Shenzhen Yudiao CNC Technology Co., Ltd.
Discussion On Static and Dynamic Accuracy of CNC Machine Tool Processing
Jul 24, 2025
Laser detection (usually refers to laser interferometer detection) is qualified, mainly to ensure the positioning accuracy and repeatability of single axis (X, Y, Z) (at a certain speed). However, the accuracy of the actual processed product is subject to an extremely complex system, and any problem in any link may lead to deviation. The following discusses the above issues.
1. Machine tool geometric accuracy (static accuracy - laser interferometer may not be fully covered)
Verticality between axes: This is the top priority! When the laser interferometer measures the accuracy of a single axis, it cannot detect the verticality error between axes. If the XY axis is not strictly vertical, the processed contour (such as a square frame, a round hole) will become a parallelogram or an ellipse. Use an electronic level, square ruler, angle ruler or professional square gauge with a micrometer for detection.
Spindle and table verticality/parallelism: The verticality of the spindle relative to the table surface in the XY plane (affects the verticality of the side wall) and the parallelism of the spindle axis and the Z-axis movement direction (affects the cylindricity of the hole and the flatness of the bottom surface). Use a precision angle ruler with a micrometer or a spindle check rod with a micrometer to check.
Table flatness: Whether the table itself is flat. Use a precision flat ruler and feeler gauge or a laser plane interferometer to check.Spindle radial runout and axial runout: The swing of the spindle during rotation will affect the roundness, surface roughness and tool life of the hole. Use a micrometer to hit the spindle nose or check rod for detection.
2. Dynamic accuracy and system rigidity Backlash (backlash): Although the lead screw has been replaced, there may also be gaps in other links of the transmission chain (coupling, bearing, nut seat, connection between the motor and the lead screw). Ensure that the backlash compensation parameters are set correctly and effectively. You can observe and measure by dialing the table with a micrometer and manually jogging the axis in the forward and reverse directions.
Servo system response and matching: Improper setting of servo gain parameters (position loop, speed loop, current loop) may lead to excessive following error, overshoot or oscillation, especially in contour processing (such as corners and arcs).
Observe the servo load and following error during processing, and ask professionals to optimize the servo parameters if necessary.
System rigidity: Machine tool structure rigidity: Deformation of base, column, saddle, worktable, etc. under cutting force. Check whether the key connecting bolts are loose and whether the structural parts are damaged or fatigued.
Spindle rigidity: Deformation of the spindle unit under cutting force.
Workpiece clamping rigidity: Is the workpiece clamped firmly? Is the fixture design reasonable? Does it deform or vibrate under cutting force? This is a very common reason!
Tool system rigidity: Tool holder (such as ER chuck rigidity is usually weaker than hydraulic tool holder or thermal expansion tool holder) and tool overhang are too long, which will significantly reduce system rigidity, resulting in tool letting, vibration and dimensional deviation.
3. Tool and process system tool compensation: This is an extremely high frequency reason!
Are the tool length compensation and radius compensation settings correct? Check whether the H/D code value in the G code is consistent with the actual measured value. Carefully re-measure the length and radius of all used tools.
Is the tool worn or chipped? Worn tools will directly lead to dimensional changes and shape errors. Check the tool status.
Tool runout: Poor tool holder quality, improper clamping or large spindle runout will cause the actual tool rotation radius to be greater than the set radius (especially in fine machining). Use a tool setting gauge or micrometer to detect the runout of the tool after installation.
Cutting parameters: Excessive cutting force (excessive cutting depth and feed) will aggravate the elastic deformation of the machine tool, workpiece, and tool system, resulting in dimensional and shape deviations. Try to reduce the cutting depth and feed and observe the effect.
Cutting vibration (chatter): Vibration will lead to poor surface quality and dimensional instability. Check whether the tool overhang is too long, whether the tool is worn, whether the cutting parameters are in the stable area, and whether the workpiece/fixture rigidity is insufficient.
Coolant and chip removal: Insufficient cooling causes thermal deformation of the tool or workpiece; poor chip removal may cause chip extrusion, scratch the machined surface or cause vibration.
4. Thermal deformation Thermal deformation of machine tools: After the machine tool has been running for a period of time, the components expand unevenly due to friction and motor heating, resulting in precision drift. Laser detection is usually performed on a cold machine or for a short time, and may not reflect the hot state accuracy. Check whether the deviation increases with the increase in startup time? Run the hot machine (idle running or light cutting) for a few hours before measuring the workpiece or lasering.
