Precision CNC Bars

Detailed explanation of the specifications and standards of precision CNC bars

In the process of modern manufacturing industry developing towards high precision and intelligence, precision CNC bars are important raw materials for CNC machine tool processing, and their specifications and standards directly affect the quality and performance of products. From the manufacturing of precision parts for aerospace to the production of micro components for electronic equipment, strict requirements are placed on the dimensional accuracy, surface quality, and material properties of precision CNC bars. Understanding and following relevant specifications and standards is the key to ensuring processing accuracy and improving production efficiency. Next, we will deeply analyze the specification and standard system of precision CNC bars from multiple dimensions.

1. Dimensional accuracy standards

1. Diameter tolerance

Diameter is a key dimensional parameter of precision CNC rods, and its tolerance range directly determines the matching accuracy of processed parts. According to the international standard ISO 286-1 and the national standard GB/T 1800.1-2009, the diameter tolerance of rods is divided into 20 tolerance grades, including IT01, IT0, IT1 to IT18. The higher the grade, the smaller the tolerance value and the higher the precision requirement.

Ordinary precision: suitable for general machining, such as agricultural machinery, general mechanical equipment parts manufacturing, diameter tolerance is usually between IT8 - IT11. For example, for a bar with a diameter of 20mm, the IT10 tolerance range is ±0.084mm.

Precision grade accuracy: Mainly used in key parts of automobile engines, medical equipment and other fields, the tolerance level is generally IT5 - IT7. Taking a bar with a diameter of 10mm as an example, the IT6 grade tolerance is only ±0.009mm, which can meet the needs of high-precision matching.

Ultra-precision accuracy: Commonly used in aerospace, semiconductor manufacturing and other fields, with tolerance levels of IT2 - IT4. For example, the diameter tolerance of bars used to manufacture the tenons of aircraft engine blades may be controlled within ±0.002mm to ensure zero-error assembly between components.

2. Length tolerance

The length tolerance of the bar also follows strict standards, usually in millimeters, and different allowable deviations are set according to the length of the bar and the requirements of use. For short-sized bars (≤100mm), the length tolerance is generally controlled at ±0.1 - ±0.3mm; for medium-length bars (100 - 1000mm), the tolerance range is ±0.5 - ±1.0mm; for long-sized bars (＞1000mm), the tolerance may be relaxed to ±1.5 - ±3.0mm. In precision machining, such as making mold ejectors and slender shaft parts, the length tolerance needs to be strictly controlled to avoid assembly difficulties or functional failures due to dimensional deviations.

3. Roundness and Cylindricity

Roundness is used to measure the roundness of the cross section of the bar, while cylindricity reflects the shape accuracy of the bar along the axis. In precision CNC machining, roundness and cylindricity are extremely demanding. The roundness error of general precision-grade bars must be ≤0.005mm, and the cylindricity error must be ≤0.01mm/m; the roundness error of ultra-precision bars is even required to be ≤0.001mm, and the cylindricity error must be ≤0.003mm/m. Through high-precision roundness and cylindricity control, it can be ensured that the processed parts have good concentricity and matching performance, reducing vibration and wear caused by shape deviation.

2. Geometric Tolerance Standards

1. Straightness

Straightness refers to the degree of straightness of the bar axis, which directly affects the linear motion accuracy of the machined parts. In precision CNC machining, the straightness error must be strictly controlled. For a bar with a length of 1000mm, the ordinary precision grade requires a straightness error of ≤0.02mm, and the ultra-precision grade requires ≤0.005mm. For example, when manufacturing the lead screw of a precision machine tool, if the straightness of the bar does not meet the standard, it will cause the moving parts of the machine tool to creep, affecting the machining accuracy.

2. Verticality and parallelism

When bars are used to manufacture parts with vertical or parallel requirements, their verticality and parallelism must meet the standards. Verticality is used to measure the verticality between the end face of the bar and the axis, and parallelism is used to control the parallel relationship between different sections of the bar. Generally, the verticality error of precision bars is ≤0.01mm, and the parallelism error is ≤0.005mm/m. In the processing of parts such as electronic equipment heat sinks and optical instrument brackets, strict control of these geometric tolerances is crucial.

