China Hardware’s Analysis on the CNC Machining Route

The ideal Cnc Machining program should not only ensure that the processing of qualified workpiece according to the drawing, but also should be able to make the function of Cnc Machine tools get reasonable application and full play. CNC machine tool is a kind of high-efficiency automation equipment, its efficiency is higher than the ordinary machine tool 2 ~ 3 times, so, to give full play to the characteristics of CNC machine tool, we must master its performance, characteristics, the use of operation method, but also must correctly determine the processing program before programming. Due to the difference of production scale, the processing plan for the same part is different, should be based on specific conditions, the choice of economic, reasonable process plan. One, the processing process is divided in the CNC machine tool processing parts, the process can be more centralized, a clamping should be as far as possible to complete the whole process. Compared with ordinary machine tool processing, processing process division has its own characteristics, commonly used process division principles have two type the following


1. The principle of ensuring the accuracy of CNC machining

CNC machining requires the process to be as centralized as possible, often rough and finish machining is completed under a clamping, in order to reduce the influence of thermal deformation and cutting force deformation on the workpiece shape, position accuracy, dimensional accuracy and surface roughness, rough and finish machining should be carried out separately. For shaft or disc parts, rough machining will be done first, leaving a small amount of finishing, to ensure the surface quality requirements. At the same time, for some box parts, in order to ensure the machining accuracy of the hole, the surface should be processed before machining the hole.


2.The principle of improving production efficiency in cnc machining

in order to reduce the number of tool change, save the time of tool change, should be used with the same knife processing parts all completed, and then change another knife to process other parts. At the same time should be as far as possible to reduce the empty stroke, with the same knife processing parts of the workpiece, should be in the shortest route to the processing parts. In practice, the cnc machining process should be considered comprehensively according to the structural characteristics and technical requirements of specific parts.



The determination of the processing route

In CNC machining, the path and direction of the tool (strictly the tool point) relative to the workpiece is called the machining path. That is, the path through which the tool moves from the tool point to the end of the machining program, including the cutting path and the non-cutting empty travel such as the introduction and return of the tool. The determination of the machining route must first ensure the dimensional accuracy and surface quality of the CNC machining parts, and then consider the numerical calculation is simple, the CNC cutting route as short as possible, high efficiency. The following examples of CNC machine tool processing parts often used in the processing route.


1.The outer cone on the CNC lathe is assumed to have a large diameter of D, a small diameter of D and a cone length of L. The processing route of the turning cone is shown in Figure 2-1. According to the step cutting route in Figure 2-1A, two rough cutting tools and the last fine cutting tools; The final distance S of two rough cutter should be calculated accurately, and the similar triangle can be obtained:

In this way, the cutting distance of the tool is shorter.


According to the diagonal-line processing route in Figure 2-1C, only the knife amount ap is determined each time without calculating the final tool distance, which is convenient for programming. But the amount of cutting back is changing in each cutting, and the cutting path of the tool is longer.


2.Analysis of the processing route of the car arc application G02 (or G03) command car arc, if the arc is processed with a knife, so that the knife is too large, easy to hit the knife. Therefore, the actual car arc, the need for multiple knife processing, the first will be most of the margin, and finally the car to the desired arc. The following introduces the common processing route of the car arc.

Figure 2-2 is the step cutting route of the car arc. That is, the first rough car into the ladder, the last knife fine car out of the arc. In this method, after determining the amount ap of each blade, the final blade distance S of the rough lathe must be calculated accurately, that is, the intersection point of the arc and the straight line. The cutting distance of this method is shorter, but the numerical calculation is more complicated.


Figure 2-3 is the concentric arc cutting route of the car arc. That is to use different radius circle to turn, and finally the desired arc processing. This method is easy to determine the starting point and end point coordinates of 90° arc after determining each cutting ap. It is simple in numerical calculation and convenient in programming. However, when processing according to Figure 2-3B, the empty travel time is longer.

