Common Sense of Selection and Use of Fixtures for CNC Machine Tools

Mechanical processing can be divided into two categories according to the production batch: single piece, multiple varieties, and small batch (referred to as small batch production). The other is small variety and large batch production. The former accounts for 70~80% of the total output value of mechanical processing and is the main body.
Why does the production efficiency of the same machine tool differ several times? The conclusion is that the fixture selected for the NC machine tool is not suitable, which significantly reduces the production efficiency of the NC machine tool. The following describes the reasonable selection and application of NC machine tool fixtures.
How can we improve the utilization rate of CNC machine tools? Through technical analysis, the use of fixtures has a great relationship. According to incomplete statistics, the proportion of unreasonable fixtures domestic enterprises use for CNC machine tools is more than 50%. By the end of 2010, the number of CNC machine tools in China had reached nearly 1 million, meaning that more than 500000 were “idle” due to unreasonable selection or improper application of fixtures. From another point of view, there is much to be done in selecting and applying NC machine tool fixtures because it contains considerable potential economic benefits.
Small batch production (preparation/waiting) time+workpiece processing time Since the “workpiece processing time” of small batch production is abridged, the length of “production (preparation/waiting) time” has a crucial impact on the processing cycle. We must find ways to shorten production (preparation/waiting) time to improve production efficiency.

1. Three types of NC machine tools and fixtures that can be prioritized for small batch production are recommended as follows:

Modular fixture
The modular fixture, or a “building block fixture,” comprises a series of machine tool fixture elements with standardized designs, functions, and specifications. Customers can quickly assemble various machine tool fixtures according to processing requirements, just like “building blocks.” Because the modular fixture saves time in designing and manufacturing unique fixtures, it dramatically shortens the production preparation time, thus effectively shortening the small batch production cycle and improving production efficiency. In addition, the modular fixture also has the advantages of high positioning accuracy, ample clamping flexibility, recycling and reuse, manufacturing energy and material saving, low use cost, etc. Therefore, modular fixtures can be preferred for small-batch processing, especially when the product shape is relatively complex.
Precision combination flat pliersPrecisionn combination flat jaw pliers belong to the “assembly” of modular fixtures. Compared with other modular fixture components, they are more versatile, standardized, easier to use, and more reliable in clamping. Therefore, they are widely used worldwide. The precision combination flat jaw pliers have the advantages of rapid installation (disassembly), rapid clamping, etc., to shorten the production preparation time and improve the efficiency of small batch production. Currently, the clamping range of precision combination flat jaw pliers commonly used internationally is 1000mm, and the clamping force is 55000 kg.
Smooth clamp base
Smooth fixture base is not widely used in China, but it is commonly used in Europe, America, and other industrial developed countries. It is the fine blank of the fixture base after finishing the positioning connection part between the element and the machine tool and the positioning surface of the part on the fixture has been completed. Users can process and make unique fixtures according to their actual needs.
It should be noted that the precision combination flat jaw pliers mentioned here are not old machine vises. The old machine vises have single functions, low manufacturing accuracy, can not be used in groups, and have a short service life, so they are not suitable for use on CNC machine tools and machining centers. The precision combination flat jaw pliers mentioned here are a series of new flat jaw pliers that originated from Europe, America, and other developed industrial countries, specially designed for the characteristics of CNC machine tools and machining centers. Such products have ample clamping flexibility, high positioning accuracy, and fast clamping. They can be used in groups and are especially suitable for CNC machine tools and machining centers.

