CNC Fixture: Types and How to Choose

At present, machining can be divided into two categories according to production batch:

One is single piece, multi variety and small batch (short for small batch production);

The other is small variety and large batch (referred to as mass production).

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Among them, the former accounts for about 70-80% of the total output value of machining and is the main body of machining.

Why is the production efficiency of the same machine tool several times different?

The conclusion is that the fixture selected by the CNC machine tool is not suitable, which greatly reduces the production efficiency of the CNC machine tool.

Today, we will introduce in detail the reasonable selection and application of CNC fixtures.

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Basic principles for selecting fixtures

The characteristics of CNC machining put forward two basic requirements for fixtures:

First, ensure that the coordinate direction of the fixture and the coordinate direction of the machine tool are relatively fixed;

Second, coordinate the dimensional relationship between parts and machine tool coordinate system.

In addition, the following four points should be considered:

1. When the processing batch of parts is not large, combined fixture, adjustable fixture and other general fixtures shall be used as much as possible to shorten the production preparation time and save the production cost.

2. Special fixtures shall be considered for batch production, and the structure shall be simple.

3. The loading and unloading of parts shall be fast, convenient and reliable to shorten the pause time of the machine tool.

4. All parts and components on the fixture shall not interfere with the machining of parts and surfaces by the machine tool, that is, the fixture shall be open and its positioning and clamping mechanism elements shall not affect the tool walking (such as collision) during machining.

How to improve the utilization of CNC machine tools?

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Through technical analysis, the use of fixtures has a great relationship.

According to incomplete statistics, more than 50% of the domestic enterprises choose unreasonable fixtures for CNC machine tools.

Small batch production cycle = production (preparation / waiting) time + workpiece processing time.

Because the “workpiece processing time” of small batch production is very short, the length of “production (preparation / waiting) time” has a crucial impact on the processing cycle.

To improve production efficiency, we must find ways to shorten the production (preparation / waiting) time.

The following three types of CNC machine tools and fixtures that can be given priority in small batch production are recommended:

① Combined fixture

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Modular fixture is also known as “building block fixture”. It is composed of a series of machine tool fixture elements with different functions and sizes through standardized design.

Customers can quickly assemble various types of machine tool fixtures according to processing requirements like “building blocks”.

Since the modular fixture saves the time of designing and manufacturing special fixture, it greatly shortens the production preparation time, thus effectively shortens the small batch production cycle, that is, improves the production efficiency.

In addition, the combined fixture also has the advantages of high positioning accuracy, large clamping flexibility, recycling and reuse, energy saving and material saving in manufacturing, and low use cost.

Therefore, for small batch processing, especially when the product shape is complex, the use of combined fixture can be given priority.

② Precision combination flat pliers

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In fact, the precision combination flat pliers belong to the “assembly” in the combination fixture.

Compared with other combination fixture elements, they are more versatile, more standardized, easier to use, and more reliable to clamp.

Therefore, they are widely used in the world.

Precision combination flat tongs have the advantages of rapid installation (disassembly) and rapid clamping, so it can shorten the production preparation time and improve the efficiency of small batch production.

At present, the clamping range of precision combination flat pliers commonly used in the world is generally within 1000mm, and the clamping force is generally within 5000Kgf.

It should be noted that the precision combination flat pliers mentioned here are not old-fashioned machine vises.

The old-fashioned machine vises have single function, low manufacturing accuracy, cannot be used in groups, and short service life, and are not suitable for use on CNC machine tools and machining centers.

The precision combined flat pliers mentioned here are a series of new flat pliers originated from developed industrial countries such as Europe and America, which are specially designed for the characteristics of CNC machine tools and machining centers.

These products have the characteristics of large clamping flexibility, high positioning accuracy, fast clamping and group use, and are especially suitable for CNC machine tools and machining centers.

③ Electro permanent magnet fixture

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The electric permanent magnet clamp is a new type of clamp designed by using neodymium iron boron and other new permanent magnet materials as the magnetic force source and using the modern magnetic circuit principle.

A large number of machining practices show that the electro permanent magnet fixture can greatly improve the comprehensive machining efficiency of CNC machine tools and machining centers.

The clamping and loosening process of the permanent magnet clamp only takes about 1 second, so the clamping time is greatly shortened;

The positioning element and clamping element of the conventional machine tool clamp occupy a large space, while the electric permanent magnet clamp does not have these space occupying elements.

Therefore, compared with the conventional machine tool fixture, the clamping range of the electro permanent magnet fixture is larger, which is conducive to making full use of the working table and processing stroke of the CNC machine tool and improving the comprehensive processing efficiency of the CNC machine tool.

The suction force of the permanent magnet clamp is generally 15 ~ 18kgf / cm2, so it must be ensured that the suction force (clamping force) is sufficient to resist the cutting force.

In general, the adsorption area shall not be less than 30cm2, that is, the clamping force shall not be less than 450kgf.

