Precise positioning during copper capillary assembly depends primarily on establishing clear and unified positioning datums, providing a reliable reference for subsequent assembly. Typically, these datums are used as the origin for positioning, either at the assembly's key functional interfaces (such as connections to valves and fittings) or on pre-defined datum surfaces. These datums are precision-machined during the copper capillary tube fabrication stage to ensure flatness and perpendicularity, preventing errors inherent in the datum from being transferred to the assembly process. For example, if the assembly connects to a valve seat in a refrigeration system, the valve seat's sealing surface is used as the datum surface. Dedicated tooling is used to ensure parallelism between the capillary tube port and the datum surface. Furthermore, the datum surface serves as a reference to determine the axial and radial position of the capillary tube in space, ensuring that fluid enters the valve seat along the predetermined path. This prevents misalignment of the port due to datum deviation, which could affect fluid flow.
The design and application of specialized assembly fixtures is a key tool for controlling the copper capillary assembly's posture and preventing misalignment during assembly. Based on the capillary's outer diameter, curvature (e.g., U-shaped, spiral), and assembly position requirements, a custom fixture with matching slots or locating pins is designed. The slot's inner diameter must precisely match the capillary's outer diameter to ensure secure grip while preventing deformation of the tube wall due to over-tightening. The locating pins correspond to the locating holes in the system's mounting position, ensuring the relative position of the fixture and the system's mounting base remains fixed, thereby constraining the capillary to a predetermined spatial trajectory.
Some fixtures also incorporate micro-adjustment mechanisms, such as adjustable set screws or sliders, to correct for minor misalignment of the capillary during assembly, ensuring perfect alignment between the port and the mating component. This is particularly useful when assembling multiple capillaries in parallel, ensuring that the fluid paths of each capillary are parallel and evenly spaced.
Pre-alignment of the component and system interface eliminates positional deviations before final assembly, ensuring precise positioning. Before installing the copper capillary assembly into the system, a pressure-free pre-assembly is performed. The capillary end is placed close to the mating interface (such as the inner bore of a connector). The coaxiality between the end and the interface is observed using an optical tool (such as a magnifying glass or microscope), or a dedicated pre-alignment pin is inserted into the positioning holes of both to gradually guide the capillary toward the correct position. If the capillary has a bend, the curvature of the bend must be carefully checked to ensure that it matches the system's reserved clearance to avoid interference with surrounding components and misalignment. For interfaces requiring welding or bonding, the pre-alignment stage also requires marking the fixing points to ensure that the capillary does not shift during final assembly due to welding heat or adhesive shrinkage during curing, thus maintaining the straightness of the fluid path or the intended curved path.
Real-time dimensional inspection and deviation correction during the assembly process are key to dynamically controlling positioning accuracy and preventing accumulated deviations. Use precision measurement tools (such as laser calipers and coordinate measuring machines) to monitor key capillary position parameters in real time, such as the port's coordinate position, distance from the reference surface, and bend radius. Compare the measured data with the designed values. If deviations are detected outside the allowable range, correct them immediately by adjusting the fixture or manually fine-tuning. For example, if radial misalignment is detected between the capillary port and the mating interface, use the radial adjustment knob on the fixture to push the capillary until the misalignment is eliminated. If the axial position is too deep or too shallow, adjust the fixture's axial positioning block to ensure the port insertion depth meets the designed requirements. This prevents excessive insertion, which could block the fluid channel, or shallow insertion, which could lead to poor sealing and fluid leakage, compromising path integrity.
A well-planned assembly sequence can prevent subsequent assembly steps from disrupting the position of the already positioned capillary assembly. In systems with multiple components, prioritize the positioning and securing of the copper capillary assembly before assembling other non-critical components. Installing larger or heavier components first can cause the capillary to be squeezed or impacted during installation, causing it to deviate from its intended position. For example, during the assembly of a refrigeration system's evaporator, the copper capillary tube connecting the evaporator is first precisely positioned and soldered securely before installing the fan, housing, and other components. This prevents mechanical impact on the capillary tube during subsequent assembly. When assembling multiple capillary tubes, the primary capillary tube, responsible for primary fluid transport, is positioned first, then used as a reference for positioning auxiliary capillaries. This ensures that the fluid paths of each capillary tube do not interfere with each other and align with the overall flow path of the system.
Controlling the elastic deformation of the copper capillary assembly is also crucial for ensuring precise positioning. Although copper possesses a certain degree of rigidity, the capillary tubes are small in diameter. If subjected to improper external forces (such as excessive pulling or squeezing) during assembly, they are prone to elastic deformation. This elastic deformation then rebounds after the external force is removed, resulting in positioning errors. Therefore, during assembly, flexible clamping tools (such as jaws wrapped in soft rubber) are required to reduce rigid compression on the capillary tube. When positioning curved capillary tubes, ensure that the clamp's support points are evenly distributed on the curved section to avoid localized excessive force that could alter the bend radius. When securing the capillary tube, appropriate fixing methods (such as multi-point clips or segmented bonding) should be selected to stabilize the capillary at multiple locations to prevent overall deformation caused by loosening at a single fixing point. This ensures that the curvature of the fluid path is consistent with the design and avoids localized throttling caused by deformation, which could affect fluid flow and pressure.
Post-assembly stability verification can further confirm whether positioning accuracy meets system fluid path requirements and prevent potential deviations. A no-load air or water flow test can be performed to observe the fluid flow within the capillary tube. If uneven flow or sudden pressure drops are observed, this may indicate channel blockage or localized narrowing due to positioning deviation, requiring disassembly and re-checking the positioning. For high-pressure systems, a pressure test should also be performed to observe leaks at the interface. Leaks often indicate poor sealing due to port positioning deviation, requiring readjustment and reinforcement. In addition, vibration tests are performed to simulate the vibration environment during system operation and check whether the capillary tube is displaced after vibration. This ensures that the positioning state is stable during long-term use and that the fluid path remains unobstructed, avoiding deviations caused by vibration that affect the normal operation of the system.
