How to Regularly Inspect the Accuracy of CNC Lathes?

I. Recommended Inspection Cycle: Manage by Usage Intensity

High-load continuous production (more than 2 shifts per day): Conduct a comprehensive accuracy inspection every 3 months.

Medium-load production (single shift): Inspect every 6 months.

Low-frequency use or precision machining equipment: Must be inspected before each major machining task, and routinely checked quarterly.

After major overhaul or relocation/installation: A full accuracy re-inspection and calibration must be performed.

II. Core Inspection Items and Methods

1. Geometric Accuracy Inspection (Static Inspection)
Performed under no-power, no-load conditions, reflecting the manufacturing and assembly quality of the mechanical structure.

 Spindle Radial Runout and Axial Movement
Using a dial indicator with a high-precision inspection mandrel (such as a BT40/50 spindle mandrel), fixed inside the spindle taper hole, measure the radial runout near the spindle end face and 300mm from the end face, as well as the axial movement of the end face boss. Acceptable standard: ≤0.01mm for both.

Guide Rail Straightness and Parallelism: Measure the straightness and parallelism of the X and Z-axis guide rails segmentally using a precision level or laser interferometer, checking in both vertical and horizontal directions. The tolerance for ordinary machine tools is 0.02mm/1000mm; higher tolerances are required for precision equipment.

Turret Indexing Repeatability: Install a dial indicator near the tool post and measure the positional deviation after multiple indexings at the same tool position. The recommended tolerance in the X-axis direction is ≤0.005mm.

2. Positioning Accuracy and Repeatability Testing (Dynamic Testing) This reflects the actual motion capability under CNC system control and must be performed under no-load conditions.

Linear Motion Positioning Accuracy: Use a dual-frequency laser interferometer as the standard measuring tool, setting multiple measurement points (usually no less than 30 points) along the entire stroke of each coordinate axis, and recording the deviation between the actual position and the commanded value. Evaluate the results according to ISO 230-2:2014 standard.

Repeatability: Perform seven rapid positioning operations at three positions: the midpoint and both ends of the stroke. Measure half of the maximum difference between the stopping positions as the indicator. The smaller this value, the better the machining consistency.

Backlash (Loss of Motion) Detection: Measure the idle distance during axis reversal using a laser interferometer or dial indicator. Backlash compensation parameters can be set in the CNC system for correction.

3. Cutting Accuracy Verification (Dynamic Comprehensive Performance): Verify the machine tool's comprehensive accuracy under actual machining conditions by test-cutting a standard workpiece.

Test Cutting Content:

Precision turning of a φ50mm outer diameter, checking roundness (≤0.005mm) and cylindricity (≤0.03mm/300mm).

Precision turning of the end face, checking flatness (≤0.025mm, concave only allowed).

Thread turning, checking pitch error within any 50mm length (≤0.025mm).

Measurement Tools: Use standard measuring tools such as a micrometer, roundness tester, and coordinate measuring machine.

III. Commonly Used Testing Tools and Accuracy Requirements The accuracy class of the testing tools must be one level higher than that of the item being measured to ensure measurement reliability.

Laser Interferometer: Used for positioning accuracy and repeatability testing, with an accuracy of ±0.5ppm. It is currently the most authoritative measurement method.

Precision Level: Used to test guide rail tilt and straightness. Resolution should be ≤0.01mm/m.

Dial Indicator + Magnetic Base + Inspection Mandrel: Used for routine testing such as spindle runout and turret accuracy.

90° Square, Straight Edge, Height Block: Used in conjunction with a dial indicator to test geometric relationships such as perpendicularity and parallelism.

✅ Note: All measuring instruments should be regularly calibrated to ensure effective traceability of measurement values.

IV. Post-Inspection Handling Recommendations

If any accuracy deviation is found, the trend should be determined by combining historical data: is it a sudden change (such as a collision accident) or a gradual deterioration (such as guide rail wear)?

Repair can be achieved by adjusting the insert gap, updating the backlash compensation parameters, and recalibrating the pitch error.

It is recommended to establish an accuracy file to record data from each inspection, facilitating the tracking of changes in equipment health status.