A Comprehensive Guide to Part Machining Accuracy Knowledge That Machine Operators Need to Master

The precision of machined parts refers to the degree to which the actual geometric parameters—specifically, size, shape, and position—of a component’s surface match the geometric specifications outlined in the engineering drawings. Ideally, for dimensions, this means the average size; for curved surfaces, it could involve shapes such as circles, cylinders, planes, cones, or straight lines. Additionally, the relative positions between surfaces may include parallelism, perpendicularity, coaxiality, symmetry, and other relationships. The deviation between the part’s actual geometric parameters and the ideal geometric specifications is known as machining error.

1. The Concept of Machining Precision

Machined part precision primarily reflects the level of product manufacturing. Both machining accuracy and machining error are terms used to evaluate the geometric parameters of machined surfaces. Machining accuracy is measured by tolerance grades—lower grade values indicate higher precision. Machining errors, on the other hand, are expressed numerically: the larger the number, the greater the error. When machining accuracy is high, machining errors are small, and vice versa.

There are 20 tolerance grades in total, ranging from IT01, IT0, IT1, IT2, IT3, all the way to IT18. IT01 represents the highest machining precision for a part, while IT18 indicates the lowest precision—typically, IT7 and IT8 fall into the medium range of machining accuracy.

The actual parameters obtained through the manufacturing process are inaccurate. However, as long as the machining error falls within the tolerance range specified on the part drawing, it is considered sufficient to ensure machining accuracy.

The quality of machined parts depends on both the processing quality of the components and the assembly quality of the machine. Specifically, the processing quality of machined parts consists of two key elements: the machining accuracy of the parts and their surface quality.

Manufacturing accuracy refers to the degree to which the actual geometric parameters (dimensions, shape, and position) of a machined part match the ideal geometric specifications. The difference between these values is known as manufacturing error. The magnitude of the manufacturing error directly indicates the level of machining accuracy—larger errors signify lower accuracy, while smaller errors correspond to higher precision.

2. Precision of machined parts related to the content

(1) Dimensional Accuracy

Indicates the degree to which the actual dimensions of a machined part coincide with the center of the part's tolerance band.

(2) Geometric Accuracy

Indicates the degree to which the actual geometry of a machined part's surface matches its ideal geometric shape.

(3) Position Accuracy

Indicates the difference in actual positional accuracy between the machined surfaces.

(4) Mutual Relationships

Typically, when designing mechanical parts and specifying part machining accuracy, note that geometric errors must be kept within positional tolerances, while positional errors should be smaller than dimensional tolerances. In other words, geometric precision requirements take precedence over positional precision requirements for critical parts or important surfaces of components—whereas positional precision itself must be prioritized over dimensional precision.

3. Methods for adjusting machined parts.

(1) Adjust the process system

(2) Reduced machine tool errors

(3) Reduce transmission chain transmission errors

(4) Reduce tool wear

(5) Reduce stress and deformation in the process system

(6) Reduce Thermal Deformation of the Process System

(7) Reduce residual stress

4. The reasons for the impact.

(1) Machining Principle Error

Processing principle error refers to the inaccuracies introduced when using an approximate tool profile or a simplified transmission mechanism during machining. This type of error frequently occurs in the machining of threads, gears, and complex curved surfaces. In general, rough machining is employed during the process; as long as the theoretical error meets the required machining accuracy, this approach helps enhance both productivity and cost-effectiveness.

(2) Adjusting Errors

The adjustment error of the machine arises from inaccuracies during the adjustment process.

(3) Machine Tool Failure

Machine errors refer to manufacturing errors, installation errors, and wear in the machine. They primarily include guideway guiding errors of machine tools, spindle rotation errors of machine tools, and transmission errors in the machine tool's drive chain.