Plot
The plot has three distinct lobes in its general shape. These are not affected by feedrate or direction but may change with the location at which the test is performed on the machine bed. A straightness type plot can be distinguished from a Tri-lobe test error type plot by its smoother transitions between lobes and better repeatability.
NOTE: When performing partial arc test, straightness errors are not displayed.
This plot shows the effect of a 'second order' error in the X axis where this axis exhibits a simple bow, as illustrated below.
If a more complex bow is present in the axis, as illustrated below, then the plot will be different. More complex straightness errors are not diagnosed by the software.
Diagnostic values
Straightness is quantified in the following format for the X and Y axes:
Straightness X -40.0 µm
Straightness Y 0.2 µm
The straightness error quoted by the diagnostic software is the peak to valley bow in the axis over the length equivalent to the diameter of the Ballbar test. For example, if the main straightness error is in the X axis, as with the figures shown above, then the straightness error is measured in microns or inches by the diagnostic software as shown in the following diagram.
In this case the bow in the X axis causes the straightness error. As can be seen, this bow has the effect of distorting the whole plot.
Cause
A straightness error is caused by a lack of straightness in the machine guideways. The guideways may be bent locally or there may be an overall guideway misalignment in the machine. This may be the result of wear in the guideways, an accident which has damaged the machine guideways or caused them to become misaligned, or poor machine foundations which may be the cause of a bowing effect in the whole machine.
Effect
The effect of a straightness error on a machine is that the accuracy of all machined parts will be poor.
Action
Check for wear or dirt in the tool cup and loose Ballbar joints. If you find any of these, then it is likely that the machine does not have a straightness error, instead it may have a Tri-lobe test error. In these circumstances take the necessary action described in the Tri-Lobe section and then repeat the Ballbar test. If a straightness type plot is still produced then the machine has a real straightness error.
If a Tri-lobe test error has been eliminated as the cause of the plot, then adjust, repair or replace the machine guideways as necessary.