Renishaw Ballbar Test - Plot Interpretation - Mills · 2020-03-06 · lubrication system....

LAST UPDATED: 11/26/2018

Renishaw Ballbar Test - Plot Interpretation - Mills

IntroductionThis document has sample ballbar plots from machines with different geometric, dynamic and test hardware errors.Each sample includes a description of the error, the possible causes of the error, the effect that the error has on themachined part, and corrective action. When you do a ballbar analysis on a machine, compare your results to thesesample plots to see what troubleshooting steps to do next.

Ballbar Analysis - Quick Check Mode

Click here for the procedure to do a BALLBAR ANALYSIS - QUICK CHECK MODE on a mill.

Compensation ParametersThe parameters that may need adjustment are listed below.

AXIS BACKLASH COMPENSATION FRICTION COMPENSATION LEAD COMPENSATION

X 13 138 229

Y 27 139 230

Z 41 140 231

Pitch, Roll and Yaw Definitions

Pitch [1] - An angular deviation possible inpositioning systems, in which the table leadingedge rises or falls as the table translates alongthe direction of travel. This represents rotationaround a horizontal axis, perpendicular to theaxis of travel.

Roll [2] - An angular deviation from ideal straightline motion, in which the positioning table rotatesaround its axis of travel as it translates along thataxis.

Yaw [3] - An angular deviation from ideal straightline motion, in which the positioning table rotatesaround the vertical axis as it translates along itstravel axis.


of 24 pages

http://www.haascnc.com/content/haascnc/en/service/troubleshooting-and-how-to/how-to/ballbar-analysis---quick-check-mode.html

http://www.haascnc.com/content/haascnc/en.html

Backlash in the X Axis

Type of backlash: Positive = Lost motion

In this example, the X-Axis lost motion is at thereversal point, and causes the ballbar to extend.The steps on both sides of the reversal points areequal.

The machine feedrate usually does not affect thesize of the step.

Probable Causes:

• A worn drive nut or ballscrew end floatcauses backlash in the drive system.

• Worn guideways cause dwell in themotion as the axis changes direction.

• Excessive strain in the ballscrew.

Effects on the Machined Part: A circularinterpolated cutter path shows a short flat.

Corrective Action:

• Remove all backlash in the drivecomponents for large backlash errors.

• Adjust the X-axis BacklashCompensation - Parameter 13 to correctfor small backlash errors.

Note: Overcompensation (Parameter 13 settoo high) shows as a step in the machined part.


of 24 pages


Positive Backlash in the Y Axis

Type of backlash: Positive = Lost motion

In this example, the Y-Axis lost motion is at thereversal point, and causes the ballbar to extend.There are equal size steps on both sides of thereversal points.

The machine feedrate usually does not affect thesize of the step.

Probable Causes:

• The drive nut or ballscrew end float isworn.

• Worn guideways cause dwell in themotion as the axis changes direction.

• Ballscrew wind-up causes too muchstrain in the ballscrew.

Effect on the Machined Part: A circularinterpolated cutter path shows a short flat.

Corrective Action:

• For large backlash errors repair thedrive components.

• For small backlash errors, adjust the Y-Axis Backlash Compensation -Parameter 27.

Note: Overcompensation (Parameter 27 settoo high) shows as a step in the machined part.


of 24 pages


Unequal Backlash in the Y Axis

Type of backlash: Unequal (bottom step is smallerthan the upper step)

You must take into account the configuration ofthe machine.

In this example, the Y-Axis lost motion is at thereversal point, and causes the ballbar to extend.There are equal size steps on both sides of thereversal points.

Possible Causes:

• Too much strain causes the ballscrew totwist. This causes an angular motionerror.

• Damage to a drive component causespitch/yaw.

Corrective Action: Inspect the drive components.


of 24 pages


Reversal Spikes in the X Axis

There is a delay in motion at the reversal points.Unlike backlash, the controller recognizes thedelay in motion and compensates.

In this example, the X Axis had a delay in motionand compensated.

The size of the step often varies with themachines feedrate.

Possible Causes:

• Tight way covers bind and causefriction.

• There is initial torque on the ballscrewat the reversal point. An inadequateamount of torque was applied by theaxis drive motor at the axis reversalpoint. It causes the axis to stickmomentarily, as the frictional forceschange direction.

• The servo response time of the machineis inadequate on BacklashCompensation. This means that themachine is unable to compensate forthe backlash in time. This causes theaxis to stop while the machine absorbsthe slack.

