Error diagnosis for poor surface quality in CNC machining: Systematic cause analysis and elimination

1. Problem description and scope

Insufficient surface quality in CNC machining is often manifested by chatter marks, excessive roughness (Rz/Ra values outside the tolerance according to DIN EN ISO 4287), burn marks or uneven surface texture. This manual addresses these critical quality issues in machining manufacturing processes such as milling, turning and drilling. Diagnosis requires a systematic examination of process variables, tool condition, machine stiffness and spindle dynamics. This guide helps maintenance technicians and machining specialists quickly isolate and correct the causes.

2. Safety precautions

WARNING: Before starting the diagnosis, the system must be switched off according to the LOTO procedure (Lockout/Tagout). When handling sharp cutting tools, cut-resistant PPE (gloves according to DIN EN 388) is mandatory. When inspecting the ongoing process, safety glasses according to DIN EN 166 must be worn. Contact with cooling lubricants (KSS) can cause skin irritation; In case of skin contact, rinse immediately with water. Always observe the applicable accident prevention regulations (UVV).

3. Required diagnostic tools

4. Initial assessment checklist

5. Systematic diagnostic flowchart

1. Symptom: Chatter marks
   • Check: Is the tool worn or incorrectly clamped?
     • YES: Change/re-clamp the tool.
     • NO: Check spindle run.
   • Check: Spindle concentricity deviation > 0.005 mm?
     • YES: Bearing damage or chuck defective -> repair.
     • NO: Check cutting parameters (vc, fz) for resonance.
2. Symptom: Excessive roughness
   • Check: Is the coolant jet directed correctly at the cutting edge?
     • NO: Align the nozzle.
     • YES: Check KSS concentration (target: 5–8%).
   • Check: Cutting feed rate too high?
     • YES: Correct feed rate (reduce fz).

6. Error-cause matrix

7. Root cause analysis for major defects

Tool wear: A worn tool massively increases the cutting forces, which leads to vibrations and chatter marks. This results from abrasive wear caused by high cutting speeds. If left untreated, this will lead to tool breakage and damage to the workpiece.

Spindle runout (TIR - Total Indicator Runout): Increased runout causes one cutting edge to be subjected to more load than the other. This causes an uneven surface profile and accelerates spindle bearing wear.

Chatter: These are self-excited vibrations between the tool and the workpiece. This often happens when the dynamic rigidity of the clamping is not sufficient. It causes microcracks in the workpiece and damages the machine spindle.

8. Step-by-step fix procedure

1. Tool change: Check cutting tool. If VBB > 0.3 mm (finishing: VBB > 0.15 mm), replace tool. Use a torque wrench for clamping screws (observe manufacturer's instructions!).
2. Spindle test: Insert the test mandrel into the spindle. Place the dial gauge at a distance of 50 mm. Turn spindle slowly. Max. concentricity: 0.005 mm. If exceeded: Check bearings.
3. Cutting value optimization: If chatter marks occur, vary the cutting speed (vc) by 10-20% to get out of the resonance range of the machine.
4. KSS adjustment: Adjust the concentration to 6% (standard steel processing) using a refractometer. Align the nozzles for maximum wetting of the cutting edge.

9. Preventive measures

10. Spare parts and components

Detailed information and ordering options can be found in our e-catalog: https://www.unitecd.com/e-catalog/

11. References

• DIN EN ISO 4287: Geometric product specification (GPS) – surface quality.
• VDI 2855: Dynamic stiffness of machine tools.
• CNC Spindle Unit Manufacturer Maintenance Manual.