Electric permanent magnet clamp
The electric permanent magnet fixture is a new type of fixture designed with neodymium iron boron and other new permanent magnetic materials as the magnetic source and the principle of modern magnetic circuits. Many machining practices show that the electric permanent magnet fixture can significantly improve the comprehensive machining efficiency of CNC machine tools and machining centers.
The clamping and loosening process of the electric permanent magnet clamp only takes about 1 second, so the clamping time is significantly shortened. Conventional machine tool jigs’ positioning and clamping elements occupy ample space, while the electric permanent magnet jigs do not have these space-occupying elements. Therefore, compared with traditional machine tool jigs, the electric permanent magnet jigs have a more extensive clamping range, which is conducive to making full use of the worktable and processing stroke of the CNC machine tool and is conducive to improving the comprehensive processing efficiency of Turning parts and machining parts. The suction of the electric permanent magnet fixture is generally 15~18Kgf/cm2, so it must be ensured that the suction (clamping force) is sufficient to resist the cutting force. Generally, the adsorption area should not be less than 30cm2; the clamping force should not exceed 450Kgf.

2. NC machine tool fixture suitable for mass processing
Mass 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 mass processing cycle, so “workpiece clamping” has become a key factor affecting the production efficiency and is also the key object of “tapping potential” of the machine tool fixture.
Therefore, unique 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:
Hydraulic/pneumatic clamp
A hydraulic/pneumatic clamp is a special clamp that uses oil pressure or air pressure as a power source to position, support, and compress the workpiece through hydraulic or pneumatic components. The hydraulic/pneumatic fixture can accurately and quickly determine the mutual position between the workpiece, machine tool, and cutter. The fixture guarantees the position accuracy of the workpiece, and the machining accuracy is high; The positioning and clamping process is fast, greatly saving the time for clamping and releasing the workpiece. At the same time, it has the advantages of compact structure, multi-position clamping, high-speed heavy cutting, automatic control, etc.
The advantages of hydraulic/pneumatic fixtures make them suitable for CNC machine tools, machining centers, and flexible production lines, especially for mass processing.
Electric permanent magnet clamp
The electric permanent magnet clamp has the advantages of fast clamping, easy multi-position clamping, multi-side machining, stable and reliable clamping, energy saving and environmental protection, and automatic control. Compared with conventional machine tool fixtures, electric permanent magnet fixtures can considerably shorten clamping time, reduce clamping times, and improve clamping efficiency. Therefore, they are suitable not only for small-batch production but also for large-batch production.
Smooth clamp base
The smooth surface fixture base can effectively shorten the cycle of manufacturing unique fixtures and reduce the production preparation time, so it can generally shorten the cycle of mass production and improve production efficiency. Simultaneously, the manufacturing cost of the sremarkablefixture can be reduced. Therefore, the smooth surface fixture base suits mass production with tight cycles.
Reasonably use clamps to tap the potential of equipment
Experience shows that to improve the processing efficiency of NC machine tools, it is not enough to “select the right” NC machine tools and fixtures but also to “use” NC machine tools and fixtures.

3. Here are three standard methods:
Multi-station method
The basic principle of the multi-station method is to shorten the unit clamping time and extend the tool’s adequate cutting time by clamping multiple workpieces simultaneously. MMulti-stationfixture refers to the 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 fixtures is increasing. Multi-station design is increasingly common in the structural design of hydraulic/pneumatic fixtures, modular fixtures, electric permanent magnet fixtures, and precision modular flat jaw pliers.
Group usage
The purpose of “multi-station” clamping can also be achieved by placing several clamps on the same workbench. The fixture involved in this method should generally go through “standardized design and high-precision manufacturing”, otherwise it is difficult to meet the requirements of NC machine tool process processing.
The method of group use can make full use of the stroke of the NC machine tool, which is beneficial 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 also be used jointly to discover the clamping of large-sized workpieces.
Local quick change method
The local quick change method quickly changes the fixture function or use mode by rapidly replacing the NC machine tool fixture’s local parts (positioning, clamping, tool set, and guide elements). For example, the quick change combination flat jaw can change the clamping function by quickly changing the jaw, such as changing the clamping square material into the clamping bar material. The clamping method can also be changed by rapidly changing the clamping elements, such as changing from manual clamping to hydraulic clamping. The local quick change method dramas fixture replacement and adjustment time and has apparent advantages in small batch production.