Figure 2-4 is the cutting route of the turning cone method of the turning arc. Namely first car a cone, then car arc. However, it should be noted that if the starting point and the end point of the car cone are not determined well, the cone surface may be damaged, or the margin may be left too large. The determination method is shown in Figure 2-4. Connect OC to cross the arc at D, and make the tangent line AB of the arc through D. According to the geometric relation CD= OC-od = -r = 0.414r, this is the maximum cutting allowance when turning cone, that is, when turning cone, the machining route cannot exceed AB line. AC=BC= 0.586r can be obtained from the relationship shown in the diagram, which can determine the starting point and the end point when exiting the cone. When R is not too large, choose AC=BC= 0.5r. The numerical calculation of this method is complicated and the cutting route is short.


 3.Analysis of axial feed distance during screw thread

When threading, the cutter feed along the thread direction should be strictly proportional to the workpiece spindle rotation. Considering the tool from the stop condition to a specified feed speed or from the specified feed speed down to zero, drive system, there will be a transition process along the axial feed processing route length, in addition to guarantee processing thread length, should be added to the delta 1 (2 ~ 5 mm) tool introduction of distance and the delta 2 (1 ~ 2 mm) in cutting out distance, as shown in figure 2-5. This ensures that when cutting the thread, the tool is brought into contact with the workpiece after the speed rise is completed, and then the speed is reduced after the tool leaves the workpiece.


4.For continuous milling contour, especially in the processing of arc, we should pay attention to arrange the cutting tool into, cut out, to try to avoid repeated processing at the junction, otherwise there will be obvious boundary traces. As shown in figure 2-6, milling outside the circle arc interpolation way, want to arrange tool from tangential into peripheral milling processing, after the entire circle is processed, not at the point of tangency return directly, and let the tool movement distance, more best along the tangent direction, so as not to cancel the cutting tool compensation, the cutting tool and the workpiece surface collided, causing workpiece scrapped. When milling the inner arc, it is also necessary to follow the principle of tangential cutting, and arrange to cut and cut the transition arc, as shown in Figure 2-7. If the tool starts from the origin of the workpiece coordinates, its processing route is 1→2→3→4→5, so as to improve the machining accuracy and quality of the inner hole surface.


5.Analysis of hole machining route with high position accuracy

For the position accuracy of high precision hole system processing, special attention should be paid to the arrangement of the hole processing sequence, the arrangement is not appropriate, it is possible to bring the reverse gap along the coordinate axis, directly affect the position accuracy. As shown in Figure 2-8, Figure A is the drawing of the part. There are two processing routes for the six holes of the same size processed on the part. When processing according to the route shown in FIG. B, because the positioning direction of holes 5 and 6 is opposite to that of holes 1, 2, 3 and 4, the reverse clearance in the Y direction will increase the positioning error, and affect the positioning accuracy of holes 5 and 6 and other holes. According to the route shown in Figure C, after finishing machining 4 holes, move up a distance to point P, and then turn back to process 5 and 6 holes. In this way, the direction is consistent, the introduction of reverse clearance can be avoided, and the position accuracy of 5 and 6 holes and other holes can be improved.



6.Analysis of machining route of CNC milling surface

CNC Milling surface, commonly used ball head knife “line cutting method” for processing. The so-called line cutting method refers to the cutting tool and parts of the contour of the tangent point track is line by line, and the distance between the lines is determined according to the requirements of machining accuracy of parts. For surface machining with open boundary, two machining routes can be adopted. As shown in Figure 2-9, for the large blade of the engine, when the processing scheme as shown in Figure 2-9A is adopted, each time the blade point is processed along a straight line, the calculation of the cutter point is simple and the program is few. The processing process conforms to the formation of ruled surface, which can accurately ensure the straightness of the bus bar. When the machining scheme as shown in FIG. 2-9B is adopted, it conforms to the data of such parts and is convenient for post-processing inspection. The accuracy of blade shape is high, but there are many procedures. Because the boundary of the curved part is open, there are no other surface restrictions, so the curved boundary can extend, the ball head knife should start processing from outside the boundary.


Above through a few examples of cnc machining in the common processing route, the actual production, the CNC machining route to determine according to the specific structural characteristics of parts, comprehensive consideration, flexible use. The general principle to determine the CNC machining route is to shorten the CNC machining route as far as possible in order to improve productivity under the condition of ensuring the machining accuracy and surface quality of parts.

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