What are the requirements for raw materials when we process self-locking tuning screws?

Self-locking tuning screw is a key fastener with the characteristics of "self-locking and anti-loosening" and "precise tuning and positioning", which is widely used in communication equipment, precision instruments, automotive electronics, aerospace and other fields (such as RF antenna tuning, sensor positioning, and medical device adjustment mechanism). Its processing raw materials should meet the four core requirements of mechanical properties, locking reliability, precision machining adaptability and environmental adaptability, as follows, which are explained in detail in combination with processing technology and application scenarios:

1. Core performance requirements: adaptive self-locking and tuning functions.

1. Mechanical strength and hardness: ensure bearing capacity and locking stability.

Self-locking tuning screws should bear assembly torque, vibration load and stress in long-term use to avoid deformation or failure. The core indicators are as follows:

Tensile strength: ordinary scene ≥800MPa, precision/high stress scene (such as aerospace) ≥ 1000 MPa;

Yield strength: ≥650MPa (avoid plastic deformation of threads caused by over tightening during assembly);

Hardness control: HRC28-35 is recommended for the hardness of finished products (if "thread rolling" process is adopted, the hardness of raw materials needs to be slightly lower, HRC22-28 ensures thread formability during rolling; If "cutting" is adopted, the hardness of raw materials can be slightly higher, HRC28-32, reducing cutting deformation);

Impact toughness: ≥ 20J/cm (dealing with vibration or impact conditions to avoid brittle fracture, especially for screws used in outdoor equipment or mobile machinery).

2. Wear resistance and bite resistance: ensure smooth tuning.

The tuning screw needs frequent rotation adjustment (such as antenna frequency fine adjustment and instrument parameter calibration), and the thread surface should have low friction coefficient and high wear resistance to avoid "seizure" (occlusion) or adjustment sticking;

Raw materials should have good wear resistance to avoid the decline of tuning accuracy caused by rapid wear of thread surface;

For materials that are easy to bite, such as stainless steel, it is necessary to control the content of sulfur, phosphorus and other impurities in raw materials (S≤0.03%, P≤0.035%), or improve the bite resistance through subsequent surface treatment (such as MoS₂ plating and nitriding), but the raw materials themselves need to be compatible with surface treatment.

3. Dimensional stability: ensure the precision tuning accuracy.

The thread accuracy of the tuning screw (usually requiring ISO 4H/5H internal thread and 6g external thread), the straightness of the stem (≤0.01mm/m) and the coaxiality of the head directly affect the tuning accuracy. The raw materials shall meet the following requirements:

Low internal stress: raw materials need to be annealed/normalized (such as complete annealing temperature of 720-780℃ and heat preservation for 2-4h) to eliminate internal stress generated during rolling and forging, and avoid dimensional deformation caused by stress release after processing (such as cutting and heat treatment);

Microstructure uniformity: grain refinement (grain size ≥6), no segregation, looseness, inclusion and other defects (inclusion content ≤0.02%), ensuring uniform cutting force during machining, high thread forming accuracy and avoiding machining errors caused by local hardness differences.

Second, the machining process adaptability requirements: meet the requirements of precision machining.

The processing technology of self-locking tuning screw is usually "raw material pretreatment → cold heading/hot heading head → thread processing (rolling/cutting) → self-locking structure processing (such as nylon locking ring groove, thread gluing groove and deformed thread) → heat treatment → surface treatment → precise detection", and the raw materials should be adapted to each process:

1. Cold working formability: suitable for cold heading/rolling process.

If the cold heading process is used to process the head (high efficiency and low cost, suitable for mass production), the raw materials should have good cold plasticity, the elongation at break δ ≥ 15% and the area shrinkage ψ≥40%, so as to avoid cracking and folding of the head during cold heading;

If the thread rolling process (superior to cutting, with high thread strength and low surface roughness Ra≤0.8μm) is adopted, the raw materials should have enough plasticity, and the hardness should be controlled at HRC22-28, so as to ensure that the thread profile is completely formed without burr and corner collapse during rolling.

2. Machinability: Adapting to complex structure machining.

Some self-locking tuning screws with high precision or special structure (such as fine thread, multi-step rod and locking groove) need cutting (turning and milling), and the raw materials should meet the following requirements:

Machinability grade: ordinary steel ≥60% (based on 45# steel), stainless steel ≥ 40%;

Avoid hard points: there must be no hard points (such as oxide and carbide inclusions) exceeding 0.05mm in raw materials to prevent the tool from wearing too fast or collapsing during cutting, which will affect the machining accuracy.

3. Heat treatment compatibility: ensure stable performance.

If the strength of the finished product needs to be improved by heat treatment (such as quenching and tempering, quenching and tempering), the raw materials should have good heat treatment responsiveness:

Hardenability: ensure the uniform hardening of the screw shaft and head to avoid local hardness deficiency;

Small heat treatment deformation: the chemical composition of raw materials is uniform (such as carbon content fluctuation ≤ 0.02%), so as to avoid bending and warping after heat treatment, which will affect the tuning accuracy.

Third, environmental adaptability requirements: according to the use of scenarios.

