1 Scientific selection of CNC tools 1.1 The principle of selecting CNC tools The tool life is closely related to the amount of cutting. When setting the amount of cutting, you should first choose a reasonable tool life, and a reasonable tool life should be based on the optimization goal. Generally, the highest productivity tool life and the lowest cost tool life are classified. The former is determined according to the minimum single working time target, and the latter is determined according to the lowest operating cost target.

The following points can be considered when selecting the tool life based on tool complexity, manufacturing and sharpening costs. Complex and high precision tool life should be chosen to be higher than single-edged tools. For the machine clamp indexable tool, due to the short tool change time, in order to give full play to its cutting performance and improve production efficiency, the tool life can be selected to be lower, generally taking 15~30min. It is more complicated for loading, changing and adjusting the knife. For multi-tool machines, combination machine tools and automated machining tools, the tool life should be chosen to be higher, especially for tool reliability. When the productivity of a certain process in the workshop limits the productivity of the entire workshop, the tool life of the process should be selected to be lower. When the total plant expenditure M is greater than the unit time of a certain process, the tool life should also be selected. Got lower. In the case of large-scale finishing, in order to ensure that at least one pass is completed and the tool is changed midway during cutting, the tool life should be determined according to the accuracy of the part and the surface roughness.

1.2 Select CNC turning tools CNC turning tools commonly used in general turning tools, sharp turning tools, circular turning tools and three categories. The forming turning tool is also called the model turning tool, and the contour shape of the machined part is completely determined by the shape and size of the turning edge of the turning tool. In CNC turning, common forming turning tools include small radius arc turning tools, non-rectangular grooving knives and thread knives. In CNC machining, the turning tool should be used with little or no use. The pointed tool is a turning tool characterized by a linear cutting edge. The cutting edge of this type of turning tool consists of straight main and auxiliary cutting edges, such as 900 inner and outer turning tools, left and right end turning tools, grooving (cutting) turning tools and various outer and inner edges with small chamfering edges. Hole turning tool. The selection method of geometrical parameters of the pointed tool (mainly geometric angle) is basically the same as that of ordinary turning, but should be comprehensively considered in combination with the characteristics of CNC machining (such as machining route, machining interference, etc.), and should take into account the tip of the tool itself. strength.

The second is a circular arc turning tool. The circular turning tool is a turning tool characterized by a circular arc-shaped cutting edge with a small degree of roundness or line contour error. Each point of the turning edge of the turning tool is the cutting edge of the circular arc turning tool. Therefore, the cutting point is not on the arc but on the center of the arc. The circular turning tool can be used for turning the inner and outer surfaces, and is particularly suitable for turning various smooth joints (concave) forming surfaces. When selecting the radius of the turning edge of the turning tool, the radius of the arc of the cutting edge of the two-point cutting tool should be less than or equal to the minimum radius of curvature on the concave contour of the part, so as to avoid the occurrence of shallow dryness. The radius should not be too small, otherwise it is not only difficult to manufacture. The tool cutter may be damaged due to the weakness of the tool tip or the poor heat dissipation of the tool body.

1.3Selecting CNC milling tools In CNC machining, the flat-bottom end mills for milling inner and outer contours of plane parts and milling planes are commonly used. The empirical data of the parameters of the tool are as follows: First, the cutter radius RD should be smaller than the minimum radius of curvature of the contour surface of the part. Rmin, generally take RD = (0.8 ~ 0.9) Rmin. Second, the processing height of the part H < (1/4 ~ 1 / 6) RD, to ensure that the knife has sufficient stiffness.

The third is to use the flat-bottom end mill to mill the bottom of the inner groove. Because the bottom of the groove needs to be overlapped twice, the radius of the bottom edge of the tool is Re=R-r, that is, the diameter is d=2Re=2 (R- r), the tool radius is taken as Re=0.95 (Rr) during programming. For the machining of some three-dimensional and variable-angle contours, spherical milling cutters, ring milling cutters, drum milling cutters, conical milling cutters and disc milling cutters are commonly used.

2Set the tool point and the tool change point from where to start moving the tool to the specified position? Therefore, at the beginning of the program execution, it is necessary to determine the position at which the tool starts moving in the workpiece coordinate system. This position is the starting point of the tool movement relative to the workpiece during program execution, so it is called the program starting point or the starting point. This starting point is generally determined by the tool setting, so this point is also called the tool point.

