Tool selection and cutting amount determination in CNC machining (2)

3. Selecting tools for CNC milling In CNC machining, the flat-end end mills for milling the inner and outer contours of plane parts and the milling plane are commonly used. The empirical data of the parameters of the tool are as follows: First, the cutter radius RD should be less than the minimum curvature of the contour surface of the part. The radius Rmin is generally taken as RD = (0.8 - 0.9) Rmin. The second is the machining height H<(1/4-1/6) RD of the part to ensure that the knife has sufficient rigidity. 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=Rr, that is, the diameter is d=2Re=2(Rr), programming The tool radius is taken as Re=0.95 (Rr). 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.

At present, most of the numerical control machine tools use serialized and standardized tools. The shank and cutter heads of the indexable machine clamps, such as external turning tools and end turning tools, have national standards and serialized models for machining centers and automatic tool changers. The machine tool of the device and the tool holder of the tool have been serialized and standardized. For example, the standard code of the taper tool system is TSG-JT, the standard code of the straight tool system is DSG-JZ, and the selected tool is selected. Before the use, the tool size must be strictly measured to obtain accurate data, and the operator inputs the data into the data system, and the machining process is completed by the program to process the qualified workpiece.

Second, set the knife point and the tool change point

From what position does the tool start moving to the specified position? So at the beginning of the program execution, the position at which the tool starts moving in the workpiece coordinate system must be determined. This position is the starting point of the tool relative to the workpiece movement during program execution. So we call the starting point of the program 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.

Third, it is determined that the amount of cutting <br> <br> NC programming, the programmer must identify the cutting parameters of each process, and written into 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.

1. Determine the spindle speed The spindle speed should be selected according to the allowable cutting speed and the workpiece (or tool) diameter. Its calculation formula is: n=1 0 00 v/7 1D

In the formula: 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—the workpiece diameter or the tool diameter, the unit is mm. Calculate the spindle speed n, and finally select the speed that the machine has or is close to.

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