Explain what coarse and fine benchmarks are, and how to choose between them

In the first process of mechanical processing, the workpiece uses the unprocessed rough surface as the positioning reference, which is called the coarse reference< br /> The selection of coarse benchmarks is as follows:
1. If it is necessary to first ensure the position requirement between the machined surface and the non machined surface on the valve workpiece, the non machined surface should be used as the rough reference. If there are many surfaces on the workpiece that do not need to be machined, the surface with higher positional accuracy requirements for the machined surface should be used as the rough reference< br /> 2. If it is necessary to first ensure a uniform margin on an important surface of the workpiece, that surface should be selected as the rough reference< br /> 3. The surface selected as a rough benchmark should be flat and free from defects such as casting, riser, or flash, in order to ensure reliable positioning< br /> 4. The coarse reference can generally only be used once, especially for the main positioning reference, which can cause significant positional errors in cotton< br /> What is a precision benchmark? What is the choice of precision benchmark< br /> Using the machined surface of the workpiece as a positioning reference is called a precision reference< br />
The selection of precise benchmarks is as follows:
1. Using process benchmarks as precise benchmarks to achieve" Datum Coincidence”, To avoid the occurrence of benchmark misalignment errors< br /> 2. When the workpiece can be conveniently machined on other surfaces using a certain set of precision benchmarks for positioning, this set of precision benchmarks should be used as much as possible in most processes to achieve" Benchmark Unification;, To reduce tooling design and manufacturing costs, improve productivity, and avoid benchmark conversion errors< br /> 3. When precision machining or finishing processes require a minimum and uniform margin, the machined surface itself should be selected as the precision reference, that is, following the" Self Benchmarking” principle. The positional accuracy between the machined surface and other surfaces is required to be ensured by the preceding process< br /> 4. In order to obtain uniform machining allowance or high positional accuracy, the principle of mutual reference and repeated machining can be followed