First, what are the influencing factors on the surface roughness of the inner bore of precision steel tubes?
The formation of the surface roughness of the inner bore of precision steel tubes is the result of multiple factors, including the plastic deformation of the material during the cutting process, the cutting marks on the tool edge, chip residue, and friction. The core influencing factors can be summarized into four categories: 1) the matching degree of the process system, including the selection of cutting methods and the rationality of process connections; 2) the state of the tool system, covering the material of the tool/grinding wheel, the accuracy of the cutting edge, and the stability of installation; 3) the adaptability of processing parameters, such as cutting speed, feed rate, and cutting pressure; and 4) external environmental interference, including cooling and lubrication effects, clamping stability, and environmental vibration. Improving the surface roughness of the inner bore essentially involves precisely controlling the above factors to reduce microscopic protrusions and defects on the surface, achieving a smooth and uniform surface morphology.

Second, what are the optimization operations for the processing technology of precision steel tubes?
The scientific nature of the process system is the foundation for improving the surface roughness of the inner bore. It requires “process refinement, method adaptation, and path optimization” to achieve a precise progression from roughing to finishing, reducing the generation of surface defects.
(1) Process Refinement and Precision Progression A graded progressive process of “roughing – semi-finishing – finishing – ultra-finishing” is adopted to avoid surface texture residue caused by process jumps. The core objectives and roughness control requirements for each stage are: ① Roughing: Efficiently remove most of the excess material, controlling surface roughness Ra≤6.3μm, avoiding material structure damage due to excessive cutting; ② Semi-finishing: Correct surface defects from roughing, initially optimize surface flatness, controlling Ra≤1.6μm, laying the foundation for finishing; ③ Finishing: Focus on improving precision, selecting turning, grinding, or honing according to material and requirements, controlling Ra≤0.4μm; ④ Ultra-finishing: For high-end requirements, use polishing, grinding, and other processes to achieve Ra≤0.2μm or even lower.
Key emphasis should be placed on improving the precision of process connections: After semi-finishing, a uniform finishing allowance must be reserved. Uneven allowance will lead to fluctuations in cutting force and uneven surface texture. Before finishing, the inner hole must be thoroughly cleaned to remove burrs, chips, and oil stains to prevent impurities from scratching the surface during subsequent processing.
(2) Precise Adaptation of Core Machining Methods
Based on the steel pipe material, size specifications, and surface roughness requirements, a suitable inner hole machining method is selected to achieve a balance between cutting effect and surface quality.
(a) CNC Precision Turning: Suitable for precision steel pipes with medium to low precision requirements (Ra≤0.8μm) and uniform wall thickness. A cutting strategy of “high speed, small feed, and small depth of cut” is adopted to reduce the extrusion deformation of the surface by cutting force.
(b) Honing: Suitable for medium to high precision requirements (Ra≤0.4μm), especially suitable for machining the inner holes of slender steel pipes with a length-to-diameter ratio >4. A “graded honing” strategy is adopted: coarse honing removes residual material using 240-320# grit honing rods, controlling Ra ≤ 1.0μm; fine honing optimizes surface quality using 400-600# grit honing rods, controlling Ra ≤ 0.4μm; ultra-fine honing targets high-end needs, using 800# and above grit honing rods to achieve Ra ≤ 0.2μm.
(c) Grinding/Polishing: Suitable for special scenarios with high precision requirements (Ra ≤ 0.2μm). Grinding uses cast iron grinding rods with grinding paste, removing microscopic protrusions on the surface through low-speed grinding; polishing uses diamond polishing paste or chromium oxide polishing paste with soft polishing tools to achieve a mirror-like surface effect. This method is less efficient and is mainly used for high-end precision component processing.
(3) Clamping and Cutting Path Optimization
Unstable clamping can easily lead to machining vibration, causing an increase in surface roughness. A clamping method of “flexible positioning + uniform force” should be adopted: For thick-walled steel pipes, an elastic expansion sleeve clamp should be used, with the clamping force controlled at 0.3-0.5 MPa to avoid roundness errors caused by a three-jaw chuck; for thin-walled steel pipes, a soft-jaw clamp with copper pads should be used to disperse the clamping force, controlling the clamping force to ≤0.3 MPa, while adding auxiliary supports to suppress vibration. The clamping positioning datum must be precision-ground, with a flatness of ≤0.005 mm, to ensure accurate axial and radial positioning.
Key points of cutting path optimization: For internal hole machining, a “from the inside out” cutting direction should be used to reduce chip accumulation in the hole; for deep hole machining, an intermittent retraction method should be used to promptly remove chips and avoid scratches caused by chip friction with the machined surface; complex internal holes (including steps and grooves) need to have special features machined before finishing to avoid subsequent machining interfering with surface quality.

