Siemens SINUMERIK Backlash and Quadrant Error Settings

This content draws on two official Siemens sources: the “Extended Functions” function manual (valid for SINUMERIK 840D sl / 840DE sl / 828D, CNC software 4.4 generation) and the “Machine Data and Interface Signals” parameter manual (valid for SINUMERIK 802D sl V1.4 SP7). SINUMERIK 840Di sl is not in the validity list of this generation and appears only in older documentation. The basic mechanical and machining symptoms described here are similar across different SINUMERIK systems. However, the availability of individual parameters and advanced compensation features depends on the control model, software version, NCU and the machine tool builder’s configuration.

Introduction

Oval circles, witness marks at direction reversals, the same dimension measuring differently depending on the approach direction, or contour distortion at high feedrates – these problems on a CNC machine are not always caused by the part program.

The following causes may be behind such defects:

Mechanical play (backlash)
Friction in the guideways and drive train
Quadrant transition error
Axis following error
Wrong or insufficient feedforward setting
Incorrectly applied compensation
Ball screw, bearing, coupling or guideway problems

On Siemens SINUMERIK systems some of these errors can be reduced with the CNC compensation functions. Compensation, however, is not a repair method that replaces fixing the mechanics. The mechanical condition must be measured first; only then should the appropriate compensation be set.

How Does the Operator Recognize These Errors on the Workpiece?

The operator must not change machine data. But by observing in which motion and in which region of the part the defect appears, the operator can provide valuable information to the service engineer.

1. Symptoms of backlash (axis play)

Backlash means that when the axis reverses direction, the motor or encoder moves while the table, slide or tool follows with a delay equal to the amount of play.

According to the Siemens documentation, on axes with an indirect measuring system the axis can travel too little or too far by the backlash amount during a direction reversal.

The operator may see the following on the workpiece:

Different results when approaching the same dimension from the right and from the left
Opposite walls of a pocket measuring differently
Circles that are not perfectly round
A small step when X or Y reverses direction
A reversal line on milled surfaces
Position deviation of hole centers
On lathes, a change in diameter after the X axis reverses
Angular position error on the C axis
The tool settling into position late

Backlash compensation becomes active in the operating modes after the reference point has been approached. The compensation value is defined in MD32450 $MA_BACKLASH.

2. Symptoms of friction and quadrant error

When an axis starts moving it needs a higher force to overcome static friction than it does while the motion continues. When the axis reverses and its velocity passes through zero, a brief sticking or delay can occur.

The Siemens documentation states that this causes contour errors on interpolating axes and is seen especially at the quadrant transitions of circular contours.

Symptoms the operator may see:

A small protrusion at the 12, 3, 6 or 9 o’clock positions of a circle
Marks in four distinct regions of the circle
A round bore coming out slightly clover-shaped
Surface defects at four points of a circular pocket
A short step at radius transitions
Tool marks at direction reversals
An error that is distinct at low feed and changes character at high feed
Quadrant lines on mold surfaces
Surface quality degrading in certain regions during circular interpolation

The circularity test plots in the documentation show clear spikes at the quadrant transitions without compensation, and a much smoother circle when the compensation is set correctly.

3. Symptoms of following error and insufficient feedforward

Following error is the difference between the position the CNC commands and the actual position of the axis.

Especially during acceleration, deceleration, corners and circular movements, if the axis follows the command with a delay, speed-dependent contour errors can occur. Dynamic feedforward is used to reduce this difference.

Symptoms the operator may see:

Correct dimensions at low feed, incorrect at high feed
Circles becoming more oval as the feedrate increases
Sharp corners rounding off
Contour deviation at corner entry or exit
Mold profiles deviating from the programmed path
Simultaneously moving axes failing to stay synchronized
The same program giving different results at different feed override values
More visible error on small radii than on large radii
Surface marks in acceleration and deceleration zones

If the feedforward setting is reduced too much, the machine may move more smoothly, but the error on curved contours such as circles can grow. The Siemens parameter description states this relationship explicitly.

What Information Should the Operator Give the Service Engineer?

