Cutting speed and feed rate of cnc vertical machining center:
1: Spindle speed = 1000Vc/πD
2: The maximum cutting speed (Vc) of general tools: high-speed steel 50 m/min; super hard objects 150 m/min; coating tools 250 m/min; ceramic and diamond tools 1000 m/min 3 Brinell hardness of alloy steel =275-325, high-speed steel tool Vc=18m/min; cemented carbide tool Vc=70m/min (cutting amount=3mm; feed amount f=0.3mm/r)
There are two calculation methods for the spindle speed. The following examples illustrate: ① Spindle speed: One is G97 S1000, which indicates that the spindle rotates 1000 revolutions per minute, which is commonly referred to as constant speed. The other is that G96 S80 is a constant linear speed, which is the spindle speed determined by the appearance of the workpiece.
There are also two feed speeds. G94 and F100 indicate that the cutting distance in one minute is 100 mm. The other is G95 F0.1, which indicates that for every revolution of the spindle, the tool feed scale is 0.1 mm. Selection of cutting tools and determination of cutting amount in CNC machining
The selection of tools and the determination of cutting parameters are important contents in the CNC machining process. It not only affects the machining power of the CNC vertical machining center, but also directly affects the machining quality. The development of CAD/CAM skills makes it possible to directly use CAD planning data in the CNC vertical machining center, especially the connection between the microcomputer and the CNC machine tool, so that the entire process of planning, process planning and programming is completed on the computer, generally not Need to output a special process file.
Nowadays, many CAD/CAM software packages provide active programming functions. These softwares generally prompt related issues of process planning in the programming interface, for example, tool selection, machining path planning, cutting amount setting, etc., and the programmer only needs to set The relevant parameters can be actively generated and transferred to the CNC machine tool for processing. Therefore, the tool selection and cutting amount determination in CNC machining are completed under the state of human-computer interaction, which constitutes a clear contrast with general machine tool processing. At the same time, it also requires programmers to master the basic criteria for tool selection and cutting amount determination. Fully consider the characteristics of CNC machining when programming. This article discusses the issues of tool selection and cutting parameters that must be faced in NC programming, gives some guidelines and propositions, and discusses the issues that should be paid attention to.
1. Types and characteristics of commonly used tools for CNC machining
CNC machining tools have the characteristics of being necessary to adapt to the high speed, high efficiency and high degree of automation of CNC machine tools. Generally, they should include general-purpose tools, general-purpose connection tool holders and a small number of special tool holders. The tool holder must be connected to the tool and mounted on the power head of the machine tool, so it has been gradually standardized and serialized. There are many ways to classify CNC tools.
According to the tool structure, it can be divided into: ①Integral type; ②Inlaid type, welding or machine clamp type connection, machine clamp type can be divided into two types: non-indexable and indexable; ③Special types, such as composite tools, reduce Vibrating knives, etc. According to the materials used to make the tools, it can be divided into: ① high-speed steel tools; ② cemented carbide tools; ③ diamond tools; ④ other materials tools, such as cubic boron nitride tools, ceramic tools, etc. From the cutting process, it can be divided into: ① turning tools, including external circle, inner hole, thread, cutting tools, etc.; ② drilling tools, including drills, reamers, taps, etc.; ③ boring tools; ④ milling tools Wait. In order to meet the requirements of CNC machine tools for tool durability, stability, easy adjustment, and exchangeability, in recent years, machine-clamped indexable tools have been widely used, reaching 30% to 40% of the entire CNC tools. Metal removal The amount accounts for 80% to 90% of the total.
Compared with the tools used on general machine tools, CNC tools have many different requirements, mainly with the following characteristics:
⑴ Good rigidity (especially for rough machining tools), high precision, low vibration resistance and thermal deformation;
⑵Good interchangeability, convenient for quick tool change;
⑶ High lifespan, stable and reliable cutting performance;
⑷The scale of the tool is easy to adjust to reduce the time of tool change adjustment;
⑸ The tool should be able to reliably break or roll chips to facilitate the removal of chips;
⑹Serialization and standardization to facilitate programming and tool management.
Second, the selection of cnc vertical machining center tools
The selection of the tool is carried out under the human-computer interaction state of CNC programming. The correct selection of tools and tool holders should be based on the processing capabilities of the machine tool, the performance of the workpiece data, the processing procedures, the cutting amount and other related factors. The general criteria for tool selection are: easy installation and adjustment, good rigidity, high durability and precision. On the premise of satisfying the processing requirements, try to choose a shorter tool holder to improve the rigidity of tool processing.
