Accurate design of steel ball positioning indexing plate for functional parts in CNC machine tools

Abstract: In this paper, the precise design of steel ball positioning indexing plate mechanism in CNC machine tools, rotary-type combined machine tools and various rotary tables is analyzed. The factors affecting the accuracy of indexing positioning are analyzed. The solution is proposed and the application examples are given. .

In CNC machine tools and turn-on combination machine tools and various rotary tables, the indexing, indexing and positioning work of the commonly used indexing table is automatically performed according to the instructions of the control system. Each time the index is rotated by a certain angle (90 °, 60°, 45°, etc.), but it is difficult to achieve the indexing accuracy of the mechanism for indexing the table. Therefore, special positioning components are required to ensure that the indexing components are often indexing tables. A key part of design, manufacturing and tuning. At present, the commonly used positioning components include pin positioning, reverse positioning, toothed disc positioning and steel ball positioning. In order to obtain high-precision indexing, a steel ball positioning indexing mechanism is widely used. It has some advantages of the positioning of the toothed disc, high self-centering and indexing precision, and simple manufacturing. The steel ball can be purchased out, especially for the small-sized high-precision indexing rotary table, which is also ideal for steel ball positioning. The positioning of the steel ball is based on the principle that the diameter of the steel ball can be pre-selected to a small range of error, thereby obtaining precise indexing accuracy. In this paper, the structural design of the steel ball positioning indexing plate mechanism is carried out, and the factors affecting the indexing accuracy of the steel ball positioning indexing plate mechanism are analyzed, and the solution is proposed.

1 Structure working principle

The structure of the steel ball positioning indexing mechanism is shown in Figure 1. It relies on a high-precision base disc 4 and a high-precision steel ball to improve the accuracy of the indexing mechanism. 120 balls of ¢8 steel balls are arranged on the base cylinder of ¢297.61238 mm without gaps, and are in contact with the same end face. The compression ring 5 is placed outside the steel ball, and a 30° oblique groove is facing the steel ball. When the compression ring is pressed, the steel ball is pressed against the base cylinder and the end surface. In addition, in order to prevent the steel ball from rotating along the circumference to break the index, a wedge-shaped cylindrical pin 12 made of high-strength steel is placed between a pair of steel balls, and the head is a 45° wedge surface to ensure High precision indexing. This mechanism can be used to machine workpieces with high-precision indexing such as the sprocket wheel (ie, the face tooth indexing plate).

During operation, when the handle 7 rotates clockwise, the lifting screw sleeve 9 drives the main shaft and the table 1 to rise, and at this time, the pressing ring 5 drives the steel ball to rise, and the table can be rotated for indexing. Finally, turning the handle 7 counterclockwise can lower the clamping sleeve 9 and clamp it. The indexing principle of the steel ball is shown in Fig. 2.

2 Factors affecting the accuracy of the indexing and solutions

2.1 Selection of steel balls

The dimensional accuracy of the steel ball is the determining factor for the accuracy of the indexing. To this end, the accuracy of the steel ball is guaranteed by a pre-selected method. With a 1 micron micrometer, 240 steel balls with a diameter error of 1 micron are preselected. The diameter error and roundness of the steel ball are preferably preselected below 0.5 micrometer.

2.2 Processing of base disc

The base circle diameter of the base disc is required to be high, and the base circle diameter is calculated as follows:

As an example, as shown in Figure 3, the diameter of the steel ball is ¢ 8 mm. It is known that the circumference is equally divided by n=120, so ∠AOB=3°, AB is the steel ball connecting the heart line, so AB=8, OC is the bisector of the angle of the ∠AOC in the triangle, so △ACO is a right triangle, and ∠ AOC=1°30', then: AO=AC/sin1o30?=4/0.02617695=152.80619 mm, therefore, the diameter d of the base cylinder of the base disc 4 is:

d=2×ao=2(AO-Aa1)=297.61238 mm

(where Aa is the radius of the steel ball, Aa = 4 mm)

Grinding and measuring the base circle is difficult. In order to solve this problem, we use the following method: first grind the hardened base disc to two planes, the parallelism is within 0.01 mm, and the surface is ground. The inner hole shall be such that the plane perpendicular to the center of the inner hole is 0.001/100 mm, and then a special mandrel is mounted, and the base circle is ground with the inner hole and the end face as a reference. Grinding the base circle while grinding the end faces. Grind the ¢297.6123 mm to a base of +0.1 mm, one at a time from 0.01 to 0.02 mm for each grinding, that is, remove the workpiece together with the mandrel (the part cannot be removed from the mandrel).

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