Table of Contents
Machine tool fixtures are commonly used in machining. Traditional fixture design, there is detailed information and mature experience for reference. CNC milling machines and machining centers have been widely used in recent years, with their high processing efficiency, high precision, and high adaptability to the workpiece so traditional machining has undergone great changes. But in the file, about CNC milling machine and machining center fixture design content is very little. In this paper, according to the processing characteristics of CNC milling machines and machining centers, to explore its fixture design methods, so that CNC milling machines and machining centers can give full play to the processing potential.
Fixture design utilizes multiple workpieces in a single clamping.
CNC milling machine to compressed air as the driving force, loosening, and clamping tool, It is easy to manually change the tool, while the machining center can realize automatic tool change. Processing tool change can realize the sequential processing of a variety of surfaces, but each time the tool change is more time-consuming, especially the automatic tool change sometimes fails (machining centers, most of the failures in the automatic tool change system). Therefore, under the premise of meeting the processing requirements, reducing the number of tool changes can save auxiliary time, can improve the reliability of CNC machine tools. In this paper, a reasonable fixture design (such as the use of clamping multiple workpieces) to properly adjust the process can minimize the number of tool changes.
Such as the workpiece shown in Figure 1, to complete the vertical machining centers to expand, reaming hole a and hole b original fixture design each time the clamping of 1 workpiece, the machining process for change hole a reaming cutter, reaming a → change hole b reaming cutter reaming b → change hole a reamer a → change hole b reamer b. From the above machining process to see that the machining of each 1 workpiece needs to be changed 4 times.
Fig1 Workpiece
Now according to the dimensions of the workpiece and machine travel, the fixture design for each clamping 8 workpieces, as shown in Figure 2, the process is as follows: change the hole a reaming cutter, to expand the workpiece 1 ~ workpiece 8 of the hole a → change hole b reaming cutter, to expand the workpiece 1 ~ workpiece 8 of the hole b → change hole a reamer, to ream the workpiece 1 ~ workpiece 8 of the hole a → change hole b reamer, to ream the workpiece 1 ~ workpiece 8 of the hole b.
Fig.2 Clamping 8 workpieces at a time.
From the above machining process, it can be seen that for machining 8 workpieces, only 4 tool changes are required, which saves a lot of auxiliary time (relative to the machine table movement time).
Multi-piece clamping fixture design points
1. Reasonable arrangement of workpieces
According to the dimensions of the workpiece, the size of the machine table, machine travel, and production batch determines the number of workpieces clamped each time and layout. The distance between the workpiece should be appropriate, such as Figure 2 to take the appropriate distance between the workpiece m and n to load and unload the workpiece and remove chips. In addition, the openness of the workpiece after installation should be good, to realize the process of centralized machining.
2. The distance between each group of positioning elements should be accurate
The distance between the positioning elements is shown in Figure 3. In Figure 3, the fixture adopts two pins on one side for positioning, and the distances between the cylindrical pins and diamond-shaped pins in each group of positioning elements are determined according to the dimensions of the workpiece. Distance between each group of positioning elements, such as the distance between the two cylindrical pins should be accurate to ensure that the moving parts of the machine tool (such as tools) according to the program to move, relative to the workpiece have an accurate position.
3. Reasonable clamping
Clamping elements as simple as possible, so that the tool movement has more safety space. Try to consider linkage clamping, and realize simultaneous clamping of multiple workpieces, to reduce the number of clamping elements [2]. As shown in Fig. 3, each platen clamps 2 workpieces at the same time. Due to the CNC milling machine’s more centralized processing, roughing and finishing are sometimes difficult to separate, so the clamping force is large clearly. This is easy to leave the pressure plate indentation on the surface of the workpiece. A simple and effective solution is to solder a layer of copper on the part of the pressure plate in contact with the workpiece, as shown in Figure 4.
Fig. 3 Distance between positioning elements
Fig.4 Platen Soldering Copper
4. Vertical CNC milling machine fixture design
In vertical CNC milling machine processing, metal chips are easy to fall on the workpiece and fixture. When designing fixtures, special consideration should be given to chip removal, which is particularly important for the process of centralized continuous processing of machining centers. Reasonable chip removal should make the chip does not affect the tool cutting automatically; so that the chip does not easily fall on the positioning elements of the fixture to ensure positioning accuracy; so that the cutting fluid is easy to flush, take away the chip; so that the compressed air to facilitate the blowing off the chip.
5. Fixture design for horizontal CNC milling machine
A horizontal CNC milling machine or machining center is generally equipped with an indexing table (or rotary table). In the process of machining, the fixture should also rotate with the table. The design of a multi-piece clamping fixture should ensure that the workpiece on the position of the same machining elements before and after the rotary can be accurately coincidentally (see examples). A fixture as close as possible to the edge of the table, so that the distance between the workpiece and the spindle taper hole end face is small, which is conducive to reducing the length of the tool holder, but also conducive to chip removal.
