Fishplate bolt hole machining accuracy requirements
- What is the diameter tolerance of fishplate bolt holes?
The diameter of fishplate holes corresponding to M24 bolts is 25±0.2mm, and for M22 bolts, it is 23±0.2mm, with the tolerance range strictly controlled within ±0.2mm. Excessively large holes (over 0.3mm above the upper limit) will cause excessive clearance between bolts and holes, bending bolts under stress; excessively small holes (over 0.3mm below the lower limit) will prevent bolt installation, and forced assembly will damage threads. After processing, use go-no-go gauges for inspection, requiring 100% qualification rate.
- What are the positional tolerance requirements for bolt holes?
The center distance tolerance of adjacent bolt holes is ±0.3mm, and the diagonal hole distance tolerance is ±0.5mm. Excessive positional deviation will cause uneven fitting between fishplate and rail, with local gaps exceeding 0.5mm, leading to stress concentration. The axis perpendicularity tolerance of the hole is 0.1mm/m; inclination will make the bolt bear additional bending moment, reducing fatigue life. A coordinate boring machine must be used during processing to ensure positional accuracy.
- How does the surface roughness of the hole affect the connection?
The surface roughness of bolt holes must be below Ra12.5μm. Excessively high roughness (Ra＞25μm) will cause additional friction during bolt installation, increasing preload fluctuation; meanwhile, rough surfaces easily accumulate moisture and impurities, accelerating bolt corrosion. After finishing, a surface roughness tester must be used for inspection, and honing is necessary if required.
- What are the differences in bolt hole processing for fishplates of different materials?
45# steel fishplates have high hardness (HB197-241). High-speed steel drills (W18Cr4V) must be used for drilling, with a feed rate 20%-30% lower than that for Q235 steel to avoid excessive drill wear. Q235 steel fishplates are easier to process, and ordinary drills can be used, but the feed rate must be controlled to prevent burrs on the hole wall. After processing, 45# steel holes must have burrs removed, and Q235 steel holes must have iron chips cleaned to prevent thread damage during assembly.
- How to detect the processing accuracy of bolt holes?
Measure the hole diameter with a vernier caliper, measuring 3 directions per hole and taking the average. Use a hole position measuring instrument to detect hole distance and positional tolerance; rework if data deviation exceeds the limit. Use a perpendicularity tester to detect hole axis inclination, with feeler gauges to measure gaps. For mass-produced fishplates, adopt sampling inspection (5% per batch), and re-inspect the entire batch if unqualified.