When selecting a tool, the dimensions of the tool must be adapted to the external dimensions of the workpiece to be processed. In production, end mills are often used for general processing of flat parts; when milling planes, carbide blade milling cutters should be selected; when processing bosses and grooves, high-speed steel end mills should be selected; the rough surface or rough machining When drilling holes, you can choose corn milling cutters with carbide inserts; for the processing of some three-dimensional profiles and general outlines with variable bevel angles, ball-end milling cutters, ring milling cutters, tapered milling cutters and disc milling cutters are often used.
When performing free-form surface processing, because the end cutting speed of the ball-end tool is zero, in order to ensure the machining accuracy, the cutting line spacing is generally very dense, so the ball-end is often used for surface finishing. The flat-end tool is superior to the ball-end tool in terms of surface processing quality and cutting power. Therefore, as long as it is guaranteed that it does not cut, whether it is roughing or finishing of curved surfaces, flat-end tools should be selected first. In addition, the durability and accuracy of the tool are greatly related to the price of the tool. It is necessary to pay attention to that, in most cases, the selected tool increases the tool cost, but the resulting processing quality and processing power The progress of this can greatly reduce the overall processing cost.
On the machining center, various tools are installed in the tool magazine, and the tool selection and tool change actions are carried out at any time according to the program regulations. Therefore, it is necessary to select standard tool holders so that the standard tools used in drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the spindle or tool magazine of the machine tool. The programmer should understand the structural dimensions, adjustment methods and adjustment scales of the tool holders used on the machine tool in order to determine the radial and axial dimensions of the tool during programming. At present, my country's machining center uses the TSG East-West system, and its tool holders have two types: straight shank (three standards) and tapered shank (four standards), including a total of 16 tool holders for different purposes.
In economical CNC machining, because the sharpening, measurement and replacement of tools are mostly performed manually, the auxiliary time is longer. Therefore, it is necessary to arrange the order of the tools reasonably. Generally, the following guidelines should be followed: ①Minimize the number of tools; ②After a tool is clamped, all processing parts that can be completed should be completed; ③Rough and finishing tools should be used separately, even if they are tools of the same standard; ④ Milling first, then drilling; ⑤ Surface finishing first, then two-dimensional general finishing; ⑥ When possible, the active tool change function of CNC machine tools should be used as much as possible to improve production power.
Third, the determination of cutting parameters for CNC machining
The criterion for reasonable selection of cutting parameters is that during rough machining, the main purpose is to increase the yield, but the economy and processing costs should also be considered; semi-finishing and finishing should take into account the cutting power under the premise of ensuring the quality of the machining. , Economy and processing cost. The detailed value should be determined according to the machine tool manual, cutting parameter manual, and combined with experience.
⑴Cutting depth t. When the rigidity of the machine tool, workpiece, and tool agrees, t is equal to the machining allowance, which is a useful way to improve the production rate. In order to ensure the machining accuracy and surface roughness of the parts, a certain margin should generally be left for finishing. The finishing allowance of CNC machine tools can be slightly smaller than that of general machine tools.
⑵Cutting width L. Generally, L is proportional to the tool diameter d and inversely proportional to the depth of cut. In economical CNC machining, the general value scale of L is: L=(0.6～0.9)d.
⑶Cutting speed v. Improving v is also a way to increase productivity, but v is more closely related to tool durability. With the increase of v, the tool durability declines sharply, so the choice of v mainly depends on the tool durability. In addition, the cutting speed is also closely related to the processing data. For example, when milling alloy steel 30CrNi2MoVA with an end mill, v can be about 8m/min; when milling aluminum alloy with the same end mill, v can be 200m/min. the above.
⑷ Spindle speed n (r/min). The spindle speed is generally selected according to the cutting speed v. The accounting formula is:
In the formula, d is the diameter of the tool or workpiece (mm).
The control panel of the cnc vertical machining center is generally equipped with a spindle speed adjustment (magnification) switch, which can adjust the spindle speed in integer multiples during the machining process.
⑸ Feed speed vF
vF should be selected based on the machining accuracy and surface roughness requirements of the parts, as well as the tool and workpiece information. The increase in vF can also increase the production power. When the roughness of the processing surface is low, the vF can be selected larger. During the machining process, vF can also be adjusted manually through the adjustment switch on the machine control panel, but the maximum feed rate is subject to the constraints of the rigidity of the equipment and the performance of the feed system.
