Precision machining accuracy of precision parts refers to the degree to which the actual geometric parameters (size, shape, and position) of the part after machining match the ideal geometric parameters specified in the drawing. The higher the degree of correspondence, the higher the machining accuracy. In machining, due to various factors, it is actually impossible to machine every geometric parameter of the part to fully match the ideal geometric parameters, and there will always be some deviations. This deviation is called machining error.

The precision achieved by the trial cutting method may be high, as it does not require complex devices. However, this method is time-consuming (requiring multiple adjustments, trial cutting, measurement, and calculation), inefficient, and relies on the technical level of workers and the accuracy of measuring instruments. The quality is unstable, so it is only used for single piece and small batch production.

As a type of trial cutting method - matching, it is a method of processing another workpiece that matches it based on the processed workpiece, or combining two (or more) workpieces for processing. The requirements for the final processed size in the matching process are based on the matching requirements with the processed parts.

(2) Adjustment method

Adjust the accurate relative positions of the machine tool, fixture, tool, and workpiece in advance with samples or standard parts to ensure the dimensional accuracy of the workpiece. Because the size is adjusted in advance, there is no need to try cutting again during processing. The size is automatically obtained and remains unchanged during the processing of a batch of parts, which is the adjustment method. For example, when using a milling fixture, the position of the tool is determined by the corresponding tool block. The essence of adjustment method is to use the fixed distance device, tool setting device, or pre adjusted tool holder on the machine tool to achieve a certain positional accuracy of the tool relative to the machine tool or fixture, and then process a batch of workpieces.

Feeding and cutting according to the dial on a machine tool is also a method of adjustment. This method requires first determining the scale on the dial according to the trial cutting method. In mass production, tool setting devices such as fixed distance stops, samples, and templates are often used for adjustment.

The adjustment method has better machining accuracy stability and higher productivity compared to the trial cutting method. It does not have high requirements for machine tool operators, but has high requirements for machine tool adjustment workers. It is commonly used in batch production and mass production.

(3) Dimensioning method

The method of using the corresponding size of the cutting tool to ensure the size of the workpiece being processed is called the fixed size method. It is processed using standard sized tools, and the size of the machined surface is determined by the tool size. Using tools with certain dimensional accuracy (such as reamers, reamers, drills, etc.) to ensure the accuracy of the machined parts of the workpiece (such as holes).

The sizing method is easy to operate, has high productivity, and relatively stable processing accuracy, almost independent of the technical level of workers. It has high productivity and is widely used in various types of production. For example, drilling, reaming, etc.

(4) Active measurement method

During the machining process, measuring the machining dimensions while processing and comparing the measured results with the design requirements, either allowing the machine to continue working or stopping the machine, is the active measurement method.

The values in the active measurement can now be displayed digitally. The active measurement method incorporates the measurement device into the process system (i.e. a unified entity composed of machine tools, cutting tools, fixtures, and workpieces), becoming its fifth factor.

To ensure the accuracy of the parts, the following aspects should be taken into consideration:

(1) Trial cutting method

That is, first try to cut a small part of the machining surface, measure the dimensions obtained from the trial cutting, adjust the position of the cutting edge relative to the workpiece according to the machining requirements, try cutting again, and then measure again. After two or three trial cuts and measurements, when the machined dimensions meet the requirements, cut the entire surface to be machined.

The trial cutting method is repeated through "trial cutting measurement adjustment trial cutting" until the required dimensional accuracy is achieved. For example, the trial boring of the box hole system.