Principles for selection of CNC machining positioning reference
(1) In the part, select the design standard as the location standard as far as possible.
Selecting the design datum as the position of the positioning datum can prevent the positioning error caused by the datum mismatch, ensure the machining accuracy and simplify the programming. When making a processing plan for a part, first select the best finishing conditions according to the principle of meeting the conditions to specify the processing path of the part. Therefore, in the initial processing process, the surface to be machined must be considered as a rough standard.
(2) When the positioning datum and design datum of the part do not match, and the machining surface and design datum are not processed simultaneously in the same installation, the drawing of the part must be carefully analyzed to determine the design function of the part design datum. Through the calculation of dimension chain, the tolerance range between positioning datum and design datum is strictly specified to ensure the machining accuracy.
(3) If the CNC milling machine cannot finish the whole surface processing including the design datum at the same time, it should be considered that the selected datum can be used for positioning, and then all the main precision parts can be processed at one time.
(4) The selection of positioning standards shall ensure the completion of as many processing contents as possible. For this reason, we must consider the positioning method that a single surface can be machined. For non-rotating parts, it is better to use the positioning scheme of one and two holes so that the tool can process another surface. If the workpiece does not have suitable holes, you can add and place machining holes.
(5) During batch processing, the part position reference should match the tool reference with the workpiece coordinate system as much as possible (the dimension value between the workpiece coordinate system origin and the position reference after processing).
During batch processing, the fixture is used to position and install the workpiece. The tool sets one workpiece coordinate system at a time, and then processes a series of workpiece. If the tool reference of the workpiece coordinate system is set to match the part positioning reference, the positioning reference is directly transferred to reduce the positioning error.
(6) If multiple installations are required, the principle of uniform standards must be followed.