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Causes of machining center accuracy variation

1、 The machining accuracy of parts is poor

Generally, the feed dynamics between the shafts are not adjusted according to the error during installation and adjustment, or the transmission chain of each axis of the machine tool has changed (such as the change of lead screw clearance, pitch error, axial movement, etc.) due to wear and tear. It can be solved by readjusting and modifying the clearance compensation. When the dynamic tracking error is too large and an alarm is given, check whether the servo motor speed is too high; whether the position detection element is good; whether the position feedback cable connector is in good contact; whether the corresponding analog output latch and gain potentiometer are in good condition; whether the corresponding servo drive device is normal.

2、 When the machine tool is in motion, it causes poor machining accuracy

It may be that the acceleration and deceleration time is too short, so the speed change time can be appropriately prolonged; or the connection between the servo motor and the lead screw is loose or the rigidity is too poor, which can appropriately reduce the gain of the position ring.

3、 Roundness out of tolerance in two axes linkage

Axial deformation of circle

This deformation may be caused by mechanical adjustment. If the positioning accuracy of the shaft is not good or the gap compensation of the lead screw is improper, the roundness error will be produced when the image limit is exceeded.

Oblique ellipse error (ellipse in 45 degree direction)

At this time, the position deviation value of each axis should be checked first. If the deviation is too large, adjust the position loop gain to eliminate. Then check whether the interface board of rotary driver or inductosyn is adjusted properly, and then check whether the mechanical transmission pair clearance is too large and whether the gap compensation is appropriate.

Check the accuracy of vertical machining center

The biggest processing attribute of vertical machining center is that the consistency of batch workpiece size should be very high. We know that the traditional processing of workpiece from one process to another has to go through a clamping process. This process is difficult to achieve the complete consistency of the processing size of each part by human operation. Of course, this does not mean that the workpiece is out of tolerance, as long as the tolerance of the workpiece is within the tolerance All parts within the band are qualified parts, which means that some workpieces may be in large difference, some parts may be in medium difference, and some parts may be in small difference, so it is difficult to ensure the consistency of workpiece size.

Therefore, at present, the utilization rate of vertical machining center is the highest in small and medium-sized processing enterprises. Of course, the vertical machining center is also the most cost-effective machine tool in CNC machine tools. It has high processing efficiency and high processing precision. From these two aspects, it also solves the problem of maintaining the size consistency of workpieces. Of course, if the numerical control parameters are not set properly or the operation is improper, the vertical machining center will also appear the phenomenon of machining out of tolerance from time to time in the actual processing. This paper briefly introduces the causes and solutions of machining out of tolerance in vertical machining center

The out of tolerance of machining accuracy of vertical machining center mainly includes: high surface roughness of inner hole, out of roundness of reamed inner hole, out of tolerance of position precision of reamed hole, obvious edge of inner surface of hole, irregularity of center line of hole after reaming, increase and decrease of hole diameter.

1、 The position accuracy of reamed hole is out of tolerance

Causes: it may be caused by the wear of the guide sleeve of the vertical machining center, the distance between the bottom end of the guide sleeve and the workpiece, the short length of the guide sleeve, the poor accuracy and the looseness of the spindle bearing.

Solution: if the position accuracy of the hole is out of tolerance during reaming of the vertical machining center, the following maintenance can be carried out according to the above-mentioned problems. The guide sleeve shall be replaced regularly and the guide sleeve shall be lengthened to improve the matching accuracy of the clearance between the guide sleeve and the reamer, the machine tool shall be maintained in time, and the spindle bearing clearance shall be adjusted.

2、 The surface roughness of inner hole is high

Causes: the surface roughness of the inner hole machined by the vertical machining center is high, and there are many reasons, which may be caused by too high cutting speed, improper selection of cutting fluid, excessive reamer main deflection angle, too large reaming allowance, uneven or small reaming allowance, unsharp edge, too wide reaming edge, poor chip removal during reaming, excessive wear of reamer, and possibly selected milling Tool and part material mismatch and other reasons.

Solution: there are many reasons for the high machining roughness, so the solution is more complex. We can start from the following aspects: reduce the cutting speed, select the cutting fluid according to the processing materials, appropriately reduce the reaming allowance, improve the position accuracy and quality of the bottom hole before reaming, or increase the reaming allowance, increase the space of chip holding groove or use the reamer with edge inclination angle to make the chip removal smooth The reamer should be changed regularly and the cutting tool should be selected reasonably. For example, when machining steel parts, the suitable tool should be selected.

3、 Increase of machining aperture

Causes: the diameter of the reamer increases gradually when machining in the vertical machining center. The causes may be: the design value of the reamer’s outer diameter is too large or the reamer edge has burr, the cutting speed is too high, the feed rate is improper or the machining allowance is too large, the reamer’s main deflection angle is too large, the reamer is bent, the reamer edge adheres to the chip, the reamer edge swing is out of tolerance during grinding, the cutting fluid is not suitable, and It may be the problem of CNC machine itself. The spindle is bent or the spindle bearing is too loose or damaged.

