Diagnostics and elimination of errors in the positioning of CNC machine tools: SHP backlash, feedback of encoders, thermal compensation and servo adjustment

1. Description of the problem and scope of application

This manual is intended for system diagnostics and troubleshooting of positioning errors that occur in CNC machine tools such as milling, turning, grinding, and EDM machines. Inaccurate positioning can lead to production defects, increased cycle time, tool and machine damage. Three main categories of positioning errors are defined by the degree of criticality:

• Critical (Critical): Positioning errors that exceed the tolerance limits of the part by more than 50% lead to a complete failure of the product, damage to the machine or tool. They demand an immediate stop of the equipment.
• Major: Positioning errors in the range of 25-50% of the allowable tolerance of the part. May result in conditionally acceptable parts or the need for rework. They require a scheduled stop for diagnostics.
• Insignificant (Minor): Positioning errors that are in the range of up to 25% of the allowable tolerance of the part, but are already observed by the operator or recorded by the monitoring system. They indicate the beginning of system degradation. They require monitoring and inclusion in scheduled maintenance.

2. Precautions

ATTENTION: Before starting any diagnostic or repair work on CNC machines, strict safety rules must be followed. Failure to follow these instructions could result in serious injury or death, or damage to the equipment.

• Locking and tagging (LOTO): Be sure to perform the locking and tagging procedure (Lockout/Tagout) in accordance with the company's internal instructions and the requirements of standards (for example, DSTU EN 1037, ISO 14118). Ensure complete shutdown of all energy sources (electrical, hydraulic, pneumatic).
• Residual Energy: Ensure that all residual energy (in capacitors, springs, hydraulic accumulators, pneumatic systems) is discharged or blocked.
• Personal protective equipment (PPE): Always use appropriate PPE: safety glasses, work gloves, safety shoes, protective clothing. Use dielectric gloves and tools when working with electrical components.
• Hot Surfaces/Components: Be careful as some components (motors, drives, spindles) can be hot even after the power is turned off.
• Moving parts: Never work with open guards while the machine is running. Avoid contact with moving parts (SHP, linear guides).

3. Necessary diagnostic tools

4. Initial evaluation checklist

Before starting a detailed diagnosis, it is necessary to collect as much information as possible about the conditions of the malfunction.

5. Systematic diagnostic algorithm

Follow this step-by-step algorithm to systematically identify the cause of the positioning error.