Spindle thermal elongation: The spindle heats up and elongates after high-speed rotation, affecting the Z-axis dimensional accuracy (especially deep hole processing).
Workpiece thermal deformation: The heat generated by cutting causes the workpiece to expand locally, and shrink and deform after processing and cooling. This is particularly obvious on thin-walled parts and aluminum alloy parts. Optimize cooling (sufficient pouring, use of internal cooling tools), rough and fine processing, and fully cool the workpiece before fine processing.
5. Workpiece coordinate system and programming tool setting accuracy: Is the workpiece coordinate system origin (G54, G55, etc.) set correctly? Is the tool setting instrument accurate? Is the tool setting process standardized (such as whether the edge finder and tool setting rod are calibrated)? Re-calibrate the tool carefully and verify the origin position.
Programming error: Is there any unexpected coordinate rotation, scaling, or mirroring in the G code? Is the tool compensation instruction (G41/G42) used correctly? Is there any error in the calculation of the program itself? Check the program carefully, especially the code segment corresponding to the feature where the deviation occurs. Perform graphic simulation on the machine tool.
Post-processor: Is the G code generated by the post-processing compatible with the machine tool control system? Is there any incorrect conversion? Try to use an extremely simple test program (such as running a square with precise dimensions) for processing verification.
6. Is the measuring tool accurate in the measurement link? Are the calipers, micrometers, and three-dimensional coordinate measuring machines within the calibration validity period? Is the measurement method correct? Are the measurement datums consistent? Are the processing datums and measurement datums unified? Is the workpiece positioned on the measuring fixture consistent with that during machining?
Systematic troubleshooting steps suggest isolating variables: Process an extremely simple test piece (such as a precision-milled square boss or cavity, a precision-bored hole). Use a brand new, rigid tool with the shortest possible overhang. Strict measurement: Use high-precision measuring tools (block gauges, micrometers, three-coordinate measuring tools) to record the deviation values and deviation directions of each key dimension of the test piece in detail. Static geometric accuracy check: According to point 1, use mechanical gauges (squares, angle rulers, micrometers, spindle gauges) to check the key geometric accuracy of the machine tool (verticality, parallelism, flatness, spindle runout). Dynamic and compensation check: Check and verify the backlash compensation value. Check whether the servo load and following error are abnormal. Re-measure and carefully enter the tool length and radius compensation values. Check the rigidity of the workpiece and tool clamping. Hot machine verification: After letting the machine tool run at normal operating speed or perform light cutting for 1-2 hours, immediately re-process the test piece in step 1 and measure it. Compare the deviations under hot and cold machine states. Program and coordinate system verification: Carefully check the test program (it is best to write a simple program manually) and re-set the workpiece coordinate system in a standardized manner. Perform graphic simulation on the machine tool. Cutting parameter adjustment: If the above is still not resolved, try to significantly reduce the cutting parameters (cutting depth, feed) to see if the deviation is reduced.
Seek professional support: Ask the machine tool manufacturer or professional maintenance personnel to use a ballbar for dynamic accuracy detection, which can effectively detect the contour accuracy (roundness test) when the two axes are linked, revealing problems such as servo matching, backlash, and verticality error. Ask professional engineers to optimize servo parameters. Use a laser tracker for more comprehensive spatial accuracy detection (higher cost).
Summary of key points Don't be superstitious about the results of the laser interferometer: It is only part of the accuracy puzzle, especially it cannot detect key geometric errors such as verticality. Geometric accuracy is the foundation: Prioritize the verticality between axes and the spindle-related accuracy. Tool compensation is a high-frequency minefield: Repeatedly and carefully confirm the tool length and radius compensation value settings. Rigidity and clamping are crucial: Ensure that the workpiece, tool, and machine tool itself are stable enough under cutting forces. The influence of thermal deformation cannot be ignored: Observe whether the deviation changes with the start-up time. Systemic problems: Check the entire link from machine tool hardware (geometry, rigidity, heat), control system (servo, compensation), tool system (compensation, wear, rigidity), process (parameters, vibration), workpiece (clamping, thermal deformation), programming measurement, etc. Solving such problems requires patience and systematic thinking to eliminate possibilities step by step. Starting with the simplest test piece, strictly controlling variables, and recording the conditions and results of each test in detail is the key to locating the problem. It is recommended to start with geometric accuracy (especially verticality) and tool compensation, the two most often overlooked and influential links. I wish you find the crux of the problem as soon as possible and restore the accuracy of the machine tool!
