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Grade 3 high accuracy styli

With the increased use of very high accuracy CMMs, the specification of the stylus ball has become a significant factor when calculating error budgets.

High precision measuring machines require precision styli. Renishaw offers grade 3 styli with a form deviation of ± 0.08 microns.

When precision counts

  • The sphericity of your stylus ball can affect your CMM measurements.
  • To ensure the accuracy of your measurements, use a minimum of AFBMA 3290 / DIN 5401-11.1993 Grade 5 balls.
  • As standard, Renishaw uses Grade 5 stylus balls with a sphericity of 0.13 µm and not the lesser Grade 10 balls that most other manufacturers use.
  • Grade 3 balls, with a sphericity of 0.08 µm, are also available.

Comparing ball grades

The table below shows the tolerances to which various ball grades must conform:

GradeNominal size in mm (over)Nominal size in mm (up to)Size deviations* μmBall dia. variation (μm) VDWSDeviation from sphere form (μm) tDWSurface roughness (μm) Ra
3-12± 5.320.080.080.010
5-12± 5.630.130.130.014
10-25± 9.750.250.250.020
16-25± 11.40.400.400.025
20-38± 11.50.500.500.032

* Values relate to the mean diameter of a ball DWM

Grade 3 ball design considerations

Investigations into the effect of the design and construction of styli using such a highly specified ball have indicated that the form of the ball can be degraded by machining a hole in it and through distortion from glueing it onto a spigot. Renishaw has researched ways to improve the design of the high accuracy Grade 3 ball styli.

All Grade 3 styli being produced by Renishaw are constructed using an undrilled ball bonded into a spherical cup. Measurements taken before and after assembly of this design have shown that the form of the ball remains well within specification throughout the process.

In order to easily identify a Grade 3 ball stylus, the stems are coated in Titanium Nitride, giving them a gold coloured finish.

Due to limits of measurement capability and bond strength, Grade 3 ball styli are only available with a minimum ball diameter of 1 mm. The styli are not serialised and are supplied with a certificate of roundness.

Nominal ball diameter DW

Nominal ball diameter

The diameter value used to identify the ball size.

Ball diameter variation VDWS

Ball diameter variation

The difference between the largest and smallest diameters of one ball.

Deviation from a spherical form tDW

Deviation from a spherical form

The greatest radial distance in any radial plane between a sphere circumscribed around the ball surface and any point on the ball surface.