How to calculate the cutting speed and feed speed of CNC machining center?

Cutting speed and feed speed of CNC machining center:

 

1: spindle speed = 1000vc / π D

 

2. Maximum cutting speed of general tools (VC): high speed steel 50 m / min; super hard tool 150 m / min; coated tool 250 m / min; ceramic diamond tool 1000 m / min 3 processing alloy steel Brinell hardness = 275-325 high speed steel tool vc = 18m / min; cemented carbide tool vc = 70m / min (draft = 3mm; feed rate f = 0.3mm / R) cnc turning part

  

There are two calculation methods for spindle speed, as shown in the following example:

 

① spindle speed: one is g97 S1000, which means that the spindle rotates 1000 revolutions per minute, that is, the constant speed. cnc machining part

 

The other is that G96 S80 is a constant linear speed, which is the spindle speed determined by the workpiece surface. machined part

 

There are also two kinds of feed speeds, G94 F100, indicating that the one minute cutting distance is 100 mm. The other is g95 F0.1, which means that the tool feed size is 0.1mm per revolution of the spindle. The choice of cutting tool and the determination of cutting amount in NC machining is an important part of NC machining technology. It not only affects the machining efficiency of NC machine tools, but also directly affects the machining quality.

 

With the development of CAD / CAM technology, it is possible to directly use the design data of CAD in NC machining, especially the connection of microcomputer and NC machine tool, which makes the whole process of design, process planning and programming complete on the computer, and generally does not need to output special process documents.

 

At present, many CAD / CAM software packages provide automatic programming functions. These software generally prompts the relevant problems of process planning in the programming interface, such as tool selection, machining path planning, cutting parameter setting, etc. the programmer can automatically generate NC programs and transmit them to the NC machine tool for processing as long as he sets the relevant parameters.

 

Therefore, the selection of cutting tools and the determination of cutting parameters in NC machining are completed under the condition of human-computer interaction, which is in sharp contrast with the ordinary machine tool machining. At the same time, it also requires the programmers to master the basic principles of tool selection and the determination of cutting parameters, and fully consider the characteristics of NC machining when programming.

 

I. types and characteristics of common cutting tools for CNC machining

 

NC machining tools must adapt to the characteristics of high speed, high efficiency and high degree of automation of CNC machine tools, generally including universal tools, universal connecting tool handles and a small number of special tool handles. The tool handle should be connected to the tool and installed on the power head of the machine tool, so it has been gradually standardized and serialized. There are many ways to classify NC tools.

 

According to the tool structure, it can be divided into:

 

① integral type;

 

(2) inlaid type, which is connected by welding or machine clamp type. Machine clamp type can be divided into two types: non transposable type and transposable type;

 

③ special types, such as composite cutting tools, shock absorption cutting tools, etc.

 

According to the materials used to manufacture the tool, it can be divided into:

 

① high speed steel cutter;

 

② carbide tool;

 

③ diamond cutter;

 

④ cutting tools of other materials, such as cubic boron nitride cutting tools, ceramic cutting tools, etc.

 

The cutting technology can be divided into:

 

① turning tools, including outer circle, inner hole, thread, cutting tools, etc;

 

② drilling tools, including drill, reamer, tap, etc;

 

③ boring tool;

 

④ milling tools, etc.

 

In order to adapt to the requirements of CNC machine tools for tool durability, stability, easy adjustment and interchangeability, in recent years, the machine clamped indexable tool has been widely used, reaching 30% - 40% of the total number of CNC tools, and the amount of metal removal accounts for 80% - 90% of the total.

 

Compared with the cutters used in general machine tools, CNC cutters have many different requirements, mainly with the following characteristics:

 

(1) good rigidity (especially rough cutting tools), high precision, small vibration resistance and thermal deformation;

 

(2) good interchangeability, convenient for quick tool change;

 

(3) high service life, stable and reliable cutting performance;

 

(4) the size of the tool is easy to adjust, so as to reduce the adjustment time of tool change;

 

(5) the cutter shall be able to break or roll chips reliably to facilitate chip removal;

 

(6) serialization and standardization to facilitate programming and tool management.

 

II. Selection of NC machining tools

 

The selection of cutting tools is carried out in the human-computer interaction state of NC programming. The tool and handle shall be selected correctly according to the machining capacity of the machine tool, the performance of the workpiece material, the processing procedure, the cutting amount and other relevant factors. The general principle of tool selection is: convenient installation and adjustment, good rigidity, high durability and precision. On the premise of meeting the machining requirements, try to choose a shorter tool handle to improve the rigidity of the tool machining. When selecting a tool, the size of the tool should be suitable for the surface size of the workpiece to be processed.

