Diagnostic Guide: Elimination of Low Quality Machined Surface in CNC Machines

1. Description of the Problem and Scope of Application

The low quality of the processed surface is one of the most frequent and critical problems in CNC metal processing, which directly affects the functionality of the part, its resource and compliance with standards. This manual covers the diagnosis and troubleshooting of inappropriate roughness, waviness, chips, burrs, or other surface defects that occur during milling, turning, and drilling on CNC machines. The main factors considered are tool wear, vibration (beating), spindle beat and optimization of cutting parameters. The scope of application includes most types of metalworking CNC machines. Severity classification: critical (defects leading to rejection of the part), major (requires additional processing or may lead to resource reduction), insignificant (aesthetic defects that do not affect functionality).

2. Precautions

WARNING! Before starting any diagnostic or repair work on the CNC machine, it is necessary to strictly follow all safety standards and company protocols. Failure to follow these instructions may result in serious personal injury or equipment damage.

• LOCKOUT/TAGOUT (LOTO): Ensure machine is completely de-energized and locked out per LOTO (Lockout/Tagout) procedure before opening guards, inspecting moving parts, or replacing components.
• STORED ENERGY: Be aware of potentially stored energy (compressed air in pneumatic systems, charged capacitors, hydraulic pressure, springs). Always release it safely before work.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear safety glasses or shield, protective gloves (appropriate type for handling sharp tools, oil or liquids), safety shoes and protective clothing. When working with noise - hearing protection.
• SHARP TOOLS: Be careful when working with cutting tools; they are extremely sharp. Use appropriate means for their replacement and disposal.
• HOT PARTS/CHIPS: Parts and chips can be very hot after machining. Use the appropriate tools to remove them.

3. Necessary Diagnostic Tools

4. Initial Evaluation Checklist

Before starting a detailed diagnosis, perform the following check:

5. Systematic Flow of Diagnostics

This section presents a decision tree for step-by-step diagnostics of a poor surface quality problem. Follow the logic of IF-THEN-ELSE.

1. Start with a visual inspection of the machined surface:
   1. What is the nature of the defect?
      • IF the defect appears as excessive roughness or microcracks throughout the surface:
        1. Check the condition of the cutting edge of the tool:
           • IF cutting edge has visible wear (dulling, chipping, burrs) THEN go to "Tool wear".
           • ELSE (edge ​​sharp, no visible defects) THEN check the cutting parameters.
      • IF the defect manifests itself as undulations, periodic traces or chatter marks:
        1. Check for vibrations:
           • IF the machine vibrates or extraneous noises are heard during processing THEN go to "Vibration (beating)".
           • ELSE (no significant vibration/noise) THEN check spindle and tool runout.
      • IF the defect manifests itself as uneven marks, large chips, or a change in diameter size:
        1. Check spindle and tool runout:
           • IF excessive spindle or tool runout is detected THEN go to ``Spindle and tool runout''.
           • ELSE (beating within normal limits) THEN check the parameters of cutting and fixing the workpiece.
      • IF the defect does not match any of the above, but the surface quality is still poor:
        1. Check the cutting parameters:
           • IF the cutting parameters do not meet the recommendations for the material/tool/machine THEN go to "Optimizing cutting parameters".
           • ELSE (parameters are normal) THEN check the quality of the MOP and its delivery.

6. Matrix of Malfunctions and Causes

7. Root Cause Analysis for Each Malfunction

Tool wear

Explanation: Tool wear is an inevitable process that occurs when cutting material. It can manifest itself as abrasive wear (wearing of the edge), adhesive wear (growths), diffusion wear (change in the properties of the tool material) or plastic deformation of the edge. A blunt tool requires more cutting force, generates more heat, which leads to the destruction of the workpiece material, increased roughness and the appearance of burrs.

How to confirm: Visual examination of the cutting edge under a microscope (magnification 50-100x). Characteristic signs: shiny wear chamfer on the back surface (more than 0.1-0.2 mm depending on the type of tool), chipping, chipping, growths of the workpiece material. An increase in temperature in the cutting zone (measured by a pyrometer > 80°C for steels) and an increase in spindle power consumption can also be observed.

Damage if not corrected: Deterioration of surface quality to an unacceptable level, increased probability of tool failure, workpiece damage, excessive load on the machine spindle assembly, increased energy consumption.

Vibration (Beat)

Explanation: Vibrations in the machine-tool-workpiece system lead to uncontrolled relative movements of the tool and workpiece, causing periodic marks on the surface known as "chatter marks" or "moon craters". Vibrations can be forced (from faulty machine components, imbalance) or self-oscillations (regenerative beating caused by the interaction of the cutting process with the dynamic characteristics of the system). Main sources: insufficient system stiffness, tool/fixture imbalance, bearing wear, incorrect workpiece clamping, suboptimal cutting parameters causing resonance.

How to confirm: Listening to a characteristic high-frequency noise during cutting. Using a vibration analyzer: measure the level of vibration on the spindle assembly, tool holder, workpiece and machine table. Normal vibration level for precision machining < 2.5 мм/с RMS (відповідно до ISO 10816). Рівні > 5 mm/s RMS indicates significant vibrations, > 10 mm/s RMS – critical. Analysis of the vibration spectrum will help determine the frequency of the source.

Damage, if not eliminated: Significant deterioration of surface quality, accelerated wear of the tool, damage to the spindle unit (bearings), reduction of the life of the machine, damage to the workpieces.

Beating Spindle and Tool

Explanation: Spindle runout (radial or axial) is a deviation of the rotation axis from the ideal one. Tool runout is the total runout that includes spindle runout, chuck/tool ​​runout, and tool runout. Excessive beating leads to the fact that the cutting edge works unevenly, which causes uneven load on the tool, uneven removal of material, waviness, increased roughness, as well as incorrect dimensions of the part (ovality, taper).

How to confirm: Using a high-precision watch-type indicator. To measure the runout of the spindle, install the indicator on the inner cone of the spindle (without a tool). Permissible radial runout of the spindle for high-precision machines < 0.003-0.005 mm (3-5 μm). To measure the runout of the tool, install the tool in the chuck and measure the runout on its working part. Allowable total tool runout < 0.01 mm (10 µm) for most applications. Measurements should be made at several points and at different angles of rotation.

Damage if not eliminated: Low machining accuracy, surface defects, accelerated tool wear (especially one edge), chuck damage, spindle bearing destruction.

Optimization of Cutting Parameters

Explanation: Incorrect selection of cutting parameters (spindle speed (V_c), feed (f_z or V_f), depth of cut (a_p), width of cut (a_e)) can be the main reason for poor surface quality. Too high a feed or too low a speed can cause excessive roughness and burrs. Too high a rotational speed or insufficient depth of cut can lead to excessive tool wear and overheating. The wrong combination of parameters can also cause vibrations.

How to confirm: Carrying out control treatments with a gradual change of one parameter. Comparison of cutting parameters with tool and material manufacturer's recommendations. Monitoring of spindle power and cutting forces (if available). Optimum parameters are usually in the range that minimizes vibration and provides stable chips.

Damage if not eliminated: Low productivity, excessive tool wear, workpiece damage, non-compliance of the part with the technical requirements.

8. Step-by-Step Troubleshooting Procedures

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