Types And Selection Principles Of Datums In Parts Machining

DALIAN, China - Dec. 26, 2022 - PRLog -- Type Of Datum In Part Machining

Datum refers to those points, lines and surfaces used to determine the geometric relationship between geometric elements on production objects. According to the different functions of the datum, the datum can be divided into two categories: design datum and process datum.

1. Design Datum

The datum adopted in the design drawing is called design datum. For the box as shown in the figure below, A and B are the dimensions of the center of the hole, and their design basis is ① and ②, which reflect lines on the figure. The design basis of aperture D is the axis, and the point is reflected in the figure.

1. Process Datum

The datum used for positioning, inspection and assembly in the processing of parts is called process datum, which includes three kinds of positioning datum, measuring datum and assembly datum. For example, for boring round holes as shown in the above figure, one installation method is to use ① and ② surfaces as the positioning datum, and the positioning datum coincides with the design datum.

Another method is to take ① and ③ planes as the positioning datum, at this time, the positioning datum does not coincide with the design datum.

In the first process, the blank surface is used as the positioning reference. The positioning reference that has not been cut is called the rough reference, which is only used once. When continuing machining, the machined surface is used as the positioning reference, and the positioning reference after cutting is called precision reference.

Selection Principle Of Datum In Part Machining

1. Selection Principle Of Rough Datum

The rough datum is the datum that is located on the blank surface in the initial processing procedure. When selecting the rough base, ensure that each surface has enough machining allowance, so that the machined surface has a proper mutual position with the non machined surface. The selection principle is:

(1) The surface of the workpiece that does not need to be machined is used as the rough datum to ensure the minimum position error between the machined surface and the non machined surface.

(2) If it is necessary to ensure that the machining allowance of an important surface of the workpiece is uniform, the surface shall be selected as the rough datum.

(3) The flat and large blank surface shall be used as the rough datum as far as possible.
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