Yearly Archive October 9, 2020

The influence of vibration of cutter teeth on the surface roughness and tool life. In order to prevent the decline of machining accuracy and tool life, the selected machining center must be equipped with a spindle with excellent dynamic balance performance, and the selected boring tool must also have high dynamic balance characteristics. Especially for the cutter tooth part of boring tools, the geometry, tool material and clamping mode suitable for high-speed cutting should be selected. In order to improve the machining efficiency, the feed rate should be increased on the premise of obtaining the same surface roughness. However, the increase of feed rate should be enough, otherwise it will increase the cutting resistance, which is not conducive to improving the processing efficiency. The cutting edge should be set with negative chamfering less than 0.1 mm, which can effectively maintain the stability of tool life.

In addition to CNC cutting method for precision machining of holes, boring and reaming can also be used for high-precision machining of holes. With the high-speed spindle of machining center, boring tools can be used for high-speed and precision machining of holes. It is reported that the cutting speed can be increased to more than 1500m / min when the diameter of boring is about 40mm on aluminum alloy. This cutting speed can also be used when using CBN sintered body as cutting edge to process steel, cast iron and high hardness steel. It is predicted that the high speed of boring will be popularized rapidly in the future.

As for the tool material, it depends on the nature of the material being processed. For example, when machining steel below 40HRC, cermet tool can be selected. Under the high-speed cutting condition of V = 300m / min, good surface roughness and long tool life can be obtained. Coated cemented carbide tools are suitable for high-speed cutting of steels below 60HRC. The tool life is very stable, but the cutting speed is slightly lower than cermet tools.

Sintered tool is suitable for machining high hardness steel, cast iron and other materials. The cutting speed can reach more than 1000m / min, and the tool life is very stable. Proper chamfering should be carried out on the cutting edge of CBN cutter teeth in CNC machining, which is very beneficial to stable high-speed cutting and prolonging tool life. In ultra-high speed cutting of nonferrous metals and non-metallic materials such as aluminum alloy, diamond sintered body tool can be selected. This kind of cutting tool is stable and has a long service life. It should be noted that when using diamond tools, the blade belt must be chamfered, which is an important condition to ensure the stability of cutting.

The power control equipment of AC motor is controlled by the frequency mode of motor power supply. In CNC machine tools, the frequency converter is mainly used to control the action of the spindle.

The blade of end mounted structure milling cutter (as shown in FIG. 10) is fixed on the cutter groove with only one screw, which is simple in structure and convenient in rotation. Although there are few tool parts in mechanical parts processing plant, it is difficult to process the cutter body. Generally, it needs five coordinate machining center for machining. Since the blade is clamped by cutting force, the clamping force increases with the increase of cutting force, so the clamping element can be omitted and the chip holding space is increased. Because the blade is installed in tangential direction, the section of cemented carbide in the direction of cutting force is large, so it can be used for large cutting depth and large cutting distance. This kind of milling cutter is suitable for heavy and medium milling.

The angle of milling cutter includes front angle, back angle, main deflection angle, auxiliary deflection angle, edge inclination angle, etc. In order to meet the different processing needs, there are a variety of angle combination types. Among the various angles, the main deflection angle and rake angle are the main ones (the main deflection angle and rake angle of the cutter are generally clearly stated in the product samples of the manufacturer).

Main deflection angle KR

The main deflection angle is the angle between the cutting edge and the cutting plane, as shown in FIG. 11. The main deflection angles of milling cutter are 90 °, 88 °, 75 °, 70 °, 60 ° and 45 ° etc.

Principal deflection angle

The main deflection angle has a great influence on the radial cutting force and cutting depth. The size of radial cutting force directly affects the cutting power and anti vibration performance of cutting tools. The smaller the main deflection angle of the milling cutter is, the smaller the radial cutting force is, the better the vibration resistance is, but the cutting depth is also reduced.

The main deflection angle of 90 ° is selected when milling the plane with shoulder, and CNC machining is not generally used for pure plane machining. This kind of tool has good versatility (not only can process step surface, but also can process plane), so it can be selected in single piece and small batch processing. Because the radial cutting force of this kind of cutting tool is equal to the cutting force, the feed resistance is large, and it is easy to vibrate. When machining the plane with shoulder, the milling cutter with 88 ° main deflection angle can also be selected. Compared with the milling cutter with 90 ° main deflection angle, its cutting performance is improved to some extent.

