What should be paid attention to in the processing of precision self-locking tuning screws?
As a high-precision fastener (widely used in communication, electronics, aerospace and other fields, it needs to meet the three core requirements of "precise size", "self-locking function" and "tuning stability" at the same time), its processing process requires extremely high process control, precision guarantee and material performance protection. The following are five key links: material pretreatment, core process control, precision detection, surface treatment and assembly adaptation, and the matters needing attention are sorted out in detail:
First, material pretreatment: to ensure the stable performance of the base material and avoid the defects in subsequent processing.
The materials of precision self-locking tuning screws are mostly high-strength alloys (such as stainless steel 304/316, titanium alloy TC4, copper alloy H62) or special engineering plastics (such as PEEK). The material pretreatment directly affects the strength, corrosion resistance and processing stability of the final product. Please pay attention to:
Material composition and state verification
Spectral analysis and hardness test (such as Rockwell Hardness HRC and Vickers Hardness HV) are required to confirm that the material composition meets the design standards (such as ensuring that the content of Cr and Ni in stainless steel reaches the standard to ensure corrosion resistance), so as to avoid "sticking to the knife" and "cracking" caused by material impurities (such as sulfur and phosphorus) during processing.
The heat treatment state (such as annealing, quenching and tempering) of metal materials should be confirmed: the hardness of annealed materials should be uniform (deviation ≤2HV) to avoid dimensional fluctuation caused by uneven hardness during processing; The internal stress of quenched and tempered materials should be controlled to prevent deformation after processing.
Material pretreatment process control
Metal materials need to undergo "stress relief annealing" (temperature 500-650℃, heat preservation for 2-4 hours) to eliminate internal stress generated during rolling/forging and avoid bending and cracking after subsequent cutting and thread processing.
Bar cutting (such as sawing and wire cutting) needs to control "notch flatness" (≤0.01mm) and "verticality" (≤0.005mm/m) to ensure that the datum plane of subsequent processing is flat and avoid eccentricity.
Second, the core process control: focus on "dimensional accuracy", "self-locking function" and "tuning smoothness"
The core processes of precision self-locking tuning screw include turning (contour machining), thread machining (self-locking structure) and groove/hole machining (tuning drive), and the errors in each link may lead to "self-locking failure" and "tuning jam", which need to be controlled emphatically:
1. Turning: Ensure the overall dimension and coaxiality.
When turning the head, stem and steps of the screw, the key points for attention are:
Benchmark selection and clamping
Adopt "double-center clamping" or "hydraulic chuck+tailstock thimble" to reduce clamping deformation (the clamping force should be uniform to avoid plastic deformation of metal materials due to excessive clamping force, resulting in out-of-tolerance of rod roundness).
Take the "center hole" of the bar as the benchmark (the center hole needs to be ground, and the roughness Ra is less than or equal to 0.8 μ m), so as to ensure the coaxiality of the head and the rod (less than or equal to 0.003 mm)-out-of-tolerance coaxiality will cause the screw to tilt during tuning and affect the self-locking stability.
Size and surface roughness control
Key dimensions (such as rod diameter and step height) shall be "gradually cut" (rough turning → semi-finish turning → finish turning), and diamond tools or cemented carbide coated tools (such as TiAlN coating) shall be selected for finish turning, and the cutting speed shall be controlled at 80-150m/min (adjusted according to the material) to avoid thermal deformation of the dimensions due to excessive cutting heat.
The surface roughness should reach the standard (Ra≤0.4μm in the rod and Ra≤0.8μm in the head). Too high roughness will increase the friction during tuning and lead to the decline of tuning accuracy.
