Published August 27, 2026 at 08:48
A coordinate measuring machine never knows where the workpiece is. It knows where the centre of the ball was at the moment the probe triggered, and then subtracts the ball radius to arrive at the point on the surface.
The entire measurement therefore rests on one assumption: that the ball is a sphere. If it isn’t, the calculated surface point ends up wrong — and in different directions depending on which part of the ball happened to make contact.
What it is
A ruby ball measuring probe for coordinate measuring machines.
- Ball diameter 3 mm, thread M2
- Overall length 50 mm, EWL 42.5 mm
- Weight 0.83 grams
Why ruby
The ball makes contact tens of thousands of times. Every measuring point is a contact, and a machine running series measurements makes hundreds of thousands of them over a stylus’s lifetime. A material that wears will flatten locally on the ball — and a ball with a flat spot no longer touches the surface at a single point, but over a small area whose position depends on the contact angle. The error becomes small, systematic and invisible.
Ruby is extremely hard and therefore keeps its shape. Equally important, it is chemically inert: soft materials — aluminium above all — readily stick to a measuring tip and build up on it over time. A build-up of a few micrometres is exactly the same as an incorrect ball radius, and it creeps in over the course of a working day without anyone noticing.
Why 0.83 grams is stated in the specification
Mass at the very end of a stylus is the enemy. It contributes in three ways, all bad: it bends the stylus when the machine accelerates, it can cause the probe to trigger from vibration rather than contact, and it forces lower speeds between measuring points to prevent the first two from happening.
A lightweight stylus is therefore not just elegant — it’s a faster machine. On a measuring routine with hundreds of points, the difference in cycle time is noticeable, and it comes entirely without sacrificing accuracy — quite the opposite.
EWL is the figure to read, not the overall length
42.5 mm out of 50 mm. Effective Working Length indicates how far the tip can actually reach before the shaft — not the ball — touches the workpiece. It is always less than the overall length, since part of the length is taken up by the mounting and the transition to the ball.
For anyone measuring at the bottom of a pocket or inside a bore, EWL is therefore the decisive dimension. Choosing a stylus based on overall length will sooner or later result in a collision between the shaft and the material — and a collision in a measuring machine is, at best, a lost stylus.
Three typical use cases
- Series production: measuring routines with many points, where both wear resistance and cycle time matter.
- Deep and narrow geometries: pockets, grooves and holes where EWL sets the limit.
- Soft materials: aluminium and plastic, where build-up on the tip would otherwise become a hidden source of error.
Why it pays off
A measuring machine costs as much as a machine. A stylus costs as much as a tool. Yet it is the stylus that determines what the machine can actually measure and how long the results remain comparable — and it is this part that wears.
There is also a source of error worth mentioning, because it’s hard to detect: a stylus that has worn or built up material still produces perfectly credible readings. Nothing triggers an alarm, the curves look normal, and the deviation only becomes visible when someone measures the same part with a different machine. Replacing a stylus in time is therefore cheaper than figuring out why two departments disagree.