Third, what are the selection criteria for tool system adaptation for precision steel pipes?
The tool system is the core carrier affecting the surface roughness of the inner hole. Its material selection, cutting-edge accuracy, and installation status directly determine the fineness of the cutting marks, requiring a precise match between the tool and the material and the accuracy requirements.
(1) Scientific Selection of Tool/Grinding
Wheel Materials Select suitable tool materials based on the characteristics of the steel pipe material to reduce surface defects caused by problems such as chip adhesion and wear:
(a) Ordinary Steel: For precision turning, use TiAlN-coated carbide tools. The coating has excellent lubricity and wear resistance, reducing chip adhesion; for honing, use CBN honing rods, which have high hardness and uniform wear, suitable for batch processing.
(b) Stainless Steel: Due to its poor thermal conductivity and strong adhesion, for precision turning, use PCD diamond tools to avoid surface roughening caused by chip adhesion; for honing, use diamond honing rods to improve cutting efficiency and surface quality.
(c) High-temperature alloy and hard chrome-plated steel pipes: Use diamond or cubic boron nitride tools to ensure sharp cutting edges and avoid surface roughness increases due to tool wear.
(2) Cutting Edge Accuracy and Condition Control
The sharpness and smoothness of the tool/grinding wheel cutting edge directly affect the cutting marks: Finishing tools require blunting treatment, with the cutting edge radius controlled at 0.01-0.02mm to avoid burrs or tearing of material from the sharp edge; honing bars need to be pre-ground before use to ensure good surface contact with the inner hole, and after pre-grinding, lightly polish the cutting edge with 400# sandpaper to remove minor burrs. A tool wear control mechanism needs to be established during batch processing: Finishing tools should be inspected every 50-100 pieces processed, and repaired or replaced promptly when the flank wear exceeds 0.2mm; honing bars should be inspected every 100-200 pieces processed, and replaced when the wear exceeds 0.3mm to avoid rough surface texture caused by tool wear.
(3) Tool Installation and Coaxiality Assurance
Insufficient tool installation precision can lead to cutting trajectory deviation, causing surface ripples or scratches. After installing the finishing tool, use a dial indicator to check the radial runout, ensuring it is ≤0.005mm. After installing the honing head, coaxiality must be calibrated, with radial runout controlled within 0.005mm, and the expansion amount adjusted to ensure uniform force distribution. For machining the inner hole of slender steel pipes with a length-to-diameter ratio >5, a guide sleeve device should be added, with the gap between the guide sleeve and the inner hole controlled at 0.01-0.02mm to suppress tool wobble.

Fourth, what are the steps to enhance the auxiliary technologies for precision steel pipe machining?
Through auxiliary technologies such as cooling and lubrication optimization and surface modification treatment, friction and defects during machining can be effectively reduced, further improving the surface roughness of the inner hole and enhancing surface properties.
(1) Cooling and Lubrication System Optimization
Insufficient cooling and lubrication can easily lead to increased cutting temperature, chip adhesion, and exacerbated surface roughness. The cooling and lubrication scheme needs to be optimized according to the machining method and material:
(a) Finish Turning: Use a high-lubricity extreme-pressure emulsion and employ an internally cooled tool to directly spray the cutting fluid onto the cutting area, ensuring timely cooling and sufficient lubrication. For difficult-to-machine materials such as stainless steel, special lubricant additives can be added to improve lubrication.
(b) Honing: Use a high-cleanliness emulsion and achieve precise cooling and chip removal through a high-pressure cooling system. The cooling flow rate should be adjusted to 25-30 L/min according to the inner diameter. Regularly filter the cutting fluid with a filtration accuracy of ≤5μm to remove impurities and prevent scratching of the inner hole surface.
(c) Routine Maintenance: Regularly test the pH and viscosity of the cutting fluid to prevent deterioration from affecting lubrication performance; replenish the cutting fluid promptly to ensure a stable fluid level and avoid surface defects caused by insufficient cooling.
(2) Pre-treatment Enhancement Before Machining
The quality of pre-treatment of steel pipe blanks directly affects the surface roughness of subsequent machining:
① Dimensional and Defect Inspection: Ultrasonic flaw detectors are used to check for internal defects in the blanks, and micrometers are used to check the uniformity of wall thickness, rejecting blanks that exceed tolerances or have defects.
② Heat Treatment Optimization: Annealing, normalizing, or solution treatment is performed according to the material to eliminate internal stress, reduce hardness, improve machinability, and avoid surface deformation due to stress release during machining.
③ Surface Cleaning: Surface oxide scale is removed by sandblasting, oil stains are cleaned with industrial alcohol, and rust-preventive oil is applied after drying to prevent impurities from affecting clamping and cutting quality.
(3) Post-Machining Surface Modification Treatment
For precision steel pipes requiring high precision, surface modification treatment can be implemented after machining to improve surface roughness while enhancing wear resistance and corrosion resistance:
① Phosphating treatment: Forms a 5-8μm phosphate film, improving surface lubricity and wear resistance, suitable for agricultural machinery and hydraulic system bushings;
② Polishing treatment: Using precision internal hole polishing tools with W1-W3 grade polishing paste, low-speed polishing for 1-3 minutes achieves a mirror finish with Ra≤0.2μm;
③ Electroplating treatment: Plating with hard chromium or nickel-phosphorus alloy, with a plating thickness of 0.01-0.03mm, reducing surface roughness to Ra≤0.1μm, while enhancing wear resistance and corrosion resistance.

