Precision machining process-part positioning and datum
1. Benchmark
Benchmark refers to the points, lines, and surfaces used to determine the geometric relationship between the geometric components of the production object. For a machine part, the datum is the point, line, and surface on which the positions of other points, lines, and surfaces on the part are determined.
In the decision-making and processing process of mechanical parts, which points, lines, and surfaces are selected as benchmarks according to different requirements are one of the main components that simply affect the processing technology of the parts, the dimensions between the surfaces, and the accuracy of the positions. Based on the difference between the results and the application, the benchmarks can be divided into two categories: decision-making benchmarks and process benchmarks.
(1) Decision-making benchmark
The decision-making benchmark refers to the benchmark adopted by marking the decision-making dimensions on the part drawing. On a mechanical part drawing, there can be one or multiple decision-making benchmarks. A and B are the decision-making benchmarks for each other; the 40mm outer circle is the decision-making benchmark for the 60mm outer circle; plane 1 is the decision-making benchmark for plane 2 and hole 3, and hole 3 is the decision-making benchmark for holes 4 and 5; the middle line is the inner hole 30mm, The gear index circle is 48mm and the top circle is 50h8mm.
(2) Process benchmark
Process benchmark refers to the benchmark adopted in the process of parts. According to the difference in use, the process reference can be sub-process reference, positioning reference, measurement reference and assembly reference.
1. Process benchmark
In the process diagram, the benchmark used to determine the size, style, and position of the processed surface of the process is called the process benchmark. , The machined surface is a D hole, and its middle line is required to be perpendicular to the A surface, and the distance between the C surface and the B surface is L1 and L2. Then A, B, and C are the process benchmarks of this process.
2. Positioning datum
During processing, the datum used to determine the correct position of the workpiece on the machine tool or in the fixture is called the positioning datum.
3. Measurement benchmark
During or after processing, the benchmark used to measure the style, position and size deviation of the workpiece is called the measurement benchmark. The dimension L1 can be measured with a depth caliper, and the end face T is the measurement standard of the end face A.
4. Assembly benchmark
During assembly, the benchmark used to determine the relative position of the parts or components on the product is called the assembly benchmark. For example, the inner hole of the gear is the assembly reference of the gear. Under normal circumstances, the decision-making reference is given on the part drawing, and the process reference is determined by the process staff based on the meticulous process selection.
When analyzing the benchmark problem, it is important to pay attention to the following two points:
1) The points, lines, and surfaces used as the reference do not necessarily exist on the workpiece in detail (such as the middle line of the hole, the axis line, etc.), but are illustrated by some detailed surfaces, which are called the base surface. For example, the middle line of the inner hole is illustrated by the inner hole surface, the middle line of the inner hole is the reference, and the inner hole surface is the base surface. Therefore, the question of choosing a benchmark is the question of choosing an appropriate base. Extraordinarily, occasionally, for the convenience of discussion, datum and base plane can be collectively referred to as datum.
2) As a datum, it can be a point and line without area or a very small surface, but the base surface representing this datum always has a bounded area. For example, a long shaft is placed on a lathe with a center point. The reference is the axis line, which has no area; the base surface is the cone surface of the center point. It has a small area but certainly has an area.