The main deflection angle of 60 ° to 75 ° is suitable for rough machining of plane milling. Due to the obvious reduction of radial cutting force (especially at 60 degrees), its vibration resistance is greatly improved, and the cutting is smooth and light. Therefore, it should be preferred in plane machining. 75 ° main deflection angle milling cutter is a general-purpose tool with wide application range; 60 ° main deflection angle milling cutter is mainly used for rough milling and semi finishing milling on boring and milling machines and machining centers.

The radial cutting force of this kind of milling cutter with 45 ° main deflection angle is greatly reduced, which is approximately equal to the axial cutting force. The cutting load is distributed on the longer cutting edge, and has good vibration resistance. It is suitable for the machining occasions with longer spindle overhang of boring and milling machine. When machining plane with this kind of cutting tool, the damage rate of blade is low and the durability is high; when machining iron castings, the edge of the workpiece is not easy to break.

Anterior angle γ

The rake angle of milling cutter can be divided into radial rake angle γ F and axial rake angle γ P. the radial rake angle γ f mainly affects the cutting power; the axial rake angle γ P affects the chip formation and the direction of axial force. When γ P is positive, the chip will fly away from the machining surface.

Anterior horn

The common combination forms of front angle are as follows:

The milling cutter with double negative rake angle and double negative rake angle usually adopts square (or rectangular) blade without back angle. The cutter has many cutting edges (generally 8), high strength and good impact resistance, which is suitable for rough machining of cast steel and cast iron. Because the chip shrinkage ratio is large, it needs a large cutting force, so the machine tool has higher power and higher rigidity. Because the forward angle of the shaft is negative, the chip can not flow out automatically. When cutting ductile materials, chip accretion and tool vibration are easy to occur.

It is suggested to use double cutting tool with negative rake angle when machining. When double positive rake angle milling cutter is used to produce chipping edge (i.e. large impact load), the double negative rake angle milling cutter should also be preferred when the machine tool allows.

The milling cutter with double positive rake angle and double positive rake angle adopts the blade with back angle, which has small wedge angle and sharp cutting edge. Because of the small shrinkage ratio of the chip, the cutting power consumed is small, and the chip is discharged in a spiral shape, which is not easy to form a chip accumulation lump. This kind of milling cutter is most suitable for cutting soft materials, stainless steel, heat-resistant steel and other materials. For the machine tools with poor rigidity (such as boring and milling machines with long spindle overhanging), low power and machining welded structural parts, double positive rake angle milling cutters should also be preferred.

Positive and negative rake angle (axial positive rake angle, radial negative rake angle) this kind of milling cutter combines the advantages of double positive rake angle and double negative rake angle milling cutter. The axial positive rake angle is conducive to the formation and discharge of chips; the radial negative rake angle can improve the edge strength and impact resistance. This kind of milling cutter has the advantages of smooth cutting, smooth chip removal and high metal removal rate, which is suitable for large allowance milling. Computer gong processing, Dongguan computer gong processing, high speed computer gong processing, mechanical parts processing, computer gong processing, Walter’s tangential tooth distribution heavy cutting milling cutter f2265 is a milling cutter with axial positive rake angle and radial negative rake angle structure.

The number of milling cutter teeth is large, which can improve the production efficiency. However, due to the limitation of chip holding space, cutter tooth strength, machine tool power and rigidity, the number of teeth of milling cutter with different diameters has corresponding regulations. In order to meet the needs of different users, there are three types of milling cutters with the same diameter: coarse teeth, medium teeth and dense teeth.

Rough tooth milling cutter is suitable for large allowance rough machining of ordinary machine tools and milling of soft materials or large cutting width; when the power of machine tool is small, in order to make cutting stable, coarse tooth milling cutter is often used.

The middle gear milling cutter is a universal series, which makes

Input shaft: forging and blank making → normalizing → finishing turning → gear rubbing → drilling → gear shaping → undercut angle → hobbing → shaving → heat treatment → grinding → meshing and finishing.

