Selection of positioning reference for hardware mechanical processing

In the design of process specifications, the correct selection of positioning benchmarks has a crucial impact on ensuring the machining requirements of parts and arranging the machining sequence reasonably.

There are two types of machining positioning benchmarks: precision benchmark and rough benchmark. The rough benchmark is used to locate the unprocessed surface of the blank. Using the processed surface as a positioning reference, this positioning reference becomes a precision reference.

When selecting a positioning reference, it is often necessary to first choose a precision reference based on the machining requirements of the part, and then reverse the process route, and then consider which set of surfaces to use as the rough reference to machine the precision reference.

1. Principles for selecting precise benchmarks

The selection of precision standards should generally follow the following principles:

(1) Principle of datum coincidence: The design datum of the machined surface should be selected as the precision datum as much as possible, which can avoid positioning errors caused by datum non coincidence.

(2) Unified benchmark principle: It is advisable to select as many surfaces as possible on the workpiece to be machined using the same set of precision benchmarks, in order to ensure the relative positional accuracy between the machined surfaces.

(3) The principle of mutual reference: When the positional accuracy between two machining surfaces on the workpiece is required to be relatively high, the method of repeatedly machining the two machining surfaces based on each other can be adopted.

(4) Self reference principle: Some surface finishing processes require small and uniform machining allowance, often using the machined surface itself as the precision reference.

The above four principles of selecting a precise benchmark cannot sometimes be met simultaneously, and the choice should be made based on actual conditions.

2. Principles for selecting rough benchmarks

(1) The first process of workpiece processing requires the use of coarse benchmarks. The correctness of the selection of coarse benchmarks is not only related to the processing of the first process, but also has a significant impact on the entire process of workpiece processing.

(2) The principle of reasonable allocation of machining allowance: In order to ensure even machining allowance of important surfaces, important surfaces should be selected as rough benchmarks.

(3) Principle of easy clamping: In order to ensure stable positioning and reliable clamping of the workpiece, it is required that the selected rough benchmark is as smooth and polished as possible, without forging burrs, casting sprue cuts or other defects, and has sufficient support area.

(4) The principle that rough benchmarks generally cannot be reused.

The principle of selecting a rough benchmark for the above four options cannot sometimes be considered simultaneously, and can only be determined based on priority.