What are the types of machine tools and fixtures?

Machine tool fixtures mainly include the following 8 types:

1. Flat tongs

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Flat tongs have great versatility and economy, and are suitable for clamping square workpieces with small size;

V-shaped groove or V-shaped block on the jaw can also clamp the cylindrical surface.

Features: 

Convenient clamping, rapid and reliable, accurate positioning, suitable for various square parts, but not suitable for clamping thin plate parts.

As shown in the figure, the commonly used flat tongs of CNC milling machines often adopt mechanical screw, pneumatic or hydraulic clamping methods.

(a) Screw clamping universal flat pliers;

(b) Hydraulic sine gauge flat pliers;

(c) Pneumatic precision flat pliers;

(d) Hydraulic precision flat tongs.

2. Pressing plate and bolt

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For the workpieces with large size or irregular surrounding, if it is impossible to clamp them with flat tongs or other clamps, they can be directly clamped with pressing plates, or the workpieces can be directly pressed with bolts through the process holes.

(a) Screw clamping universal flat pliers;

(b) Hydraulic sine gauge flat pliers;

(c) Pneumatic precision flat pliers;

(d) Hydraulic precision flat tongs.

Characteristic:

It takes a lot of time to clamp and align. The position and height of the pressing plate should be considered in the processing programming to avoid interference with the knife.

The method of bolt penetration through the process hole occupies a small space and is easy to give way.

However, the hole position should coincide with the position of the T-groove of the machine tool. Otherwise, it is troublesome to manufacture a backing plate for bolt penetration.

They are most suitable for special-shaped parts, large-sized parts and parts that cannot be clamped by flat tongs.

3. Chuck

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According to the number of jaws, it can be divided into two jaw chuck, three jaw self centering chuck, four jaw chuck and six jaw chuck.

Three jaw self centering single action chuck and four jaw chuck are widely used in CNC milling machines and machining centers.

In particular, the three claw self centering chuck has the characteristics of automatic centering and simple clamping.

Therefore, when small and medium-sized cylindrical workpieces are processed on a CNC milling machine or machining center, the three claw self centering single action chuck is often used for clamping.

Chuck clamping has mechanical screw type, pneumatic type or hydraulic type and other forms.

Features: convenient and reliable clamping, automatic centering, suitable for cylindrical parts.

4. 3R fixture system

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Characteristic:

The clamping is convenient and reliable, and the repeated positioning accuracy is high.

It is especially suitable for the production of die electrodes.

5. Magnetic chuck

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Features: easy and reliable clamping, wide application range, but the parts must be made of magnetic absorbing materials.

6. Vacuum fixture

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It is suitable for workpieces with large positioning plane or large sealable area.

Some CNC milling machines (such as wall plate milling machines) are equipped with a general vacuum platform.

When installing workpieces, special rubber strips (hollow or solid circular sections with certain size requirements) can be directly inserted into the sealing groove of the fixture for rectangular workpieces with regular shape.

Then, the workpieces can be clamped by placing them on the surface and starting the vacuum pump.

For rough workpieces with irregular shape, it is not suitable to use rubber strips.

Putty (commonly used rubber putty) must be applied around them for sealing.

This is not only troublesome, but also takes a long time and has low efficiency.

In order to overcome this difficulty, a special transition vacuum platform can be used, which is superimposed on the general vacuum platform.

7. Indexing head

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When many mechanical parts, such as spline, clutch, gear and other parts are processed on the machining center, the method of dividing by dividing plate is often used to divide each tooth slot equally, so as to process qualified parts.

8. Special fixture, combined fixture and Group Fixture

When small and medium batches of workpieces are processed on the machining center, combined fixtures can be used for clamping.

When processing large quantities of workpieces, special fixtures or group fixtures are mostly used for clamping, but the machining center is more suitable for processing single and small batches of workpieces.

Therefore, this kind of fixture is rarely used in CNC machine tools.

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CNC machine tool fixture suitable for mass processing

Batch processing cycle = processing waiting time + workpiece processing time + production preparation time.

“Processing waiting time” mainly includes the time for workpiece clamping and tool replacement.

The “workpiece clamping time” of the traditional manual machine tool fixture can reach 10-30% of the cycle of mass processing, so “workpiece clamping” has become a key factor affecting production efficiency and is also the key object of “tapping the potential” of the machine tool fixture.

Therefore, special fixtures for rapid positioning and rapid clamping (loosening) should be used for mass processing, and the following three types of machine tool fixtures can be given priority:

① Hydraulic / pneumatic clamp

Hydraulic / pneumatic clamp is a special clamp that uses oil pressure or air pressure as the power source and realizes the positioning, supporting and pressing of the workpiece through hydraulic or pneumatic elements.

The hydraulic / pneumatic fixture can accurately and quickly determine the mutual position between the workpiece, the machine tool and the tool.

