The non-standard double-head screws (M5/M6/M8) in the 12.9-level CNC tool turning tool bar accessories are mainly used for high-strength connections between the turning tool bar and the blade, tool holder, or other accessories. Its surface treatment needs to take into account the corrosion resistance, wear resistance, and high strength requirements, and at the same time, needs to adapt to the harsh environment of the cutting working conditions. The following are key treatment solutions and technical points.
1. Core technology of surface treatment
Process: alkaline high-temperature oxidation or room-temperature chemical oxidation to form a dense Fe₃O₄ film.
Advantages: low cost, rust prevention (neutral environment), no effect on size.
Limitations: weak corrosion resistance (needs to be used with anti-rust oil), suitable for M5/M6 small-size non-standard parts.
Galvanizing (zinc-nickel alloy)
Electrogalvanizing: conventional coating (5-15μm), salt spray test for more than 72h, color/black passivation is required to enhance protection.
Zinc-nickel alloy (recommended): nickel content 12-15%, salt spray 500-1000h, hardness >=400HV, suitable for M8 high load scenarios.
Process: zinc-aluminum sheet + chromate coating, no risk of hydrogen embrittlement, salt spray up to 1000h.
Advantages: high temperature resistance (300℃), suitable for complex thread structure, but color is limited (silver gray).
Coating: TiN (gold), CrN (silver gray), film thickness 2-5μm, hardness 2000-3000HV.
Applicable scenarios: screw contact surfaces that require wear resistance and friction reduction, high cost, mostly used for high-end tool accessories.
2. Special requirements and precautions
12.9 grade screws need to be dehydrogenated at 200℃×4h after electroplating (by ISO 4042).
Thread accuracy: PVD/Dacromet needs to control the film thickness to avoid affecting the thread fit (it is recommended to reserve a 2-4μm margin).
High temperature environment: Dacromet or oxidation + high temperature lubricating coating (such as molybdenum disulfide).
3. Adaptability of non-standard parts
Double-head thread protection: Use hanger shielding or sandblasting post-treatment to ensure uniform coating.
Marking requirements: Laser marking must be completed before surface treatment to avoid damaging the coating.
12.9 grade non-standard screws are preferably zinc-nickel alloy or Dacromet, taking into account both corrosion resistance and strength; if the budget is sufficient and wear resistance is required, local PVD strengthening can be performed. Pre-treatment and post-treatment must be strictly controlled (to ensure the synergistic performance with the turning tool bar base.
