Diagnosis and elimination of poor surface quality during CNC machining: tool wear, vibration, spindle knocking and optimization of cutting modes

1. Description of the problem and scope of application

This diagnostic manual is designed to systematically identify and eliminate the causes of poor surface quality on CNC machines. The low quality of the surface is manifested in the form of increased roughness, visible traces of the tool, waviness, streaks or other defects that do not meet the established technical requirements.

The problem can occur on various types of CNC equipment, including milling centers, lathes, grinders, and multifunction machines. It directly affects the functionality, durability and aesthetic appearance of the final product, leads to an increase in the number of defects, additional proofing operations and significant financial losses.

Classification of severity:

• Critical: Processed parts do not meet the minimum functional requirements, which leads to a complete failure of the batch or equipment failure. Immediate cessation of production.
• Significant: The surface quality is below standard, requiring additional processing (grinding, polishing), which increases the cycle time and cost. Decreased productivity.
• Minor: Small deviations from ideal quality that do not affect functionality, but may be aesthetically unacceptable or complicate further processes.

2. Precautions

SAFETY FIRST: Before starting any diagnostic or repair work on a CNC machine, lockout/tagout (LOTO) procedures must be strictly followed in accordance with DSTU EN ISO 12100. This prevents unauthorized equipment start-up and protects personnel from injury. Always use appropriate personal protective equipment (PPE), such as safety glasses (PPE EN 166), protective gloves (PPE EN 388), hearing protection (PPE EN 352) and protective clothing.

STORED ENERGY WARNING: Hydraulic systems, pneumatic actuators, capacitors, and spring mechanisms can contain significant amounts of stored energy even after power is turned off. Make sure all these systems are de-energized or locked out before starting work. Be careful with hot surfaces (cutting area, spindle motors) and sharp cutting edges of the tool.

3. Necessary diagnostic tools

The following list of specialized tools is required for accurate diagnosis of the causes of poor surface quality:

4. Initial evaluation checklist

Before starting a detailed diagnostic search, perform a preliminary inspection and gather information about the current condition of the machine and the machining process. This data is critical to quickly and accurately identify the root cause.

5. Systematic diagnostic algorithm

Follow this step-by-step algorithm to systematically identify the root cause of poor surface quality:

