I. Determining Acceptance Standards Based on Application Scenarios
1. Hydraulic/Pneumatic Piston Rods: Ra ≤ 0.4μm is required to ensure smooth sliding of the sealing ring and reduce the risk of leakage.
2. Automated Guide Shafts: Ra 0.2～0.8μm is acceptable, balancing operational stability and lubrication performance.
3. High-Precision Transmission or Mirror-Finish Roller Shafts: Ra ≤ 0.2μm, or even lower, is required to achieve ultra-precision fit.
4. Ordinary Support Structures: Ra ≤ 1.6μm meets general industrial needs.
✅ Most high-quality chrome-plated rods have a factory control standard of Ra 0.2–0.4μm, which is a standard used by manufacturers such as Kunyuan Machinery and Rizhao Steel Machinery.
II. Commonly Used Testing Methods and Operating Points
1. Profilometer Method (Most Authoritative): Uses a diamond stylus to slide along the surface, automatically calculating the Ra value.
Complies with national standard GB/T 3505, high accuracy, suitable for factory inspection and quality arbitration.
Operating suggestion: Select 3–5 cross-sections along the entire length, measure 2–3 paths at each point, and take the average value to eliminate local errors.
2. Comparison Method (Quick On-Site Judgment): Visually or tactilely compare the surface to be tested with a standard roughness sample.
No equipment required, suitable for initial workshop inspection, but relies on experience and should only be used as a reference.
3. Optical Section Microscopy Method: Utilizes the principle of light cutting to obtain a surface profile image, measuring Rz (ten-point height), not the Ra value.
Suitable for soft or fragile surfaces that cannot be touched.
4. Interference Method (Non-Contact High Precision): Based on the principle of light wave interference, it can detect nanoscale undulations and is used for optical-grade chrome-plated parts.
The equipment is expensive and mostly used in laboratories or high-end manufacturing environments.
5. Imprint Method (Indirect Measurement): Surface morphology is replicated using silicone or a low-melting-point alloy, and then the replica is measured.
Suitable for deep holes, internal cavities, and other areas difficult for instruments to access.
III. Suggested Practical Judgment Steps
1. Clarify Equipment Requirements: Consult equipment drawings or original manufacturer's instructions to confirm the required Ra value.
2. Choose an Appropriate Method: For factory acceptance, an electric profilometer is recommended; for on-site inspection, a combination of sample comparison and a portable roughness tester can be used.
3. Average Multiple Measurements: Avoid single-point misjudgments and improve the reliability of results.
4. Verify the Test Report: Reputable suppliers should provide a factory report containing the measuring instrument model, standard reference, and measured data.
✅ If "orange peel texture," "tool marks," or "pockmarks" are found, even if the Ra value meets the standard, it may affect sealing performance and lifespan; a comprehensive visual judgment is required.
