1. Concept of Machining Accuracy

Machining accuracy refers to the degree of precision achieved during the manufacturing process; both machining accuracy and machining error are terms used to evaluate the geometric parameters of a machined surface. Machining accuracy is measured by tolerance grades—the lower the grade value, the higher the accuracy. Machining error is expressed numerically—the larger the value, the greater the error. High machining accuracy implies low machining error, and vice versa.

There are 20 tolerance grades, ranging from IT01, IT0, IT1, IT2, and IT3 up to IT18. IT01 represents the highest level of machining accuracy, while IT18 represents the lowest; IT7 and IT8 generally represent intermediate levels of accuracy.

No machining method yields absolutely precise parameters; from a functional standpoint, machining accuracy is considered satisfied as long as the machining error falls within the tolerance range specified on the part drawing.

The quality of a machine depends on both the machining quality of its parts and the quality of its assembly. Part machining quality comprises two main components: machining accuracy and surface quality.

Machining accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of a machined part correspond to the ideal geometric parameters. The discrepancy between them is known as machining error. The magnitude of the machining error reflects the level of machining accuracy: the greater the error, the lower the accuracy, and the smaller the error, the higher the accuracy.

2. Aspects of Machining Accuracy

(1) Dimensional accuracy refers to the degree to which the actual size of a machined part corresponds to the center of the specified tolerance zone. (2) Shape accuracy refers to the degree to which the actual geometric shape of a machined surface corresponds to the ideal geometric shape. (3) Positional accuracy refers to the degree of precision regarding the actual relative positions of the machined part’s surfaces. (4) Interrelationships: When designing machine parts and specifying machining accuracy, shape errors should generally be controlled within the limits of positional tolerances, and positional errors should be kept within the limits of dimensional tolerances. In other words, for precision parts or critical surfaces, the requirement for shape accuracy should be stricter than that for positional accuracy, and the requirement for positional accuracy should be stricter than that for dimensional accuracy.

3. Adjustment Methods

(1) Adjust the machining system (2) Reduce machine tool errors (3) Reduce transmission errors in the drive train (4) Reduce tool wear (5) Reduce deformation of the machining system under load (6) Reduce thermal deformation of the machining system (7) Reduce residual stress

4. Causes of Errors

(1) Machining principle errors: These are errors resulting from the use of approximate cutting-edge profiles or approximate transmission relationships during machining. Such errors frequently occur in the machining of threads, gears, and complex curved surfaces. In practice, approximate machining methods are often employed to enhance productivity and cost-efficiency, provided that the theoretical error remains within the limits required for machining accuracy. (2) Adjustment errors: These are errors arising from inaccurate machine tool settings or adjustments. (3) Machine tool errors: These refer to errors stemming from the manufacturing, installation, and wear of the machine tool itself. Key examples include guideway guidance errors, spindle rotation errors, and transmission errors within the machine tool’s drive train.

5. Measurement Methods

Various measurement methods are employed depending on the specific aspects of machining accuracy and the required precision levels. Generally, these methods fall into the following categories:

(1) Classification based on whether the parameter is measured directly: Direct measurement and indirect measurement. Direct measurement: The parameter is measured directly to obtain the dimension (e.g., using calipers or a comparator). Indirect measurement: Geometric parameters related to the target dimension are measured, and the dimension is subsequently calculated. Clearly, direct measurement is more intuitive, whereas indirect measurement is more complex. Indirect measurement is typically adopted when direct measurement is impractical or fails to meet accuracy requirements. (2) Classification based on whether the reading directly indicates the dimension value: Absolute measurement and relative measurement. Absolute measurement: The reading directly indicates the magnitude of the dimension (e.g., using a vernier caliper). Relative measurement: The reading indicates the deviation of the dimension from a standard reference value. For instance, when measuring a shaft diameter with a comparator, the instrument is first zeroed using gauge blocks; the measured value represents the difference between the shaft diameter and the gauge block size—this constitutes relative measurement. Generally, relative measurement offers higher precision but involves a more cumbersome process. (3) Classification based on contact between the measured surface and the measuring instrument’s probe: Contact measurement and non-contact measurement. Contact measurement: The probe makes physical contact with the surface being measured, involving a mechanical contact force (e.g., measuring a part with a micrometer). Non-contact measurement: The measuring head does not touch the surface of the part being measured; this avoids the influence of measuring force on the results. Examples include projection methods and optical interference methods. (4) Classification based on the number of parameters measured at once: single-parameter measurement and comprehensive measurement. Single-parameter measurement: Each parameter of the part is measured individually. Comprehensive measurement: A composite indicator reflecting relevant part parameters is measured. For instance, when measuring threads with a toolmaker’s microscope, one can separately measure the actual pitch diameter, thread flank angle error, and cumulative pitch error. Comprehensive measurement is generally more efficient and more reliable for ensuring part interchangeability; it is frequently used for the inspection of finished parts. Single-parameter measurement allows for the determination of errors for each specific parameter and is typically used for process analysis, in-process inspection, and the measurement of specific designated parameters. (5) Classification based on the role measurement plays during machining: active measurement and passive measurement. Active measurement: The workpiece is measured during the machining process, and the results are used directly to control that process, thereby preventing the production of scrap parts in a timely manner. Passive measurement: Measurement performed after the workpiece has been machined. This type of measurement only determines whether the part is acceptable; it is limited to identifying and rejecting scrap parts. (6) Classification based on the state of the part during measurement: static measurement and dynamic measurement. Static measurement: The measurement involves relative stillness (e.g., measuring diameter with a micrometer). Dynamic measurement: The surface being measured and the measuring head undergo relative motion during measurement, simulating actual operating conditions. Dynamic measurement methods reflect the part’s condition under near-service states and represent the future direction of measurement technology.

CNC Machined Metal Parts

CNC machined metal parts are precision components created by computer-controlled cutting tools that remove material from a solid metal block or bar.

Main Processes

CNC Milling: Uses rotating cutting tools to shape stationary blocks into complex 3D geometries.

CNC Turning: Rotates the metal workpiece against a single-point tool to manufacture precise cylindrical parts like shafts and bushings.

Drilling & Boring: Creates accurate hole patterns, internal threading, and counterbores.

Common Metals Used

Aluminum (6061, 7075): Lightweight with high machinability, ideal for brackets, housings, and UAV components.

Stainless Steel (304, 316): Strong and corrosion-resistant, frequently used for medical or marine instruments.

Brass: Easy to machine and well-suited for tight-tolerance connectors and fittings.

Titanium: Delivers an exceptional strength-to-weight ratio for demanding aerospace uses.