Denchmark
Parts are composed of several surfaces, and each surface has certain size and mutual position requirements.
The relative position requirements between the surfaces of the parts include two aspects: the distance dimensional accuracy between the surfaces and the relative position accuracy (such as coaxiality, parallelism, perpendicularity and circular runout, etc.) requirements.
The study of the relative positional relationship between the surfaces of the parts is inseparable from the datum, and the position of the part surface cannot be determined without a clear datum. In its general sense, the datum is the part used to determine other points, lines,
The point, line, or area on which the position of the face is based. According to their different functions, benchmarks can be divided into two categories: design benchmarks and process benchmarks.
1. Design Dasis
The datum used to determine other points, lines and surfaces on the part drawing is called the design datum. For the piston, the design datum refers to the centerline of the piston and the centerline of the pin hole.
2. Process Denchmark
The datum used by the parts in the process of machining and assembly is called the process datum. According to different uses, process benchmarks are divided into positioning benchmarks, measurement benchmarks and assembly benchmarks.
1) Positioning datum: The datum used to make the workpiece occupy the correct position in the machine tool or fixture during processing is called the positioning datum. According to the different positioning components, the most commonly used are the following two categories:
Automatic centering and positioning: such as three-jaw chuck positioning.
Positioning sleeve positioning: The positioning element is made into a positioning sleeve, such as the positioning of the stop plate.
Others include positioning in a V-shaped frame, positioning in a semicircular hole, etc.
2) Measurement datum: The datum used to measure the size and position of the machined surface of the star during parts inspection is called the measurement datum.
3) Assembly datum: The datum used to determine the position of the part in the component or product during assembly is called the assembly datum.