At present, machining can be divided into two categories according to the production batch: one is a single piece, multiple varieties, small batches (referred to as small batch production), and the other is a small variety and large batches (referred to as mass production). Among them, the former accounts for about 70~80% of the total output value of machining, and is the main body of machining.
The same machine tool, why the production efficiency is several times different? The conclusion is that the fixture selected by the CNC machine tool is not suitable, so that the production efficiency of the CNC machine tool is greatly reduced. Today, the editor of metal processing will give you a detailed introduction to the reasonable selection and application of CNC machine tool fixtures.
How to 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 selected by CNC machine tools in domestic enterprises is as high as more than 50%. By the end of 2010, China's CNC machine tool ownership of nearly one million units, that is to say, there are more than 500,000 CNC machine tools due to unreasonable fixture selection or improper application, and there is a "nest worker" phenomenon;
Small-batch production cycle = production (preparation/waiting) time + workpiece processing timeSince the "workpiece processing time" is very short in low-volume production, the length of the "production (preparation/waiting) time" has a crucial impact on the processing cycle. In order to improve productivity, it is necessary to find ways to reduce production (preparation/waiting) time.
Here are three types of CNC machine fixtures that can be prioritized for low-volume production:
Combination fixtures
The combination fixture is also known as the "building block fixture", which is composed of a series of machine tool fixture elements with different functions and different specifications and sizes after standardized design, and customers can quickly assemble various types of machine tool fixtures according to the processing requirements. Because the combination fixture eliminates the need for design and manufacturing special fixtures, the production preparation time is greatly shortened, thus effectively shortening the production cycle of small batches, that is, improving production efficiency. In addition, the combined fixture also has the advantages of high positioning accuracy, large clamping flexibility, recycling and reuse, manufacturing energy saving and material saving, and low use cost. Therefore, for small batch processing, especially when the product shape is more complex, the use of combination fixtures can be preferred.
Precision combination vise
Precision combination vises are actually the "fits" in the combination fixture, and compared with other combination fixture elements, they are more versatile, more standardized, easier to use, and more reliable in clamping, so they are widely used worldwide. Precision combination vises have the advantages of quick installation (disassembly) and quick clamping, so they can shorten production preparation time and improve the efficiency of small batch production. At present, the clamping range of precision combination flat nose 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 of flat nose pliers mentioned here is not the old-fashioned machining vice, the old-fashioned machining vices have a single function, 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 of flat nose pliers here is a series of new flat nose pliers originated in developed industrial countries such as Europe and the United States, specially designed for the characteristics of CNC machine tools and machining centers, such products have the characteristics of large clamping flexibility, high positioning accuracy, fast clamping, and can be used in groups, especially suitable for CNC machine tools and machining centers.
Electro-permanent magnet fixtures
The electric permanent magnet fixture is a new type of fixture designed by using the principle of modern magnetic circuit with new permanent magnet materials such as NdFeB as the magnetic force source. A large number of machining practices show that electric permanent magnet fixtures can greatly improve the comprehensive processing efficiency of CNC machine tools and machining centers.
The clamping and unclamping process of electric permanent magnet fixture only takes about 1 second, thus greatly shortens the clamping time; the positioning element and the clamping element of conventional machine tool fixture occupy a large space, and the electric permanent magnet fixture does not have these elements that occupy space, therefore compared with conventional machine tool fixture, the clamping range of electric permanent magnet fixture is larger, this is conducive to making full use of the working table and processing stroke of CNC machine tool, is conducive to improving the comprehensive processing efficiency of CNC machine tool. The suction power of the electric permanent magnet fixture is generally 15~18Kgf/cm2, so it is necessary to ensure that the suction (clamping force) is enough to resist the cutting force, under normal circumstances, the adsorption area should not be less than 30cm2, that is, the clamping force is not less than 450Kgf.
