The ideal machining procedure should not only ensure that qualified workpieces conform to the drawings, but also enable the functions of the CNC machine tool to be reasonably applied and fully utilized. CNC machine tools are a kind of high-efficiency automation equipment. Its efficiency is 2 to 3 times higher than that of ordinary machine tools. Therefore, to make full use of this feature of CNC machine tools, one must master its performance, characteristics, and operation methods. The machining plan must be correctly determined before programming.
In the processing of CNC machine tools, due to the complex and diverse processing objects, in particular the shape and position of the profile curve, and the influence of various factors such as different materials, different batches, etc., a specific analysis should be performed when formulating a specific part. Treated differently and differently. Only in this way can the formulated processing plan be reasonable so as to achieve the objectives of high quality, high efficiency and low cost.
After careful and careful analysis of the processing technology, the general principles for the development of the processing plan are first rough after finishing, first near and far, first inside and outside, the least program segment, and the shortest route, due to the difference in production scale. The processing plan for the same part is different, and an economic and reasonable process plan should be selected according to the specific conditions.
1, the processing process division
In the processing of parts on CNC machine tools, the process can be relatively concentrated, and one installation should be completed as much as possible. Compared with the ordinary machine tool processing, processing process division has its own characteristics, the common process division principle has the following two.
1.1 Principle of Guaranteed Accuracy
CNC machining requires that the processes be as focused as possible. Rough and finish machining is often done in a single fixture. In order to reduce the effect of thermal deformation and cutting force deformation on the shape, position accuracy, dimensional accuracy, and surface roughness of the workpiece, rough and finish machining should be performed separately. For shaft or disc parts, rough machining will be done first, leaving a small amount of fine finishing to ensure surface quality requirements. At the same time, for some box parts, in order to ensure the hole's machining accuracy, the surface should be machined first and then the hole should be machined.
1.2 Principle of Increasing Productivity
In the numerical control processing, in order to reduce the number of tool changes and save the time of tool change, all the machining parts that need to be machined with the same tool should be completed, and then another tool should be used to process other parts. At the same time, the idle stroke should be minimized. When machining multiple parts of the workpiece with the same knife, the shortest route should be used to reach each processing part.
In practice, the numerical control processing process should be comprehensively considered based on the structural characteristics and technical requirements of specific parts.
2, the determination of processing routes
In NC machining, the path and direction of movement of a tool (strictly a tool location) with respect to a workpiece is called a machining path. That is, the tool passes from the tool-cutting point until the end of the machining program, including the cutting path and the non-cutting idle stroke such as the tool introduction and return. There are many factors that affect the cutting path, such as process methods, workpiece materials and their status, machining accuracy and surface roughness requirements, workpiece rigidity, machining allowance, tool rigidity, durability and status, machine type and performance, etc. The determination must first ensure the dimensional accuracy and surface quality of the part being machined, and then consider the simple numerical calculation, as short as possible route, high efficiency and so on.
The following is an example of the analysis of commonly used processing routes for machining CNC machined parts.
2.1 analysis of the processing route of the car cone
The outside cone of a numerically-controlled lathe is assumed to have a large diameter of a cone D, a diameter of d, a length of a cone L, and a processing path of the cone of the vehicle is shown in FIG.
Figure 1 Processing route of the car cone
According to the ladder cutting route in Fig. 1(a), two-knife roughing and last-smooth finishing; the final blade distance S of the two-knife roughing shall be precisely calculated, and similar triangles may be obtained:
In this type of processing route, when the roughing, the amount of back knife is the same, but when finishing the car, the amount of backing knife is different; at the same time, the cutting course of the cutting tool is the shortest.
According to the similar slash cutting route in Fig. 1(b), the final vehicle distance at the roughing stage S must also be calculated, which can also be calculated from similar triangles:
According to this processing route, the tool cutting movement distance is short.
According to the slash processing route shown in Fig. 1(c), only the amount of a back knife a is determined, and it is not necessary to calculate the final tool distance, which is convenient for programming. However, the amount of backing knife is changed in each cutting, and the cutting movement of the tool is longer.
2.2 Analysis of processing arcs of car arc
Use the G02 (or G03) command car arc, if you use a knife to process the arc, so eat a knife is too large, easy to hit the knife. Therefore, when the actual vehicle arc, it needs to be multi-tooled, first remove the large excess amount, and finally get the desired arc.
