What Is Common Size Analysis?

The positioning size is used to mark the specific position of the part in a large structure. For example, when a cylindrical hole is dug in a cuboid, the distance between the center axis of the hole and the boundary of the cuboid is the positioning size.

In order to ensure processing accuracy, improve production efficiency, reduce labor intensity, and meet the needs of mass production, special fixtures are often used in the manufacturing process of mechanical parts. "One side, two pins" is a common positioning method for box and bracket parts. In the past, fixtures designed in the past often took a lot of time and effort when assembling and debugging. The common problem is that the workpiece is not easy to install and needs to be re-positioned. In this way, not only the manufacturing cycle of the jig is long, but also because the positioning pin is repeatedly disassembled and assembled, its assembly accuracy is reduced, thereby affecting the maintainability of the processing accuracy.

When the workpiece is positioned with "two holes on one side", the corresponding positioning elements on the fixture are "one side and two pins". Among the "two pins", one is a cylindrical pin and the other is a chamfered pin. The positioning dimensions are the working diameter of the two pins and the center distance between the two pins. The reasonable design of the positioning size is the key to the design of the entire fixture.

Conventional design method

The conventional design method is: the basic size of the positioning pin is taken from the basic size of the positioning hole on the workpiece, and the tolerance band is determined according to the method of dimensional tolerance selection, that is, the small gap fit H / g or H / f is selected for the fit nature. The height of the hole is 1 or 2 levels; the tolerance between the center distance of the two positioning pins is 1/5 to 1/3 of the tolerance of the center distance of the workpiece hole. The fixture designed by this method will have difficulty in mounting the workpiece in actual use, that is, the workpiece cannot be mounted on the fixture well. The solution is generally to grind the working diameter of the positioning pin when debugging the fixture.

Routine design analysis

The fixture designed and manufactured according to the conventional method often interferes with the positioning hole on the workpiece and the positioning pin on the fixture during use. The main reasons are as follows:

(1) The positioning hole has a shape error

The actual shape of the positioning hole on the workpiece is not ideal. The error of the shape of the actual hole mainly includes errors of roundness and cylindricity. The dimensional accuracy of the two holes used for positioning on the workpiece is generally guaranteed. However, due to processing errors and internal stress release of the workpiece, the shape of the positioning hole has some errors, which greatly reduces the actual fitting clearance between the positioning pin and the positioning hole. If the minimum clearance left in the design is small, the actual installation It will become an interference fit, which makes installation difficult.

(2) The verticality error between the positioning hole and the positioning plane

The included angle between the axis of the actual positioning hole and the positioning plane is not an ideal 90 °. There is a certain verticality error between the axis of the hole actually obtained and the positioning plane. This non-verticality will reduce the actual size of the positioning hole, resulting in a theoretically compatible pin and hole, which is difficult to install uniformly during actual assembly.

(3) The axis of the two positioning holes on the workpiece has a parallelism error

The actual axis of the two holes used for positioning on the workpiece is not ideally parallel. There is a parallelism error between the actual axes of the two holes. This non-parallelism will reduce the actual center distance of the two holes, which will cause The positioning hole interferes with the positioning pin on the clamp.

It can be seen that, when designing the fixture, if only the dimensional fit of the positioning dimensions is considered, and the shape and position error of the positioning surface is not considered, there must be some problems in the design. This design will bring trouble to the manufacture of the fixture, and if the fixture is put into use without being adjusted, the operator will force the fixture to use it, which will result in actual over-positioning, which will cause the positioning pin to deform and cause force. Increasing positioning errors increase machining errors.

The design of the positioning size should not only meet the processing accuracy requirements, but also make the workpiece easy to load and unload. In order to meet the processing accuracy, the matching gap between the positioning hole and the positioning pin must be as small as possible or no gap; To meet the convenience of workpiece loading and unloading, there must be sufficient clearance between the positioning hole and the positioning pin. Consistent with the center distance of the two holes. Only by comprehensively considering the above factors can a reasonable positioning size be designed.

Design the diameter of the positioning pin

The effective dimension refers to the dimension that is effective when the part is assembled, and is formed by the combination of the actual size and the shape and position error. Because a set of clamps is applicable to all the same kind of parts, the design must meet the interchangeability requirements. The design of the positioning pin must consider the effective size of the hole, and it must ensure good assembly in the limit state. This limit state is the maximum solid effective state, and the corresponding size is the maximum solid effective size, which is equal to the minimum limit size of the positioning hole diameter minus the shape and position tolerance of the hole axis.

Determine the actual size and shape tolerance of the positioning hole

For workpieces such as connecting rods that are marked with the shape and position tolerances of the positioning holes on the part drawing, the positioning dimensions can be obtained simply by calculating according to the requirements of the part drawing. However, for some workpieces that are positioned with process holes such as bolt holes, such as various boxes, the actual size and shape and position tolerances of the positioning holes cannot be known from the drawings. Although the dimensions of the positioning holes are written on the process file, the holes are generally not marked The shape and position tolerance can only be determined by measuring the actual hole size and shape and position error on the workpiece, and replacing the theoretical size and shape and position tolerance with the actual size and shape and position error. This method is feasible. Because for the same kind of workpiece, the processing method of the positioning hole and positioning surface is determined, the structural shape of the workpiece is also consistent, and the size and shape error of the hole also change according to the same law ^[2] .