Output shaft: forging billet → normalizing → finishing turning → rolling gear → shaving → heat treatment → grinding → meshing and finishing.

1. Specific process flow

(1) Forging billet

Hot die forging is a widely used blank forging process for automobile gear parts. Computer gong processing, Dongguan computer gong processing, high speed computer gong processing, mechanical parts processing, computer gong processing

Hot forging and cold extrusion are widely used in the past. In recent years, cross wedge rolling technology has been widely used in shaft machining. This technology is especially suitable for the production of complex stepped shafts. It not only has high precision, small machining allowance, but also has high production efficiency.

(2) Normalizing

The purpose of this process is to obtain the hardness suitable for subsequent gear cutting and to prepare the microstructure for the final heat treatment, so as to effectively reduce the heat treatment deformation. Due to the influence of personnel, equipment and environment, the normal normalizing can not control the cooling rate and cooling uniformity of workpiece, resulting in large hardness dispersion and uneven microstructure, which directly affects machining and final heat treatment.

(3) Finish turning

In order to meet the positioning requirements of high-precision gear machining, CNC lathe is used for the finish turning of gear blank. First, the inner hole and locating end face of the gear are machined, and then the machining of the other end face and outer diameter is completed synchronously. It not only ensures the perpendicularity requirement between the inner hole and the locating end face, but also ensures the small size dispersion in the mass production of gear blanks. Thus, the accuracy of gear blank is improved and the machining quality of subsequent gear is ensured.

There are mainly three ways of positioning datum and clamping for shaft parts processing

Positioning by the center hole of the workpiece: in the machining of the shaft, the coaxiality of the outer circle surface and end face of the part, and the perpendicularity of the end face to the rotating axis are the main items of their mutual position accuracy. The design basis of these surfaces is generally the center line of the shaft. If two center holes are used for positioning, it is in line with the principle of datum coincidence.

1. Outer circle and central hole as the positioning reference (one clip and one top): Although the centering accuracy is high, the rigidity is poor, especially when machining heavy workpieces, and the cutting parameters cannot be too large.

In rough machining, in order to improve the rigidity of the parts, the cylindrical surface of the shaft and a central hole can be used as the positioning reference. This positioning method can bear large cutting torque and is the most common positioning method for shaft parts.

1. Take two cylindrical surfaces as positioning datum: when machining the inner hole of hollow shaft (for example: machining the inner hole of Morse taper on the machine tool), the central hole cannot be used as the positioning reference, and the two outer cylindrical surfaces of the shaft can be used as the positioning reference. When the workpiece is the spindle of machine tool, the two supporting Journal (assembly reference) is often used as the positioning reference, which can ensure the coaxiality requirement of the taper hole relative to the supporting journal, and eliminate the error caused by the non coincidence of the reference.

CNC machining refers to the processing with CNC machining tools. CNC index controlled machine tool is programmed by NC machining language, usually G code. NC machining G code language tells CNC machine tool which Cartesian position coordinates, and controls the tool feed speed and spindle speed, as well as tool converter, coolant and other functions. Compared with manual machining, CNC machining has great advantages, such as the parts produced by CNC machining are very accurate and repeatable; CNC machining can produce parts with complex shapes that cannot be completed by manual processing. CNC machining technology has been widely promoted, most of the machining workshops have the ability of NC machining. The most common NC machining methods in typical machining workshops are CNC milling, CNC lathe and CNC EDM wire cutting (WEDM). The tool for NC milling is called CNC milling machine or CNC machining center. CNC turning lathe is called CNC lathe center. NC machining G code can be manually programmed, but usually the machining workshop uses CAM software to automatically read CAD file and generate G code program to control NC machine tool. Leading CNC machine tool brands include Hass, DMG (Deckel MAHO GILDEMEISTER), Mazak, Mori Seiki, fadal and wasino.