1. Initial assessment of surface quality.
   • Measure the roughness using a profilometer (Ra, Rz).
   • Visually assess the nature of defects (stripes, waviness, pores).
   • If the surface quality does not meet the requirements, go to step 2.
2. Checking the condition of the cutting tool.
   1. Visual inspection of the tool:
      • Remove the tool from the spindle.
      • Carefully examine the cutting edge using an optical microscope (x50 - x200 magnification). Search for:
      • Wear on the back surface (wear chamfer): The normal chamfer width for a carbide tool is 0.1-0.3 mm. If > 0.3 mm, the tool is worn.
      • Chips: Small or large edge chips.
      • Processes: Adhesion of the processed material to the cutting edge.
      • Signs of overheating: Changing the color of the tool.
   2. Evaluation of tool overhang:
      • Too much tool overhang can reduce rigidity and cause vibrations. Recommended runout < 3-5 tool diameters.
   3. If significant wear, chips, or burrs are found:
      • Probable root cause: Tool wear or suboptimal tool/mode selection.
      • Go to the section "7. Root Cause Analysis: Tool Wear".
   4. If the tool is OK or replacing it did not solve the problem:
      • Go to step 3.
3. Diagnostics of vibration (ringing noise).
   1. Visual-acoustic inspection:
      • During processing, pay attention to unusual noises (ringing, grinding), visually evaluate the vibrations of the workpiece, tool, and spindle.
   2. Using the vibration analyzer:
      • Install the vibration analyzer accelerometer as close as possible to the cutting area (for example, on the spindle housing or workpiece fixture).
      • Take measurements of vibration velocity (mm/s RMS) and acceleration (g RMS) between 0 and 5000 Hz.
      • Compare with the permissible thresholds: For a spindle assembly, the norm is up to 2.8 mm/s RMS. Values ​​between 2.8 and 4.5 mm/s RMS indicate moderate wear or a potential developing problem. Readings above 4.5 mm/s RMS are considered dangerous and require immediate intervention, indicating a high probability of vibration or critical bearing wear.
      • Pay special attention to the spectral analysis to detect vibration peaks at frequencies related to the speed of rotation of the spindle, tooth frequencies of the tool, resonant frequencies of the system.
   3. If a high level of vibration or characteristic ringing noise is detected:
      • Probable root cause: Vibration (ringing noise), insufficient system stiffness, bearing wear.
      • Go to the section "7. Root Cause Analysis: Vibration (Ringing Noise)”.
   4. If the vibration is normal or the problem is not resolved:
      • Go to step 4.
4. Spindle and tool runout check (radial and axial).
   1. SAFETY: Before starting measurements, make sure the spindle is completely stopped and the system is locked (LOTO).
   2. To measure spindle runout:
      • Mount the IGT on the magnetic stand so that the measuring tip touches the inner cone of the spindle (without a tool or chuck).
      • Slowly rotate the spindle by hand through 360°. Record the maximum and minimum readings.
      • Permissible radial runout of the inner cone of the spindle: no more than 0.005 mm (5 μm) for precision work, up to 0.010 mm (10 μm) for general work.
   3. To measure chuck/mandrel runout:
      • Install the chuck/mandrel into the spindle. Measure the runout of the outer surface of the chuck/mandrel.
      • Permissible chuck/mandrel runout: Usually up to 0.010-0.015 mm (10-15 μm).
   4. To measure the runout of the tool:
      • Install the tool in the chuck/mandrel. Measure the blow of the working part of the tool (at a distance of 1-2 mm from the cutting edge).
      • Tolerable tool runout: up to 0.015-0.020 mm (15-20 µm) for most applications. For high-precision work, less than 0.010 mm is desirable.
   5. If runout exceeds acceptable values:
      • Probable root cause: Spindle, chuck, tool runout, or taper contamination.
      • Go to the section "7. Root Cause Analysis: Spindle/Tool Runout”.
   6. If the beat is normal:
      • Go to step 5.
5. Evaluation and optimization of cutting modes.
   1. Comparison with recommended parameters:
      • Compare the current cutting speeds (Vc), feed per tooth/revolution (Fz/Fn) and depth of cut (ap, ae) with the recommendations of the tool and workpiece material manufacturer.
      • Refer to datasheets or software to calculate optimum modes.
   2. Swarf analysis:
      • Assess the shape, size and color of the chips. An ideal chip should be compact, homogeneous, without signs of overheating.
      • A long, curled chip may indicate insufficient feed or incorrect geometry. Fine, powdery - for excessive tool wear or very high speed.
   3. Temperature control:
      • Measure the temperature in the cutting area using a thermal camera. Excessive heating (over 200°C on the chip) indicates incorrect modes.
   4. If significant deviations from optimal modes are found:
      • Probable root cause: Suboptimal cutting modes.
      • Go to the section "7. Root Cause Analysis: Suboptimal Cutting Modes".
   5. If all previous checks have not revealed obvious problems:
      • Consider other possible causes: problems with workpiece/fixture stiffness, coolant quality, condition of machine guides, backlash in feed drives, workpiece material defects. Carry out their additional diagnostics.

6. Matrix "Failure-Cause"

This matrix provides a quick overview of common symptoms, their likely causes, diagnostic methods, and expected outcomes.

7. Root cause analysis for each malfunction

7.1. Wear of the cutting tool

• Why it happens: Tool wear is a natural process, but its acceleration can be caused by several factors: abrasive wear (hard inclusions in the workpiece material), adhesive wear (sticking of material to the edge), diffusion wear (interaction of materials at high temperatures), oxidative wear, thermomechanical breakdown (due to cyclic thermal and mechanical loads). Incorrect choice of tool material, its coating or geometry for a specific workpiece material and cutting modes significantly accelerates wear. Insufficient concentration or improper supply of coolant also contributes to overheating and wear.
• How to confirm: A detailed microscopic analysis of the cutting edge of the tool will reveal the nature of the wear: the width of the wear chamfer (norm: 0.1-0.3 mm, critical: >0.3 mm), the presence of chips, cracks, growths. An increase in cutting force and spindle power are also signs of wear.
• What damages it causes, if not eliminated: Excessive consumption of electricity, increased heat generation in the cutting zone, which can lead to deformation of the workpiece and changes in its microstructure. A worn tool generates increased vibrations, which accelerates the wear of spindle bearings and guides. Ultimately, this leads to tool failure, workpiece damage, and in some cases, chuck or spindle damage.

