Selection of the benchmark of the hottest mechanic

Selection of machine parts benchmark

the relative position between the tables of machine parts includes two sides: one side is the position and dimensional accuracy between the tables; On the other hand, it is relative position accuracy. The dimensional accuracy and position accuracy requirements between tables cannot be separated from the reference basis - datum

benchmark is a widely used concept in mechanical manufacturing, It is used to determine what elements are on the production object (parts, machines) (other points, lines, and beauties are based on those points, lines, or and.

1 Classification of datum

according to the function of datum, it can be divided into design datum and process datum.

1.1 design datum

when designing parts, the designer determines the starting position of dimensioning (or angle) according to the assembly relationship of parts in the assembly structure and the mutual position relationship between structural elements of parts themselves. The starting position of these dimensions (or angles) is called the design basis; That is, the datum used to determine other points, lines, and positions on the part engineering drawing, which is also called the design datum

the selection principle of design basis is: in the process of component design, in order to ensure the accuracy after mechanical assembly, single parts and components usually use the center line of shaft and hole and the center plane of symmetrical parts as the design basis

1.2 process benchmark

the benchmark used in the processing, measurement and assembly of parts is called process benchmark. Process datum can be divided into positioning datum, measurement datum and assembly datum according to the actual occasions of mechanical manufacturing

(1) positioning datum. The datum used to determine the correct position of the workpiece relative to the machine tool or tool during part processing is called positioning datum. According to the selection order and function in the process of mechanical manufacturing, the positioning datum can be divided into coarse datum, fine datum and auxiliary datum in order to reduce the weight of the aircraft

rough datum refers to the positioning datum of the workpiece that has not been machined during the initial processing of the part

precision datum refers to the datum of positioning by using the machined table in the machining of parts

auxiliary datum is a datum specially designed for the purpose of facilitating the stability of installation and machining positioning on parts, or for the process needs of realizing datum unification

(2) measurement benchmark. During assembly, processing or after processing, the benchmark used to measure the shape, position and dimensional errors of parts and the mutual position, perpendicularity and parallelism between parts is called measurement benchmark

the measurement basis of parts, generally select the design basis; The position accuracy measurement benchmark between equipment or machine parts generally selects the assembly benchmark

(3) assembly benchmark. In the assembly process, the datum used to determine the relative position of parts or components in the product is called assembly datum

the mutual position accuracy in assembly, including the parallelism, perpendicularity, coaxiality and various runout of the friction between the relevant parts and components, which will increase the friction between the moving parts. There are accuracy requirements for relative motion direction and relative speed direction between parts with relative motion in the product. The accuracy of motion direction is mostly expressed as the parallelism and perpendicularity of relative motion between parts; The linear motion accuracy or circular motion accuracy of components is the basis of relative motion accuracy

reasonable selection of appropriate benchmarks plays a vital role in achieving the design performance of equipment or machines. For example, the Journal of shaft parts, the inner hole of gear parts and the bottom of the box are often used as assembly benchmarks

2 selection of positioning datum

positioning datum is the direct datum to obtain the size of parts in the process of parts processing, which is very important to the dimensional accuracy and position accuracy of parts

2.1 selection principle

(1) select the table with the largest size for installation, the table with the longest distance for guidance, and the table with the smallest size for support

(2) first consider to ensure the spatial position accuracy, and then consider to ensure the dimensional accuracy

(3) try to choose the main table of the part as the positioning datum, because the main table is the dimensional datum that determines other tables of the part, that is, the important design datum

(4) the positioning datum is conducive to clamping and indwelling, and is stable and reliable in the processing process

2.2 selection of rough datum

(1) the table that does not need to be processed but has higher position accuracy requirements with the processing table should be used as the datum to ensure the mutual requirements between the processing table and the non processing table; If each table on the workpiece needs to be machined, the table with the smallest allowance shall be used as the phase reference to ensure that each machining table has sufficient allowance

(2) it should be ensured that each machining table has sufficient machining allowance, and the table with small and uniform machining allowance should be used as the phase reference. For example, the excircle machining should be based on the axis

(3) as far as possible, use the blank middle ruler, J 'and the position to be reliable, flat and smooth. At the same time, you can process multiple tables under one clamping as the phase reference

(4) the phase reference is not allowed to be reused, which is because the phase reference is generally very rough. If it is reused, it will cause large position error and cannot guarantee the design requirements after processing

2.3 selection of precision datum

mainly starts from ensuring the machining accuracy of parts, while considering convenient clamping and simple fixture structure

(1) adopting the principle of single reference in the processing of parts can reduce the type and quantity of fixtures or facilitate automatic production, which is conducive to ensuring the mutual position requirements of each table, simplify fixture design and manufacturing, shorten the production preparation cycle, and avoid the error caused by reference transformation

for example, when machining shaft parts, especially step shaft parts, generally two tip holes are used as a unified precision benchmark to process all the cylindrical surfaces and ends on shaft parts, so as to ensure the coaxiality between the cylindrical surfaces and the perpendicularity of the ends to the axis line

(2) the greater the sliding resistance of the original table converted from the processing table to the positioning table. It is generally applicable to the precision machining and smooth grinding of some parts with high spatial position accuracy requirements. For example, there are strict coaxiality requirements between the front and rear support journals of the earth shaft of the lathe and the taper hole of the earth shaft. Usually, the taper hole of the earth shaft is used as the benchmark first, and the front and rear support journals of the earth shaft are ground; Then, the front and rear bearing journal tables are used as the benchmark, without protection, with long service life, and the tapered hole of the grinding shaft; Finally, the coaxiality requirements specified on the drawing are met

(3) try to choose the design datum as the positioning datum to achieve the unification of datum, so as to avoid the error caused by the non coincidence of datum

(4) self benchmarking principle. During precision machining of parts, some tables with high accuracy requirements. In order to ensure the machining accuracy, the machining allowance is required to be small and uniform, so it is often positioned by the machining itself. After clamping, the positioning element is removed and then processed. This can improve the accuracy and appearance quality of the processing itself

2.4 auxiliary datum

auxiliary datum is a structure specially designed or processed according to the needs of machining process, which is not on the part itself. Auxiliary datum does not work in the actual use of parts, but is set for the needs of machining. After all parts are processed, they are generally cut off. For example, the positioning of the tip hole of the commonly used items of shaft parts, the boss of the casting with

set due to the needs of the casting process, the process hole of some parts, the central hole of shaft parts, etc

figure L is an example of a typical auxiliary benchmark. The lathe bed is not processed but has a certain radian. When processing the guide rail, the lathe bed cannot be leveled and aligned because it is inclined. This problem will not be considered in design, but processing alignment should be considered in process countersignature. For processing and positioning, when casting the bed, a boss auxiliary datum A is added to facilitate the alignment of the processing guide rail

3 conclusion

whether in the assembly process or the processing process of parts, whether in the assembly benchmark or positioning benchmark, its stability and reliability are guaranteed; Try to select the positioning datum from the parts list with direct position requirements, so as to reduce the error caused by improper selection of datum; In addition, it is necessary to ensure that the parts processing is convenient for the loading, unloading and clamping of workpieces, the structure of tooling and

fixture is simple, the mutual position accuracy between parts is convenient for assembly and measurement, and there is enough measurement space