Check	Information to record
Type of defect	Protrusion, ovality, step, surface mark or dimensional difference
Axis	X, Y, Z, C or another axis
Direction of motion	Positive-to-negative or negative-to-positive reversal
Programmed feedrate	For example F500, F2000
Feed override	25%, 50% or 100%
Type of operation	Circle, pocket, radius, straight line or positioning
Repeatability	Does it occur at the same spot on every part?
Cold/warm machine	Does it change as the machine warms up?
Air cut/cutting	Is it also visible during non-cutting motion?
Measurement	Circle diameter, wall dimension or direction-dependent difference

This information helps separate whether the problem originates from the program, the mechanics or the axis control.

What the Operator Must Not Do

The following machine data must not be changed by trial and error:

MD32450 $MA_BACKLASH
MD32452 $MA_BACKLASH_FACTOR
MD32490 $MA_FRICT_COMP_MODE
MD32500 $MA_FRICT_COMP_ENABLE
MD32510 $MA_FRICT_COMP_ADAPT_ENABLE
MD32520 $MA_FRICT_COMP_CONST_MAX
MD32530 $MA_FRICT_COMP_CONST_MIN
MD32540 $MA_FRICT_COMP_TIME
MD32550–32570 acceleration characteristic points
MD32610 $MA_VELO_FFW_WEIGHT
MD32620 $MA_FFW_MODE
MD32630 $MA_FFW_ACTIVATION_MODE
MD32810 $MA_EQUIV_SPEEDCTRL_TIME

Incorrect compensation can amplify the existing error in the opposite direction instead of reducing it. Especially on short positioning movements, a value determined from the circularity test can over-compensate the axis.

Diagnosis and Setup Section for Siemens Service Engineers

1. Check the mechanics first

Before starting any compensation setup, the following must be checked:

Ball screw nut
Fixed and floating bearings
Motor coupling
Gearbox or gear train
Guideway lubrication
Linear guide carriages
Hydraulic counterbalance
Motor brake
Encoder connection
Difference between the linear scale and the motor encoder
Physical play at the table or slide

If the mechanical play keeps growing, simply increasing MD32450 is not a permanent solution.

2. Measuring backlash and MD32450

The backlash value is entered separately for each axis or spindle:

MD32450 $MA_BACKLASH

According to the documentation:

a positive value is used in the normal case where the encoder leads the machine part,
a negative value is used where the encoder lags behind the machine part.

If a second measuring system exists, a separate value must be defined for each measuring system.

Measurement method:

Reference the axis.
Mount a dial indicator securely and rigidly.
Approach the measuring point from one direction.
Zero the indicator.
Move the axis in the opposite direction with small increments.
Measure how far the CNC position changes while the indicator does not move.
Repeat the test at different positions along the axis.

If the play varies significantly along the axis, the problem may not be fully corrected with a single backlash value.

3. Backlash factor

MD32452 $MA_BACKLASH_FACTOR

allows the backlash value to be weighted per parameter set – for example when different gear stages have different amounts of play.

Do not assume that this function is supported in the same way on every control variant.

4. Friction compensation mode

Basic method selection:

MD32490 $MA_FRICT_COMP_MODE

0 = No friction compensation
1 = Conventional compensation with constant or adaptive characteristic
2 = Neural QEC using a learned characteristic

Enabling the function on the axis:

MD32500 $MA_FRICT_COMP_ENABLE

Neural QEC (quadrant error compensation with a neural network) is defined in the documentation specifically as a SINUMERIK 840D sl option; it must not be assumed to exist on every 802D sl or older 840Di sl model. If neural QEC is to be used, the function must not be enabled before a valid characteristic has been learned.

5. Identifying quadrant error with the circularity test

The circularity test records the actual axis positions during circular motion and plots the deviation from the programmed radius. The documentation explicitly describes this function as a commissioning tool.

