The correct selection and programming skills of CNC machine tools
1 Introduction
At present, many CAD/CAM software packages provide automatic programming functions. These softwares usually prompt the relevant problems of the process planning in the programming interface, such as tool selection, machining path planning, cutting amount setting, etc. The programmer only needs to set relevant parameters. , you can automatically generate the NC program and transfer it to the CNC machine to complete the processing.
Therefore, the selection of the tool and the determination of the cutting amount in the numerical control machining are completed under the human-machine interaction state, which is in sharp contrast with the ordinary machine tool processing. At the same time, the programmer is required to master the basic principles of tool selection and cutting amount determination. When programming, the characteristics of CNC machining are fully considered, and the cutting tool and cutting amount can be selected correctly.
2. The number and characteristics of commonly used knives for processing
CNC machining tools must adapt to the characteristics of high-speed, high-efficiency and high-degree of automation of CNC machine tools. Generally, they should include general-purpose tools, universal connection tool holders and a small number of special tool holders. The tool holder is connected to the tool and mounted on the power head of the machine, so it has been gradually standardized and serialized.
2.1 Classification of Numerical Control Tools There are multiple methods
a. According to the tool structure can be divided into
(1) Integral;
(2) Inlaid type, using welding or clip-type connection, machine clip type can be divided into non-inverted and indexable;
(3) Special types, such as compound cutters, shock-absorbing cutters, etc.
b. According to the materials used to manufacture the tool, it can be divided into:
(1) High-speed steel cutters;
(2) Carbide cutting tools;
(3) diamond tools;
(4) Other materials such as cubic boron nitride tools, ceramic tools, etc.
c. From the cutting process can be divided into:
(1) Turning tools, such as outer circle, inner hole, thread, cutting tool, etc.;
(2) Drilling tools, including drills, reamers, taps, etc.;
(3) Boring tools;
(4) Milling tools.
In order to meet the requirements of CNC machine tools for durable, stable, easy-to-adjust, convertible and other tools, in recent years, the tool-type indexable cutting tools have been widely used, reaching 30% to 40% of the entire number of CNC tools in the number of metal cutting. The amount accounts for 80% to 90% of the total.
2.2 Compared with the tools used on ordinary machine tools, numerical control tools have many different requirements. The main features are as follows:
(1) Good rigidity (especially rough machining tools), high precision, low vibration and thermal deformation, good interchangeability, and quick tool change;
(2) High life, stable and reliable cutting performance;
(3) The size of the tool is easy to adjust to reduce the time for tool change adjustment;
(4) The tool should be able to reliably chip or chip, in order to facilitate the removal of chips;
(5) Serialized standardization to facilitate programming and tool management.
3. CNC machining tool selection
The selection of the tool is performed in the CNC-programmed human-machine interaction. It should be based on the processing capability of the machine tool, the performance of the workpiece material, plus
The correct selection of tools and shanks for the cutting process and other related factors. The general principle of tool selection is: easy installation and adjustment, good rigidity, high durability and high precision. In the premise of meeting the processing requirements, try to choose a shorter tool holder to improve the rigidity of the tool processing.
(1) When selecting a tool, make the size of the tool compatible with the surface size of the workpiece being machined. In production, the peripheral contours of flat parts are often machined with end mills. When milling planes, carbide insert cutters are selected. When machining bosses or grooves, high-speed steel end mills are selected; surface roughing or rough machining is performed. In the case of holes, corn milling inserts with carbide inserts can be selected. For the machining of some three-dimensional profiles and variable bevel profiles, ball-end cutters, ring cutters, taper cutters, and disc-type cutters are often used.
(2) When free-form (die) machining is performed, the end cutting speed of the ball-end cutting tool is zero. Therefore, in order to ensure the machining accuracy, the cutting pitch is generally the top-pitch, so the ball head is often used for finishing the surface. The flat-blade cutter is superior to the ball-end cutter in terms of surface machining quality and cutting efficiency. Therefore, as long as it is not overshadowed, the flat cutter must be selected first regardless of whether the surface is rough or finished. In addition, the tool's durability and precision are highly related to the tool's price. It must be noticed that, in most cases, selecting a good tool increases the tool cost, but the resulting increase in machining quality and machining efficiency. , you can greatly reduce the cost of the entire process.
