How to detect the thread quality of self-locking tuning screw?
The core of detecting the thread quality of self-locking tuning screws is "full coverage, accuracy and quantification", which should be carried out in the order of "appearance → dimensional accuracy → meshing performance → functional reliability", combined with special tools and standard procedures. The specific methods are as follows:
1. Appearance quality inspection: check structural defects.
Visual inspection+magnifying glass inspection (basic screening)
Tools: naked eye+10-20 times magnifying glass (or microscope).
Test contents: the thread tooth type has no broken teeth, missing teeth or broken teeth, and there are no scratches, pits and scales on the tooth side; Self-locking structure (nylon ring, embossed teeth) has no shedding, cracking and lodging; There are no burrs and steps in the transition section at the end of the thread.
Criteria: No visible structural defects, and slight scratches will not affect the integrity of tooth profile (depth ≤0.01mm).
Surface treatment quality inspection
Tools: 100-gauge knife, adhesive tape, coating thickness gauge.
Test contents: Measure the coating thickness of the thread surface (such as 8-12μm galvanized) with a tolerance of 1 μ m with a coating thickness gauge; After being marked with a grid knife (1mm×1mm), it is torn by adhesive tape, and no coating falls off (grid test ≥4B level); No missing plating, bubbles and peeling.
Second, dimensional accuracy detection: quantification of core indicators
Diameter detection (the most critical indicator)
Tools: thread micrometer, three-pin measurement method (precision detection), smooth limit gauge.
Detection method:
Thread micrometer directly measures the thread pitch diameter, and the reading value is compared with the standard pitch diameter (for example, M3 pitch diameter is 2.650mm), and the error should be within the range of IT6-IT8 (±0.015~±0.025mm).
Three-needle measurement method: Select measuring needles with corresponding specifications (such as 0.577mm measuring needle for M3 thread), measure the distance between measuring needles with micrometer, and calculate the actual pitch diameter to ensure that the difference between single pitch diameter and virtual pitch diameter is ≤ 0.015mm..
Pitch detection
Tools: pitch gauge, tool microscope (batch/precision inspection).
Detection method: use the pitch gauge with corresponding pitch to fit the thread, and if there is no gap, the pitch is qualified; The pitch error measured by tool microscope is ≤±0.01mm per 25mm length, and the total cumulative error is ≤ 0.03 mm..
Large diameter/small diameter detection
Tools: outside micrometer (large diameter), inside micrometer (small diameter), limit plug gauge.
Inspection standard: the tolerance of large diameter is IT8-IT10, and the tolerance of small diameter is IT7-IT9. When the limit gauge is used for inspection, the gauge can pass smoothly and the gauge can't pass.
Third, form and position tolerance detection: ensure assembly adaptation
Thread coaxiality detection
Tools: deflectometer, dial indicator.
Detection method: jack up and fix both ends of the screw, touch the thread surface with the probe of the dial indicator, rotate the screw once, and read the runout value. The coaxiality error is ≤ 0.02 mm..
Tooth profile angle detection
Tools: tool microscope, tooth angle template.
Detection standard: the error of tooth profile angle (e.g. 60) is ≤ 1, and the arc radii of tooth top and tooth bottom meet the design requirements.
4. Detection of engagement and functional reliability: to verify the actual performance.
Meshing smoothness detection
Tools: standard threaded holes/nuts and torque wrenches.
Detection method: screw the screw into the matching threaded hole manually, without jamming and obvious resistance fluctuation; Measure the screwing torque with a torque wrench, and the fluctuation of the same batch is ≤ 15%.
Self-locking torque detection
Tools: Torsion meter (accuracy ±0.1N・cm).
Detection method: screw the screw into the matching threaded hole to the specified pre-tightening force, and measure the minimum torque (self-locking torque) when unlocking, which needs to be within the set range (for example, M3 screw 5 ~ 15 ncm), and the fluctuation of the same batch is ≤ 20%.
Wear resistance and anti-loosening detection
Wear-resistance test: use special equipment to simulate the tuning action (reciprocating screwing/screwing ≥1000 times). After testing, the thread wear is ≤0.01mm, and the meshing is still smooth.
Anti-loosening test: Put the locked screw into a vibrating table (10~50Hz, 30min), and there is no looseness after vibration, and the thread clearance is not obviously increased.
Five, batch inspection and sampling rules
Sampling standard: according to GB/T 2828.1, general inspection level II, AQL value (qualified quality level): dimensional accuracy AQL 1.5, appearance defect AQL 2.5, and functional defect AQL 0.65.
Batch screening: use thread check gauge to quickly screen unqualified products, and randomly check 10% (not less than 20 pieces) of key dimensions (pitch and pitch) in each batch. If unqualified products are found, double sampling will be conducted, and if they are still unqualified, they will be fully inspected.
VI. Summary of Common Testing Tools
Accuracy requirements of common tools for testing projects
Appearance defects are 10-20 times larger. Magnifier and microscope can identify defects ≥ 0.01 mm.
The measuring accuracy of micrometer, three-pin and gauge with medium diameter/large diameter/small diameter thread is 0.001 mm.
The measuring accuracy of pitch gauge and tool microscope is 0.005 mm.
Self-locking torque high-precision torque meter accuracy ±0.1N・cm
Coating quality The coating thickness measurement accuracy of coating thickness gauge, grating knife and adhesive tape is ±0.1μm m.