• There are loose bearings.• There is twist on the ballscrew.• The is a problem with the axis

lubrication system.

Note: Before you use the FrictionCompensation parameter to remove theproblem, remove all other possible causes.

Effect on the Machined Part: The effect of areversal spike is that a circular interpolated cutterpath shows a small flat followed by an inwardrecovery step.


of 24 pages


Reversal Spikes in the Y Axis

There is a delay in motion at the reversal points.Unlike backlash, the controller recognizes thedelay in motion and compensates.

In this example, the Y Axis had a delay in motionand compensated.

The size of the step often varies with themachines feedrate.

Possible Causes:

• Tight way covers bind and causefriction.

• There is initial torque on the ballscrewat the reversal point. The axis drivemotor does not apply enough torque atthe axis reversal point. It causes the axisto stick momentarily, as the frictionalforces change direction.

• The servo response time of the machineis inadequate on backlashcompensation. This means that themachine is unable to compensate forthe backlash in time. This causes theaxis to stop while the machine absorbsthe slack.

• There are loose bearings.• There is twist on the ballscrew.• The is a problem with the axis

lubrication system.

Note: A Friction Compensation parameteradjustment may get rid of the problem, buteverything else must be ruled out.

Effect on the Machined Part: A reversal spikecauses a circular interpolated cutter path to showa small flat followed by an inward recovery step.


of 24 pages


Plot Rotation Error

Spiral Errors

These types of errors start to appear as backlash in only one side or unequal backlash.

True backlash mirrors the plot when performing two runs.

If the ballbar hardware is worn, loose or dirty, you cannot repeat the plot.

Probable Causes:

• There is a test error or hardware error.• Loose ballbar hardware. The sphere may be moving within the cup, this changes the position of the center.

Corrective Actions: Inspect the hardware and do the test again.

Reversal spikes that occur at 45º are most likelycaused by tester error since both axes are at thesame speed at those points and there is nochange in direction.

Probable Causes:

• There is a test error.• The ballbar hardware is loose.

Corrective Action: Inspect the hardware and dothe test again.


of 24 pages


Squareness Error

The plot displays an oval peanut shape distortedat the 45º or 135º diagonal.

The axis of distortion is the same for bothdirections of the run (CCW and CW). The amountof distortion is unaffected by the feedrate.

Probable Causes:

• The ballscrew may be bent locally orthere may be an overall axismisalignment in the machine.

• The machine is not level. A machine thatis not level shows a symmetrical, flat-sided plot in the shape of an oval, orpeanut, tilted at a 45° angle.

• The axes of the machine droops. Thiscauses the axis to mis-align at certainlocations.

Corrective Actions:

• Repeat the test at various locations onthe machine to find out whether thesquareness error is local to a certainarea on the machine or if it affects thewhole machine.

• If a squareness error affects the wholemachine, then realign the machine axesif possible. If the guide ways are wornthey may require replacement.


of 24 pages


Servo Mismatch

Squareness and Servo Mismatch Error

Servo mismatch is quantified as the time inmilliseconds by which one of the machine's axisservos leads the other.

Probable Causes:

• The servo loop gains of the axes aremismatched. This causes one axis tolead over the other, which shows anoval plot. The leading axis is the axiswith the higher loop gain.

• Poor drive performance• The linear guide trucks do not move

freely.

Corrective Action: Adjust the machine controllerto balance the loop gains of the axis servos.Either turn up the gain of the axis which islagging, or turn down the gain of the axis which isleading.

In run two, both errors cancel each other and theresulting plot in Run 2 may look OK.

Probable Causes: Refer to the Squareness Errorand Servo Mismatch error descriptions.

Corrective Action: Refer to the Squareness Errorand Servo Mismatch error descriptions.


of 24 pages


Scaling Mismatch

Squareness and Scaling Mismatch

To calculate the scaling mismatch, the plotsubtracts the X-Axis diameter from the Y-Axisdiameter.

The temperature of the machine effects scalingmismatch plots.

Probable Causes:

• A ballscrew that is damaged or too hotcauses a pitch error.

• The axis linear guides are not straight.• The ballbar is not calibrated correctly.

Note: To identify the actual cause, it is a goodidea to do another plot.

Corrective Actions:

• Adjust the axis Lead Compensationparameter.

• Do the test with the tester elevatedabove the table. If the mismatch getsworse, then the error is caused by pitch.