(3) At the machining center, various tools are installed on the tool magazine, and the tool selection and knife action are performed at any time according to the procedure. Therefore, a standard tool holder must be used so that the standard tool used for drilling, boring, expanding, milling, etc. can be quickly and accurately mounted on the spindle or tool magazine of the machine tool. The programmer should know the structural dimensions, adjustment methods, and adjustment ranges of the tool shank used on the machine tool to determine the radial and axial dimensions of the tool during programming. At present, China's machining center adopts the TSG tool system, and its handle has two kinds of straight shank (three kinds of specifications) and taper shank (four kinds of specifications), including a total of 16 kinds of handles for different purposes.
(4) In the machining process of economical CNC machine tools, since the sharpening, measurement, and replacement of the tools are mostly performed manually, and the time for assisted use is long, the order of the tools must be arranged properly. Generally should follow the following principles: 1 reduce the number of tools as much as possible; 2 After a tool is clamped, it should complete all the processing steps it can perform; rough finishing of the tool should be used separately, even the same size of the tool; 4 first Milling after drilling; 5 first surface finishing, after two-dimensional outline finishing; 6 where possible, should use the automatic tool change function of CNC machine tools, in order to increase production efficiency.
4. Determination of cutting amount during processing
The principle of reasonable choice of cutting amount is: When rough machining, the general improvement is to increase productivity. However, economical efficiency and processing cost should also be taken into account. When semi-finishing and finishing, the cutting efficiency should be taken into account under the premise of ensuring the quality of processing. Economics and processing costs. The specific value should be based on the machine tool manual for cutting consumption, combined with experience.
Specifically consider the following factors:
(1) Cutting depth ap. In the case of machine tool, workpiece and tool rigidity, ap is equal to the machining allowance, which is an effective measure to increase productivity. In order to ensure the machining accuracy and surface roughness of the parts, a margin should generally be left for finishing. The finishing allowance of CNC machine tools can be slightly smaller than that of ordinary machine tools.
Cutting width L. In general, L is proportional to the tool diameter d and inversely proportional to the cutting depth. In the processing of economical CNC machine tools, the general L range is L = (0.6-0.9)d.
(2) Cutting speed V. Raising V is also a measure to increase productivity, but v has a closer relationship with tool life. As v increases, the tool life drops sharply, so the choice of v depends mainly on tool life. In addition, the cutting speed and processing materials are also very important, for example, milling with an end mill just 30CrNi2MoVA alloy, V can be used about 8m/min; and with the same endmill milling aluminum alloy, V can be selected more than 200m/min .
Spindle speed n (r/min). The spindle speed is generally selected according to the cutting speed v. The formula is: V=pnd/1000. CNC machine tool control panel is generally equipped with spindle speed adjustment (magnification) switch, the spindle speed can be adjusted during the whole multiple adjustment.
(3) Feed rate Vf. Vf should be selected according to the machining accuracy and surface roughness requirements of the part and the tool and workpiece material. The increase in Vf can also increase production efficiency. When the surface roughness requirement is low, Vf may be selected to be larger. In the process, Vf can also be manually adjusted through the trim switch on the machine control panel, but the maximum feed rate is limited by the equipment stiffness and feed system performance.
Conclusion
With the wide application of numerical control machine tools in production practice and quantification of the production line, numerical control programming has become one of the key issues in numerical control processing. In the process of NC programming, it is necessary to select the tool and determine the cutting amount in real time under the condition of human-machine interaction. Therefore, programmers must be familiar with the selection method of cutting tools and the principle of determining the cutting amount, so as to ensure the processing quality and processing efficiency of parts, give full play to the advantages of CNC machine tools, and improve the economic efficiency and production level of enterprises.
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