Cyclic error (machine error)
Plot
The plot has a cyclic sinusoidal error, varying in both frequency and amplitude around the plot. In this plot cyclic error is shown in the Y axis only. Dy, the wavelength as measured along the Y-axis, is approximately constant around the circle.
Diagnostic values
Cyclic error is quantified in the following format for the X and Y axes:
Cyclic error (µm)
X 0.0 0.0
Y 9.5 9.5
The values quoted are the peak to peak amplitude of the cyclic error in that axis moving in the direction indicated by the arrow, where specifies forward motion, that is positively increasing in position, and specifies reverse motion, that is positively decreasing in position. These values are shown on the plot.
This measurement technique is necessary since a cyclic error does not always effect the whole plot. Therefore it is necessary to specify where on the plot the cyclic error is located.
The pitch of the cyclic error is also available. Cyclic pitch is quoted as a value in millimetres or inches, depending on the units used, in both the X and Y axes as follows:
Cyclic Pitch X 4.0 mm
Cyclic Pitch Y 6.0 mm
Cause
An axis ballscrew (in this case, the Y axis) has a cyclic error problem. There are a number of possible causes for this:
The axis ballscrew thread is 'Drunk' causing the axis to move in a sinusoidal manner rather than at a uniform rate.
The encoder mountings may be eccentric.
The ballscrew mountings may be eccentric.
Badly adjusted resolvers or inductosyns.
If the cause is the axis ballscrew then the plot is unaffected by direction. Clockwise and counter-clockwise plots will appear similar, if not identical.
If the cyclic error is occurring in a vertical axis in one direction only when the axis is moving up or down, then the counterbalance mechanism of the machine is likely to be at fault.
If the cause is a faulty counterbalance then the plot will be affected by direction, therefore the clockwise and counter-clockwise plots will be different. In general, the counterbalance will cause a cyclic error when the machine is moving vertically upwards. The clockwise and counter-clockwise plots for a cyclic error caused by such a counterbalance problem may be as shown below.
The counterbalance problem is clearly shown as only one half of each plot exhibits a cyclic error.
Effect
The effect of any type of cyclic error is that parts cut on the machine will exhibit dimensional errors.
Action
Using diagnostic analysis, view the clockwise and counter-clockwise plots individually to identify whether the cause is a ballscrew problem or a counterbalance problem.
If the suspected cause of the cyclic error is the ballscrew then adjust the ballscrew or encoder mountings to eliminate the cyclic error.
If the suspected cause of the cyclic error is the counterbalance mechanism of the machine, adjust the counterbalance mechanism to eliminate the cyclic error.
If necessary the pitch of the cyclic error can be calculated manually as follows:
Pitch = x test radius x 2
This gives the pitch of the cyclic error (where Dy and y are measured with a ruler directly off the plot, ignoring the axis scale). This tells you how far the axis has travelled for each cycle of error. This value can be compared with the pitch of the leadscrew. The amplitude (peak to peak) of the error is given by e.
NOTE: Correct diagnosis of cyclic errors relies on feedrate, particularly where counterbalance is the problem. If you can identify any element of feedrate error in a cyclic error diagnosis then the diagnosis may be incorrect. In these circumstances carry out tests at various feedrates in order to identify whether the machine has a cyclic error problem.