• Do the test at a different location on thetable, if the mismatch gets worse, thenthe error is caused by yaw.

Note the angle of the oval shape, the axis of theshape is not at the 45º. The scaling mismatchcauses the angle to lean towards the longer axis.

Probable Causes: Refer to the Scaling Mismatcherror descriptions.

Corrective Action: Refer to the Scaling Mismatcherror descriptions.


of 24 pages


Tri-lobe

Y-Axis Straightness on X-Axis Error

The plot has three distinct lobes in its generalshape. These lobes may change size andorientation between adjacent clockwise andcounter-clockwise runs. A tri-lobe plot has sharptransitions between lobes; a straightness errorplot has more gradual transitions.

This is a test error.

Probable Causes:

• The hardware is loose.• The ball may be moving on the holding

cup.

Corrective Action: Inspect the ballbar hardwareand do the test again.

The plot has three distinct lobes in its generalshape. These are not affected by feedrate ordirection but may change with the location atwhich the test is performed on the machine bed.A straightness type plot can be distinguishedfrom a Tri-lobe test error type plot by itssmoother transitions between lobes and betterrepeatability.

Probable Causes:

• The number one cause for straightnesserror is roll.

• The linear guides are bent.• Overall linear guide misalignment.• A foundation that is not stable causes

the machine to distort.

Corrective Actions:

• Check for loose ballbar hardware anddo the test again, if the error does notrepeat it is not a straightness error.

• Inspect the machine's linear guides.• Use a granite standard to determine

which axis has the error.


of 24 pages


X-Axis Straightness on Y-Axis Error

Straightness Error

The plot has three distinct lobes in its generalshape. These are not affected by feedrate ordirection but may change with the location atwhich the test is performed on the machine bed.A straightness type plot can be distinguishedfrom a Tri-lobe test error type plot by itssmoother transitions between lobes and betterrepeatability. The number one cause forstraightness error is roll.

Probable Causes:



Corrective Actions:


• Inspect the machine's linear guides.

The plot has three distinct lobes in its generalshape. These are not affected by feedrate ordirection but may change with the location atwhich the test is performed on the machine bed.A straightness type plot can be distinguishedfrom a Tri-lobe test error type plot by itssmoother transitions between lobes and betterrepeatability. The number one cause forstraightness error is roll.

Probable Causes:



Corrective Actions:


• Inspect the machine's linear guides.


of 24 pages


Cyclic Error on X Axis

Cycles are more pronounced as the faulty axisapproaches the reversal point. They will appearsmaller at the point where the faulty axis movesat maximum speed and the frequency does notchange. Measure the cycles and determine thepitch for each component in the drive system.

Probable Causes:

• The ballscrew is off-center.• The ballscrew is damaged.• The motor mount is defective.• The coupler is damaged.• The bearing pack is damaged.

Corrective Actions:

• Adjust the off-center ballscrew in thefaulty axis.

• Adjust or replace the faulty motormounts.

• Replace the faulty coupler.• Replace the bearing pack.


of 24 pages


Cyclic Error on Y Axis

Offset Change

Cycles are more pronounced as the faulty axisapproaches the reversal point, They will appearsmaller at the point where the faulty axis movesat maximum speed and the frequency does notchange. Measure the cycles and determine thepitch for each component in the drive system.

Probable Causes:

• The ballscrew is off-center.• The ballscrew is damaged.• The motor mount is defective.• The coupler is damaged.• The bearing pack is damaged.

Corrective Actions:

• Adjust the off-center ballscrew in thedamaged axis.

• Adjust or replace the damaged ordefective motor mounts.

• Replace the damaged coupler.• Replace the bearing pack.

The center of each plot is at a different location.

Probable Causes:

• There is a tester error.• The ballbar hardware is loose.

Corrective Action: Inspect the ballbar hardwareand do the test again.


of 24 pages


Different Diameter Plots

Stick Slip on Y-Axis

The center for each plot is at the same location,but the diameter of the plots are different.

Probable Causes:

• There is a tester error.• The ballbar hardware is loose.

Corrective Action: Inspect the ball bar hardwareand do the test again.

This error will appear on the 90º of the axis thathas it as it travels at low federate.

Friction forces are greater as the axis slows down.

Probable Causes:

• The quantity of lubrication is notsufficient.

• The linear guides do not move freely.

