Analyze the Technical Requirements for Flat Sealing Surfaces
Taking the example of a 6", 2500LB coal chemical prototype, the sealing surfaces of the valve body and seat parts are made of STL21 and NI-WC, respectively, with a hardness of ≥45HRC. The requirements specify a surface finish of Ra0.1 and a flatness of 0.01. Particularly challenging is the valve body's flat sealing surface, located at the bottom of an inner hole, presenting the following main technical challenges:
- High Flatness Requirement: Utilizing turning operations under ideal cutting conditions can maintain a precision of about 0.02 (grinding is not possible due to the location of the sealing surface).
- High Roughness Requirement: Conventional cutting methods achieve a roughness of about Ra0.8, which does not meet the requirement of Ra0.1.
Ultra-Precision Lapping Processing
By analyzing the factors influencing each stage of the ultra-precision lapping process, based on traditional lapping techniques, we have found several factors influencing the ultra-precision lapping process.
Abrasive Agent
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Abrasive Material: Boron carbide (B4C), one of the three hardest known materials, is characterized by its high hardness, high wear resistance, and relatively low brittleness. It retains certain sharpness after fracturing, making it the preferred abrasive for hard materials, especially for NI-WC sealing surfaces with hardness ≥63HRC.
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Lapping Machine: Wet lapping places high demands on the composition of the liquid abrasive agent, which primarily ensures the even suspension of the abrasive in the agent, providing dilution, lubrication, and cooling. Ingots oil offers excellent cooling properties, rust and corrosion protection, while paraffin provides the necessary viscosity to keep the abrasive suspended in the liquid. A suitable mix of ingot oil, paraffin, and abrasive, adjusted according to environmental temperature differences, meets the needs of ultra-precision lapping.
Lapping Tools
Ductile iron, with its spheroidal graphite structure, enhances plasticity, toughness, and wear resistance compared to other cast irons, making it suitable for ultra-precision lapping tools. However, it must be free from casting defects such as shrinkage cavities, porosity, and inclusions, with hardness controlled between 170HB~220HB.
Following the principle of replicating the tool's surface quality on the workpiece, when the tool's surface is larger than the workpiece's, the accuracy of the workpiece's shape largely depends on the tool itself. Considering the use of platforms or plates as lapping tools, a flatness requirement of 0.01 is met by employing three precisely ground platforms in a mutual lapping method to achieve true flat surfaces.
When the workpiece is stationary, and the tool moves, the tool's structural design should prioritize rigidity, ease of handling, lightweight, and symmetry to ensure stable motion. The tool's surface should include at least four grooves to store excess abrasive, preventing accumulation that could affect processing accuracy, and to hold the chips produced during lapping, preventing scratches on the workpiece surface. These grooves also enhance cutting capacity and cooling during processing.
Lapping Pressure, Speed and Time
- Lapping Pressure: Lapping pressure is a crucial parameter affecting the lapping process. The force applied to the lapping tool acts through the abrasive particles on the surface being lapped. The lapping pressure influences the choice of abrasive concentration within the agent.
- Lapping Speed: Lapping speed is an important technical parameter that controls the rate of material removal and the quality of the lapped surface. For obtaining a specified surface roughness, the lapping time required may be greater than the time needed for material removal, necessitating an appropriate reduction in lapping speed.
- Lapping Time: Usually, the sealing surface is pre-machined to a certain precision before lapping. During the initial stage of lapping, due to the sharpness of the abrasive particles, geometric shape errors and roughness of the part being lapped are quickly corrected. As lapping time increases, the abrasive particles become blunt, reducing the micro-cutting action, not only preventing any improvement in machining accuracy but also causing a decrease in surface quality due to increased heat. Thus, coarse lapping is conducted under specified process parameters with coarser abrasives, higher speeds, and pressures to quickly eliminate geometric shape errors and remove larger amounts of material, typically completing the rough lapping phase. Fine lapping time is generally about 5min~10min, with diminishing returns beyond 10min.
Requirements for Workpiece Lapping
To improve the quality and efficiency of ultra-precision lapping, define the workpiece's planar geometric tolerance, roughness, and lapping allowance before lapping:
- Geometric Tolerance: Ensure flatness within a 0.02 range. Ultra-precision lapping primarily improves surface roughness, with auxiliary correction for geometric tolerance.
- Roughness: Control roughness within Ra0.8. Higher roughness increases the lapping volume and efficiency but reduces the lapping tool's lifecycle and does not guarantee the quality of the lapped sealing surface.
- Lapping Allowance: Since lapping almost involves no cutting action, controlling the lapping allowance is crucial, typically maintained between 0.02mm~0.04mm.
Key Points in the Lapping Process
- Clean the part's surface and the plate before lapping, apply the lapping agent evenly on the part's surface to be repaired, and place it on the lapping plate.
- During lapping, lightly press the part with your hand to create cross-hatched marks (no need when your machine has platen spindle installed), improving the surface roughness grade. After lapping for a while, rotate the part to a certain angle and continue lapping. Add lapping agent timely and in appropriate amounts to ensure continuous flow.
Conclusion
By analyzing each step of the ultra-precision lapping process and combining it with traditional lapping techniques, we have developed a synergistic process that meets the needs of ultra-precision flat lapping, ultimately achieving the technical requirements of ultra-precision sealing surfaces. This provides important technical assurance for the development of new products in coal chemical engineering and lays a solid foundation for the company to undertake orders under stringent working conditions.
