Scaling error (mismatch) (machine error)
Plot
The plot has an oval or peanut shape distorted along the 0° or 90° axis. The axis of distortion is unaffected by the direction of data capture, i.e. clockwise or counter-clockwise. The amount of distortion caused by Scaling Error is usually unaffected by machine feedrate.
Scaling Error is the difference in the measured travels of the axes during the test. For example, if the machine is performing a circle in the XY plane; the X and Y axes should move over exactly the same distance. If they do not, the difference in their movements is the scaling mismatch error. In the plot below this is the difference between a and b.
Note: When performing partial arc tests, scaling errors and scale mismatch are not displayed.
Diagnostic values
The diagnostic values quoted for Scaling Error depend on whether a calibrated Ballbar has been used for data capture. If a calibrated Ballbar has been used then Scaling Error is quoted in both the X and Y axes as the exact distances these axes have travelled. If an uncalibrated Ballbar has been used, Scaling Error can only be quoted as a mismatch value between the X and Y axes. In these circumstances a value is quoted for Scaling Mismatch instead of Scaling Error.
Scaling Error (if applicable) is quantified in the following format:
Scaling Error X 49.5 ppm
Scaling Error Y 39.7 ppm
Scaling Error is normally quoted in ppm (parts per million). Other units are available by clicking on the units button. A positive value for Scaling Error means that there is over travel in the direction of that axis, a negative value means there is under travel. The amount of under or over travel can be calculated by multiplying the ballbar test circle diameter by the ppm error.
For example, if the Scaling Error in the A axis direction is -25 ppm, and the test radius was 150 mm, the travel in the X direction is short by:
25/1,000,000 x 2 x 150 mm = 0.0075 mm = 7.5µm
(i.e: the measured travel was 299.9925 mm instead of 300.00 mm)
Notes:
The accuracy of the Scaling Error results depends heavily on the correct calibration of the ballbar length and on the correct application of thermal compensation. Refer to the Temperature Compensation section of the manual for more details. Failure to carry out the calibration and compensation correctly is likely to result in an error diagnosis that falsely indicates large axis Scaling Errors.
Scaling Error results are sensitive to the effects of servo-following errors, which can cause the actual radius followed by the machine to shrink as the feedrate is increased. Tests carried out at higher feedrates can produce large axis Scaling Error values. Hence it is important to carry out tests for Scaling Errors at an appropriate feedrate. It is suggested that a feedrate of 10% of the machine’s maximum feedrate is used. Alternatively, select a feedrate close to that used for finishing cuts.
Scaling Mismatch (if applicable) is quantified in the following format:
Scaling Mismatch 9.8 µm
Scaling Mismatch is quoted in microns. It is calculated by taking the X axis diameter from the Y axis diameter, as measured from the plot. If the value quoted for Scaling Mismatch is a positive value then the X axis is over travelling the Y axis. If it is a negative value then the Y axis is over travelling the X axis.
In the Diagnostic Printout, values for both Scaling Error and Scaling Mismatch are provided if a calibrated Ballbar has been used. If an uncalibrated Ballbar has been used then only Scaling Mismatch is provided. However, in the Diagnostic Printout only Scaling Mismatch is listed as a percentage because only the mismatch between the Scaling Errors in each axis contribute to non-circularity.
On a 220° test a large centre offset can result in inaccurate diagnosis of scale errors.
Cause
One of the machines axes is either over travelling or under travelling relative to the other. There are a number of possible causes for this:
If linear error compensation parameters are being used, these may be set incorrectly.
The axis tape scale may be either over-tensioned or under-tensioned.
The axis ballscrew may be faulty or overheating causing a ballscrew pitch error.
The machine may be subject to an angular error, causing the X or Y axis to pitch out of the test plane as it moves. This is because the axis guideways are not straight or are not sufficiently rigid.
Effect
The effect of a Scaling Error is that parts cut on the machine will exhibit dimensional errors.
Action
Check that any linear error compensation values being used are set correctly.
Check that the axis tape scales are tensioned correctly.
Check that the ballscrew is in good condition and is not overheating. If necessary maintain or renew it.
Check that the machine guideways are straight and in good condition. If necessary realign or renew the guideways.
Angular errors can be identified by repeating t