Example of Multi-Part Clamping Fixture Design
This fixture is used on a horizontal machining center for machining 2 end faces and 2 coaxial holes of the workpiece shown in Figure 5.
Fig. 5 Workpiece 2
1. Workpiece processing requirements
Face C, face D, and hole 2-ϕ15H7 ( 0+0.018 ) of the workpieces have already been machined. This process involves milling face A, and face B, reaming, and boring coaxial hole 1 and coaxial hole 2. Because of the small diameters of the holes in the middle of coaxial hole 1 and coaxial hole 2, machining needs to be done from both ends.
2. Clamp Functions
The workpiece face C (D) and hole 2-ϕ15H7 ( 0+0.018 ) are selected as the locating datum, and the fixture uses two pins on one side (a cylindrical pin and a rhombic pin) to locate the workpiece. Through the clever design of the fixture, a set of fixtures can be used in the same process to complete the processing of the workpiece and can realize the installation of six workpieces at a time. The horizontal machining center fixture is shown in Figure 6.
3. Machining process with fixtures
When the workpiece is mounted, face C is in contact with the fixture-locating surface. Now the face A on the workpiece is located at the edge of the fixture.
The machining process is: milling is located in the fixture Ⅰ – Ⅰ edge of the workpiece face A → proceed in sequence expanding, boring the workpiece with the coaxial of the hole 1 and hole 2, in which the length of the central hole should be processed more than half the size of the hole(the length of the small hole in the middle should be processed more than half.).
The table turns 180°. At this time, the fixture Ⅰ – Ⅰ and Ⅱ – Ⅱ side of the interchange, Ⅱ – Ⅱ side to reach the processing area. After rotation, the end faces of the workpiece and the position of the coaxial holes 1 and 2 are the same as before rotation.
Next, the face A of each workpiece on the II-II side of the fixture, as well as the coaxial holes 1 and 2, are machined, with half of the length of the small hole in the center being machined. This completes the machining of face A and the hole from the end of face A.
Release the workpiece and turn it over by 180°. Face D of the workpiece is now in contact with the fixture locating surface, face B, at the edge of the fixture. Next, face B and the hole from face B are machined. The table is turned 180° in the same way as before. The small hole in the center should be connected to the previously machined part.
4. Key points of fixture design
1) Set up the rotary center and side alignment datum. The rotary center of this fixture is ϕ30H5 hole, when installing, ϕ30H5 hole coincides with the rotary center of the indexing table. Select the long side of the fixture with high precision as the calibration reference when the fixture is installed.
2) Determine the position of the cylindrical pin and rhombic pin. The cylindrical pin and rhombic pin position determine the position of the workpiece in the fixture. Workpiece installation, to meet the following requirements:(1) the center of the hole to the center of the rotation distance should be symmetrical and consistent.
Such Figure 6, the distance was X1 、X2、X3. (2) the processing end face (hole end face) to the center of rotation distance should be symmetrical and consistent. As in Figure 6, the distance at the same time for. This makes the corresponding point on the workpiece to the center of the rotary distance equal, such as in Figure 6, two dimensions l. This ensures that the table rotates 180 °, the workpiece on the position of holes 1, holes 2, and end face and rotary before the complete coincidence (one of the benefits is to facilitate the preparation of CNC programming and call subroutines). Design, through the appropriate cylindrical pin, diamond pin position to meet the above requirements. As can be seen in Figure 6, to meet these requirements, the distance from the locating pin to the center of rotation in the X and Y directions is marked with tolerances.
Fig. 6 Horizontal machining center fixture
3) The edge of the workpiece extends beyond the edge of the fixture. As shown in Fig. 6, the edge of the workpiece extends 4mm in the Y-direction to facilitate the machining of the end face and the removal of chips.
5. Clamp positioning accuracy analysis
This process is part of the processing requirements as shown in Figure 5 (due to the subsequent rigid boring and reaming process and floating grinding process, the requirements of this process are not high). The diameter of the two positioning holes is ϕ15H7 ( 0+0.018 ), the center distance is 140±0.03, the center distance of the two pins is 140±0.01, the diameter of the cylindrical pin is ϕ15 -0.014-0.006, and the diameter of the rhombic pin is ϕ15-0.023-0.015, as shown in Fig.6.
As shown in Fig.5, among the process dimensions, the hole diameter dimension (not shown) is guaranteed by the fixed-size tool, which is not affected by the fixture positioning accuracy. Hole depth size (not shown) and end face dimensions are low-precision, no tolerance, and fixture positioning accuracy has little effect on it. Now analyze the fixture positioning error on the process size of 35 ± 0.1 and 85 ± 0.15.