1. Initial check (without power):
   1. Visual inspection:
      • Check cables of motors, encoders, sensors for damage, bends, connection reliability.
      • Inspect the couplings between the engine and the gearbox for backlash, damage, loose screws.
      • Check the presence of foreign objects on the SHP, linear guides.
      • Inspect the protective covers for damage that may interfere with movement.
   2. Manual mechanical check:
      • Disconnect the servo motor clutch from the SHV. Try to turn the SHVP manually. IF The valve rotates tightly or with noticeable resistance THEN check the valve bearings, the presence of dirt, damage to the valve nut (See 7.1.3).
      • Scroll the servo motor manually. IF spins stiff or noisy THEN check motor bearings.
      • Move the axis of the machine manually (with the remote control disconnected or the engine turned off). IF movement occurs with jerks or with great effort THEN check the linear guides, their lubrication, the presence of mechanical damage.
2. Diagnostics of ball screw backlash (Ball Screw Backlash):
   1. Measurement of ball screw backlash using an indicator:
      • Install a clock-type indicator on the bed of the machine, rest its leg against the moving part of the axis (for example, a table).
      • Switch to MDI mode in the CPC.
      • Move the axis to the position where the error occurs.
      • Make a small movement (eg 10mm) in one direction (eg +X). Record the indicator reading.
      • Make a small movement (eg 10 mm) in the opposite direction (-X). Record the indicator reading.
      • IF the difference in the readings of the indicator exceeds 0.02 mm THEN the likely cause is the backlash of the valve stem.
      • IF backlash is 0.005 mm - 0.02 mm THEN this is a sign of initial wear that requires monitoring or scheduled maintenance.
      • Especially critical: backlash > 0.05 mm.
   2. Checking the bearings of the valve:
      • Check the axial displacement of the valve support using the indicator.
      • IF the axial displacement exceeds 0.005 mm THEN the bearings of the SHV need to be replaced or adjusted.
      • Use a vibration analyzer. Measure the vibration on the supports of the SHVP. IF the total vibration level exceeds 4.5 mm/s (RMS) or characteristic frequencies of the bearings are observed THEN the bearings are worn.
   3. Checking the fastening of the screw nut:
      • Inspect the screw nut fastening to the moving part for loosening.
      • Tighten the fasteners to the recommended torque (see OEM documentation).
3. Diagnostics of encoder feedback (Encoder Feedback):
   1. Encoder cable integrity check:
      • ATTENTION: Turn off the CNC power supply!
      • Using a multimeter, check the conductivity of each conductor of the encoder cable. Resistance should be <1 ohm.
      • Check the insulation of the cable for a short circuit to the body or between itself. Resistance should be >1 MΩ.
      • IF an open or short circuit is detected THEN replace the encoder cable.
   2. Analysis of encoder signals with an oscilloscope:
      • Connect the oscilloscope to the outputs A, B, Z of the encoder (differential signals, if any).
      • Move the axle manually or at low speed.
      • IF quadrature signals (A and B) do not have a phase shift of 90° ± 10° or have uneven amplitude (more than 10% of nominal) THEN faulty encoder or dirty ruler.
      • IF signal Z (reference mark) missing or unstable THEN faulty encoder or dirty ruler.
      • IF signals have significant noise or distortion THEN check cable shielding, system grounding.
      • Example of threshold values: for TTL encoders, the amplitude of the signals should be in the range of 4.5-5.5 V. For sinusoids - 0.5-1.2 V peak-peak.
   3. Check for contamination or damage to the encoder line:
      • CAUTION: Turn off the power supply of the CNC!
      • Inspect the optical ruler or magnetic tape of the encoder for dirt, dust, grease, and scratches.
      • Carefully clean the ruler with a special agent for optics (without abrasives) or isopropyl alcohol.
      • IF the ruler is damaged (deep scratches, chips) THEN replace the ruler or the encoder.
   4. Checking the offset of the encoder/scale ruler:
      • Check the reliability of the attachment of the encoder reading head and the ruler itself.
      • Check the recommended gap between the reading head and the ruler (usually 0.1 - 0.2 mm). Adjust as needed.
4. Diagnostics of thermal compensation (Thermal Compensation):
   1. Temperature monitoring:
      • Use a thermal imager to monitor the temperature of the headstock, bed, bearings, and motors during machine operation.
      • Compare the temperature with the nominal values ​​or with the temperatures of similar, working nodes.
      • IF the temperature of the SHVP in the central part differs from the edges by more than 5°C after 30 minutes of operation THEN probable thermal deformation.
   2. Checking the temperature sensors:
      • ATTENTION: Turn off the power to the PDA!
      • Check the resistance of the thermistors or thermocouples, if they are installed on the SVP or other components. Compare with table data or readings of a working sensor.
      • Check the integrity of the wiring to the temperature sensors.
   3. Setting the parameters of the CPC:
      • Check whether the thermal compensation function is activated in the CPC parameters.
      • Check compensation parameters (expansion coefficients, measuring points). If necessary, consult the machine manufacturer's documentation.
5. Servo Tuning Diagnostics:
   1. Servo Error Analysis:
      • Use servo diagnostic software (such as DriveMonitor) to read the error log and servo parameters.
      • Pay attention to positioning errors, current errors, speed errors.
   2. Gain Parameters check:
      • Check the proportional (P), integral (I) and differential (D) gain (PID controller) values ​​for the corresponding axis.
      • IF values ​​are very different from the factory settings or from the settings of other healthy axes THEN probably, the servo is out of order.
      • Autotuning the servo, if possible, following the manufacturer's instructions.
   3. Monitoring the servo signal with an oscilloscope:
      • Connect the oscilloscope to the output current of the servo amplifier and monitor the waveform during axis movement.
      • IF current waveform has oscillations, significant overshoot, or quiescent current is significantly higher than rated THEN servo needs tuning or there is a mechanical problem (rubbing, high friction).
   4. Mechanical vibrations:
      • Using a vibration analyzer, measure vibrations on the engine, clutch, and gearbox.
      • IF at certain frequencies of the axis movement there are significant vibrations (> 3 mm/s RMS) that resonate with the servo frequencies THEN the servo setting may conflict with the mechanical resonance of the system.