7.2. Vibration (ringing noise)

• Why this happens: CNC machining vibration (especially chatter) is a self-excited phenomenon where the deformation caused by cutting changes the thickness of the chip, which in turn causes further deformation. The main reasons:
• Insufficient rigidity of the system: The workpiece, tool, fixture, spindle, machine bed may have insufficient rigidity, which allows them to oscillate at certain frequencies (resonance).
• Imbalance: Unbalanced rotating masses (tool, chuck, spindle) create centrifugal forces that cause vibration, especially at high speeds.
• Mismatch: Inaccurate installation of drive parts, motors or spindle.
• Spindle bearing wear: Worn or damaged bearings lose their stiffness and precision, allowing the spindle to vibrate.
• Inappropriate cutting modes: Certain combinations of speed, feed, and depth of cut may excite system resonance frequencies.
• How to confirm: A vibration analyzer with spectral analysis is critical for vibration diagnosis. It allows you to identify vibration frequencies and their amplitudes, which indicates the source of the problem (imbalance, misalignment, bearings, resonance). Acoustic analysis (by ear) is also the first sign.
• What damage it causes if not removed: Greatly accelerated tool and equipment wear. Deterioration of surface quality, reduction of dimensional accuracy, increased noise level. It can lead to material fatigue in critical machine parts, causing costly breakdowns and extended downtime.

7.3. Spindle and tool runout (Runout)

• Why this happens: Whipping is a deviation from the central axis of rotation.
• Spindle runout: The main cause is wear or damage to the precision spindle bearings. Other causes include: soiling or damage to the spindle seat cone, inaccurate assembly of the spindle assembly, thermal deformation of the spindle.
• Instrument beating: Can be caused by:
• Inaccuracy or wear of the chuck/mandrel.
• Contamination or damage to the tool shank or chuck cone.
• Incorrect installation of the tool in the chuck (for example, uneven tightening of the collets).
• Deformation of the tool itself.
• How to confirm: Measurement of radial and axial runout using IGT on the inner taper of the spindle, on the mandrel/chuck and on the working part of the tool is the only accurate method.
• What damages it causes, if not eliminated: Uneven chip thickness, which leads to uneven load on the cutting edge, rapid one-sided tool wear, deterioration of surface quality (spiral marks, waviness), low accuracy of dimensions and shape of the workpiece. Leads to increased vibration and accelerated wear of spindle bearings.

7.4. Suboptimal cutting modes

• Why this happens: Cutting modes (spindle rotation speed, feed, depth of cut) must be optimized for a specific workpiece material, tool material, its geometry and system stiffness.
• Cutting speed that is too high: Leads to rapid thermomechanical wear of the tool, overheating of the cutting zone, growths, burns.
• Cutting speed that is too low: Can cause material to stick to the tool, increase machining time, and reduce chipping and surface quality.
• Too much feed: Increases the load on the tool, can cause it to chip, break, vibrate, and increase roughness.
• Feed too small: Leads to friction, overheating, rapid wear on the back surface, polishing instead of cutting, which deteriorates the surface quality.
• Cutting depth too large/small: Incorrect cutting depth can cause vibrations or inefficient material removal.
• How to confirm: Analysis of chips, sound during cutting, temperature in the cutting zone, and comparison of current parameters with tool manufacturer's recommendations and specialized manuals.
• What damage it causes if not removed: Accelerated tool wear and breakage, low productivity, significant deterioration of surface quality, overheating of the workpiece, which can lead to deformation and changes in material properties.

8. Step-by-step troubleshooting procedures

8.1. Elimination of wear of the cutting tool

1. Tool replacement:
   • Action: Replace the worn tool with a new one.
   • Check: Select a tool with the appropriate material, coating (e.g. AlTiN for hard alloys, PVD for viscous materials) and geometry that is optimal for the workpiece (according to ISO 513).
   • Verification: After replacement, check the quality of the surface using a profilometer.
2. Optimizing cutting modes:
   • Action: Adjust cutting speed (Vc) and feed per tooth (Fz). If wear is rapid, reduce cutting speed by 10-20% or increase feed by 5-10%.
   • Verification: Carry out trial machining, evaluate surface quality and chip character.
3. Coolant control:
   • Action: Check the concentration of the emulsion (according to the refractometer, it should meet the manufacturer's recommendations, usually 5-10%). Check the coolant pressure and flow, ensure that it is supplied directly to the cutting area.
   • Check: Replace or add coolant if necessary. Check the operation of the pump.

