1. Selection of rough datum

In order to make all the machined surfaces have sufficient machining allowance and ensure that each machined surface has a certain position accuracy to the unmachined surface, the following principles should be followed in the selection of rough datum. Rough datum of lathe bed

(1) Select the unprocessed table on the workpiece as the rough reference

(2) Select an important surface or the surface with the smallest machining allowance as the rough reference

(3) Choose a relatively smooth and clean surface with a large area as the rough reference

(4) Rough benchmarks can generally only be used once, try to avoid repeated use

2. Selection of fine benchmark The selection of fine benchmark should abide by the following principles:

(1) The principle of datum coincidence is to choose the design datum as the positioning datum as much as possible to avoid the positioning error caused by the misalignment of the datum.

(2) The principle of the same datum should make as many surface processing as possible adopt the same fine datum.

(3) Generally, when selecting a fine reference, it is required that the positioning is accurate, stable and reliable, and the fixture structure is simple and easy to operate.