+86 18924600025 Frank Zhou

News

Company news
your present position: Home » News  » Company news

Precision machining process - part positioning principle

2018-07-13 Administrator home station clicks:

(I) six-point positioning principle

As shown in figure 1-1, any free rigid body, the space has six degrees of freedom (degree of freedom is completely determine the geometrical position of objects in space the number of independent coordinates required), namely the space coordinate axis x, y, z three directions of movement and around the three axes of rotation (on the x -, y -, z - and x, y, z).

20171010164823552355.jpg

               图1-1


Figure 1-2 shows the localization of a rectangular workpiece in the spatial coordinate system.

20171010165186808680.jpg

 Figure 1-2

In setting 3 x  y plane bearing (not on a straight line), the workpiece on the three bearing, can limit the artifacts -, y, z x3 degrees of freedom; In setting two x  z plane bearing (two attachment cannot be parallel to the z axis), put the workpiece on the two bearing, two degrees of freedom can limit - > y, z; In setting a y  z plane bearing, depend on the support of components, limits the x - > the degrees of freedom. In this way, all six degrees of freedom of the workpiece are restricted, and the position of the workpiece in space is completely determined.

The essence of workpiece positioning is to limit the workpiece's freedom so that the workpiece has a certain correct machining position in the fixture.

In this spatial coordinate system, the six supports set are called positioning support points, which are actually positioning elements. In this way, the problem of positioning analysis is very simple and clear.

In the fixture, six positioning support points are arranged reasonably to contact the positioning reference of the workpiece to limit the six degrees of freedom of movement of the workpiece.

When applying the six-point positioning principle to realize correct positioning of workpiece in jig, the following points should be noted:

1) the plane of three positioning support points is limited to one moving degree of freedom and two rotational degrees of freedom, which is called the main positioning surface. The surface selected as the main positioning of the workpiece should try to make the area as large as possible, while the distribution of the three positioning support points should be as far away from each other as possible, and should never be distributed on a straight line, so as to bear large external forces and improve the positioning stability.

2) set the plane limit of two degrees of freedom for two positioning support points, which is called the guiding positioning surface. The surface selected for guiding positioning on the workpiece should try to make the area narrow and long, and the distribution of two positioning support points should be as far away from each other as possible in the longitudinal direction of the plane, so as to make the guiding function better and improve the positioning stability.

3) set the plane of a positioning support point to limit one degree of freedom, and call the assumed positioning surface or anti-rotation positioning surface. Whether it is thrust or anti-rotation depends on whether the degree of freedom restricted by the positioning support point is moving or rotating.

4) a positioning support point can only limit one degree of freedom.

5) the positioning supporting point must always be in close contact with the positioning reference of the workpiece. Once separated, the positioning support point loses the effect of limiting the freedom of workpiece.

6) the number of degrees of freedom that the workpiece needs to be limited in positioning and which degrees of freedom are determined by the processing requirements of the workpiece in this process. Specific analysis should be made according to the actual situation, and the number and distribution of positioning support points should be reasonably set.

7) the degree of freedom limited by positioning support point shall not be repeated or contradictory in principle.

Complete positioning and incomplete positioning

All 6 degrees of freedom of the workpiece are restricted and occupy a completely certain unique position in the fixture, which is called complete positioning. As shown in FIG. 1-3, a closed hole is processed on a rectangular workpiece. In order to meet all processing requirements, 6 degrees of freedom of the workpiece must be limited.

There is no full limit of 6 degrees of freedom of the workpiece, but it can also meet the processing requirements of positioning, known as incomplete positioning. As shown in FIG. 1-4, a cut slot is processed on a rectangular workpiece, and only 5 degrees of freedom of the workpiece is required to meet all processing requirements, while the y-degree of freedom of the workpiece can be unrestricted, which is incomplete positioning. It is particularly important to note that, in order to facilitate the bearing of cutting force, clamping force, or to ensure the feeding length of a batch of workpieces is consistent, the freedom of freedom without machining requirements is sometimes limited. For example, the y-degree of freedom of workpiece shown in FIG. 1-4 is also limited. Actual machining is allowed and sometimes necessary.

20171010165236743674.jpg

Figure 1-3 localization analysis of the drilling process of rectangular workpiece figure 1-4 localization analysis of the milling process of rectangular workpiece


Underpositioning and overpositioning

According to the processing requirements, the workpiece must limit the degree of freedom does not reach the full limit localization, called underlocalization. Underpositioning inevitably leads to failure to properly guarantee the processing requirements specified in the process. As shown in FIG. 1-5, when there is no groove in a certain shaft in milling, only 4 degrees of freedom of the workpiece are limited, while x - to - degree of freedom is not limited, so the length and size of the groove processed cannot be guaranteed consistent. Therefore, underpositioning is not allowed.

20171010165484828482.jpg

 Figure 1-5 positioning analysis of milling groove process


When the workpiece is positioned in the fixture, if several positioning support points are repeatedly limited to the same degree of freedom or several degrees of freedom, it is called overpositioning. Whether overpositioning is allowed or not shall be specifically analyzed according to different processing conditions of the workpiece. Generally, overpositioning is not allowed when the workpiece takes the blank surface with low shape accuracy and position accuracy as the positioning reference. In order to improve the stability and stiffness of the workpiece positioning, the workpiece positioning is allowed under certain conditions when the machined or high precision blank surface is used as the positioning reference. As shown in figure 1-6, when milling the upper plane of a rectangular workpiece, the workpiece takes the bottom plane as the positioning reference. When 3 positioning support points are set, as shown in FIG. 1-6a, they are incomplete positioning and are reasonable solutions.

20171010165891499149.jpg

FIG. 1-6 localization analysis of a milling rectangular part

When 4 positioning support points are set, as shown in FIG. 1-6b, they are over-positioning. If the bottom plane is rough or the positioning analysis of 4 rectangular workpieces in FIG. 1-6 are not on the same plane, and there are only 3 contact points, the position of the workpiece is uncertain or the position of a batch of workpieces is inconsistent, which is an unreasonable scheme. If the bottom plane has been processed to ensure that four positioning support points are on the same plane, then the position of a batch of workpieces in the fixture is basically the same. The increased positioning support points can make the workpiece positioning more stable, which is more conducive to ensuring the machining accuracy of the workpiece. Since the four positioning support points are on the same plane, three positioning support points actually limit the three degrees of freedom of the workpiece, which is in line with the positioning principle.

If there is serious interference and conflict between positioning support points that repeatedly restrict the same degree of freedom, resulting in deformation of workpiece or fixture, thus obviously affecting positioning accuracy, such over-positioning must be strictly prohibited. Table 1-1 lists the common typical positioning methods and their positioning analysis.

20171010170176167616.jpg


20171010170156885688.jpg