 

In production, the end milling cutter is often used to process the peripheral contour of plane parts; when milling plane parts, the carbide blade milling cutter should be selected; when machining boss and groove, the high-speed steel end milling cutter should be selected; when machining blank surface or rough machining hole, the corn milling cutter with carbide blade can be selected; for the processing of some three-dimensional profile and contour with variable bevel angle, the ball head milling cutter and ring milling are often used Cutter, taper cutter and disc cutter. In the process of free-form surface machining, because the end cutting speed of the ball head cutter is zero, in order to ensure the machining accuracy, the cutting line spacing is generally very dense, so the ball head is often used for surface finishing. The flat head cutter is superior to the ball head cutter in surface machining quality and cutting efficiency. Therefore, the flat head cutter should be selected preferentially as long as the rough machining or finish machining of the curved surface is guaranteed.

 

In addition, the durability and accuracy of cutting tools have a great relationship with the price of cutting tools. It must be noted that, in most cases, the selection of a good cutting tool increases the cost of cutting tools, but the resulting improvement of processing quality and efficiency can greatly reduce the whole processing cost.

 

In the machining center, all kinds of tools are installed on the tool magazine, and they can select and change tools at any time according to the program. Therefore, the standard tool handle must be used so that the standard tools for drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the spindle or magazine of the machine tool. The programmer shall know the structural dimension, adjustment method and adjustment range of the tool handle used on the machine tool, so as to determine the radial and axial dimensions of the tool when programming. At present, TSG tool system is used in machining centers in China. There are two kinds of tool shanks: straight shanks (three specifications) and taper shanks (four specifications), including 16 kinds of tool shanks for different purposes. In the economical NC machining, because the grinding, measuring and replacing of cutting tools are mostly done manually, which takes up a long time, so it is necessary to arrange the order of cutting tools reasonably.

 

Generally, the following principles shall be followed:

 

① minimize the number of tools;

 

② after a tool is clamped, all machining parts that it can carry out shall be completed;

 

③ the tools for rough and finish machining shall be used separately, even those with the same size and specification;

 

④ milling before drilling;

 

⑤ finish the surface first, then finish the two-dimensional contour;

 

⑥ if possible, the automatic tool change function of CNC machine tools should be used to improve production efficiency.

 

III. determination of cutting parameters for CNC machining

 

The principle of reasonable selection of cutting parameters is that in rough machining, productivity is generally improved, but economy and machining cost should also be considered; in semi-fine machining and finishing, cutting efficiency, economy and machining cost should be considered on the premise of ensuring machining quality. The specific value shall be determined according to the machine tool manual, cutting parameters manual and experience.

 

(1) cutting depth t. When the rigidity of machine tool, workpiece and tool is allowed, t is equal to machining allowance, which is an effective measure to improve productivity. In order to ensure the machining accuracy and surface roughness of parts, a certain margin should be reserved for finishing. The finishing allowance of CNC machine tools can be slightly less than that of ordinary machine tools.

 

(2) cutting width L. Generally, l is directly proportional to the tool diameter D and inversely proportional to the cutting depth. In economic NC machining, the value range of L is generally L = (0.6-0.9) d.

 

(3) cutting speed v. Increasing V is also a measure to improve productivity, but V is closely related to tool durability. With the increase of V, the tool durability decreases sharply, so the choice of V mainly depends on the tool durability. In addition, the cutting speed also has a great relationship with the processing materials. For example, when milling 30crni2mova with end milling cutter, V can be about 8m / min; when milling aluminum alloy with the same end milling cutter, V can be more than 200m / min.

 

(4) spindle speed n (R / min). The spindle speed is generally selected according to the cutting speed v. The calculation formula is: where D is the diameter of tool or workpiece (mm). Generally, the control panel of CNC machine tools is equipped with the spindle speed adjustment (multiple) switch, which can adjust the spindle speed in the process of machining.

 

(5) the feed speed vfvfvf shall be selected according to the requirements of the machining accuracy and surface roughness of the parts as well as the materials of the tools and workpieces. The increase of VF can also improve the production efficiency. When the surface roughness is low, VF can be selected larger. In the process of machining, VF can also be adjusted manually through the adjustment switch on the control panel of the machine tool, but the maximum feed speed is limited by the rigidity of the equipment and the performance of the feed system.