Fifth. Process Control and Anomaly Handling of Precision Steel Pipes
Establish a full-process quality control system to promptly identify and resolve surface roughness issues during machining, ensuring stable quality in mass production.
(1) Standardized Inspection Process
A closed-loop inspection process is implemented: “Trial grinding – First piece inspection – Batch sampling inspection – Finished product re-inspection”:
① Trial grinding stage: After processing 1-2 pieces, surface roughness is checked with a roughness tester, and inner hole roundness is checked with a roundness tester. Batch processing begins after confirmation of compliance.
② Batch sampling inspection: Sampling is conducted every 10-20 pieces processed, focusing on surface roughness and defects. The sampling rate is no less than 5%.
③ Finished product re-inspection: Finished products must be visually inspected with a magnifying glass for surface scratches, burrs, and other defects. A comprehensive roughness test is used to ensure compliance with requirements.
(2) Solutions to Common Surface Quality Problems
1. Surface ripples: Often caused by tool vibration, excessive speed, or poor guidance. Solutions: Calibrate tool coaxiality and reduce machining speed; add guide sleeves and reinforce tooling to suppress vibration; check tool wear and replace tools promptly.
2. Surface roughness/scratches: Originate from residual chips, excessive impurities in cutting fluid, or burrs on the tool edge. Solutions: 1. Optimize chip removal path and increase intermittent tool retraction frequency; replace cutting fluid and clean the filter system; re-sharpen tool edges and remove burrs.
3. Unstable surface roughness: This is often caused by uneven tool wear, parameter fluctuations, or changes in clamping force. Solutions: Establish a tool wear log and replace tools regularly; lock CNC system parameters to avoid misadjustment; check tooling fixtures to ensure uniform and stable clamping force.
(3) Environmental and Equipment Status Management The machining environment and equipment status have a significant impact on surface roughness:
① Environmental control: Maintain a constant temperature in the machining area to avoid thermal deformation of the workpiece and equipment due to temperature changes; control workshop humidity to prevent workpiece corrosion.
② Equipment maintenance: Regularly calibrate machine tool accuracy and check spindle radial runout and guide rail positioning accuracy; perform vibration damping treatment on the equipment foundation to avoid vibration transmission from surrounding equipment and ensure machining stability.

Sixth, Conclusion
Improving the surface roughness of the inner bore of precision steel pipes is a systematic project that requires focusing on four core aspects: “process optimization, tool adaptation, auxiliary strengthening, and process control.” This involves achieving refined management throughout the entire process, from blank pretreatment to finished product inspection. In actual production, it is necessary to dynamically optimize processing techniques and parameters based on the steel pipe material, dimensions, and application scenarios’ precision requirements, balancing processing accuracy, efficiency, and cost. By scientifically applying technologies such as graded processing, precise tool matching, and efficient cooling and lubrication, medium-to-high precision requirements with Ra≤0.4μm can be stably achieved. For high-end demands, further combining polishing and surface modification technologies can achieve a mirror finish with Ra≤0.2μm, providing core component assurance for the reliable operation of various precision equipment.