2. Thread processing: ensure self-locking performance and matching accuracy.
The threads of self-locking tuning screws are mostly "fine thread" (such as M3×0.5 and M4×0.7) or "special self-locking thread" (such as triangular locking thread and serrated thread). Please pay attention to the following when machining:
Precise control of thread parameters
The tolerance of pitch diameter, top diameter and bottom diameter of thread should strictly comply with the design requirements (usually IT5-IT7 grade). For example, the tolerance of pitch diameter of M3×0.5 thread is 0.010mm, which should be detected in real time by thread micrometer and three-pin measurement method to avoid insufficient self-locking force caused by too large pitch diameter or assembly jamming caused by too small pitch diameter.
Pitch error should be ≤0.005mm/100mm. Excessive pitch deviation will lead to "inconsistent displacement per turn" during tuning, which will affect the tuning accuracy (if the designed tuning accuracy is 0.001mm/turn, the pitch error will be directly superimposed on the tuning error).
Machining details of self-locking structure
If it is a "crushed self-locking thread" (self-locking is realized through local deformation of the thread), the size of the "crushed area" should be controlled (for example, the crushing height is 1/3-1/2 of the thread height), and a special molding tool should be used during machining to avoid thread breakage caused by excessive crushing or self-locking failure caused by insufficient crushing.
If it is a "coated self-locking thread" (e.g. coated with PTFE coating), it should be coated after the thread processing is completed, and the coating thickness should be uniform (5-15μm) to avoid that the coating is too thick to affect the thread fit or too thin to shorten the self-locking life.
3. Slot/hole machining: ensure the smoothness of tuning drive.
Slots (such as straight slots, cross slots and hexagon slots) or holes (such as positioning holes and weight-reducing holes) at the head of the screw are the driving parts during tuning, so attention should be paid to processing:
Position accuracy of groove/hole
The "coaxiality" between the center of the groove/hole and the screw axis should be ≤0.005mm, and the "symmetry" should be ≤0.003mm (for example, the symmetry of the two grooves of the cross groove). The position deviation will lead to uneven stress on the tuning tools (such as screwdriver and Allen wrench), resulting in "slipping" or "stuck".
Size and edge quality of groove/hole
The tolerance of groove width and groove depth should be controlled within ±0.005mm (for example, the dimension tolerance of the opposite side of the hexagonal groove is ±0.003mm), so as to avoid the tool from being unable to adapt due to dimension deviation.
The edge of the groove/hole needs to be rounded (R0.05-R0.1mm) or deburred (burr height ≤0.002mm). Sharp edges will scratch the tuning tool or cause debris to remain, which will affect the tuning stability.
Third, the accuracy test: control the whole process to avoid the unqualified products flowing into the downstream.
The precision of precision self-locking tuning screws is extremely high (the dimensional tolerance is mostly μm), so it is necessary to establish the whole process inspection system of "inter-process inspection+final inspection". The key points for attention are as follows:
Inter-process inspection: timely intercept defects.
Inspection after turning: focus on the roundness (≤0.002mm), coaxiality and surface roughness of the rod, and use laser caliper and roundness meter for automatic inspection (higher efficiency and smaller error).
Inspection after thread processing: In addition to measuring the pitch diameter with three needles, the "thread gauge" (general gauge and stop gauge) should be used to verify the compatibility-the general gauge should be able to screw in smoothly, and the screw-in depth of the stop gauge should be ≤2 teeth (to avoid being too loose or too tight); At the same time, observe whether the thread tooth profile is complete (no tooth collapse or missing angle) with optical microscope.
Inspection of groove/hole after machining: use an image measuring instrument (accuracy 0.001mm) to inspect the position, size and symmetry of groove/hole to ensure that it meets the design drawings.
Final inspection: verify the overall performance.
Full inspection of dimensions: all key dimensions (such as total length, rod diameter, thread middle diameter and groove width) are 100% inspected, and three-dimensional dimension scanning is realized by coordinate measuring machine (CMM, accuracy 0.0005mm) to avoid missing hidden deviation.