6. Matrix of malfunctions and causes

7. Root cause analysis for each malfunction

7.1. Ball Screw Backlash (Ball Screw Backlash)

7.1.1. Wear of the screw nut:

• WHY: The most common reason. Wear occurs as a result of friction of the balls against the raceways of the nut and the shaft of the SHV. Accelerates with insufficient lubrication, overload or presence of abrasive particles. The wear of the nut leads to an increase in the gap between the balls and the tracks, which manifests itself as backlash.
• HOW TO CONFIRM: Measure the backlash of the SHV using a laser interferometer (according to ISO 230-2) or a watch-type indicator. Moving the axis of the machine by hand with the motor disconnected: considerable free movement will be felt.
• DAMAGE IF NOT REMOVED: Positioning inaccuracy, taper, non-roundness of parts. Increased vibration, which accelerates the wear of other mechanical components (bearings, linear guides). Damage to the tool and the machine due to excessive dynamic loads.

7.1.2. Wear or damage to the bearings of the SHVP:

• WHY: Spherical thrust bearings (usually radial thrust) ensure rigidity and accuracy of axial positioning. Their wear, improper preload or damage (for example, from shocks) leads to axial play of the SHV and radial runout transmitted to the axle.
• HOW TO CONFIRM: Measurement of the axial displacement of the supports of the SHVP with an indicator. Analysis of bearing vibration. Friction or a characteristic noise may be felt when rotating the valve by hand.
• DAMAGE IF NOT FIXED: Unstable positioning, vibration, increased noise, overheating of bearings, which can lead to the destruction of the support and the SHV.

7.1.3. Weakening of the servomotor-SHV clutch:

• WHY: The clutch transmits the torque from the servo motor to the servo motor. Its weakening, wear of damping elements or splines leads to the loss of synchronization between the rotation of the engine and the movement of the axis, creating backlash.
• HOW TO CONFIRM: Visually inspect the coupling for backlash when attempting to turn it by hand. Checking the tightening of the fastening screws.
• DAMAGE IF NOT FIXED: Positioning errors occurring randomly or with rapid changes in direction. Excessive load on the servomotor and the servomotor, which can lead to their premature wear.

7.2. Encoder feedback problems

7.2.1. Contamination or damage to the ruler/encoder head:

• WHY: Dust, grease, coolant, or metal shavings that get on the optical ruler or magnetic tape prevent correct reading. Scratches or mechanical damage to the ruler/head will also distort the signal.
• HOW TO CONFIRM: Visual inspection of the ruler and encoder head. Analyzing signals with an oscilloscope - signals may be missing, distorted, or the Z mark may "disappear".
• DAMAGE IF NOT FIXED: Chaotic positioning errors, impossibility of exiting to the reference point, CNC malfunctions, movement of axes with "jerks". May cause collision, damage to tool and machine.

7.2.2. Faulty encoder cable or bad contact:

• WHY: Cables laid in cable channels are prone to mechanical wear and kinks. A conductor break, a short circuit between the conductors or to "earth", or oxidation of the contacts leads to loss or distortion of signals.
• HOW TO CONFIRM: Checking the integrity of the cable with a multimeter (resistance, insulation). Analysis of signals with an oscilloscope - signals can be of low amplitude, very noisy or completely absent.
• DAMAGE IF NOT FIXED: Loss of axis control, uncontrolled movements, CNC alarms (eg "encoder error"). High risk of collision and accidental damage to equipment.

7.2.3. Failure of the encoder itself:

• WHY: The internal electronic components of the encoder (optical elements, photoreceptors, microcircuits) can fail due to aging, overheating, voltage drops, vibrations or mechanical shocks.
• HOW TO CONFIRM: After excluding other causes (cable, ruler), the encoder malfunction is confirmed by the absence or incorrect output signals when powering and moving.
• DAMAGE IF NOT FIXED: Complete loss of axis feedback resulting in machine stoppage.