Corrective Action: Inspect and repair thecomponents that cause friction.


of 24 pages


Odd Shapes

Worn, damaged, loose or chip-covered ballbar components cause distorted plots.

The wrong feedrate causes a plot that does not meet at the beginning and the end.

In most cases, a false reading is obvious. Be aware, however, that dirty equipment or a component that has worndown to the point where it is just marginal, produces plots that appear to be normal but are actually out of round orflat-sided.


of 24 pages


Other Plots from Tested Machines


of 24 pages


Other examples of distorted plots from machines with bad ballbar or other components are shown below:


of 24 pages


PLOT CAUSE

Cause: The servo amplifier is bad.

Cause: The X-Axis motor housing isbad.


of 24 pages

https://service.haascnc.com/sites/default/files/Locked/2/Bad_servo_amplifier.png

https://service.haascnc.com/sites/default/files/Locked/2/Bad_servo_amplifier.png

https://service.haascnc.com/sites/default/files/Locked/2/Bad_X_Axis_Motor_Housing.png

https://service.haascnc.com/sites/default/files/Locked/2/Bad_X_Axis_Motor_Housing.png


Cause: The Y-Axis servo motor is bad.

There is noise on the top and thebottom of the plot at or near thetransition point.

Cause: The motor or the amplifier isbad.

Corrective Action: Change the LowPass Filter Setting to ON to verify thatthe problem is electrical.


of 24 pages

https://service.haascnc.com/sites/default/files/Locked/2/Bad_Y_Axis_Servo_Motor.png

https://service.haascnc.com/sites/default/files/Locked/2/Bad_Y_Axis_Servo_Motor.png

https://service.haascnc.com/sites/default/files/Locked/2/Low_Pass_Filter_Setting.png

https://service.haascnc.com/sites/default/files/Locked/2/Low_Pass_Filter_Setting.png


Cause: The Y-Axis ballscrew is tightnear the motor.

Corrective Action: Realign the Y-Axisballscrew.

Cause: The X-Axis Servo Motor is bad.


of 24 pages

https://service.haascnc.com/sites/default/files/Locked/2/Y_Axis_Ballscrew_Tight.png

https://service.haascnc.com/sites/default/files/Locked/2/Y_Axis_Ballscrew_Tight.png

https://service.haascnc.com/sites/default/files/Locked/2/X_Axis_Servo_Motor.png

https://service.haascnc.com/sites/default/files/Locked/2/X_Axis_Servo_Motor.png


Cause: Backlash errors in Y axis.There is drag in the way coversandthe linear guides are misaligned.

Cause: There is an electrical problemwith the ballbar equipment. Theballbar equipment is not correctlygrounded or there is a badconnection.


of 24 pages

https://service.haascnc.com/sites/default/files/Locked/2/48_Backlash_Comp.png

https://service.haascnc.com/sites/default/files/Locked/2/48_Backlash_Comp.png

https://service.haascnc.com/sites/default/files/Locked/2/Electrical_Problem_with_Ballbar.png

https://service.haascnc.com/sites/default/files/Locked/2/Electrical_Problem_with_Ballbar.png


Cause: There is excessive friction onthe way covers [1].There is backlash inthe linear guides in both X and Y axes[2].

Cause: There is backlash in the Y-axisbearing pack.

Corrective Action: Change the Y-axisbearing pack.


of 24 pages

https://service.haascnc.com/sites/default/files/Locked/2/Drag_on_way_cover.png

https://service.haascnc.com/sites/default/files/Locked/2/Drag_on_way_cover.png

https://service.haascnc.com/sites/default/files/Locked/2/Excessive_Slop.png

https://service.haascnc.com/sites/default/files/Locked/2/Excessive_Slop.png


Cause: The pre-load is not adjustedcorrectly. The nut on end of ballscrewis not tight.

Cause: There is a loose ballbarcomponent. The coupler nut is loose.


of 24 pages

https://service.haascnc.com/sites/default/files/Locked/2/Preload_not_adjusted.png

https://service.haascnc.com/sites/default/files/Locked/2/Preload_not_adjusted.png

https://service.haascnc.com/sites/default/files/Locked/2/Loose_Component.png

https://service.haascnc.com/sites/default/files/Locked/2/Loose_Component.png


Renishaw Ballbar Test - Plot Interpretation - Mills · 2020-03-06 · lubrication system....

Documents

Transcript of Renishaw Ballbar Test - Plot Interpretation - Mills · 2020-03-06 · lubrication system....