Self-locking performance test: Apply "pre-tightening torque" (for example, the pre-tightening torque of M3 screw is 0.5-0.8 nm) through a torque tester, and then measure "loosening torque" (80% or more of the pre-tightening torque) to ensure that the self-locking force reaches the standard; At the same time, carry out "fatigue test" (such as 1000 tuning cycles) to detect whether the self-locking force is attenuated.
Tuning accuracy test: install the screw in the simulation fixture, drive it with a precision motor (step angle 0.9), and measure the displacement of each turn with a laser displacement sensor (accuracy 0.1μm), and the error shall be less than or equal to 5% of the design value (if the design is 0.001mm/turn, the actual error shall be less than or equal to ≤0.00005mm).
Fourth, surface treatment: give consideration to corrosion resistance, lubricity and appearance.
Surface treatment not only affects the service life of the screw, but also may affect the tuning smoothness. Please pay attention to:
Coating Selection and Thickness Control
Corrosion resistance requirements: stainless steel can be passivated (chromate passivation, thickness 1-3μm), titanium alloy can be anodized (thickness 5-10μm), and copper alloy can be nickel plated (thickness 3-5 μ m)-the coating thickness should be uniform, so as to avoid out-of-tolerance caused by local thickness (for example, too thick thread coating will affect the fit).
Lubrication requirements: If low friction is required during tuning, a "solid lubrication coating" (such as MoS₂ coating with a thickness of 2-5μm) or a "dry lubricant" (such as PTFE spray) can be applied, but it is necessary to ensure that the coating does not fall off (through the adhesion test, such as the grid test ≥4B level).
Surface quality control
After coating, the "appearance" (no pinholes, bubbles and scratches) and "corrosion resistance" (such as neutral salt spray test, stainless steel should be rustless for ≥48 hours and titanium alloy should be rustless for ≥100 hours) should be tested.
Avoid "hydrogen embrittlement" during surface treatment (such as "dehydrogenating treatment" after electroplating at 180-220℃ for 2-4 hours). Hydrogen embrittlement will cause the screw to break under stress, especially for high-strength alloy materials (such as stainless steel with tensile strength ≥800MPa).
V. Assembly adaptation: ensure compatibility with downstream components.
Precision self-locking tuning screws need to be used together with the base, nuts and other parts, and "assembly clearance" and "functional adaptation" should be considered when machining:
Fit clearance control
The fit with the threaded hole of the base should be "transition fit" or "small clearance fit" (such as H6/g5 fit). Too large clearance will lead to shaking during tuning, while too small clearance will lead to difficult assembly. During machining, the adjustment screw thread pitch diameter should be measured according to the actual size of the threaded hole of the base (samples should be obtained in advance) to ensure that the fit clearance is within the range of 0.002-0.005 mm.
Tuning stroke adaptation
The "effective thread length" of the screw should be matched with the tuning stroke (for example, the tuning stroke is 10mm, the effective thread length should be ≥12mm, and a safety margin of 2mm should be reserved), so as to avoid the tuning "top dead" caused by insufficient effective length or interference caused by excessive length during machining.
Cleanliness control
After machining, it is necessary to carry out "precise cleaning" (such as ultrasonic cleaning, using ethanol or isopropanol solvent) to remove machining debris and cutting fluid residues (residues will lead to tuning jam or corrosion), and the cleanliness should reach "ISO 16232-10" (particle size ≤5μm, quantity ≤10 pieces/piece).
summary
The machining core of precision self-locking tuning screw is "controllable precision, reliable function and stable performance", which needs to be strictly controlled from the whole chain of materials, processes, detection, surface treatment and assembly. Any slight deviation (such as μm size error and local burr) may lead to product failure. Therefore, it is necessary to combine "automatic equipment (such as CNC lathe and five-axis machining center)", "high-precision testing instruments (such as CMM and laser caliper)" and "standardized process SOP" in the process of machining, and strengthen the skill training of operators to ensure that the products can meet the needs of high-end fields.