1. Machine tool geometric accuracy (static accuracy - laser interferometer may not be fully covered)
Verticality between axes: This is the top priority! When the laser interferometer measures the accuracy of a single axis, it cannot detect the verticality error between axes. If the XY axis is not strictly vertical, the processed contour (such as a square frame, a round hole) will become a parallelogram or an ellipse. Use an electronic level, square ruler, angle ruler or professional square gauge with a micrometer for detection.
Spindle and table verticality/parallelism: The verticality of the spindle relative to the table surface in the XY plane (affects the verticality of the side wall) and the parallelism of the spindle axis and the Z-axis movement direction (affects the cylindricity of the hole and the flatness of the bottom surface). Use a precision angle ruler with a micrometer or a spindle check rod with a micrometer to check.
Table flatness: Whether the table itself is flat. Use a precision flat ruler and feeler gauge or a laser plane interferometer to check.Spindle radial runout and axial runout: The swing of the spindle during rotation will affect the roundness, surface roughness and tool life of the hole. Use a micrometer to hit the spindle nose or check rod for detection.
2. Dynamic accuracy and system rigidity Backlash (backlash): Although the lead screw has been replaced, there may also be gaps in other links of the transmission chain (coupling, bearing, nut seat, connection between the motor and the lead screw). Ensure that the backlash compensation parameters are set correctly and effectively. You can observe and measure by dialing the table with a micrometer and manually jogging the axis in the forward and reverse directions.
Servo system response and matching: Improper setting of servo gain parameters (position loop, speed loop, current loop) may lead to excessive following error, overshoot or oscillation, especially in contour processing (such as corners and arcs).
Observe the servo load and following error during processing, and ask professionals to optimize the servo parameters if necessary.
System rigidity: Machine tool structure rigidity: Deformation of base, column, saddle, worktable, etc. under cutting force. Check whether the key connecting bolts are loose and whether the structural parts are damaged or fatigued.
Spindle rigidity: Deformation of the spindle unit under cutting force.
Workpiece clamping rigidity: Is the workpiece clamped firmly? Is the fixture design reasonable? Does it deform or vibrate under cutting force? This is a very common reason!
Tool system rigidity: Tool holder (such as ER chuck rigidity is usually weaker than hydraulic tool holder or thermal expansion tool holder) and tool overhang are too long, which will significantly reduce system rigidity, resulting in tool letting, vibration and dimensional deviation.
3. Tool and process system tool compensation: This is an extremely high frequency reason!
Are the tool length compensation and radius compensation settings correct? Check whether the H/D code value in the G code is consistent with the actual measured value. Carefully re-measure the length and radius of all used tools.
Is the tool worn or chipped? Worn tools will directly lead to dimensional changes and shape errors. Check the tool status.
Tool runout: Poor tool holder quality, improper clamping or large spindle runout will cause the actual tool rotation radius to be greater than the set radius (especially in fine machining). Use a tool setting gauge or micrometer to detect the runout of the tool after installation.
Cutting parameters: Excessive cutting force (excessive cutting depth and feed) will aggravate the elastic deformation of the machine tool, workpiece, and tool system, resulting in dimensional and shape deviations. Try to reduce the cutting depth and feed and observe the effect.
Cutting vibration (chatter): Vibration will lead to poor surface quality and dimensional instability. Check whether the tool overhang is too long, whether the tool is worn, whether the cutting parameters are in the stable area, and whether the workpiece/fixture rigidity is insufficient.
Coolant and chip removal: Insufficient cooling causes thermal deformation of the tool or workpiece; poor chip removal may cause chip extrusion, scratch the machined surface or cause vibration.
4. Thermal deformation Thermal deformation of machine tools: After the machine tool has been running for a period of time, the components expand unevenly due to friction and motor heating, resulting in precision drift. Laser detection is usually performed on a cold machine or for a short time, and may not reflect the hot state accuracy. Check whether the deviation increases with the increase in startup time? Run the hot machine (idle running or light cutting) for a few hours before measuring the workpiece or lasering.
Spindle thermal elongation: The spindle heats up and elongates after high-speed rotation, affecting the Z-axis dimensional accuracy (especially deep hole processing).