1)Positioning error on the size of 35 ± 0.1. Positioning error ΔD, including datum non-coincidence error ΔB and datum displacement error ΔY. Dimension 35 ± 0.1 of the process reference and positioning reference coincide, so ΔB ＝0. Reference displacement error on the size of 35 ± 0.1 of the impact of the following three factors is analyzed as follows.
Fig. 7 Datum displacement for the positioning of a cylindrical pin and a rhombic pin
(1) Due to the positioning pin and positioning holes between the existence of clearance, so that the benchmark displacement, results in the benchmark displacement error. The cylindrical pin and rhombic pin positioning of the benchmark displacement shown in Figure 7, consider the extreme case, in the cylindrical pin at the existence of the maximum gap X1max, and the rhombic pin at the existence of the maximum gap X2max respectively.
Where: D1max with the cylindrical pin with the maximum diameter of the workpiece hole, D1max ＝15＋0.018; D2max with the rhombic pin with the maximum diameter of the workpiece hole, D2max ＝15＋0.018; d1min for the minimum diameter of the cylindrical pin, d1min ＝15-0.014; d2min for the minimum diameter of the rhombus pin,d2min＝15—0.023; X1max is the maximum clearance at the cylindrical pin, X1max ＝（15＋0.018）—（15—0.014）＝0.032; X2max is the maximum clearance at the rhombic pin, X2max ＝（15＋0.018）—（15—0.023）＝0.041.
In the dimension 35±0.1 direction, the maximum clearance that exists results in a reference displacement error ΔY. The datum displacement error ΔY includes linear displacement error ΔY1 and angular displacement error ΔY2. The linear displacement error ΔY1 is the maximum clearance between X1max and X2max The angular displacement error ΔY2 is caused by the corner error ± Δα： tan Δα ＝ （X1max＋ X2max ）／（2L）Where: Δα for the corner error, can be deflected in both directions, so ± Δα; L for the center distance, L = 140.
Since the clearance at the rhombic pin is large, the resulting error is large, so calculate the datum displacement error at the rhombic pin. The datum displacement errors ΔY‘ are largest at face B on the workpiece near the end of the diamond pin:
Where: ΔY‘ is the reference displacement error generated at face B, ΔY1 is the linear displacement error, at the rhombic pin, ΔY1 ＝X2max ＝0.041; ΔY2 is the angular displacement error, considering the deflection in both directions, at face B, ΔY2 ＝ 2 L1 tan Δα; L1 is the distance from face B to the rhombic pin, L1＝ 15.
(2)Due to errors in the distance from the positioning pin to the center of the table rotary table 180 ° rotary table, positioning reference displacement (the program determines tool position, is unchanged), resulting in the benchmark displacement error o f ΔY“ 。ΔY“ size for the positioning pin to the table rotary center of the size of the tolerance, as shown in Figure 6, X-direction size of the deviation of ± 0.01, so ΔY“ ＝0.02.
Fig. 8 Table rotation angle error generates datum displacement.
(3)due to the table rotary angle error, resulting in the rotation, positioning reference displacement, resulting in the benchmark displacement error ΔY“‘。 Study and rotary center of the largest distance from the positioning pin. Figure 6, from the center of the rotary X direction of the largest size of 215, Y direction of 165. now the table rotary error for ±2“. Table rotary angle error generated benchmark displacement shown in Figure 8, after the ideal position in the rotary pointＯ, the maximum deviation from the position in the pointＯ′and pointＯ″. Benchmark displacement error ΔY“ forＯ′Ｏ″ in the 35±0∙1. The square direction of the projection, that is, theＯ′Ｎof the Length.
The reference displacement error ΔY that affects dimension 35 ± 0.1 is the sum of the reference displacement errors from the three factors:
The positioning error ΔD, which affects dimension 35 ± 0.1, is the sum of the datum non-recombination error ΔB and the datum displacement error ΔY :
The positioning error ΔD is calculated to be approximately 1/3 of the 35 ± 0.1 tolerance of the process dimension, which meets the requirements. The calculation of the positioning error here is obtained by analyzing the geometrical relationships. It is also possible to get an accurate drawing using CAD software.
2) The effect of positioning error on dimension 85 ± 0.1 is similar to that on 35 ± 0.1.
Conclusion
The fixture design of the CNC milling machine is different from that of the traditional fixture design. According to the processing characteristics of the CNC milling machine and machining center, reasonable design fixture, try to use a clamping multiple workpieces, to make full use of the processing potential of the machine tool.
Keyword: CNC machining parts