When programming, it is necessary to correctly select the position of the tool point. The principle of setting the tool point is to facilitate numerical processing and simplify programming. Easy to align and easy to inspect during processing; resulting in small machining errors. The tool point can be set on the machined part, or on the fixture or on the machine. In order to improve the machining accuracy of the part, the tool point should be set as much as possible on the design basis or the craft base of the part. When the machine is actually operated, the tool position of the tool can be placed on the tool point by manual tool setting operation, that is, the “tool point” and the “tool point” coincide. The so-called "tool point" refers to the positioning reference point of the tool, and the tool point of the turning tool is the tool tip or the center of the tool nose arc. The flat end mill is the intersection of the tool axis and the bottom surface of the tool; the ball end mill is the ball center of the ball head, and the drill bit is the drill tip. With manual tool setting, the tool setting accuracy is low and the efficiency is low. In some factories, optical pairing mirrors, tool setting instruments, and automatic tool setting devices are used to reduce tooling time and improve tool setting accuracy. When a tool change is required during machining, the tool change point should be specified. The so-called "tool change point" refers to the position when the tool holder is rotated and the tool change point is set. The tool change point should be set outside the workpiece or the clamp, and the workpiece and other components are not affected when the tool is changed.

3 Determine the amount of cutting CNC programming, the programmer must determine the amount of cutting for each process, and write the program in the form of instructions.

The cutting amount includes the spindle speed, the amount of backfeed and the feed rate. For different processing methods, different cutting amounts are required. The selection principle of cutting amount is to ensure the machining accuracy and surface roughness of the parts, to maximize the cutting performance of the tool, to ensure reasonable tool durability, and to fully utilize the performance of the machine to maximize productivity and reduce costs.

3.1 Determining the spindle speed The spindle speed should be selected according to the permissible cutting speed and the workpiece (or tool) diameter. The calculation formula is: n=1000v/71D where: v? cutting speed, the unit is m/m movement, determined by the durability of the tool; n one spindle speed, the unit is r/min, D? workpiece diameter or tool Diameter, the unit is mm. Calculate the spindle speed n, and finally select the speed that the machine has or is close to.

3.2 Determining the feed rate The feed rate is an important parameter in the cutting amount of CNC machine tools. It is mainly selected according to the machining accuracy and surface roughness requirements of the parts and the material properties of the tools and workpieces. The maximum feed rate is limited by the machine stiffness and the performance of the feed system. The principle of determining the feed rate: When the quality requirements of the workpiece can be guaranteed, in order to increase the production efficiency, a higher feed rate can be selected. Generally selected in the range of 100 ~ 200mm / min; when cutting, processing deep holes or processing with high-speed steel tools, should choose a lower feed rate, generally in the range of 20 ~ 50mm / min; when processing accuracy, surface When the roughness requirement is high, the feed rate should be selected smaller, generally in the range of 20 ~ 50mm / min; when the tool is empty stroke, especially when the distance is "return to zero", the setting of the CNC system can be set. Maximum feed rate.

3.3 Determine the amount of back-feeding knife The amount of back-feeding knife is determined according to the rigidity of the machine tool, the workpiece and the tool. Under the condition of the rigidity, the amount of the back-feeding knife should be equal to the machining allowance of the workpiece as much as possible, so that the number of passes can be reduced. Increase productivity. In order to ensure the quality of the machined surface, a small amount of finishing allowance can be left, generally 0.2 to 0.5 mm. In short, the specific value of the cutting amount should be determined by analogy according to the machine tool performance, relevant manuals and practical experience.

At the same time, the spindle speed, depth of cut and feed rate can be adapted to each other to form the optimum cutting amount.

The cutting amount is not only an important parameter that must be determined before the machine tool is adjusted, but also whether the value is reasonable or not has a very important influence on the processing quality, processing efficiency, production cost and the like. The so-called "reasonable" cutting amount refers to the cutting amount that utilizes the cutting performance of the tool and the dynamic performance of the machine tool (power, torque) to obtain high productivity and low processing cost while ensuring the quality.