Solution: according to the actual situation, the following countermeasures can be made: according to the specific situation, reduce the reamer outer diameter, reduce the cutting speed, adjust the feed rate or reduce the machining allowance, properly reduce the main deflection angle, straighten or scrap the bending reamer that can not be used, select the cutting fluid with good cold performance, adjust or replace the main shaft bearing, or replace the whole spindle.

4、 The reamed inner hole is not round

Causes: the machining of the inner hole is not round and the roundness is not enough. It is mainly due to the following factors: the reamer is too long, the rigidity is not enough, the vibration is generated during reaming, the reamer’s main deflection angle is too small, the reaming allowance is too small, there are gaps on the inner hole surface, cross holes, especially sand holes and air holes on the surface of the hole, as well as the looseness of the spindle bearing of the machine tool.

Solution: we can start from the following aspects: the installation of reamer should adopt rigid connection, select qualified reamer, control the hole position tolerance of pre-processing process, adopt unequal pitch reamer, select qualified blank, timely adjust the spindle clearance of machine tool, etc.

5、 After reaming, the center line of the hole is not straight

Causes: when the vertical machining center is machining holes, the center line is not straight. The possible reasons may be: the hole is inclined when drilling, especially when the diameter is small, Due to the poor rigidity of reamer, the original bending can not be corrected, the reamer’s main deflection angle is too large and the guide is poor, so that the reamer is easy to deviate from the direction in reaming, the inverted cone of the cutting part is too large, and the reamer is displaced intermittently at the gap in the middle of the hole.

Solution: increase reaming or boring process to correct the hole, reduce the main deflection angle, adjust the appropriate reamer, replace the reamer with guide part or lengthening cutting part, and pay attention to correct operation.

Down milling and up milling of CNC milling machine

1、 What is up milling and up milling of CNC milling machine

The rotation direction of the contact part between the milling cutter and the workpiece is the same as the feed direction of the workpiece, and vice versa. There is always a large or small gap between the nut and the lead screw of the milling machine. With the change of the horizontal milling component force, the table and the lead screw will move left and right. This periodic movement makes the table movement very unstable and easy to cause damage to the cutter teeth. However, the vertical milling component force of the forward milling presses the workpiece to the worktable, and the sliding and friction phenomenon between the cutter tooth and the machined surface is small, which reduces the friction The tool tooth wear, reducing work hardening and surface roughness are favorable. Therefore, when the clearance between the screw and nut of the worktable is adjusted to less than 0.03mm or when milling thin and long workpieces, it is better to use the down milling. In reverse milling, the workpiece is lifted up by the vertical force of milling, and the friction between the cutter tooth and the machined surface is increased. However, the horizontal force of milling is helpful for screw and nut to stick tightly, so that the table movement is more stable, and the cutter tooth wear caused by milling casting and forging is also small. Therefore, the general milling is usually up milling.

2、 Characteristics of down milling and up milling of CNC milling machine

Features of down milling:

In down milling, the cutting thickness of each tool changes gradually from small to large. When the cutter tooth is just in contact with the workpiece, the cutting thickness is zero. Only when the cutter tooth slides over a certain distance on the cutting surface left by the previous cutter tooth, and the cutting thickness reaches a certain value, the cutter tooth can really start cutting. Up milling makes the cutting thickness change from large to small, and the sliding distance of cutter teeth on the cutting surface is also very small. Moreover, the distance of the cutter teeth on the workpiece is shorter than that of the reverse milling. Therefore, under the same cutting conditions, the tool is easy to wear when using up milling. In down milling, the cutter teeth are cut from the surface of the workpiece each time, so it is not suitable for processing the workpiece with hard skin. The average cutting thickness is large and the cutting deformation is small during the down milling, and the power consumption is less than that of the up milling.

Features of up milling:

Because the horizontal cutting force acting on the workpiece by the milling cutter is opposite to the feed movement direction of the workpiece, the screw rod and nut of the worktable can always keep one side of the thread tightly fit. However, it is not the case in down milling. Because the direction of horizontal milling force is consistent with the direction of workpiece feed motion, when the force of cutter teeth on the workpiece is large, due to the existence of the gap between the table screw rod and nut, the table will move, which not only destroys the stability of the cutting process, but also affects the processing quality of the workpiece, and in serious cases, it will damage the cutter. Due to the large friction between the cutter teeth and the workpiece, the cold and hard phenomenon on the machined surface is serious.

Conclusion: both forward milling and reverse milling have their own advantages and determination. The specific milling method should be based on the requirements of the workpiece to be processed. If the surface smoothness is required to be high, the forward milling should be used, and the reverse milling should be used.