3. Surface quality standards

1. Surface roughness

Surface roughness is an important indicator to measure the microscopic geometric shape error of the bar surface, usually expressed as Ra (arithmetic mean deviation of the profile). In precision CNC machining, different application scenarios have significantly different requirements for surface roughness:

Ordinary processing: such as rough processing of mechanical structural parts, the Ra value is generally 1.6 - 6.3μm.

Semi-finishing: Suitable for parts with certain requirements on surface quality, such as the intermediate processing stage of automotive parts, the Ra value is controlled at 0.8 - 1.6μm.

Finishing: In the fields of precision molds, medical devices, etc., the Ra value needs to reach 0.2 - 0.8μm.

Ultra-finishing: used in semiconductor chip manufacturing, optical lens processing, etc., the Ra value can be as low as 0.02-0.1μm. Rods with low surface roughness can not only reduce cutting resistance during processing, but also improve the wear resistance and corrosion resistance of parts.

2. Surface defects

The surface of precision CNC bars is not allowed to have defects that affect performance, such as cracks, folds, scars, slag inclusions, etc. There are also strict limits on the depth and length of minor scratches, pits and other defects. For example, the depth of scratches on the surface of ordinary precision bars shall not exceed 1/4 of the diameter tolerance, and the length shall not exceed 5% of the length of the bar; ultra-precision bars require no defects visible to the naked eye on the surface, and need to be inspected with the help of a microscope.

4. Material performance standards

(1) Chemical composition

The chemical composition of the bar directly determines its physical and mechanical properties. Taking alloy steel as an example, the content of different alloying elements (such as chromium, molybdenum, vanadium, etc.) must be strictly controlled within the standard range. For example, in 40Cr alloy steel, the chromium content should be between 0.80% and 1.10%, and the carbon content should be between 0.37% and 0.44%. Exceeding the range will affect the strength, toughness and heat treatment properties of the material. In key fields such as aerospace and nuclear power, the detection accuracy of the chemical composition of the bar is extremely high, and precision equipment such as spectrometers are required for analysis.

2. Mechanical properties

Mechanical properties include tensile strength, yield strength, elongation, hardness and other indicators. Rods for different purposes have different requirements for mechanical properties:

Structural bars: They need to have high tensile strength and yield strength to ensure the stability of the structure, such as high-strength rebar for construction.

Rods for cutting: On the basis of meeting a certain strength, good cutting performance is required, which is usually achieved by adjusting the hardness of the material (such as HRC 20 - 30).

Bars for precision transmission: In addition to high strength, they must also have high toughness and fatigue strength to prevent breakage under alternating loads.

5. Special Industry Standards

1. Aerospace industry

The aerospace industry has extremely stringent requirements for precision CNC bars. In addition to complying with general standards, they must also comply with the AS9100 aviation quality management system requirements. Bars must have an ultra-high strength-to-weight ratio, high temperature resistance, and fatigue resistance, and their dimensional accuracy, surface quality, and material purity must all reach a high level. For example, titanium alloy bars used to manufacture aircraft engine turbine blades have a diameter tolerance of ±0.003mm, a surface roughness of Ra≤0.1μm, and must undergo rigorous non-destructive testing to ensure that there are no internal defects.

2. Medical device industry

Rods for medical devices must meet the requirements of biocompatibility, corrosion resistance and high precision. The materials must comply with ISO 10993 biosafety standards and the surface must be specially treated to prevent bacterial attachment. For example, 316LVM medical stainless steel rods used to manufacture orthopedic implants must not only ensure dimensional accuracy (diameter tolerance ±0.005mm), but also pass strict human safety tests.

The specification standard of precision CNC rods is a comprehensive system covering dimensional accuracy, geometric tolerances, surface quality and material properties. Different industries and application scenarios have different and stringent requirements for them. Only by strictly following the relevant standards and accurately controlling the entire process from raw material production to processing and manufacturing can we produce high-quality rods that meet the needs of modern manufacturing and promote precision processing technology to new heights.