Accuracy in CNC machining is not a single concept but a combination of different dimensions of precision. Understanding these accuracy types is critical for design, machining, and quality control.
1. Machine Tool Accuracy
This is the foundation of accuracy, referring to the precision a CNC machine achieves when running without a workpiece or tool. It is mainly determined by the machine builder and defines the grade of the machine.
Positioning Accuracy
Definition: The maximum deviation between the commanded position (e.g., X100.0mm
) and the actual achieved position (e.g., X99.998mm
or X100.001mm
).
Meaning: Reflects the accuracy of point-to-point movements.
Influencing Factors: Ball screw and guideway errors, and the CNC system's compensation ability.
Repeatability
Definition: The maximum spread of actual positions when the machine repeatedly moves to the same commanded point under identical conditions.
Meaning: Shows the machine’s stability. For example, repeating 10 times at X100.0mm
may result in a variation range of 0.001mm.
Importance: Even if positioning accuracy is slightly off, high repeatability ensures errors can be compensated, enabling high-precision part production.
2. Workpiece Accuracy
This refers to the precision of the actual machined part, influenced not only by the machine’s base accuracy but also by tooling, process, and environmental factors.
Dimensional Accuracy
Definition: The degree to which actual part dimensions (hole size, slot width, shaft diameter) meet the drawing’s specified tolerances. Example: shaft machined to Ø10 ±0.01mm.
Geometric Accuracy (GD&T)
Definition: The conformity of part features (points, lines, surfaces) to their ideal geometry or positional relationships.
Types: Form tolerances (straightness, flatness, roundness, cylindricity) and positional tolerances (parallelism, perpendicularity, coaxiality, true position, runout).
Surface Finish (Roughness)
Definition: Microscopic irregularities on the part’s surface measured in Ra or Rz (µm).
Impact: Affects friction, wear, sealing, fatigue life, corrosion resistance, and appearance.
Summary of Accuracy Types
Accuracy Type | Core Definition | Main Factors |
---|---|---|
Positioning Accuracy | Absolute movement accuracy | Ball screw, guideways, CNC system |
Repeatability | Stability of repeated movements | Transmission rigidity, system stability |
Dimensional Accuracy | Part dimension tolerance | Machine accuracy, tool wear, heat |
Geometric Accuracy | Shape & position tolerance (GD&T) | Machine geometry, fixturing, cutting |
Surface Roughness | Smoothness of machined surface | Tool sharpness, cutting parameters |
Key Factors Affecting Machining Accuracy
- Machine tool: Base precision, rigidity, thermal stability.
- Cutting tools: Tool accuracy, wear, balance, clamping rigidity.
- Workpiece: Material hardness, residual stress, fixturing.
- Process: Cutting parameters, coolant, toolpath strategy.
- Environment & operator: Temperature variation, vibration, skill level.
How to Decide Which Accuracy to Prioritize?
Functional & assembly needs:
- If parts fit together → focus on dimensional accuracy + GD&T. If not, tolerances can be relaxed.
- Moving or load-bearing parts → strict GD&T and surface finish.
- Sealing parts → high flatness and surface finish.
- Batch production → repeatability is key for consistency.
- One-off/small runs → positioning accuracy and operator skill matter most.
Rule of thumb: Specify only the tolerances needed for functionality. Unnecessary high accuracy increases cost and time.
Practical Impact of Each Accuracy Type
Positioning accuracy: Affects absolute location of features (e.g., hole patterns).
Repeatability: Determines process stability and batch consistency.
Dimensional accuracy: Decides if parts fit and assemble correctly.
GD&T (geometric accuracy): Ensures smooth function and long service life.
Surface roughness: Influences friction, sealing, fatigue strength, corrosion, aesthetics.
Final Recommendation
When preparing drawings, always use GD&T standards to clearly indicate required accuracies. Avoid vague notes like “precision machining.” Clear tolerances communicate design intent, help manufacturers choose the right processes, and control costs effectively.