When the workpiece is machined, the surface for determining the relative position of the workpiece to the machine tool and the tool is called the position basis. The position basis used in the initial process is the unprocessed surface of the rough, which called rough datum . The position basis used in subsequent process machining is the machined surface, which called fishing datum.
1. Rough datum selection principle
1) Selection of the rough datum should be easy to locate, clamping and CNC machining, and make the fixture structure simply.
2) If it is necessary first to ensure the positional accuracy between the workpiece CNC machining surface and non-machining surface, it should be to the non-machining surface as a rough datum.
3) To ensure that the roughing allowance of some important surfaces is small and uniform, this surface should be selected as a rough basis.
4) In order to make the CNC machining allowance of multiple surfaces on the blank relatively uniform, the rough basis should select the rough surface that can evenly divide the position error of the remaining rough surface to the selected rough basis.
5) The rough datum surface should be smooth, without the defects of gate, riser or flying edge so as to make the location reliable.
6) The rough datum can only be used once, so as to avoid greater position errors.
2. Fishing datum selection principle
1) The selected positioning datum shall be easy to locate, clamp and process, and have enough positioning accuracy.
2) Benchmark unification principle. When the workpiece is positioned on a set of finishing datum, it is easier to process most of the remaining surfaces, the method of locating the same set of reference shall be used in the CNC machining of these surfaces. this can reduces tooling design and manufacturing, avoids baseline conversion errors, and improves productivity.
3) Standard coincidence principle. When the final finishing of the surface needs to ensure the positional accuracy, the design reference shall be chosen as the positioning benchmark, which is called the standard coincidence principle. In the process of finishing the surface where the position accuracy is not guaranteed, the standard coincidence principle must be adopted.
4) Self reference principle. When some surface finishing processes require small and uniform allowance, the machined surface itself can be used as a method of locating the reference, which is called the self - reference principle. And the position accuracy at this time shall be guaranteed by the current procedure.