General test sequence:

Run the first test with friction compensation switched off: MD32500 $MA_FRICT_COMP_ENABLE = 0
Record the deviations at the quadrant transitions.
Select conventional friction compensation: MD32490 $MA_FRICT_COMP_MODE = 1
Enable the compensation on the relevant axis: MD32500 $MA_FRICT_COMP_ENABLE = 1
Switch adaptation off for the initial setup: MD32510 $MA_FRICT_COMP_ADAPT_ENABLE = 0
Adjust the amplitude and time constant according to the circularity plot: MD32520 $MA_FRICT_COMP_CONST_MAX and MD32540 $MA_FRICT_COMP_TIME
Repeat the test at different radii and speeds.

The documentation requires the changes to be verified with measured and plotted circles.

6. How to recognize incorrect friction compensation

Amplitude too low: the spike at the quadrant transition is not sufficiently removed.

Amplitude too high: over-correction in the opposite direction can appear at the quadrant transition.

Time constant too short: the deviation recovers briefly and then grows again immediately.

Time constant too long: the effect of the correction continues along the contour longer than needed after the quadrant transition.

On the workpiece these outcomes can produce hollows or protrusions, distorted radii, inverse marks in the four quadrant regions of a circle, and surface defects extending after a direction reversal.

7. Dynamic feedforward setup

Feedforward reduces the following error that occurs especially during acceleration and on curved contours.

In this documentation generation the basic selection is:

MD32620 $MA_FFW_MODE

0 = No feedforward
3 = Velocity feedforward
4 = Combined torque/velocity feedforward

Note: in classic 840D/810D documentation the values of this parameter are 1 (velocity) and 2 (torque); the 840D sl generation uses values 3 and 4. Always follow the parameter manual of your own control version.

If control from the part program is permitted, the commands

FFWON
FFWOF

can be used. For this, MD32630 $MA_FFW_ACTIVATION_MODE must be set accordingly. Siemens requires the switchover to be performed while the axis or spindle is stationary so that no abrupt motion is generated.

One of the key values for velocity feedforward is:

MD32810 $MA_EQUIV_SPEEDCTRL_TIME

The documentation requires this time constant to be determined correctly from the speed setpoint step response.

Which Alarms Can Be Seen?

The source files behind this content are primarily function and parameter manuals; they do not contain a direct alarm number list. For that reason, no specific alarm numbers are given here.

These problems can, however, result in the following alarm groups:

Following error / axis monitoring
Contour monitoring
Positioning monitoring
Standstill monitoring
Speed setpoint limitation
Measuring system difference
Encoder or axis reference loss

For exact alarm numbers, consult the Siemens Diagnostics Manual for the same control version. Do not assume that legacy 840D alarm numbers apply directly to 840D sl, 828D, 802D sl or other software versions.

Operator or Service Engineer?

Task	Operator	Service
Spotting marks and dimensional errors on the workpiece	Yes	Yes
Recording in which axis direction the defect occurs	Yes	Yes
Recording feedrate and override values	Yes	Yes
Measuring backlash with a dial indicator	No	Yes
Changing MD32450	No	Yes
Setting up the circularity test	No	Yes
Running the QEC learning process	No	Yes
Determining the feedforward time constant	No	Yes
Servo and compensation optimization	No	Yes
Mechanical maintenance and encoder checks	No	Yes

Conclusion

Backlash, friction, quadrant and following errors can produce machining defects that look very similar. An oval circle does not always mean backlash alone; reversal friction, feedforward setup, servo dynamics or a mechanical problem can contribute to the same result.

The correct diagnostic sequence is:

Classify the defect on the workpiece
Identify the axis and the direction of motion involved
Check the mechanical system
Measure the backlash
Run the circularity test
Change compensation and feedforward settings in a controlled way
Measure again under identical conditions

Disclaimer

This content is prepared to help CNC operators recognize machining symptoms and to explain the diagnostic approach used by Siemens service personnel. Machine data, servo, encoder, feedforward and compensation settings must not be changed without taking the necessary backups and reviewing the machine builder’s procedures. The same parameters can behave differently on different SINUMERIK models and software versions; the machine builder’s and Siemens’ official manuals always take precedence.