The application environment of self-locking tuning screws is quite different (such as indoor precision instruments, outdoor communication equipment, high temperature/corrosive environment), and the raw materials should meet the environmental requirements:

1. Corrosion resistance: dealing with different media environments.

Ordinary indoor scenes (such as office equipment and household appliances): high-quality carbon steel (such as 45# and S45C) or low-alloy steel (such as 40Cr) can be selected, and then the corrosion resistance can be improved by electroplating (zinc plating and nickel plating);

Outdoor/humid environment (such as base station antenna and automotive electronics): Stainless steel (such as 304 and 316) is recommended, among which 316 stainless steel has stronger salt spray corrosion resistance (neutral salt spray test ≥500h), which is suitable for seaside and high humidity areas;

High-temperature environment (such as engine periphery and industrial furnace equipment): heat-resistant alloy steel (such as Inconel 600 and 1Cr18Ni9Ti) shall be selected, and the continuous service temperature shall be ≤600℃ to ensure that the strength and locking performance will not be attenuated.

2. Low temperature stability: avoid brittle fracture at low temperature.

If it is used in low-temperature environment (such as outdoor low-temperature areas, aerospace equipment, temperature ≤-40℃), raw materials should have good low-temperature toughness to avoid brittle fracture;

It is recommended to choose materials with excellent low-temperature toughness (such as 304L stainless steel and 12CrNi3A alloy steel), with low-temperature impact toughness (-40℃) ≥ 15J/cm;

Avoid using materials with too high carbon content (C≥0.45%) or high phosphorus and sulfur impurities, which have high risk of brittleness at low temperature.

3. Non-magnetic/low-magnetic requirements (special scene)

The tuning screws in communication equipment and medical instruments (such as MRI nuclear magnetic resonance instrument) should avoid magnetic field interference, and the raw materials should meet the following requirements:

Non-magnetic materials: austenitic stainless steel (304, 316) and titanium alloy (TC4) are selected, and the magnetic permeability μ ≤ 1.01× 10h/m;

Avoid using ferrite stainless steel, common alloy steel and other magnetic materials, or through subsequent demagnetization treatment, but the raw materials themselves need to have low magnetic characteristics.

IV. Selection of Common Raw Materials and Applicable Scenarios

In combination with the above requirements, the following are the mainstream raw materials and adaptation scenarios of self-locking tuning screws, which can be selected according to actual needs:

Suitable scenarios for processing technology adaptation of core performance highlights of raw material brands

45#/S45C (high-quality carbon steel) has low cost, good machinability and moderate strength. Cutting and cold heading (hardness needs to be controlled) are common indoor equipment and low stress scenes (such as toys and common instruments).

40Cr (alloy structural steel) has high strength, good hardenability and excellent wear resistance. Cold heading, thread rolling and quenching and tempering treatment are medium and high stress scenarios (automotive electronics, industrial sensors)

304 stainless steel has good corrosion resistance, no magnetism and excellent plasticity. Cold heading, rolling and cutting are used to process outdoor wet environment and ordinary precision equipment (such as base station antenna).

316 stainless steel is resistant to salt fog, chemical corrosion and good toughness at low temperature. Cutting and rolling (requires special tools) seaside equipment, medical equipment and corrosive environment (such as chemical instruments)

12CrNi3A (high-quality alloy steel) has high strength, high toughness and good dimensional stability. Cutting, heat treatment, tempering, high stress and vibration scenes (aerospace equipment, precision sensors)

Cutting TC4 titanium alloy with light weight, high strength and excellent corrosion resistance (requires special tools) High-end scenes (aerospace, medical instruments, lightweight equipment)

Additional requirements for raw materials:

Material certification: a material certificate (such as EN 10204 3.1B) is required to ensure that the chemical composition and mechanical properties are up to standard;

Surface quality: there are no cracks, scratches and scales on the surface of raw materials, and the roughness Ra is ≤ 3.2 μ m to avoid affecting the subsequent processing and surface treatment effects;

Diameter tolerance: h8-h10 is recommended for the diameter tolerance of raw material bar (for example, φ5mm bar tolerance ±0.015mm), so as to ensure uniform machining allowance and reduce dimensional fluctuation.

V. Key points for attention

Matching of self-locking structure with raw materials: If self-locking of "nylon locking ring" is adopted, the hardness of raw materials should not be too high (HRC≤35) to avoid the nylon ring from being crushed during assembly; If "deformed thread" (such as triangular locking thread) is used, the raw materials should have certain plasticity to ensure that the thread still maintains strength after deformation;

Avoid excessive pursuit of high strength: too high hardness (HRC > 38) will lead to the increase of thread brittleness, and thread fracture is easy to occur during tuning, so it is necessary to balance strength and toughness according to actual load;

Environmental compatibility is preferred: in outdoor or corrosive environment, stainless steel or corrosion-resistant alloy is preferred to avoid tuning sticking or self-locking failure due to corrosion;

Consistency of batch production: raw materials should come from stable suppliers to ensure consistency of chemical composition, hardness and dimensional tolerance among batches, and avoid batch nonconformity during processing.

To sum up, the selection of raw materials for self-locking tuning screws needs to focus on "functional requirements → process adaptation → environmental adaptation": 45# steel and 40Cr can be selected for ordinary scenes, 304/316 stainless steel can be selected for corrosion resistance, and alloy steel or titanium alloy can be selected for high-end and high-stress scenes. The core is to balance the mechanical properties and machinability, so as to ensure that the screw can realize precise tuning and maintain self-locking reliability for a long time.