Parts are composed of a number of surfaces, each surface has a certain size and mutual position requirements. The relative position requirements between the surface of the part include two aspects: the distance between the surface size accuracy and relative position accuracy (such as the same axis, parallelism, perpendicularity and circular runout, etc.) requirements. The study of the relative position between the surfaces of the parts can not be separated from the benchmark, without a clear benchmark can not determine the position of the surface of the parts. Benchmark in its general sense, is the part used to determine the position of other points, lines, surfaces based on the points, lines, surfaces. Benchmark according to its role is different, can be divided into two categories of design benchmark and process benchmark.

1, design benchmark

In the parts diagram to determine other points, lines, surface benchmark, known as the design benchmark, for the piston, the design benchmark refers to the piston centre line and pin hole centre line.

2, process benchmark

Parts in the processing and assembly process used in the benchmark, known as the process benchmark. Process benchmark according to different uses, and is divided into positioning benchmark, measurement benchmark and assembly benchmark.

(1) positioning datum: processing so that the workpiece in the machine tool or fixture to occupy the correct position used in the datum, known as the positioning datum. According to the different positioning elements, the most commonly used are the following two types.

Automatic centring positioning: such as three-jaw chuck positioning.

Positioning sleeve positioning: the positioning element is made into a positioning sleeve, such as the stopper disc positioning.

Others are positioned in the V-frame, positioned in the semi-circular hole, etc.

(2) measurement benchmark: parts inspection, used to measure the size and position of the machined surface benchmark, known as the measurement benchmark.

(3) assembly benchmark: assembly to determine the position of the part in the component or product benchmark, known as the assembly benchmark.