7.3. Problems of thermal compensation

7.3.1. Thermal deformation of the SHVP or bed frame:

• WHY: The heating of the SHP during operation (due to friction) leads to its thermal expansion. If the machine does not have an effective cooling system of the SHP or activated thermal compensation, the expansion leads to a change in the actual position relative to the set point. The bed can also heat up, causing deformations.
• HOW TO CONFIRM: Measurement of the temperature of the SHP with a thermal imager or contact thermometers. Detection of the discrepancy between the specified and the actual position, which increases with the time of operation of the machine. Measurement of linear accuracy with a laser interferometer after "warming up" the machine.
• DAMAGE IF NOT FIXED: Errors in the dimensions of parts that change during a work shift. Reduction of accuracy and quality of processing.

7.3.2. Malfunction of temperature sensors or cooling system:

• WHY: If the machine is equipped with temperature sensors for compensation, their malfunction (break, short circuit, failure) leads to incorrect data for the CNC. A pump malfunction or contamination of the filters of the cooling system of the SHP leads to its overheating.
• HOW TO CONFIRM: Checking the readings of the temperature sensors in the CHPC diagnostic system. Checking the integrity of sensor wiring. Checking the performance of the cooling system (fluid flow, pressure).
• DAMAGE IF NOT REMOVED: Same as for heat warp. In addition, overheating can accelerate the wear of the valve stem and bearings.

7.4. Servo setup problems

7.4.1. Incorrect setting of PID controller parameters:

• WHY: The servo gain parameters (P, I, D) determine the system's response to control signals and disturbances. Incorrect values ​​(such as P-gain too high or I-gain too low) can result in oscillations, overshoots, slow response or inaccurate position holding.
• HOW TO CONFIRM: Analysis of motion and motor current oscillograms using servo diagnostic software. Observing the response of the axis to the movement command (is there oscillation, readjustment).
• DAMAGE IF NOT REMOVED: Uneven movement of the axis, vibration, overheating of the motor, reduction of processing accuracy, rapid wear of mechanical components due to increased dynamic loads.

7.4.2. Mechanical resonance:

• WHY: Every mechanical system has its own resonant frequencies. If the servo's frequencies (especially with incorrect tuning) coincide with these resonant frequencies, significant vibrations occur that degrade positioning accuracy.
• HOW TO CONFIRM: Frequency vibration analysis using a vibration analyzer. Detection of vibration peaks at frequencies that coincide with the operating frequencies of the servo drive.
• DAMAGE IF NOT FIXED: Excessive vibration leading to rapid wear of bearings, mechanical seals, loosening of fasteners, and significant degradation of machined surfaces.

8. Step-by-step troubleshooting procedures

8.1. Elimination of the backlash of the SHP

1. Switch nut replacement:
   1. SAFETY: Perform LOTO procedure.
   2. Disconnect the screw nut from the moving part of the axle.
   3. Remove the nut from the PTO shaft (a special device may be required to prevent the balls from falling apart).
   4. Install a new valve stem nut following the manufacturer's instructions. Make sure the orientation is correct.
   5. Fasten the nut to the moving part with the recommended tightening torque (for example, 80-120 Nm for M10).
   6. CHECK: Repeat the backlash measurement with an indicator or a laser interferometer. Allowable backlash <0.01 mm.
2. Replacing or adjusting the bearings of the SHV support:
   1. SAFETY: Perform the LOTO procedure.
   2. Dismantle the supports of the SHVP.
   3. Replace the bearings with new ones (usually a set of radial thrust bearings with preload). Use a special tool for installation.
   4. Install the supports in place, observing the recommended tightening torque of the fastening bolts.
   5. CHECK: Measure the axial displacement of the SHVP supports with an indicator. It should be <0.003 mm. Carry out a vibration analysis.
3. Tightening or replacement of servomotor-SHV coupling:
   1. SAFETY: Perform LOTO procedure.
   2. Check the tightening of the clutch screws. Tighten to the torque recommended by the manufacturer (for example, 15-25 Nm for shaft-mounted couplings).
   3. If the coupling is worn or damaged, replace it with a new one.
   4. CHECK: Visual control of the absence of backlash. Test run of the axis at low speeds.

