Positioning and Clamping: Basics You Should Know

This is a summary of fixture design by people in the industry, but it is far from so simple.

In the process of contacting various schemes, we have found that there are always some positioning and clamping problems in the preliminary design that are not well solved.

In this way, any innovative scheme will lose practical significance.

Only by understanding the basic knowledge of positioning and clamping can we fundamentally ensure the integrity of fixture design and processing scheme.

Locator knowledge

1. Basic principles of positioning from the side of the workpiece

When positioning from the side of the workpiece, like the supporter, the three-point principle is the most basic principle.

This is the same as the principle of the support, which is called the three-point principle, and is derived from the principle of “three points not on the same line determine a plane”.

Among the four points, three points can determine a face, so a total of four faces can be determined, but no matter how to locate, it is quite difficult to make the fourth point in the same plane.

Three principles

For example, when four fixed height locators are used, only three points somewhere can contact the workpiece, and the possibility that the remaining fourth point does not contact the workpiece is still very high.

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 configuring the positioner, it is necessary to confirm the direction of applying processing load in advance.

The direction of processing load is the travel direction of the tool handle / tool.

Configuring a locator at the end of the feed direction can directly affect the overall accuracy of the workpiece.

Generally, when positioning the workpiece blank surface, the bolt type adjustable positioner is used, and when positioning the workpiece processing surface, the fixed positioner (the workpiece contact surface is ground) is used.

2. Basic principles of positioning from workpiece hole

When using the holes processed in the previous process of the workpiece for positioning, it is necessary to use a pin with tolerance for positioning.

By matching the accuracy of the workpiece hole with the accuracy of the pin shape and combining according to the matching tolerance, the positioning accuracy can meet the actual requirements.

In addition, when using pins for positioning, usually one uses straight pins and the other uses diamond pins, so it will become more convenient to assemble and disassemble the workpiece, and the workpiece and pin will rarely get stuck.

Use pin positioning

Of course, it is also possible to use straight pins for both pins by adjusting the fitting tolerance.

For more accurate positioning, it is usually the most effective to use a straight pin and a diamond pin.

When using a straight pin and a diamond pin, the connection line of the configuration direction of the diamond pin (at the contact with the workpiece) is usually 90 ° perpendicular to the connection line of the straight pin and the diamond pin.

This configuration method is for angular positioning (workpiece rotation direction).

Relevant knowledge of clamp

1. Classification of clamps

According to the clamping direction, it is generally divided into the following categories:

Classification of clamps

Next, let’s take a look at the characteristics of various clamps.

1. Clamp pressed from above

The clamping device that compresses from the top of the workpiece has the least deformation when clamping and is the most stable in workpiece processing, so in general, the first consideration is to clamp from the top of the workpiece. The most common fixture pressed from the top of the workpiece is a manual mechanical fixture.

For example, the following figure is called the “pine leaf” clamp.

The clamp composed of pressing plate, stud bolt, Jack and nut is called “pine leaf” clamp.

Clamp pressed from above

In addition, according to the shape of the workpiece, different shapes of pressing plates can be selected to correspond to different shapes of workpieces.

different shapes of pressing plates

The relationship between the torque and clamping force of the pine leaf clamp can be calculated by the propulsion force of the bolt.

The simple calculation formula of clamping force is as follows:

F(kN) = T(N·m)/0.2d

(d refers to the nominal diameter of the bolt)

In addition to the pine leaf clamp, there are also the following similar clamps for clamping from above the workpiece.

2. Clamp clamping from the side

Originally, the clamping method of clamping the workpiece from the top is the most stable in accuracy and the processing load of the workpiece is also the smallest.

However, when it is impossible to clamp from the top of the workpiece due to various conditions such as machining above the workpiece or clamping from the top of the workpiece is not very appropriate, you can choose to clamp from the side of the workpiece.

But relatively speaking, the workpiece will produce an upward force when clamped from the side. How to eliminate this force must be paid attention to when designing the fixture.

There are also clamps for clamping from the side, as shown in the above figure.

When the side generates thrust, there is an oblique downward force, which can effectively prevent the workpiece from floating.

There are also the following similar clamps for clamping from the side.

3. Clamp for tightening the workpiece from the pull-down

When processing the upper surface of a thin plate workpiece, it is not only impossible to clamp it from the top, but also unreasonable to compress it from the side.

The only reasonable clamping method is to tighten the workpiece from the bottom.

When tightening the workpiece from below, if it is made of iron, a magnet clamp can usually be used.

For non-ferrous metal workpieces, vacuum suction cups can generally be used for tightening.

In the above two cases, the clamping force is directly proportional to the contact area between the workpiece and the magnet or vacuum chuck.

If the processing load is too large when processing small workpieces, the processing effect will not be ideal.

In addition, when using a magnet or vacuum chuck, the contact surface with the magnet and vacuum chuck needs to be smooth to be safe and normal.

4. Clamp clamped with hole

When using a 5-axis machine for multi-facet simultaneous processing or mold processing, in order to prevent the fixture and tools from affecting the processing, it is generally appropriate to choose the way of hole clamping.

Compared with the way of clamping from the top and side of the workpiece, the way of hole clamping produces less load on the workpiece, which can effectively deform the workpiece.

Machining directly with holes

Machining directly with holes

Set the pull pin for clamping

2. Pre clamping

The above mainly refers to the fixture of workpiece clamping. How to improve operability and use pre clamping is also crucial.

When the workpiece is set vertically on the base, the workpiece will fall downward due to gravity.

At this time, you must press and hold the workpiece by hand while operating the gripper.

Pre clamping

If the workpiece is heavy or most of them 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 kind of spring pre clamping product can make the workpiece operate the gripper in a stationary state, greatly improve the operability and reduce the clamping time of the workpiece.

3. Precautions when selecting the clamp

When multiple types of clamps are used in the same tooling, the tools for clamping and loosening must be unified.

For example, as shown in the left figure below, when a variety of tool wrenches are used for clamping operation, the overall burden of the operator will increase, and the overall clamping time of the workpiece will also increase.

For example, in the figure on the right, the tools and wrenches are unified, and the bolt size is also unified, which is convenient for on-site operators.

Operability of workpiece clamping

In addition, when configuring the clamp, it is necessary to consider the operability of workpiece clamping as much as possible.

If the workpiece needs to be clamped obliquely during clamping, it is very inconvenient to operate. This situation needs to be avoided when designing fixture tooling.

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