The order of processing between the various surfaces of the part has a great influence. When the workpiece is mounted by the fixture, the choice of the positioning reference will also affect the complexity of the fixture structure. Therefore, the choice of positioning reference is a very important process issue.
What I share today is the principle of selection of rough benchmarks.
When selecting a rough reference, the main requirement is to ensure that each processing surface has sufficient margin so that the position between the machined surface and the unmachined surface conforms to the pattern requirements, and special attention is paid to obtaining the fine base surface as soon as possible. The following principles should be considered when making specific choices:
(1) Select the important surface as the rough reference to ensure that the machining allowance of the important surface on the workpiece is small and uniform, then the surface should be selected as the rough reference. The so-called important surface is generally the surface with high processing precision and surface quality requirements, such as the guide surface of the bed and the spindle hole of the headstock of the lathe, all of which are important surfaces. Therefore, when machining the bed and the headstock, the guide surface or the spindle hole should be used as the rough reference. As shown in Figure 1.
(2) Selecting the unmachined surface as the rough reference In order to ensure the positional requirements between the machined surface and the unmachined surface, the unmachined surface should generally be selected as the rough reference. If there are multiple unmachined surfaces on the workpiece, the unmachined surface with higher requirements on the position of the machined surface should be selected as the rough reference to ensure the requirements and make the shape symmetrical. As shown in Figure 2, the eccentricity between the blank hole and the outer circle is large. The outer circle that is not machined should be selected as the rough reference. The workpiece is clamped in the three-jaw self-centering chuck, and the coaxiality of the blank is set. The error is removed at the time of the pupil to ensure a uniform wall thickness.
(3) Select the surface with the smallest machining allowance as the rough reference. If there is no requirement to ensure that the important surface machining allowance is uniform, if each surface on the part is to be machined, the surface with the smallest machining allowance should be selected as the rough reference. In order to avoid the surface being left in the processing due to insufficient margin, the surface of the blank is discarded.
(4) Select a smooth and smooth surface with a large processing area as a rough reference for reliable workpiece positioning and easy clamping.
(5) The rough reference can only be used once in the same size direction. Because the rough reference itself is an unmachined blank surface, the surface is rough and the precision is low. If it is repeated, it will cause a large error.
