In the final analysis, the fixture is: positioning and clamping, with only one purpose: not to deform. This is the summary of people in the industry when they summarize the fixture design, but things are far from that simple. In the process of contacting various solutions, we found that there are always some positioning and clamping problems in the preliminary design that are not well solved. Even the most innovative solutions will lose their practical significance. Only by understanding the basic knowledge of positioning and clamping can we fundamentally ensure the integrity of the fixture design and processing plan.
Locator knowledge
1. Basic principles for positioning from the side of the workpiece
When positioning from the side of the workpiece, just like the support, the 3-point principle is the most basic principle. This is the same principle as the support, which is called the 3-point principle. It is derived from the principle of "three points that are not on the same straight line determine a plane." Among the 4 points, 3 points can determine a surface, so a total of 4 surfaces can be determined. However, no matter how it is positioned, it is quite difficult to make the 4th point in the same plane.
Picture ▲ 3-point principle
For example, when using four fixed-height positioners, only three points somewhere can contact the workpiece, and there is a high possibility that the remaining fourth point does not contact the workpiece.
Therefore, when configuring the locator, it is generally based on three points, and the distance between these three points should be increased as much as possible.
In addition, when arranging the positioner, it is necessary to confirm the direction in which the processing load is applied in advance. The direction of the machining load is the direction of travel of the tool holder/tool. Arranging a positioner at the end of the feed direction can directly affect the overall accuracy of the workpiece.
Generally, when positioning the rough surface of the workpiece, a bolt-type adjustable positioner is used, and when positioning the machined surface of the workpiece, a fixed type (the workpiece contact surface is ground) positioner is used.
2. Basic principles of positioning from workpiece holes
When positioning using the holes processed in the previous process of the workpiece, pins with tolerances need to be used for positioning. By matching the accuracy of the workpiece hole with the accuracy of the pin shape, and combining them according to the fit tolerance, the positioning accuracy can meet actual requirements.
In addition, when using pins for positioning, generally one uses a straight pin and the other uses a diamond pin, so it will be more convenient to assemble and disassemble the workpiece, and the workpiece and the pin will rarely get stuck.
picture
▲Use pin positioning
Of course, you can also use straight pins for both pins by adjusting the fit tolerance. For more precise positioning, it is usually most effective to use a straight pin and a diamond pin.
When using a straight pin and a rhombus pin, usually the line connecting the arrangement direction of the rhombus pin (where it contacts the workpiece) is 90° perpendicular to the line connecting the straight pin and the rhombus pin. This arrangement is for the sake of angle. Positioning (direction of workpiece rotation).
Clamp related knowledge
1. Classification of clamps
According to the clamping direction, it is generally divided into the following categories:
picture
Next, let's take a look at the characteristics of various clamps.
1. Clamp pressed from above
A clamp that clamps from above the workpiece has the least amount of deformation during clamping and is the most stable during workpiece processing. Therefore, in general, the first consideration is to clamp the workpiece from above. The most common clamp that is pressed from above the workpiece is a manual mechanical clamp. For example, the picture below is called a "pine leaf" clamp. The clamp composed of pressure plate, stud bolt, jack and nut is called "loose leaf" clamp.
picture
In addition, according to the shape of the workpiece, different shapes of pressure plates can be selected to correspond to various shapes of workpieces.
picture
The relationship between the torque and the clamping force when clamping with a loose leaf clamp can be calculated based on the pushing force of the bolt.
picture
In addition to the loose leaf type clamp, there are also the following similar clamps that clamp the workpiece from above.
picture
2. Clamp that clamps from the side
Originally, the clamping method of clamping the workpiece from above has the most stable accuracy and the smallest processing load on the workpiece. However, if the upper part of the workpiece needs to be processed, or clamping from the upper part of the workpiece is not very suitable for various situations, and it cannot be clamped from the upper part of the workpiece, you can choose to clamp from the side of the workpiece. However, relatively speaking, when the workpiece is clamped from the side, a floating force will be generated. How to eliminate this force must be paid attention to when designing the fixture.
picture
There are also clamps that clamp from the side, as shown in the picture above. While generating thrust from the side, there is an oblique downward force. This kind of clamp can effectively prevent the workpiece from floating up.
Clamps that clamp from the side also include the following similar clamps.
picture
3. The clamp that tightens the workpiece from below
When processing the upper surface of a thin-plate workpiece, not only cannot it be clamped from above, but it is also unreasonable to clamp it from the side. The only method of tightening the workpiece from below is a reasonable clamping method. When tightening a workpiece from below, if it is made of iron, a magnet type clamp can usually be used. When working with non-ferrous metal workpieces, vacuum suction cups can generally be used to tighten them.
In the above two cases, the clamping force is proportional to the contact area between the workpiece and the magnet or vacuum chuck. If the processing load when processing small workpieces is too large, the processing effect will not be ideal.
picture
In addition, when using magnets or vacuum suction cups, the contact surfaces with the magnets and vacuum suction cups need to be smooth to a certain degree for safe and normal use.
4. Clamp using hole clamping
When using a 5-axis processing machine for simultaneous multi-face processing or mold processing, in order to prevent fixtures and tools from affecting the processing, it is generally more appropriate to use hole clamping. Compared with clamping from above and from the side of the workpiece, hole clamping exerts less load on the workpiece and can effectively deform the workpiece.
picture
▲Use holes for direct processing
picture
▲Set rivets for clamping
2. Pre-clamping
The above mainly talks about the workpiece clamping fixture. How to improve the operability and the use of pre-clamping are also crucial. When the workpiece is placed vertically on the base, the workpiece will fall downward due to gravity. At this time, you must hold the workpiece with your hands while operating the clamp.
picture
▲Pre-clamping
If the workpiece is heavy or multiple are clamped at the same time, the operability will be greatly reduced and the clamping time will be very long. At this time, using this spring-type pre-clamping product can allow the workpiece to operate the clamp while remaining stationary, greatly improving operability and reducing workpiece clamping time.
3. Things to note when choosing a clamp
When using multiple types of clamps in the same tooling, be sure to use the same tools for clamping and loosening. For example, in the picture on the left below, when multiple tool wrenches are used for clamping operations, the overall burden on the operator will increase, and the overall clamping time of the workpiece will also become longer. For example, in the picture on the right below, the tool wrenches and bolt sizes are unified to facilitate on-site operators.
picture
▲Operationability of workpiece clamping
In addition, when configuring the clamp, it is necessary to consider the operability of the workpiece clamping as much as possible. If the workpiece needs to be tilted during clamping, the operability will be very inconvenient. This situation needs to be avoided when designing the fixture tooling.
