1. Types of benchmarks in parts processing
Datum refers to those points, lines, and surfaces used to determine the geometric relationship between geometric elements on the production object. According to the different functions of benchmarks, benchmarks can be divided into two categories: design benchmarks and process benchmarks.
1. design basis
The basis used on the design drawings is called the design basis. As shown in the box below, A and B are the dimensions of the center of the hole, and the design basis is plane ① and plane ②, which are reflected in the diagram as lines. The design basis of the aperture D is the axis, and what is reflected on the diagram is a point.
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2. Process benchmark
The reference used for positioning, detection and assembly in the process of part processing is called process reference, which includes three types: positioning reference, measurement reference and assembly reference. For example, when boring a round hole as shown in the above figure, one installation method is to use the ① and ② surfaces as the positioning reference, and the positioning reference and the design reference coincide;
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Another method is to use the ① and ③ planes as the positioning datum, at this time the positioning datum does not coincide with the design datum
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The first process uses the blank surface as a positioning reference. This positioning reference that has not been cut is called a rough reference, and the rough reference is only used once. When continuing to process, the machined surface is used as the positioning reference. This kind of positioning reference after cutting is called fine reference.
2. The selection principle of datum in parts processing
1. The selection principle of rough datum
The rough datum is the datum that is positioned on the surface of the blank in the initial machining process. When selecting a rough datum, it should be ensured that each surface has sufficient machining allowance, so that the machined surface has a suitable mutual position with respect to the unprocessed surface. The selection principle is:
1) Use the surface of the workpiece that does not need to be processed as the rough reference to ensure that the position error between the processed surface and the non-processed surface is the smallest.
2) If it is necessary to ensure that the machining allowance of an important surface of the workpiece is uniform, this surface should be selected as the rough reference.
3) A flat and sufficiently large blank surface should be used as a rough reference as much as possible.
4) The rough datum cannot be used repeatedly, because the surface accuracy of the rough datum is low, and it cannot guarantee that the workpiece will maintain the same position in the two installations.
2. The selection principle of fine benchmark
In the subsequent processes, the processed surface must be used as a positioning reference, which is called a fine reference. The choice of fine datum directly affects the mutual position accuracy of each surface of the part. Therefore, when choosing a fine datum, it is necessary to ensure the machining accuracy of the workpiece and the convenience and reliability of clamping. The principles for selecting a fine benchmark are:
1) The principle of datum coincidence: use the design datum as the fine datum as much as possible to avoid positioning errors caused by datum misalignment.
2) The principle of the same benchmark: as much surface processing as possible should use the same precise benchmark to reduce the error caused by changing the positioning benchmark and make the fixture structure unified. For example, the center hole is used as the fine datum in the processing of shaft parts, and it is always used as the fine datum in the processes of turning, milling, grinding, etc., so that the coaxiality between the journals of each section can be guaranteed. It can also increase productivity. Another example is that when machining gears, the inner hole is usually processed first, and then the inner hole is used as the fine reference.
3) The principle of mutual reference: use two surfaces on the workpiece with mutual position accuracy requirements alternately as the positioning reference. For example, to process a short sleeve, in order to ensure the coaxiality between the hole and the outer circle, the outer circle should be used as the positioning reference to grind the hole first, and then the outer circle should be ground with the ground hole as the positioning reference.
4) It is easy to install and makes the structure of the fixture simple.
5) Try to choose a surface with simple shape and large size as the fine reference to improve the stability and accuracy of installation.