8.2. Encoder feedback troubleshooting

1. Cleaning the ruler and encoder head:
   1. SAFETY: Perform the LOTO procedure.
   2. Carefully clean the ruler and optical elements of the encoder head with a soft, lint-free cloth moistened with isopropyl alcohol or a special agent for optics. Do not use abrasive materials!
   3. CHECK: After cleaning, analyze the signals with an oscilloscope. Signals must be clean, noise-free, with correct phase and amplitude.
2. Encoder cable replacement:
   1. SAFETY: Perform LOTO procedure.
   2. Disconnect the old cable. Lay a new cable, following the original route and bend radii. Avoid tension and mechanical damage.
   3. Connect the new cable, making sure that each conductor is correctly connected (pin-to-pin).
   4. CHECK: Check the integrity of the new cable with a multimeter. Analyze encoder signals with an oscilloscope.
3. Encoder replacement:
   1. SECURITY: Perform the LOTO procedure.
   2. Disassemble the faulty encoder.
   3. Install the new encoder, ensuring proper alignment and the recommended gap between the head and the ruler (usually 0.1 – 0.2 mm).
   4. Fasten the encoder with the recommended torque.
   5. CHECK: After installation, analyze the signals with an oscilloscope. Perform the procedure for setting the reference point (Zero Point) of the CPC.

8.3. Elimination of thermal compensation problems

1. Checking and activating thermal compensation in the CPC:
   1. Enter the CPC parameters. Check the state of the thermal compensation function (ON/OFF). If disabled, activate.
   2. Check the compensation parameters (for example, the coefficients of temperature expansion for a specific SHVP). If necessary, adjust according to the machine tool manufacturer's documentation or with a laser interferometer after measuring the strain.
   3. CHECK: After making changes, warm up the machine and measure the positioning accuracy using a laser interferometer.
2. SHP cooling system diagnosis and repair:
   1. SAFETY: Perform the LOTO procedure.
   2. Check the coolant level, the condition of the pump, filters and heat exchanger.
   3. Clean filters, replace fluid, repair or replace pump/heat exchanger as needed.
   4. CHECK: Monitoring of the temperature of the heat exchanger with a thermal imager or built-in sensors. The temperature should not exceed the nominal values ​​(for example, +5°C from the ambient temperature) after an hour of operation.

8.4. Servo settings

1. Executing servo autotuning:
   1. Start servo diagnostic software (for example, Servus, DriveMonitor).
   2. Select the auto-tuning function. Follow the software and servo manufacturer's instructions. The machine will perform test movements during auto-adjustment.
   3. CHECK: After auto-tuning, check the gain parameters. Test start of the axis, observing the movement and the absence of oscillations.
2. Manual adjustment of PID controller parameters:
   1. If auto-adjustment did not give results or is unavailable, perform manual adjustment.
   2. Sequentially adjust the gain parameters P, I, D, starting with P. Increase P until the moment of oscillations, then decrease a little. Next, adjust I and D to optimize response and eliminate static error.
   3. CHECK: Use the oscilloscope function in the servo diagnostic software to monitor the positioning error and motor current. The movement should be smooth, without adjustments and oscillations.
3. Identifying and eliminating sources of mechanical resonance:
   1. Using a vibration analyzer, identify components that vibrate at resonance frequencies.
   2. Check the rigidity of the fastening of the motors, the supports of the SHVP, and the linear guides. Tighten all loosened fasteners.
   3. If the vibration is due to an imbalance, balance the rotating parts (for example, the engine flywheel).
   4. VERIFY: Re-analysis of vibration. The vibration level should be within normal limits (typically < 2.8 mm/s RMS for new machines, < 4.5 mm/s RMS for worn ones).

9. Preventive measures

10. Spare parts and components

To order quality spare parts and components from leading manufacturers, visit our unitec electronic catalog.

11. Links

• DSTU EN 1037: Machine safety. Prevention of unexpected start.
• ISO 14118: Machine safety. Prevention of unexpected start.
• ISO 230-2: Test methods for machine tools. Part 2: Determination of positioning accuracy and repeatability of CNC machines.
• ISO 10816: Mechanical vibration. Evaluation of machine vibration by measurements on non-rotating parts.
• Operation and maintenance manuals from machine tool manufacturers (eg DMG MORI, MAZAK, OKUMA).
• Documentation for servo systems (eg FANUC, Siemens, Bosch Rexroth).