Six Point Positioning——Basic Theory of Coordinate System

In the space rectangular coordinate system, a rigid body has six degrees of freedom, that is, three degrees of freedom moving along the X, Y, and Z axes and three degrees of freedom rotating around these three axes. Having six reasonably distributed support points restricts the six degrees of freedom of the workpiece, so that the workpiece occupies the correct position in the fixture, which is called the six-point positioning rule.

The basic concept of clamping

Clamping: the process of positioning and clamping the workpiece on the machine tool or fixture.

Positioning: Make a batch of workpieces in the correct processing position relative to the tool on the machine tool or fixture

Operation process.

Clamping: After the workpiece is positioned in the fixture, press it tightly and clamp it firmly, so that the workpiece is in the process of processing.

Always maintain the correct machining position obtained during positioning.

The role of clamping : to keep the workpiece in the correct processing position obtained by positioning in the fixture, so that the workpiece

During processing, the correct positioning that has been obtained will not be damaged due to external forces such as cutting force.

The difference between positioning and clamping : Positioning is to make the workpiece occupy a correct position, and clamping is to keep the workpiece in this correct position. Positioning and clamping are two very important concepts in fixture design. The two are closely related and indispensable, but there are strict differences in concept: The role of positioning is to determine that the workpiece is in the fixture relative to the tool. Correct processing position, and the role of clamping is to ensure that the workpiece always maintains the correct processing position determined by positioning during processing. Clamping cannot replace positioning.

The purpose of workpiece positioning : to make the same batch of workpieces have the correct processing position on the machine tool or fixture.

Fixture positioning involves three levels of relationships: (1) the positioning of the workpiece on the fixture; (2) the positioning of the fixture relative to the machine tool; (3) the positioning of the workpiece relative to the machine tool-indirectly guaranteed by the fixture. This chapter mainly discusses the positioning principle of the workpiece on the fixture.

After the workpiece is positioned, the clamping force must be generated by a certain device to keep the workpiece in an accurate position. This kind of device that generates clamping force is the clamping device.

Positioning of the workpiece in the fixture

Benchmark

The points, lines, and areas on the part used to determine the positions of other points, lines, and areas.

Design basis

It is used to determine the datum of the point, line and surface position on the part drawing.

Positioning datum

Datum used in part processing, measurement and assembly process. Divided into process benchmarks, positioning benchmarks, measurement benchmarks and assembly benchmarks.

Six-point positioning principle

A free object has six possibilities for three mutually perpendicular coordinate systems, three of which are movement and three are rotation. It is customary to call the possibility of such activities as degrees of freedom, so any free object in space has six degrees of freedom.

Six supporting points arranged according to certain rules are adopted to limit the six degrees of freedom of the workpiece, so that the workpiece occupies the correct position in the machine tool or fixture.

③Three supporting points are arranged in the XOY plane, and the restricted degrees of freedom of the workpiece are three:

Constrained two rotational degrees of freedom (rotation around X and Y axes) and one translational degree of freedom

(Move along the Z axis).

②Two supporting points are arranged in the XOZ plane, and the restricted degree of freedom of the workpiece is two:

Z axis rotation and Y axis translation.

①A supporting point is arranged in the YOZ plane, and the restricted degree of freedom of the workpiece is one:

X axis translation.

At this time, the six degrees of freedom are completely locked.