8.2. Elimination of vibration (ringing noise)

1. SAFETY: Be sure to follow LOTO procedures before doing any work that requires access to moving machine parts!

2. Increasing system stiffness:
   • Action: Check the reliability of fastening the workpiece. Use additional supports, clamps, special clamping devices. Reduce tool overhang to the minimum possible (<3-5D). Check chuck/mandrel fitment in spindle.
   • Verification: Carry out repeated vibration measurements with a vibration analyzer after adjustment.
3. Optimization of cutting modes:
   • Action: Gradually change the spindle rotation speed by ±10-20% of the current one. Often this allows you to get out of resonance. Reduce depth of cut (ap) and/or width of cut (ae).
   • Verification: Monitoring of vibration and surface quality.
4. Dynamic Balancing:
   • Action: If vibration is significant and related to rotational speed, perform dynamic balancing of the tool and/or spindle.
   • Verification: Reduction of vibration amplitude.
5. Inspection and replacement of spindle bearings:
   • Action: If the vibration analysis shows signs of bearing wear (harmonics, rolling noises), conduct a more detailed diagnosis. Replace the spindle bearings according to the manufacturer's recommendations (ISO P4/ABEC 7).
   • Verification: IGT spindle beat measurement and repeated vibroanalysis.

8.3. Elimination of spindle and tool runout

1. SAFETY: All spindle assembly disassembly and assembly work requires special training and strict adherence to LOTO.

2. Cones Cleaning:
   • Action: Thoroughly clean the spindle seat cone, tool shank and chuck/mandrel cone of chips, dirt, grease. Use special cleaners and lint-free wipes.
   • Verification: Repeated measurement of IHT beat.
3. Checking and replacing the chuck/mandrel:
   • Action: If the runout of the tool exceeds the norm, but the runout of the chuck is normal, the problem may be in the tightening or in the chuck itself. If the runout of the cartridge exceeds 0.010-0.015 mm, replace it with a new one (ISO high-precision cartridges HSK-A63, DIN 69871-AD/B).
   • Verification: Measurement of the beating of a new cartridge and tool.
4. Spindle assembly diagnosis and repair:
   • Action: If the runout of the spindle inner taper exceeds 0.005 mm, this indicates a problem with the spindle. This can be wear of bearings, deformation of the shaft, damage to the landing cone. Professional intervention is required: dismantling of the spindle, defects, replacement of bearings or complete replacement of the spindle.
   • Verification: After repairing or replacing the spindle, perform a full cycle of runout and vibration measurements.

8.4. Optimization of cutting modes

1. Recommendation Consultation:
   • Action: Always start with the cutting modes recommended by the tool manufacturer for the specific material and operation. Consult their handbooks or online calculators.
2. Incremental optimization:
   • Action: Change only one parameter at a time to evaluate its impact.
   • If the roughness is too high: Increase the feed (F) by 10-20% or decrease the cutting speed (Vc) by 10-20%.
   • If rapid tool wear or burn-through is observed: Reduce the cutting speed (Vc) by 10-20% and/or reduce the depth of cut (ap/ae).
   • If vibration occurs: Try changing the spindle rotation speed by ±10%.
   • Verification: Trial cutting, surface quality measurement, chip analysis, cutting sound monitoring.
3. Using simulation and CAM systems:
   • Action: Modern CAM systems and simulation programs can help optimize toolpaths and cutting modes before machining begins.

9. Precautions

Implementation of these preventive measures will significantly reduce the likelihood of surface quality problems.

10. Spare parts and components

The following spare parts and components must be in stock for quick troubleshooting of surface quality issues. All of the above can be found in UNITEC-D E-Catalog.

To order and get a detailed overview of the assortment, visit UNITEC-D E-Catalog.

11. Links

• DSTU ISO 4287:2018 (ISO 4287:1997; Amd 1:2009, IDT). Geometric characteristics of products (GPS). Surface roughness. Profile method. Terms, definitions and parameters of roughness.
• DSTU EN ISO 13565-1:2018 (EN ISO 13565-1:1998, IDT). Geometric characteristics of products (GPS). Surface roughness. Profile method. Surfaces that have functional properties.
• DSTU ISO 10816-3:2004 (ISO 10816-3:1998, IDT). Vibration is mechanical. Evaluation of machine vibration based on the results of measurements on non-rotating parts. Part 3. Industrial machines with a rated power of more than 15 kW and a rated speed of 120 rpm to 15000 rpm when operating under conditions of installation on rigid or elastic supports.
• ISO 513:2012. Classification and application of hard-metal inserts for machining.
• Operation and maintenance manuals (OEM manuals) for a specific CNC machine.
• Related UNITEC-D maintenance manuals (e.g. "Diagnosis and Maintenance of Precision Bearings").