It is advisable to process CNC machine tool fixtures in large quantities
Mass processing cycle = machining waiting time + workpiece processing time + production preparation time "Processing waiting time" mainly includes the time for workpiece clamping and tool change. The "workpiece clamping time" of the traditional manual machine tool fixture can reach 10~30% of the mass processing cycle, so that the "workpiece clamping" has become a key factor affecting the production efficiency, and it is also the key object of the "potential" of the machine tool fixture.
Therefore, it is advisable to use special fixtures for rapid positioning and quick clamping (loosening) for large-scale processing, and the following three types of machine tool fixtures can be given priority:
Hydraulic/pneumatic clamps
Hydraulic/pneumatic fixture is a special fixture that uses oil pressure or pneumatic pressure as the power source to realize the positioning, support and pressing of the workpiece through hydraulic components or pneumatic components. The hydraulic/pneumatic fixture can accurately and quickly determine the mutual position between the workpiece and the machine tool and the tool, and the position accuracy of the workpiece is guaranteed by the fixture, and the machining accuracy is high; the positioning and clamping process is fast, 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 it particularly suitable for use in CNC machine tools, machining centers, and flexible production lines, especially for large-scale processing.
Electro-permanent magnet fixtures
The electric permanent magnet fixture has the advantages of fast clamping, easy to realize multi-station clamping, multi-sided processing at one clamping, stable and reliable clamping, energy saving and environmental protection, and automatic control. Compared with conventional machine tool fixtures, electric permanent magnet fixtures can greatly shorten the clamping time, reduce the number of clamping, and improve the clamping efficiency, so it is not only suitable for small batch production, but also suitable for large batch production.
Smooth fixture base
The smooth fixture base is not widely used in China, but it is widely used in industrially developed countries such as Europe and the United States. It is actually the fine blank of the fixture matrix that has been finished, and the positioning connection part between the component and the machine tool and the positioning surface of the part on the fixture have been finished. Users can process and produce special fixtures according to their actual needs.
The smooth fixture base can effectively shorten the cycle of manufacturing special fixtures, reduce production preparation time, so that the cycle of mass production can be shortened overall, improve production efficiency, and reduce the manufacturing cost of special fixtures. Therefore, the smooth gripper base is particularly suitable for high-volume production with tight cycles.
Rational use of fixtures to tap the potential of equipment
Experience shows that in order to improve the processing efficiency of CNC machine tools, it is not enough to "choose" the CNC machine tool fixture, but also to work the "good" CNC machine tool fixture.
Here are three common methods:
Multi-station method
The basic principle of the multi-station method: by clamping multiple workpieces at one time, the purpose of shortening the unit clamping time and prolonging the effective cutting time of the tool is achieved. A transfer fixture is a fixture that has multiple positioning clamping positions.
With the development of CNC machine tools and the needs of users to improve production efficiency, there are more and more applications of multi-station fixtures. Multi-station design is becoming more and more common in the structural design of hydraulic/pneumatic clamps, combination fixtures, electric permanent magnet fixtures and precision combined vises.
Use in groups
The same fixtures can be used on the same working table, and the purpose of "multi-station" clamping can also be realized. The fixtures involved in this method should generally undergo "standardized design and high-precision manufacturing", otherwise it is difficult to meet the requirements of CNC machine tool processing.
The group use method can make full use of the CNC machine tool stroke, which is conducive to the balanced wear of the machine tool transmission parts, and the relevant fixtures can be used independently to achieve multi-piece clamping, and can be used jointly to achieve large-size workpiece clamping.
Local quick-change method
The local quick change method is to quickly change the function or use of the fixture by quickly replacing the parts (positioning elements, clamping elements, tool setting elements and guiding elements) of the CNC machine tool fixture. For example, the quick-change combination vise can change the clamping function, such as from clamping cube to clamping bar, by quickly changing the jaws, and the clamping method can also be changed by quickly changing the clamping element, such as from manual clamping to hydraulic clamping. The local quick-change method greatly shortens the time to replace and adjust the fixture, and has obvious advantages in small batch production.