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The position accuracy of the workpiece is guaranteed by the fixture, and the machining accuracy is high;

The positioning and clamping process is rapid, which greatly saves the time of clamping and releasing the workpiece;

At the same time, it has the advantages of compact structure, multi station clamping, high-speed heavy cutting and automatic control.

The above advantages of hydraulic / pneumatic fixtures make them particularly suitable for use in CNC machine tools, machining centers and flexible production lines, especially for mass processing.

② Electro permanent magnet fixture

The electric permanent magnet fixture has the advantages of fast clamping, easy to realize multi station clamping, one clamping can be multi-faceted processing, stable and reliable clamping, energy saving and environmental protection, and automatic control.

Compared with the conventional machine tool fixture, the electric permanent magnet fixture can greatly shorten the clamping time, reduce the clamping times, and improve the clamping efficiency.

Therefore, it is not only suitable for small batch production, but also suitable for large batch production.

③ Smooth clamp base

Smooth fixture base is not widely used in China, but it is widely used in Europe and America and other industrial developed countries.

In fact, it is a fine blank of the fixture base after finishing processing.

The positioning connecting part of the components and the machine tool and the positioning surface of the parts on the fixture have been finished.

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Users can manufacture special fixtures according to their actual needs.

The smooth surface fixture base can effectively shorten the cycle of manufacturing special fixtures and reduce the production preparation time, so it can shorten the cycle of mass production on the whole and improve the production efficiency;

And the manufacturing cost of the professional fixture can be reduced.

Therefore, the smooth fixture base is particularly suitable for large-scale production with tight cycle.

Reasonable use of fixtures to tap the potential of equipment

Experience shows that in order to improve the machining efficiency of CNC machine tools, it is not enough to only “select the right” CNC machine tools and fixtures, but also to “use” CNC machine tools and fixtures.

Here are three common methods:

① Multi station method

The basic principle of the multi station method is to shorten the unit clamping time and prolong the effective cutting time of the tool by clamping multiple workpieces at once.

Multi station fixture is a fixture with multiple positioning and clamping positions.

With the development of CNC machine tools and the need of users to improve production efficiency, the application of multi station fixture is more and more.

In the structural design of hydraulic / pneumatic clamp, combination clamp, electro permanent magnet clamp and precision combination flat clamp, multi station design is more and more common.

② Group use method

Putting the same several clamps on the same workbench can also achieve the purpose of “multi station” clamping.

The fixtures involved in this method should generally be subject to “standardized design and high-precision manufacturing”, otherwise it is difficult to meet the requirements of NC machine processing.

The group use method can make full use of the travel of the CNC machine tool, which is conducive to the balanced wear of the transmission parts of the machine tool;

At the same time, the relevant fixtures can be used independently to realize the clamping of multiple pieces, and can be used jointly to realize the clamping of large-sized workpieces.

③ Local quick change method

The local quick change method is to rapidly change the function or use mode of the fixture by quickly changing the parts of the fixture (positioning element, clamping element, tool setting element and guide element) of the CNC machine tool.

For example:

Quick change combined flat jaw pliers can realize the change of clamping function by quickly changing the jaws, for example, from clamping square materials to clamping bar materials;

The clamping mode can also be changed by quickly changing the clamping element, for example, from manual clamping to hydraulic clamping.

The local quick change method greatly shortens the time for replacing and adjusting fixtures, and has obvious advantages in small batch production.

Requirements for fixture of CNC machining center

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Fixture is an indispensable part of CNC machining center.

Each CNC machining center will use fixtures, and the fixtures used are different according to the products processed, and the requirements for fixtures are different.

However, the requirements for fixtures of CNC machining centers are the same, and the requirements for fixtures of special workpieces are somewhat different.

What are the requirements of CNC machining center for fixture?

High precision fixture

As we all know, CNC machining centers have very high accuracy, and are generally used to process parts or molds requiring high accuracy.

Therefore, CNC machining centers also put forward requirements for high-precision positioning devices of fixtures, as well as requirements for indexing and positioning accuracy.

The fixture should be convenient and fast when clamping the workpiece.

In order to meet the requirements of high-speed machining fixtures of CNC machining centers, hydraulic and pneumatic quick locking forces are selected, especially for the locking of workpieces with a long processing time.

Generally, the hydraulic clamping system is equipped with a reservoir, which can make up for internal leakage and avoid loosening of workpieces on the fixture.

The clamp shall have excellent openness

The CNC machining center is processed by the tool touching the workpiece, and the workpiece should be clamped by the fixture to provide a relatively wide operating space for the fast moving tool and the rapid action of tool change.

Especially for workpieces with complex machining and multiple tool changes, the structure of the fixture shall be as simple, convenient, and open as possible to make the tool easy to enter and exit and avoid the collision between the tool and the fixture workpiece during the machining movement.

The fixture shall have high adaptability

We all know that CNC machining center has mobility and multi deformation during machining, so it is required that the fixture should be highly adaptable to different workpieces and different clamping.

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