The difference between over the top positioning and cross positioning, what are the positioning meth

The so-called over-the-table positioning and cross positioning often refer to the parts when the fixture is designed.

Typically, a part has six degrees of freedom, and all six degrees of freedom are constrained when designing fixtures. If the degree of freedom of the design constraint is less than six during the design, it is cross-positioning; If there are more than six, it is a pass-through positioning.

In general, cross positioning can occur when the normal use of the fixture is no longer affected. However, overpositioning is generally not allowed.

For example, when designing a fixture for grinding a flat surface for a train connecting rod, one of the two holes is a cylindrical pin and the other is a flat pin to ensure that there is no underpositioning or only positioning.

Generally, there are functions such as fence alarm, power failure, vibration and so on.

An object has six degrees of freedom in space.

That is, the movement in three directions and the rotation in three directions, if you limit the six degrees of freedom with six points when positioning, we call it full positioning.

Sometimes the processing of goods does not require six degrees of freedom, and full positioning is sufficient. We call this positioning incomplete positioning.

If two or more points are used to locate the same degree of freedom, it is called overpositioning.

The degree of freedom that should be restricted, and which is not restricted, is called under-positioning.

Complete positioning means that the six degrees of freedom of the positioning of the workpiece in the fixture are all limited by the positioning element in the fixture, and occupy a completely definite position in the fixture, which is called complete positioning.

Incomplete positioning is a positioning method, in the positioning process, according to the processing requirements, there is no need to limit all the degrees of freedom of the workpiece, allowing one or several degrees of freedom not to be limited by positioning, called incomplete positioning. Although the workpiece is limited to less than six degrees of freedom, it can guarantee the machining requirements. In actual production, the number of degrees of freedom of the workpiece is generally limited to no less than three.

In machining, a part usually has six degrees of freedom, when the fixture is designed to constrain these six degrees of freedom, and underpositioning is that the degrees of freedom that should be limited according to the processing requirements are not all limited, and the positioning of insufficient constraint points is called underpositioning. Undertargeting is not allowed.

Full Positioning The six degrees of freedom of the workpiece are all limited by the positioning element in the fixture, and the only position that occupies a completely certain position in the fixture is called complete positioning. 2) Incomplete positioning According to the different processing requirements of the workpiece processing surface, the number of positioning bearing points can be less than six. Some degrees of freedom have an impact on the machining requirements, and some have no effect on the machining requirements, and this positioning situation is called incomplete positioning.

Incomplete positioning is allowed, 3) Underpositioning Positioning that is not limited by the degrees of freedom that should be limited according to the processing requirements is called underpositioning. Undertargeting is not allowed. Because the lack of positioning does not guarantee the processing requirements.

4) Overpositioning The positioning of one or several degrees of freedom of the workpiece is repeatedly limited by different positioning elements is called overpositioning. When over-positioning causes deformation of the workpiece or positioning element and affects the machining accuracy, it should be strictly prohibited. However, when over-positioning does not affect the machining accuracy, but is beneficial to improve the machining accuracy, it can also be used.

The so-called over-positioning and under-positioning often refer to the selection of the degree of freedom of the parts in the design of the fixture.

2. Incomplete positioning.

If 1 or more of the 6 degrees of freedom of the workpiece are not restricted, it is called incomplete positioning. Incomplete targeting is not the same as incomplete targeting.

If the workpiece itself is perfectly symmetrical with respect to a certain point or line, the degree of freedom of rotation of the workpiece around this point or line cannot be limited (even if it is limited, it is meaningless). For example, the rotation of a sphere around the axis of the spherical mandrel, the rotation of a cylinder around its own axis, etc.

The machining requirements of the workpiece do not require the restriction of one or a few degrees of freedom. If the upper surface of the flat plate is processed and the thickness of the flat plate and the parallelism with the lower plane are required, it is sufficient to limit only 3 degrees of freedom.

3. Lack of positioning.

The degree of freedom that must be limited during workpiece machining is not completely limited, and it is called underpositioning. Underpositioning does not guarantee the correct installation of the workpiece and is therefore not allowed.

4. Over-positioning.

Overpositioning - A certain degree of freedom (or several degrees of freedom) of the workpiece is constrained by two (or more) constraint points, which is called overpositioning.

Whether over-targeting is allowed or not depends on the specific situation:

1) If the positioning surface of the workpiece is machined, and the shape, size, and position accuracy are high, overpositioning is allowed. Sometimes it is still necessary, because reasonable overpositioning not only does not affect the machining accuracy, but also plays a role in strengthening the stiffness of the process system and increasing the positioning stability.

2) On the contrary, if the positioning surface of the workpiece is a blank surface, or although it has been machined, but the machining accuracy is not high, then overpositioning is generally not allowed, because it may cause inaccurate positioning, or unstable positioning, or positioning interference.