Several groups of common instructions for CNC machining

1、 Pause instruction g04x (U)_ /P_ It refers to the tool pause time (feed stop, spindle does not stop), the value after address P or X is the pause time. The value after X should have a decimal point. Otherwise, it should be calculated by one thousandth of this value in seconds (s). The value after P cannot have a decimal point (i.e., an integer), and the unit is Ms. For example, G04 x2.0; or G04 X2000; pause for 2 seconds G04 P2000; but in some hole series processing instructions (such as g82, G88 and g89), in order to ensure the roughness of the hole bottom, when the tool is machined to the hole bottom, there must be a pause time, which can only be expressed by the address P. if the address x is used, the control system thinks that x is the x-axis coordinate value for execution. For example, g82x100.0y100.0z-20.0r5.0f200p2000; drilling (100.0100.0) to the bottom of the hole for 2 seconds; g82x100.0y100.0z-20.0r5.0f200x2.0; drilling (2.0100.0) to the bottom of the hole will not be suspended.

2、 The difference and connection of M00, M01, M02 and M30 are program unconditional pause instructions. When the program is executed, the feed stops and the spindle stops. To restart the program, you must first return to the og state, press CW (spindle forward rotation) to start the spindle, and then return to the auto state and press the start key to start the program. M01 is the program selective pause instruction. The op stop key on the control panel must be opened before the program is executed. The effect after execution is the same as that of M00. Restart the program as above. M00 and M01 are often used in the inspection or chip removal of workpiece size in the process of machining. M02 is the end instruction of main program. Execute this command, feed stop, spindle stop, coolant off. But the program cursor stops at the end of the program. M30 is the main program end instruction. The function is the same as that of M02, except that the cursor returns to the program head position, regardless of whether there are other program segments after M30.

3、 The meaning of the address D and H is the same. The tool compensation parameters D and H have the same function and can be interchanged arbitrarily. They all represent the address name of the compensation register in the CNC system, but the specific compensation value is determined by the compensation number address behind them. However, in order to prevent errors in the machining center, generally, h is the tool length compensation address, the compensation number is from No. 1 to No. 20, D is the tool radius compensation address, and the compensation number starts from No. 21 (tool magazine of 20 tools). For example, g00g43h1z100.0; g01g41d21x20.0y35.0f200;

4、 Mirror instruction mirror processing instruction M21, M22, M23. When only the x-axis or y-axis is mirrored, the cutting sequence (forward milling and reverse milling), cutter compensation direction and circular interpolation steering will be opposite to the actual program. When the x-axis and y-axis are mirrored at the same time, the tool feed sequence, tool compensation direction and circular interpolation direction remain unchanged. Note: after using the mirror instruction, M23 must be used to cancel, so as not to affect the following program. In G90 mode, if mirror or cancel command is used, it can be used only after returning to the origin of workpiece coordinate system. Otherwise, the NC system can not calculate the motion track behind, which will lead to the phenomenon of random tool walking. At this time, it is necessary to implement manual origin reset operation to solve the problem. The spindle rotation does not change with the mirror command.

5、 Circular interpolation instruction G02 is clockwise interpolation, G03 is counter clockwise interpolation, in XY plane, the format is as follows: G02 / g03x_ Y_ I_ K_ F_ Or G02 / g 03 x_ Y_ R_ F_ Where x and y are the coordinates of the end point of the arc, I and j are the increment values from the starting point of the arc to the center of the circle on the X and Y axes, R is the radius of the arc, and F is the feed rate. In circular arc cutting, it should be noted that Q ≤ 180 ° R is positive; Q > 180 ° R is negative; I and K can also be specified by R. when both are specified, R command has priority, I and K are invalid; R cannot be used for whole circle cutting, and can only be programmed with I, J and K, because there are countless circles with the same radius passing through the same point. When I and K are zero, they can be omitted; regardless of G90 or G91 mode, I, J, K are programmed according to relative coordinates; when circular interpolation, tool compensation instruction G41 / G42 cannot be used.

6、 The advantages and disadvantages between G92 and G54-G59 are the coordinate system set before machining, while G92 is the coordinate system set in the program. If G54-G59 is used, there is no need to use G92, otherwise G54-G59 will be damaged

In the process of NC machining, it is easy to appear over cutting, undercutting and other phenomena. At the same time, the damage of cutting tools, or the processing of waste products, the interference and collision between parts and tools, tools and fixtures, tools and worktables, will cause a certain cost waste. The NC machining simulation system can simulate the related program well and reduce the loss to the greatest extent.

