The core and cavity of the mold often have various free-form surfaces, which is very suitable for processing on CNC machine tools. The CNC machining technology is quite different from the ordinary machining technology. This article combines the numerical control processing technology of the mold core of children's product decoration.
The numerical control processing technology refers to the sum of various methods and technical means used when processing parts with a numerical control machine tool, which is applied to the entire numerical control processing technology process. CNC machining has the characteristics of high processing efficiency, stable quality, relatively low technical requirements for workers, and the ability to complete the processing of complex curved surfaces in one clamping. The more important. The quality of CNC process design will directly affect the dimensional accuracy and surface quality of CNC machining dimensions, the length of processing time, the cost of materials and labor, and even directly affect the safety of processing. The following examples analyze the CNC machining technology of typical mold forming parts.
1. Product analysis
The product exemplified in this article is a children's product decoration, the material is ABS. The structure of the product is relatively simple, the surface is flat, there are semi-circular holes on the side, and there are multiple circular holes on the top. Because the product is an ornament, it is not a precise structural part, so the appearance quality of the product is high, and the dimensional tolerance is not strict.
2. Structure and analysis of molded parts
After obtaining the physical shape or engineering drawing of the product, the mold can be designed using the CAD function in Pro / ENGINEER, NX, or MasterCAM. The designed mold core is shown in Figure 2.
The mold core has the following characteristics:
(1) The size of the core blank is 200 × 170 × 65mm, the size after processing is 160.8 × 126.6 × 35.8mm, and the material is S136 steel.
(2) The core height of the core is 35.8mm, and the position where the ellipse and the triangle intersect is rounded, only R1mm. It is more difficult to directly machine these positions with a milling cutter, and electrical discharge machining can be used to meet the requirements.
Because the dimensional tolerance requirements of the product are not high, the core can be directly processed using CNC machine tools.
3. Process analysis
There is a certain difference between CNC machining technology and traditional machining technology. Since most CNC machine tools do not have the ability to process, every detail of the processing process must be determined in advance, and the processing is automatically completed according to the compiled program. Therefore, the processing technology must be analyzed in detail before programming and the corresponding design must be designed. Processing operations.
1. Process benchmark selection
CNC machining mostly adopts the principle of process concentration. Therefore, when selecting the process reference, the appropriate reference elements should be selected as much as possible to reduce the number of clampings and improve the processing efficiency and processing accuracy. At the same time, when selecting the positioning reference, you must refer to the It is required to make the process benchmark coincide with the design benchmark to reduce the error caused by the non-coincidence of the benchmark.
In this example, the workpiece blank is a rectangular parallelepiped blank that has been ground, and the parallelism, perpendicularity, and dimensional accuracy have been ensured. Therefore, the opposite surface of the length and width directions can be used as the reference for the horizontal direction (XY direction); As a reference in the height direction (Z direction). At the same time, find a pair of tool references on the machine tool to ensure that the programmed height reference, that is, the Z0 point of the workpiece coordinate system, can still be accurately found after tool change. These reference planes are no longer processed in the CNC machining process, as a processing reference can ensure the accuracy of the reference and unity before and after.
2. Selection of clamping method
During milling, the clamping method of the workpiece is generally in the form of clamping with a pressure plate and bolts, clamping with machine flat-nose pliers, and clamping with special clamps. The mold core is produced in a single order and generally does not use a special fixture; the size of the mold core is 200 × 170 × 65mm, which is a small workpiece. Therefore, the machine uses flat-nose pliers for clamping.
When using machine flat-nose pliers for clamping, the height of the core should be considered to be 35.8mm. Therefore, the height of the top surface of the blank after clamping should be greater than 35.8mm, and the bottom surface can be filled with equal-height iron pads.
3. Arrangement of processing sequence
Parts processed on CNC machine tools are generally divided according to the principle of process concentration, that is, each process should include as much processing content as possible. The process division methods include division by tool used, division by installation frequency, division by rough finishing, division by machining part, etc. This example is a mold core, which belongs to single-piece production. Therefore, when arranging the processing sequence, the principle of process concentration is used to reduce the number of tool changes and improve processing efficiency.
Whether the processing order is arranged reasonably directly affects the processing quality, processing efficiency and processing cost. When selecting the processing sequence, according to the blank condition and part structure, combined with the positioning reference and clamping method of the part, it is important to consider that the rigidity of the workpiece during the processing is not damaged, reduce the deformation, and ensure the processing quality.
The rough material of the core of the mold is a rectangular parallelepiped, and the cutting amount of the parting surface of the product is large. It must be roughed first, and then completed by semi-finishing and finishing. After processing the parting surface, finish the top surface, bumping surface and glue position in sequence.
4. Tool selection
The choice of tool is one of the important contents in the CNC machining process. Processing tools not only affect the processing efficiency of machine tools, but also directly affect the processing quality of parts. The spindle speed and range of CNC machine tools are much higher than ordinary machine tools, and the output power of the spindle is larger. Therefore, compared with traditional machining methods, higher requirements are placed on CNC machining tools, which require high precision and strength. It has the characteristics of good rigidity and high durability, and requires stable dimensions and easy installation and adjustment. CNC tools are one of the prerequisites for improving processing efficiency. Its choice depends on the geometry of the parts to be processed, the state of the material, the rigidity of the jig and the tool selected by the machine tool.
The material of the core of this mold is S136 steel, and the hardness is about 220HB; the blank clamping adopts flat-nose pliers, which provides sufficient rigidity. Therefore, the carbide core cutter is used for the processing of this core, and the tool parameters are as follows:
(1) Round nose knife with a diameter of 30mm and a radius of 5mm;
(2) Round nose knife with a diameter of 16mm and a radius of 0.5mm;
(3) A ball head knife with a radius of 5mm;
(4) Flat-bottom end mill with a diameter of 8mm.
4. CNC machining process based on MaterCAM
MaterCAM software is a CAD / CAM system based on the PC platform. Due to its low hardware requirements, flexible operation, easy to learn and use, it is widely used in mechanical processing, mold manufacturing, automotive industry and aerospace industry. Design, three-dimensional surface design, tool path simulation and machining entity simulation and other functions. This example uses MasterCAM 9.1 for tool path programming. The processing sequence is shown in Table 1.
The above processing steps have been verified by actual processing, which proves that the processing effect is good, and the dimensional accuracy and surface processing quality meet the requirements of the drawings.
V. Analysis and summary
(1) CNC machine tools have obvious advantages in the processing of molded parts with complex curved surfaces. As long as the process design is reasonable, more than 80% of the processing volume can be completed.
(2) The CNC machining of the mold is generally a single piece of small batch production, so it is usually used to use a common fixture for clamping, and it is usually a clamping to complete the processing of multiple processes. Keep the process as concentrated as possible, and at the same time, reduce the number of tool changes as much as possible to reduce standby time and improve machine tool utilization.
(3) Molded parts can generally be completed through three processes: roughing → semi-finishing → finishing. In the process design, we must pay attention to the selection of positioning benchmarks, try to ensure that the positioning benchmarks and design benchmarks coincide, and reduce the errors caused by the inconsistency of the benchmarks. At the same time, the selection of the positioning reference should make the setting of the workpiece coordinate system simple.
(4) When choosing a tool, comprehensive consideration should be taken according to many factors such as the machine tool, workpiece material and design requirements. The setting of tool parameters should be based on the parameters raised by the tool manufacturer, and be modified accordingly according to the processing conditions to maximize the potential of the tool. Under the premise of ensuring that the tool does not interfere with the workpiece, the length of the tool is shortened as much as possible to reduce tool vibration, extend tool life, and improve machining accuracy.
