How to test used CMM

Author:J           Date:2025-09-26

1. Pre-Test Preparation: Clarify Standards and Conditions

Agreed Acceptance Criteria: The buyer and seller must agree in writing in advance on the standards to be used for testing, most commonly the ISO 10360 series. This will serve as the sole basis for determining equipment compliance, avoiding disputes.

Check Environmental Conditions: Ensure the testing environment meets the used CMM's operating requirements. Key requirements include constant temperature and humidity (typically 20 ± 1°C with acceptable humidity), a stable foundation, clean and dry air supply, and a stable power supply. If the environment does not meet these standards, any accuracy testing will be invalid.

Prepare Standards: According to ISO 10360, the following gauges or standards are required:

Step Gauges or Gauge Blocks: Used to check length measurement error (EL, MPEE).

Standard Balls: Used to check probing error (P, MPEP) and scanning performance (if any).

2. Step-by-Step Testing Process: From the outside in, from static to dynamic

Appearance and Mechanical Inspection:

Visual Inspection: Carefully inspect the guide rails and work surface for scratches, rust, and signs of impact. Check the air bearing for wear.

Manual Test: Manually move each axis to feel its smoothness. There should be no sticking, unusual noise, or noticeable "sucking." Check that the air bearing air pressure is stable.

Verification of Basic Control System and Software Functionality:

Power-On and Home Return: Power on the system and observe that each axis returns to home smoothly without any alarms.

Software Operation: Open the measurement software and verify that all authorized modules are functioning properly. Create basic elements (such as planes, circles, and cylinders) to verify software responsiveness and the absence of freezes.

Core Accuracy Performance Test (according to ISO 10360): This is the core of the test and is recommended to be performed by a professional engineer.

Probing Error Test:

Secure a standard sphere on the workbench.

Use the probe to probe at least 25 points at different locations on the sphere (avoiding the extreme points).

The software calculates a fitted sphere. The maximum radial deviation is the probing error (P or MPEP value). This value should be better than the standard tolerance.

Length Measurement Error Test:

Place the step gauge at multiple spatial orientations and angles (e.g., parallel to the X, Y, and Z axes, and along diagonals).

Measure each known length on the step gauge.

Calculate the difference between each measured value and the reference value. The maximum difference is the length measurement error (EL or MPEE). This value is the most critical indicator of CMM accuracy.

Repeatability and Scanning Performance Test (optional but important):

Repeatability: Measure the same feature (e.g., a hole) multiple times and calculate the standard deviation of its position or diameter. Lower values ​​indicate better repeatability.

Scanning Test (if using a scanning probe): Scan a standard sphere at high speed, observing the smoothness of the scan path and the software's ability to process data, checking for any abnormal fluctuations.

III. Final Verification and Document Verification

Physical Verification: Measure a complex workpiece with known precise dimensions (e.g., a gear or impeller). Compare the results with an authoritative measurement report or results from higher-precision equipment. This is the most intuitive verification.

Document review: Request and verify the latest accuracy calibration report to confirm whether the test results match the reported data. Also check whether the equipment manual, software authorization certificate, etc. are complete.

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