Before the actual machining, the programmer can confirm whether the finished cutting product is consistent with the original design drawing through the simulation system, and can make better detailed analysis.

collision detection

In the actual processing, five axis CNC machining has more advantages than three axis CNC machining, can achieve a wider range, has faster material and efficiency, can reduce processing time and improve surface accuracy. However, due to the high degree of freedom of the two additional rotating axes, it is easy to cause collision and damage to the machine tool.

Therefore, before the actual processing, it is necessary to detect the collision with the help of NC machining simulation system. The main detection algorithms are as follows:

The machine tool includes many parts, but the collision detection algorithm between different parts is similar. The computer gong machining simulation system takes the cutter, workpiece and fixture as the main research object to detect the possible global collision.

1. According to the tool location, the grid voxel model of the tool head scanning body and the grid voxel model of the tool holder scanning body are solved respectively. The tool holder scanning body is used to detect whether there is a global collision between the tool, workpiece and fixture, and the tool head scanning body is used to calculate the intersection between the tool and the workpiece.
2. A feature attribute of voxel model is cuboid envelope box. The envelope box is used to make rough judgment. If the envelope box does not intersect, the objects enclosed in the envelope box will not intersect. At this time, the position data of the object envelope box will be updated, and other information will not change. It will prepare for the generation of the next tool point scanner and collision detection. Rough judgment can speed up the detection Speed.
3. Once a collision occurs, the system stops reading data and reports the error message and the precise location of the collision. If there is no collision, the system continues to carry out the teaching operation between the tool head and the workpiece.

A circle passing through the same point

When I and K are zero, they can be omitted; regardless of G90 or G91 mode, I, J, K are programmed according to relative coordinates; when circular interpolation, tool compensation instruction G41 / G42 cannot be used.

1. Advantages and disadvantages between G92 and G54-G59

G54-G59 is the coordinate system set before machining, while G92 is the coordinate system set in the program. If G54-G59 is used, there is no need to use G92 again, otherwise G54-G59 will be replaced and should be avoided, as shown in Table 1.

Table 1 difference between G92 and working coordinate system`

Note: (1) once G92 is used to set the coordinate system, G54-G59 will not work unless the system is restarted after power failure or G92 is used to set the required new workpiece coordinate system. (2) After using G92 program, if the machine tool does not return to the original point set by G92, the program will be started again, and the current position of the machine tool will become the new workpiece coordinate origin, which is prone to accidents. Therefore, we hope that readers will use it carefully.

1. Compile tool changing program.

In the machining center, tool change is inevitable. However, there is a fixed tool change point when the machine tool leaves the factory. If it is not in the position of tool change, it is impossible to change the tool. Moreover, before changing the tool, the tool compensation and circulation must be cancelled, the spindle is stopped and the coolant is turned off. There are many conditions. It is not only easy to make mistakes but also inefficient to ensure these conditions before each manual tool change. Therefore, we can compile a tool change program to save the memory and MDI status

Under the call of M98, the tool change action can be completed at one time.

Taking pmc-10v20 machining center as an example, the program is as follows:

O2002; (program name)

G80g40g49; (cancel fixed cycle, cutter compensation)

M05; (spindle stop)

M09; (coolant off)

G91g30z0; (Z axis returns to the second origin, i.e. tool change point)

M06; (tool change)

M99; (end of subroutine)

When the tool needs to be changed, just type “t5m98p2002” in MDI state to replace the required tool T5, thus avoiding many unnecessary mistakes. According to the characteristics of their own machine tools, the majority of readers can work out the corresponding tool change program.

1. Others

The sequence number of the program segment is represented by address n. Generally, the memory space of NC device itself is limited (64K). In order to save storage space, the sequence number of program segment is omitted. N only means the segment label, which is convenient to find and edit the program, and has no effect on the processing process. The sequence number can be increased or decreased, and the continuity of the value is not required. However, some loop instructions, jump instructions, calling subroutines and mirror instructions can not be omitted.

1. In the same program segment, the later occurrence of the same instruction (the same address symbol) or the same set of instructions takes effect.

For example, tool change procedure, t2m06t3; instead of T2, T3 is replaced;