate of thermal conductivity ensure a low temperature sensitivity and allow use at normal working temperatures with minimal error.

Spheric’s tungsten carbide reference balls are supplied with a NAMAS Certificate of Calibration, which lists the absolute diameter and sphericity of the ball as measured in three planes. Roundness traces are provided for each plane and all measurements are traceable back to National Standards.

There are two tolerance levels:

Grade 10: measured in three planes, the ball may exhibit a maximum deviation from a true sphere of 10µin.

Grade 5: measured in three planes, the ball may exhibit a maximum deviation from a true sphere of 5µin.

For reference tooling balls, AISI 440C stainless steel is sometimes specified for less arduous applications where the properties and cost of tungsten carbide are not warranted or justified, As with all steel gauges, they must be handled and used with care and replaced or recalibrated more frequently than their tungsten carbide counterparts.

• Spheric Engineering
• Tel: 01293 541377
• Fax: 01293 511921
• Contact: Julian Pratt