I. Core Concepts: Precision and Tolerance

1. Precision

Processing precision is a key indicator for measuring the manufacturing quality of products in the field of mechanical processing. It directly affects the fit precision, working performance, and service life of mechanical parts. In definition, processing precision usually refers to the degree of conformity between the actual size, shape, and position of the processed parts and the ideal design size, shape, and position. Its significance lies in that high-precision processing can ensure that parts meet the expected performance standards during assembly and use, reduce wear, extend the service life of equipment, and at the same time reduce maintenance costs and improve production efficiency. Precision is a broad concept that describes the comprehensive technical level of processed parts. It is not a single numerical value but a collection of multiple dimensional features, mainly including:

Dimensional precision: It refers to the degree of closeness between the actual size after processing and the theoretical design size. The closer the degree, the higher the dimensional precision.

Geometric precision: It refers to the accuracy of the macroscopic geometric shape and relative position of the parts, also known as geometric tolerance. For example, straightness, flatness, roundness, cylindricity, as well as parallelism, perpendicularity, coaxiality, etc.

Surface quality: It mainly refers to the microscopic geometric characteristics of the part surface, that is, surface roughness. The smaller the roughness value, the smoother the surface, and the higher the perception of precision.

2. Tolerance

The definition of tolerance is: the difference between the maximum and minimum allowable dimensions of the nominal size (basic size).

The purpose of tolerance is to acknowledge that absolute precision is unattainable in reality and not economical, thereby defining an "acceptable error range". It is the digital representation of accuracy on the drawing.

For example, the designed diameter of an axle is Ø20 mm, and the tolerance is marked as ±0.01 mm. This means that the diameter of all qualified products must be between 19.99 mm and 20.01 mm. This 0.02 mm interval is the tolerance zone.

3. The Relationship Between the Two

Accuracy is the result after processing, while tolerance is the target set during the design stage. The manufacturing goal is to ensure that the product falls within the tolerance range. In the field of mechanical processing, the relationship between accuracy and tolerance is the key to ensuring product quality and performance. Accuracy refers to the degree to which the dimensions, shapes, and positions of the processed part are close to the ideal values, while tolerance is the maximum range within which the dimensions, shapes, and positions of the processed part can deviate from the ideal values. For example, in precision processing, the diameter tolerance of a part may be set at ±0.01 millimeters, which means the actual diameter of the part must be within the range of ±0.01 millimeters of the designed size to ensure its functionality and interchangeability. Tolerance is a quantitative concept, and it is the allowable variation in size and geometry that is clearly marked by the design engineer on the drawing.

II. How to ensure high-quality parts through process control?

The core of ensuring high-quality parts is to implement a systematic process control method to ensure that every part produced can consistently and stably meet the tolerance requirements specified in the drawings.

1. Design and process planning stage (pre-control)

Reasonable tolerance design: Designers must mark the tolerances based on the functional requirements of the part to avoid unnecessary strict tolerances, in order to control the manufacturing cost.

Process route planning: Process engineers need to formulate a scientific processing sequence (such as roughing first and then finishing, primary parts first and secondary parts later), and select appropriate machines, tools, and fixtures to ensure that the process capability is sufficient to meet the requirements.

2. Manufacturing process control (in-process control)

This is the most critical quality control stage.

Equipment capability: Use high-precision and stable processing equipment, and strictly implement regular maintenance and calibration procedures to prevent loss of accuracy due to machine wear or aging.

Tool management: The geometric parameters of the tool have a significant impact on the machining accuracy. The rake angle, clearance angle, helix angle, etc. of the tool directly affect the size of the cutting force, the formation of chips, and the distribution of cutting temperature, which in turn affect the roughness and dimensional accuracy of the machined surface [3]. For example, a larger rake angle can reduce the cutting force, but it will also reduce the strength of the tool, leading to accelerated tool wear and affecting the machining accuracy.

Fixture and clamping: Design precise and reliable fixtures to ensure accurate positioning and firm clamping of the workpiece during the machining process, avoiding vibration or deformation, which is the basis for ensuring repeatability accuracy.

Cutting speed, feed rate, and depth of cut

These three factors are the key factors affecting machining accuracy. Increasing the cutting speed can increase the material removal rate, but if it exceeds the tolerance limit of the material or the tool, it will lead to increased tool wear, thereby affecting the smoothness and dimensional accuracy of the machined surface.

Environmental Control: In the mechanical processing technology, the thermal expansion coefficient of the material is one of the key factors affecting the processing accuracy. The thermal expansion coefficient describes the sensitivity of the material's size change with temperature variations, and is crucial for ensuring the dimensional stability of the processed parts. For instance, the thermal expansion coefficient of aluminum alloy is approximately 23.6×10-6/K, while that of steel ranges from 10.8 to 12.0×10-6/K. This means that under the same temperature change, the size change of aluminum alloy will be much more significant than that of steel. Therefore, in precision processing, choosing the appropriate material and controlling the temperature of the processing environment become particularly important.

3. Inspection and Feedback (Post-event Control and Closed-loop)

Precise Measurement: Use precise measuring tools that match the tolerance requirements (such as micrometers, coordinate measuring machines CMM, etc.). All measuring tools must be regularly calibrated to ensure their own accuracy is much higher than the tolerance of the workpiece.

Process Quality Control (SPC): Adopt statistical process control methods. This is not a simple post-event inspection, but predictive control. By sampling and measuring at regular intervals during the processing and drawing control charts, real-time monitoring of the process stability is conducted. Abnormal trends can be detected before parts exceed tolerance limits and adjustments can be made promptly to avoid batch waste, which is the core for achieving high quality and efficiency.

Feedback and Adjustment: Establish a "Measurement-Feedback-Adjustment" closed-loop system. The inspection results should be promptly fed back to the operators so that operations such as tool compensation can be performed on the machine, continuously optimizing the process.