Aiming at the problems of difficult processing of spatial arc surfaces, many processing elements, high processing precision and high surface roughness requirements in cover processing, through the analysis of machine tool systems, part manufacturability, processing tools and programming methods, a formula for processing the machining center was formulated. The process implementation plan introduces the application method of ball-nose tools in the process of processing the housing space curved surface.
1 Preface
The cover is generally used as a seal. Before assembly, it needs to undergo gas, water and other pressure tests to ensure that the product will not leak and ensure the airtightness of its assembly and use. Most of them are integral castings or welded parts with complex shapes and multiple structures. Variable, different sizes, cavity-shaped interior, thin and uneven walls. In production and manufacturing, there are not only hole systems, sealing grooves and planes with high precision requirements, but also many special-shaped fillets, bosses and irregular curved surfaces, which are difficult to process and manufacture [1].
2 Part structure and process analysis
2.1 Part structure analysis
The cover is a box-type part. It is a semi-enclosed polyhedron with uneven cavities and inner walls and mostly irregular structures. It is mainly used to ensure the cleanliness of the body and reduce the noise generated by the body during work. At the same time, it can Play a role in beautifying the appearance. In mechanical processing, there are many processing elements, large processing volume, and irregular structure, which makes the process complex [2].
Figure 1 Cover process requirements
2.2 Process analysis
Cover: The blank is a solid cast iron part with strict surface quality requirements, the material is difficult to process, the tool wears quickly, and it is difficult to process spatial curved surfaces. The cover parts are shown in Figure 1. There are left and right arcs on the back of the flange, separated by 14mm ribs in the middle. The left and right are symmetrical structures, with one side left on the upper and lower sides. The surface roughness value Ra=1.6μm .
2.3 Difficulty analysis
The cover is a box type part. The material QT{{0}} is ductile iron casting, which has high strength and good toughness. It has the characteristics of wear resistance, vibration absorption and oxidation resistance, but its cutting performance is poor. According to the drawing requirements, the back side of the connecting flange needs to be fully processed. The arcs are symmetrically distributed on the left and right, separated by ribs in the middle. The arc surface is processed perpendicular to the tool axis. When processing the curved surface, the geometric dimensions of the tool need to be fitted to the surface tool path to ensure that the shape of the final curved surface meets the process requirements. . As shown in Figure 1, the thickness of the rib plate is (16±0.025) mm, (14±0.02) mm, and the root fillet R (82.5±0.025) mm. The processing accuracy is high and the surface quality requirements are strict. Since the back of the flange is separated by ribs, interference will occur when using a three-sided milling cutter or lathe, making it impossible to process [3].
3. Process flow and CNC machining methods
3.1 Processing methods
Although the arc surface of this part is a surface of revolution, its shape and structure are box-type parts (see Figure 2), so it is not suitable for turning machine tools. The back side of the flange is separated by three ribs, with a rounded root transition. The back and front sides require high dimensional accuracy and surface roughness, and can be processed on three-axis and multi-axis milling machines. In multi-axis machining, since the mutual positions of the tool and the workpiece change at any time during processing, all processing can be completed in one clamping to obtain optimal processing conditions. However, its procurement cost and software cost are much higher than those of three-axis, maintenance and upkeep costs are too high, and the requirements for operators' operating skills are also high, resulting in high labor costs. In the three-axis machine tool, the tool axis vector remains unchanged and is processed in the normal plane of the Z-axis. The use of linkage fitting can complete spatial surface processing and achieve better system rigidity. Since this product is produced in large quantities and in small batches, there is no need to customize tooling. The production needs of this product can be met by using existing universal equal-height pads and downward pressure plates for positioning and clamping. After on-site measurement of the machine tool milling head and analysis of the housing processing factors, a ball-end milling cutter can be used to create a curved surface fillet in the ZY plane along the Z-axis direction in order to obtain better surface processing accuracy, quality and efficiency. And the best value for money.
Figure 2 Cover blank
3.2 Tool concept
The selection of tools and the determination of cutting amount are important contents in CNC machining technology. They not only affect the processing efficiency of CNC machine tools, but also directly affect the processing quality, and at the same time change the entire processing cost. Combined with the characteristics of the machine tool, the performance of the workpiece material, clamping and process requirements, three-sided edge milling cutters, end mills and ball end mills are selected for processing. Since the three sections of ribs on the back of the flange are evenly spaced at 90 degree , there is a lot of residue at the root of the ribs when back-milled with a three-sided edge milling cutter, and the end mill can be used to process all the ribs along the arc direction. The root arc surface is a three-dimensional surface formed from bottom to top. A ball-end tool with a radius less than or equal to the minimum curvature radius of the surface should be used for interpolation milling. It is measured that the 6mm margin on one side of the blank is large. In order to ensure processing rigidity and efficiency, the specifications shown in Figure 3 are φ20mm×80mm×150mm×4F (YT) end mill and R10mm×80mm×150mm (YT) ball end milling cutter. knife.