Workpiece thermal deformation: The heat generated by cutting causes the workpiece to expand locally, and shrink and deform after processing and cooling. This is particularly obvious on thin-walled parts and aluminum alloy parts. Optimize cooling (sufficient pouring, use of internal cooling tools), rough and fine processing, and fully cool the workpiece before fine processing.
5. Workpiece coordinate system and programming tool setting accuracy: Is the workpiece coordinate system origin (G54, G55, etc.) set correctly? Is the tool setting instrument accurate? Is the tool setting process standardized (such as whether the edge finder and tool setting rod are calibrated)? Re-calibrate the tool carefully and verify the origin position.
Programming error: Is there any unexpected coordinate rotation, scaling, or mirroring in the G code? Is the tool compensation instruction (G41/G42) used correctly? Is there any error in the calculation of the program itself? Check the program carefully, especially the code segment corresponding to the feature where the deviation occurs. Perform graphic simulation on the machine tool.
Post-processor: Is the G code generated by the post-processing compatible with the machine tool control system? Is there any incorrect conversion? Try to use an extremely simple test program (such as running a square with precise dimensions) for processing verification.
6. Is the measuring tool accurate in the measurement link? Are the calipers, micrometers, and three-dimensional coordinate measuring machines within the calibration validity period? Is the measurement method correct? Are the measurement datums consistent? Are the processing datums and measurement datums unified? Is the workpiece positioned on the measuring fixture consistent with that during machining?
Systematic troubleshooting steps suggest isolating variables: Process an extremely simple test piece (such as a precision-milled square boss or cavity, a precision-bored hole). Use a brand new, rigid tool with the shortest possible overhang. Strict measurement: Use high-precision measuring tools (block gauges, micrometers, three-coordinate measuring tools) to record the deviation values and deviation directions of each key dimension of the test piece in detail. Static geometric accuracy check: According to point 1, use mechanical gauges (squares, angle rulers, micrometers, spindle gauges) to check the key geometric accuracy of the machine tool (verticality, parallelism, flatness, spindle runout). Dynamic and compensation check: Check and verify the backlash compensation value. Check whether the servo load and following error are abnormal. Re-measure and carefully enter the tool length and radius compensation values. Check the rigidity of the workpiece and tool clamping. Hot machine verification: After letting the machine tool run at normal operating speed or perform light cutting for 1-2 hours, immediately re-process the test piece in step 1 and measure it. Compare the deviations under hot and cold machine states. Program and coordinate system verification: Carefully check the test program (it is best to write a simple program manually) and re-set the workpiece coordinate system in a standardized manner. Perform graphic simulation on the machine tool. Cutting parameter adjustment: If the above is still not resolved, try to significantly reduce the cutting parameters (cutting depth, feed) to see if the deviation is reduced.
Seek professional support: Ask the machine tool manufacturer or professional maintenance personnel to use a ballbar for dynamic accuracy detection, which can effectively detect the contour accuracy (roundness test) when the two axes are linked, revealing problems such as servo matching, backlash, and verticality error. Ask professional engineers to optimize servo parameters. Use a laser tracker for more comprehensive spatial accuracy detection (higher cost).
Summary of key points Don't be superstitious about the results of the laser interferometer: It is only part of the accuracy puzzle, especially it cannot detect key geometric errors such as verticality. Geometric accuracy is the foundation: Prioritize the verticality between axes and the spindle-related accuracy. Tool compensation is a high-frequency minefield: Repeatedly and carefully confirm the tool length and radius compensation value settings. Rigidity and clamping are crucial: Ensure that the workpiece, tool, and machine tool itself are stable enough under cutting forces. The influence of thermal deformation cannot be ignored: Observe whether the deviation changes with the start-up time. Systemic problems: Check the entire link from machine tool hardware (geometry, rigidity, heat), control system (servo, compensation), tool system (compensation, wear, rigidity), process (parameters, vibration), workpiece (clamping, thermal deformation), programming measurement, etc. Solving such problems requires patience and systematic thinking to eliminate possibilities step by step. Starting with the simplest test piece, strictly controlling variables, and recording the conditions and results of each test in detail is the key to locating the problem. It is recommended to start with geometric accuracy (especially verticality) and tool compensation, the two most often overlooked and influential links. I wish you find the crux of the problem as soon as possible and restore the accuracy of the machine tool!
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