The following research and analysis of the car arc commonly used processing route.
Figure 2 Form of Arc Cutting Route
In FIG. 2 , the a diagram is represented as a concentric circular pattern, the b diagram is represented as a circular arc of the same diameter (a different center), the c diagram is a triangular pattern, and the d diagram is a trapezoidal pattern. Different types of cutting routes have different characteristics. Understanding their respective characteristics is conducive to rationally arranging their route. The above several cutting routes are analyzed: the least number of blocks is the concentric circular type and the equal diameter circular type; the shortest path is the concentric circular type, and the rest is the triangular trapezoid and the equal diameter circular type; the simplest calculation and programming are Equal-diameter circular type (program circulation function is available), followed by concentric circles, triangles and trapezoids: the metal removal rate is the highest, and the cutting force distribution is the most reasonable in the trapezoidal form; the balance of the finished car is the concentric circular type.
Figure 3 The arc of the ladder cutting route
Figure 3 shows the step cutting path of the car arc. That is to say, the first rough car becomes a ladder, and the last one is a circular arc. After this method determines the amount of knife a), the final tool distance S of the roughing vehicle must be accurately calculated, that is, the intersection point between the arc and the straight line. In this method, the tool cutting movement distance is short, but the numerical calculation is more complicated.
Figure 4 Concentric arc cutting route car arc
Figure 4 shows the concentric arc cutting path of the car arc. That is, turning with different radius circles and finally machining the required arc. This method determines the starting point and end point coordinates of the 90° arc after each time the amount of knife a is determined. The numerical calculation is simple and the programming is convenient and often used. However, in the case of processing according to Figure 4b, the idle travel time is longer.
Figure 5 Car Cone Cutting Route Arc
Figure 5 shows the car cone cutting course of the car arc. That is, a car first cone, and then car arc. However, it should be noted that if the determination of the starting point and the end point of the cone is not good, the conical surface may be damaged, or the excess may be left too large. Determine the method as shown in Figure 5, connect OC to arc in D, over D to make tangent to the arc AB.
From the geometric relationship of CD = OC-OD = one day = 0.4148, this is the maximum cutting allowance when the cone, that is, when the cone, the processing route can not exceed the AB line. From the graphic relationship, AC=BC=0.5868 can be obtained, which can determine the start and end of the cone. When R is not too large, it is desirable that AC=BC=0.5R. The numerical calculation of this method is more complicated and the cutting path of the tool is shorter.
Figure 6 Incoming and outgoing distances when cutting threads
3. Analysis of axial feed distance when threading
When threading, the feed of the tool in the direction of the thread should maintain a strict speed ratio with the spindle rotation of the workpiece. Considering that the tool reaches the specified feed speed from the stop state or from the specified feed rate to zero, the drive system must have a transitional process. The length of the processing path along the axial feed must be maintained in addition to the length of the machined thread. Increase the cutter introduction distance of δ1 (2mm~5mm) and the tool cutting distance of δ2 (1mm~2mm), as shown in Fig.6. This ensures that when the thread is cut, the tool is brought into contact with the workpiece after the speed has been raised, and the tool is decelerated after it leaves the workpiece.
4. Analysis of contour milling processing route
For continuous milling contours, especially when machining circular arcs, it is necessary to pay attention to arrange the cutting and cutting out of the cutters, and to avoid repeated processing at the junctions, otherwise there will be obvious boundary marks. When circular interpolation is used to mill the outer circle, the tool must be arranged to enter the circumferential milling process from the tangential direction. When the complete circle is processed, do not directly retract the tool at the tangent point, and let the tool move a certain distance, preferably along the Tangential direction, so as not to cancel the tool compensation, the tool and the workpiece surface collision, resulting in scrapped parts. When milling the inner arc, we must also observe the principle of cutting from the tangential direction, and arrange to cut in and cut out the transition arc to improve the machining precision and quality of the inner hole surface.
5. Analysis of Porous Processing Routes
For the machining of the hole system where the position accuracy requires high precision, special attention should be paid to the arrangement of the processing sequence of the hole. If the arrangement is improper, it is possible to bring along the backlash along the coordinate axis and directly affect the position accuracy.
