Spiral error (test error)
Plot
A Spiral plot may spiral inwards or outwards such that the plot fails to join at the data start/stop position. See illustration below.
Diagnostic values
This type of plot could be caused by a test error or unequal backlash.
Cause
If the plot is characterised by a deviation in the trace then:
The cause is usually the Ballbar changing in temperature during the sweep. Such temperature fluctuations can cause the Ballbar to change length.
If using a QC10, the Ballbar cable may be catching somewhere during the sweep causing the Ballbar to dislodge from the toolcup.
The centre mount may be becoming unscrewed. This can cause the Ballbar to move out of its working range.
The machine spindle may be unlocked and rotating.
The Ballbar balls may be coming unscrewed.
The Part Program may not match the software setup. Check the capture and overshoot arc settings
In some cases, what appears to be a Spiral plot can actually be Unequal Backlash.
Effect
Plots are misleading and not represented as they should be. Automatic calculations such as radial and circular deviation will be incorrect. The machine error diagnosis made by the diagnostic software is likely to be inaccurate or incorrect.
Action
If temperature fluctuation is suspected as the cause, then check for any heat sources or sudden draughts that could be affecting the test by changing the Ballbar temperature. When you have removed all possible causes of temperature fluctuation, allow the Ballbar to come to temperature before performing data capture again.
If using a QC10 and cable interference is suspected as the cause, then make sure the Ballbar cable does not interfere with the Ballbar or the tool cup during the course of data capture by snagging on the machine or winding tightly around the centre mount.
If it is suspected that any part of the Ballbar assembly has become loose then check the tightness of all Ballbar joints. Check also that the centre mount is located correctly and the machine spindle is locked.
If the Part Program does not match the software setup, rewrite the Part Program.
When all possible causes of the test error have been eliminated, recapture data and perform diagnostic analysis again. Refer to Testing hints and tips for a reminder of the test procedures which should be used to ensure effective plot diagnosis.