Figure 3 End mill (bottom) and ball end mill (top)
3.3 Cutting plan
In the cutting process, according to the actual processing conditions of the workpiece, in order to ensure the accuracy and surface roughness of the rounded curved surface, climb milling is used from bottom to top. Separate tool start points and tool set points. Under the premise of ensuring safety, the starting point of the tool should be as close as possible to the workpiece to reduce idle tool travel, shorten the feed path, and save execution time during the machining process. Since the blank margin is large, the cyclic processing method should be used to mill in sequence as shown in Figure 4, gradually removing the margin in the YZ direction, and leaving a 0.2mm margin for finishing. During this period, it should be noted that the feed and retract points should be perpendicular to In the Z-axis direction, the feed speed cannot be "G0", and the "G0" command should avoid "Y, Z" moving at the same time.
The tool cutting parameters are selected: φ20mm end mill. The tool material supports a linear speed vc of 90~120m/min, a back cutting amount ap of 0.3~2mm, and a feed fz of 0.07~0.3mm/z.
R10mm×80mm×150mm (YT) ball end milling cutter, the tool material supports linear speed vc of 120~150m/min, back engagement ap of 0.3~0.8mm, and feed fz of 0.11~0.18mm/z.
Because the blank is a solid casting, affected by the casting process, the surface of the blank may occasionally have hard spots, pores, and sand inclusions. In order to reduce quality risks and ensure cutting stability, after debugging and verification of the test piece, the final cutting parameters of the φ20mm end mill were selected as vc=92m/min, n=1465r/min, ap=1.5mm, fz=0.07mm/z, vf =410mm/min; the cutting parameters of R10mm ball end mill are selected as vc=130m/min, n=2070r/min, ap=0.5mm, vf=228mm/min. After processing 12 pieces in a batch, using the above cutting parameters, the processing quality and stability are good, and the tool is durable.
Figure 4: Tool path
3.4 Programming
According to the geometric dimensions of the part drawing, the tool center running trajectory data is calculated. Since the arc surface is in the YZ plane, when using a ball end milling cutter, it is necessary to calculate the coordinates of the contact point and complete R82.5mm arc milling through point approximation. The ultimate goal of numerical calculation is to obtain all relevant position coordinate data required for programming. Calculate the Y and Z coordinate values through trigonometric functions according to Figure 5: Y=Rcos , Z=Rsin .
Figure 5 Coordinate calculation principle
When programming the Heidenhain CNC program, set Q1=17 as the starting angle, Q2=0.1 as the angle increment, Q3=+76.5 as the ending angle, Q4=92.5 (R=82.5+10) as the arc radius, Q1=Q1 +Q2 adds a variable for angle. After the program is compiled, the program operation must be checked before it is officially used for production and processing. In special cases, trial machining inspection of parts is also required. According to the inspection results, the program is modified and adjusted, and it is often repeated many times until a program that fully meets the processing requirements is obtained.
56 TOOLCALL "D20-QTD" Z S500
57L Z+100 R0 FMAX
58L X-50 Y-150 R0 FMAX
59L Z+26R0 FMAX
60 L X+32 R0 F1000
61 L Y-88.771
62 FN 0:Q1 =+17; starting angle
63 FN 0:Q2 =+0.1; angle increment
64 FN 0:Q3 =+76.5; final angle
65 FN 0:Q4 =+92.5; arc radius
66 FN 0:Q5 =+0
67 FN 0:Q6 =+0
68 LBL.2
69 Q1=Q1+Q2; angle increases variable
70 Q5=Q4×COS Q1; loop calculation of Y value
71Q6=Q4×SIN Q1; loop calculation of Z value
72 L Y-Q5 Z+Q6 R0 F1000
73 FN 12: IF+Q1LT+Q3 GOTO LBL 2; loop judgment
74L Y-21 Z+90.085
75L Z+100 FMAX; knife retraction
76 M0
4 Debugging, processing and inspection
The processing origin of the surface fillet in the program is the center of the flange, that is, X{{0}}, Y0, and Z0 in G54 are on the upper surface of the flange. After using the edge finder to center in the X and Y directions, input the mechanical coordinates into the corresponding G54. After the Z-direction mandrel or reference knife fits the outer circle of the flange, calculate the Z value and input it into G54. Before processing, let the machine tool run dry to check the correctness of the tool motion trajectory. During debugging, the spindle speed and feed rate during processing can be appropriately adjusted according to the actual situation (see Figure 6 for the processing process) to achieve the best cutting performance. After the first piece is completed, it is sent to a three-coordinate measuring instrument to measure linear dimensions, geometric tolerances and surface roughness. The test results meet the process requirements.
Figure 6 Surface fillet processing
5 Conclusion
Through the special use of ball-end milling cutters, after many attempts and tests, the process plan for processing the cover surface was finally determined, successfully solving the problem of difficult processing of the arc surface of the cover space, many processing elements, high processing accuracy and surface roughness. Strict requirements and other difficult issues. It ensures the correctness of cover processing, improves the controllability and stability of processing quality, and ultimately forms mass production capabilities. At the same time, this method has wide practicability and can provide help and reference for similar surface processing applications.
