Now, many CAD/CAM software packages provide automatic programming functions. These software generally prompt the related issues of process planning in the programming interface, such as tool selection, machining path planning, cutting amount setting, etc. The programmer only needs to set the relevant issues. parameters, the NC program can be automatically generated and transmitted to the CNC machine tool to complete the processing. Therefore, the tool selection and cutting amount determination in CNC machining are completed in the state of human-computer interaction, which is in sharp contrast with ordinary machine tool processing. At the same time, programmers must master the basic principles of tool selection and cutting amount determination. Fully consider the characteristics of CNC machining when programming. In this paper, the problems of tool selection and cutting amount determination that must be faced in NC programming are discussed, some principles and suggestions are given, and the problems that should be paid attention to are discussed.
Types and characteristics of commonly used tools for CNC machining
CNC machining tools must adapt to the high speed, high efficiency and high degree of automation of CNC machine tools, and generally include general 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 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
②Inlaid type, using welding or machine clamp connection, machine clamp type can be divided into two types: non-indexable and indexable
③ Special types, such as compound tools, shock-absorbing tools, etc.
According to the material used to manufacture the tool, it can be divided into:
①High-speed steel tool
②Carbide tool
③Diamond tool
④ Other material tools, such as cubic boron nitride tools, ceramic tools, etc.
From the cutting process, it can be divided into:
① Turning tools, including external circles, internal holes, threads, cutting tools, etc.
②Drilling tools, including drills, reamers, taps, etc.
③ Boring tool
④ Milling tools, etc.
In order to meet the requirements of CNC machine tools for tools that are durable, stable, easy to adjust, and replaceable, machine-clamped indexable tools have been widely used in recent years, reaching 30% to 40% of the entire CNC tools. The amount accounts for 80% to 90% of the total.
Compared with the tools used on ordinary machine tools, CNC tools have many different requirements, mainly including the following characteristics:
(1) Good rigidity (especially roughing tools), high precision, small vibration resistance and thermal deformation
⑵ good interchangeability, easy to change the tool quickly
(3) Long life, stable and reliable cutting performance
⑷The size of the tool is easy to adjust to reduce the tool change adjustment time
⑸The tool should be able to break or roll chips reliably to facilitate the removal of chips
⑹Serialization and standardization to facilitate programming and tool management.
Selection of CNC machining tools
The tool selection is carried out under the man-machine interaction state of NC programming. The tool and tool holder should be correctly selected according to the processing capacity of the machine tool, the performance of the workpiece material, the processing procedure, the cutting amount and other related factors.
The general principle of tool selection is: easy installation and adjustment, good rigidity, high durability and precision. On the premise of meeting the processing requirements, try to choose a shorter tool holder to improve the rigidity of tool processing.
When selecting a tool, the size of the tool should be adapted to the surface size of the workpiece to be machined.
In production, end mills are often used to process the peripheral contours of flat parts. When milling the plane, the carbide insert milling cutter should be selected. When machining bosses and grooves, choose high-speed steel end mills. When machining the surface of the blank or roughing the hole, you can choose a corn milling cutter with a carbide insert. For the processing of some three-dimensional profiles and variable bevel profiles, ball-end milling cutters, annular milling cutters, conical milling cutters and disc milling cutters are often used.
When machining free-form surfaces, since the cutting speed of the end of the ball nose tool is zero, in order to ensure the machining accuracy, the cutting line spacing is generally very dense, so the ball nose is often used for the finishing of the surface. The flat-end tool is superior to the ball-end tool in terms of surface processing quality and cutting efficiency. Therefore, as long as the premise of ensuring that it does not cut, whether it is roughing or finishing of the curved surface, the flat-end tool should be preferred. In addition, the durability and accuracy of the tool have a great relationship with the price of the tool. It must be noted that in most cases, although the choice of a good tool increases the tool cost, the resulting improvement in processing quality and processing efficiency , the entire processing cost can be greatly reduced.
On the machining center, various tools are installed on the tool magazine, and the tool selection and tool change can be performed at any time according to the program regulations. Therefore, a standard tool holder must be used, so that the standard tools for drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine. Programmers should understand the structure size, adjustment method and adjustment range of the tool holder used on the machine tool, so as to determine the radial and axial dimensions of the tool during programming.
In economical CNC machining, since the sharpening, measurement and replacement of tools are mostly performed manually, it takes a long time to assist. Therefore, the arrangement order of the tools must be reasonably arranged. Generally, the following principles should be followed:
① Minimize the number of tools
② After a tool is clamped, it should complete all the processing parts it can perform
③ The rough and finishing tools should be used separately, even if the tools are of the same size
④ Milling first and then drilling
⑤ Perform surface finishing first, and then perform 2D contour finishing
⑥ When possible, the automatic tool change function of CNC machine tools should be used as much as possible to improve production efficiency.
In economical CNC machining, since the sharpening, measurement and replacement of tools are mostly performed manually, it takes a long time to assist. Therefore, the arrangement order of the tools must be reasonably arranged. Generally, the following principles should be followed:
① Minimize the number of tools
② After a tool is clamped, it should complete all the processing parts it can perform
③ The rough and finishing tools should be used separately, even if the tools are of the same size
④ Milling first and then drilling
⑤ Perform surface finishing first, and then perform 2D contour finishing
⑥ When possible, the automatic tool change function of CNC machine tools should be used as much as possible to improve production efficiency.
Determination of Cutting Quantity in CNC Machining
The principle of reasonable selection of cutting amount is that in rough machining, the main purpose is to improve productivity, but economy and processing cost should also be considered. Economics and processing costs. The specific value should be determined according to the machine tool manual, cutting quantity manual, and experience.
⑴ cutting depth t.
When the rigidity of the machine tool, workpiece and tool allows, t is equal to the machining allowance, which is an effective measure to improve productivity. 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 ordinary machine tools.
⑵ cutting 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 range of L is: L=(0.6～0.9)d.
⑶ cutting speed v.
Increasing v is also a measure to improve productivity, but v is closely related to tool durability. As v increases, the tool durability drops sharply, so the choice of v mainly depends on the tool durability. In addition, the cutting speed is also closely related to the material to be processed. For example, when milling the alloy 30CrNi2MoVA with an end mill, v can be about 8m/min. When milling aluminum alloy with the same end mill, v can be selected to be more than 200m/min.
(4) Feed speed vF
vF should be selected according to the machining accuracy and surface roughness requirements of the part, as well as the tool and workpiece material. An increase in vF can also improve production efficiency. When the surface roughness requirement is low, vF can be selected to be larger. During the machining 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 rigidity of the equipment and the performance of the feed system.