There are many problems to be considered in tool selection, and there are many principles, such as efficiency principles, machining accuracy principles, stability principles, economic principles, and so on.
First talk about the efficiency principle. The principle of efficiency is inseparable from other principles, especially the principle of economics. The main purpose of requiring efficiency is to ensure the economics of the entire process. But efficiency is especially important, so separate it and discuss it separately.
The efficiency principle is first and foremost an efficiency that guarantees acceptable processing accuracy and acceptable stability. Without this basic condition, efficiency cannot be discussed. Just as we want our vehicles (such as cars) to bring us faster speeds, but safety is first. Once a plane crashes, many people will carefully consider whether they will continue to choose to travel, and airlines will revisit existing security policies. Without safety, the aircraft will not be the preferred means of transportation. The same is true for the choice of tool.
Secondly, we will not emphasize efficiency under all conditions, and there are some constraints on the pursuit of efficiency. The improvement of the processing efficiency of one part needs to be compatible with the efficiency of other parts, and the efficiency improvement of one process needs to be compatible with the efficiency of other processes. If you neglect these constraints and blindly pursue efficiency, you will be thankless. Just like the train from Shanghai to Beijing, it is now about 8 o'clock in the evening and arrives at 10 o'clock the next morning. If you can arrive at 8 o'clock in advance, it may be the most popular; but if you advance to 6 o'clock, perhaps the popularity will decline. Because the conductor will arrange the passenger bed one hour or two hours before arriving at the station, most passengers will not be too happy to get up at 4 or 5 in the morning. The same is true for factories, especially under the conditions of production lines. What we need to solve is the “bottleneck†process in the entire pipeline. As long as the production capacity of this process is increased, it is possible to increase the production capacity of the entire production line, increase the production capacity of the entire product, and shorten the manufacturing cycle, which is expected by many companies. The requirements for stand-alone or flexible manufacturing systems are different. They are less constrained, that is, less relevant to other processes. Due to the flexibility, the shortening of the manufacturing cycle of a part or a process often means that the device can be put into the production of other parts or other processes, thereby creating more benefits.
I believe that in today's increasingly fierce market competition, the company's expectations of process engineers are not to solve simple process problems, but to expect process engineers to make greater contributions to the company. If our process engineers can make a contribution from the overall situation of the enterprise and contribute to the improvement of the manufacturing process, we will definitely get the approval and approval of the business owner.
As for the data surveyed by foreign modern metal processing companies, the proportion of the tool itself in manufacturing costs is not very high, usually between 2% and 4%, and the high is about 7%. But the impact of tools on machining efficiency is enormous. When I participated in the "Research on the Revitalization of Mechanical Products in 2000" organized by the Ministry of Machinery more than 20 years ago, I was deeply impressed. The work experience of foreign companies in the past 10 years made me have a deeper understanding of this point. More specific feelings. That is the ability of a so-called $100,000 device to play its due role, often depending on a few dollars of knives. An example of cost analysis I have ever seen shows that a 30% reduction in purchase price (meaning that there is no change in tool performance) or a 50% increase in tool life (usually dependent on the tool maker's technological advances) can only reduce manufacturing costs. % or so - because tool costs only account for 4% of total manufacturing costs. However, if the processing parameters can be increased by 20%, the manufacturing cost can be reduced by about 15% - although if the cutting speed is increased by 20%, the tool cost will increase by 50%, but the total cost will be greatly reduced because of the shortened processing cycle.
I have visited a Shanghai mold company. This is a company that once had a very advanced technology. The mold of a mechanical department that we have saved in the quality of the medals is what they do. But when I went to visit as a tool sales engineer for a foreign company, they were already in the doldrums and had no competition. I walked into their workshop and the scene in front of me shocked me. They imported a 5-axis linkage machining center from Germany's Maho Corporation, which is worth several million RMB, but processed it at these machining centers with high-speed steel milling cutters that are extremely low-cost and extremely inefficient. I seem to see the problem of their decline, of course, this may not be the only reason. The backwardness of processing concepts and management concepts has led to their inefficient production; high equipment depreciation has made their manufacturing costs completely uncompetitive. Therefore, from this point of view, how to choose a tool, whether to pay attention to processing efficiency, may be related to the survival of the enterprise.
In addition to the principle of machining efficiency, the influence of the tool on the machining accuracy is also considered, especially in applications where machining accuracy and surface quality are required.
Under rough processing conditions, we generally adopt the efficiency priority principle. At this stage, the quick removal of the machining allowance on the workpiece blank and the rapid approach to the "net size" state of the finished part is the first factor in our consideration of tool selection and machining parameters.
However, under the conditions of finishing, the situation will be very different. When finishing, we should adopt the principle of precision priority, that is, firstly ensure the dimensional accuracy, surface roughness and surface quality of the processing.
Now a typical tool that prioritizes accuracy is favored by many foreign tool manufacturers. This is an end mill that is close to the perfect 90° lead angle. Mathematically, if a plane (the rake face for the tool) is used to cut a cylindrical surface (the ideal cutting edge is formed around the tool axis), only the plane contains the cylindrical axis (ie When the tool's axial rake angle is zero, its intersection line will be a straight line. However, the force on the blade is usually not ideal, and a positive axial rake angle is often required to improve the cutting performance of the tool. However, this results in the shape accuracy of the rotating surface: a staggered straight line (cutting edge) that rotates around the tool axis produces a cylindrical surface instead of a hyperboloid. Only when the cutting edge becomes part of the ellipse will it form a cylindrical surface as a result of its rotation about the tool axis. So some foreign tool companies have developed such tools: Kenner Metal is called Mill 1, Sandvik Coromant is called R390, and Walter is called F4042. The nature of these tools is the same, they use a curved cutting edge to form a nearly perfect cylindrical surface. Although different diameter milling cutters should have different curves, and the economical requirements of blade production do not allow this, each factory uses different axial rake angles to improve the difference. The idea of ​​this product development is worth learning by domestic manufacturers. Studying the needs of users, analyzing existing problems, and then trying to solve these problems for customers is an effective means for enterprises to continuously innovate, continuously improve, and continuously meet the growing needs of customers.
There are also some tools that have been modified to achieve the final quality requirements in one pass. That is to say, the precision improvement scheme is introduced on the tool originally used for roughing, so that the first machining can obtain better precision and surface quality. Wiper blades for turning and three-edged drills for aluminum drilling (such as Kennametal's TF drills) are examples of this. If the same feed rate (eg 0.05mm) is used, the surface roughness of conventional inserts can be five times higher than that of Wiper inserts. This processing accuracy is guaranteed, and in many cases it can even be worn by a car.
At the International Conference of High Speed ​​Machine (ICHSM) held in Suzhou in May 2006, the PTW Institute of the University of Darmstadt in Germany presented some of their research results on drilling. Part of this is about the impact of bit asymmetry on drilling accuracy. Studies have shown that the two cutting edges of a symmetrical drill bit form a complete circle. If the core is asymmetrical, it will form a non-circular shape. We know that it is called an equiaxed curve. The characteristic of the equiaxed curve is that the dimensions of any pair of sides are equal. This results in loss of precision in the holes (especially the drilled portion) drilled by the drill bit. Some inexpensive drill bits and inexpensive sharpening equipment have contributed to or contributed to this phenomenon. The end user may have to add a reaming process to correct this shape accuracy error. Therefore, the principle of precision that emphasizes blade selection can also help us improve our competitiveness.
Efficiency principle and precision principle of tool selection
In metal cutting, tool selection is almost a problem every process engineer must face.