Automatic tool changing device of machining center

The machining center has been developed from the basic CNC milling machine. It combines all the features and functions of the CNC milling machine. The only difference from the CNC milling machine is that the machining center has automatic tool changing device, which can realize the automatic tool changing function during processing. But NC milling machine has no automatic tool changing device, which can not realize the function of automatic tool change, which is the only difference between CNC milling machine and machining center.

The composition of automatic tool changing device in machining center

The automatic tool changing device of machining center is composed of tool magazine, manipulator and driving mechanism. The tool magazine is used to store the tools needed in the process. There are many types of tool banks, mainly classified according to the shape, such as the Douli type magazine, disc magazine and chain magazine, etc. the capacity of these repositories is several to hundreds of tools. In the market, there are two kinds of tool storehouses used in the common processing centers, namely, the Douli type and the disc type. However, chain type tool base is not used by any manufacturer because of its high price. Here is a small edition to introduce the way of tool change.

The way of changing the tool of the Douli type magazine

The tool changing method of the Douli type magazine is relatively simple, and there is no mechanical arm for such magazine, so it is not necessary to use the mechanical arm to complete the tool change. The tool changing mode of its magazine is that the magazine moves to the spindle to realize the tool change. This kind of tool has the advantages of high cost performance, convenient maintenance and simple structure, and the disadvantage is that the speed of tool change is slow.

The way of changing the tool of disc magazine

The tool changing mode of disc magazine is more complicated, mainly by the manipulator to complete the tool change action, and the manipulator takes the tool 180 ° from the magazine and rotates it into the spindle to complete the tool change. This kind of tool magazine has the advantage of fast tool changing, and its disadvantage is complex structure, inconvenient maintenance and high failure rate.

Although the tool changing mode, tool selection mode and tool structure of these tool banks are different, they are controlled by numerical control system, and the selection and exchange of tools are realized by motor, air pressure or hydraulic and manipulator.

Precautions for tool magazine of CNC machining center

As we all know, the structure of the tool magazine of machining center is very complex, especially the disc magazine. And in the normal work of the machining center, the tool magazine frequently transfers tools. Therefore, the machining center is prone to failure. According to statistics, 50% of the machine tool failures are related to the tool magazine. Therefore, when using the machining tool magazine, we must pay attention to some matters, and try to minimize the fault of the tool magazine of the machining center.

Precautions for tool magazine of machining center

It is forbidden to load the overweight and extra long tools into the tool magazine, which can prevent the tool from falling off or collision between the tool and workpiece and fixture during tool change.
When installing the tool into the tool magazine, we must pay attention to the correct installation according to the sequence of the tool magazine. After the installation, let the machining center idle to see whether the tool magazine replaced is consistent with the required tool, so as to prevent the adverse consequences caused by the wrong replacement
When the tool needs to be installed manually, make sure that it is installed in place and firmly. Check whether the locking device on the cutterhead is started and ensure these factors, otherwise unnecessary accidents will occur.
The operator of the machining center should check whether the tool magazine returns to the correct position at any time, simply return to zero, and then check whether the spindle of the machining center returns to the tool change position. If one of the two problems is found, it should be timely sorted out and fed back, otherwise the tool change can not be completed, affecting the work efficiency of the machining center.
The tool, handle and sleeve should be cleaned and maintained regularly. The tool, handle and sleeve must be kept clean.
When the machining center is started, if you want to make the machining center run at idle, check whether the tool magazine, manipulator and other moving parts are normal, especially if the travel switch and solenoid valve are working normally. The other is to check whether the hydraulic pressure value of the manipulator is within the normal range, whether the cutting tool is locked on the manipulator. If the above problems are found, they should be immediately fed back and dealt with in time.

Tool magazine of five axis machining center

1、 What tool magazine does five axis machining center use

Among all kinds of machining centers, five axis machining center is the most technical machining center. In the 1980s and 1990s, the five axis machining center was listed as a strategic material, and there were extremely strict restrictions on export and import. We can imagine how high the status of five axis machining center was at that time. Today, the five axis machining center still has a strong influence, and is still in an irreplaceable position in the processing of various high-end scientific and technological parts.

Although there are no clear requirements for the selection of tool magazine for five axis machining center, we will always choose a better tool magazine when selecting tool magazine of five axis machining center. This is based on the features of the five axis machining center. The workpieces processed by 5-axis machining center are generally very complex, and the requirements for precision are very high. The selection of tool magazine and tool directly affects the machining efficiency and the quality of workpiece, so the selection of tool magazine for five axis machining center cannot be ignored.

When selecting tool magazine for five axis machining center, we need to choose better tool magazine and more precise tool. A good magazine is based on the type and brand of the magazine. For example, for a five axis vmc850 machining center, we can choose the tool magazine types of hat type and tool arm type, so we’d better choose to use the tool arm type tool library. As for the magazine and the brand of the tool, there are different opinions.

2、 What are the tool magazine of five axis machining center

The five axis machining center can be a large vertical machining center or a small vertical machining center. However, the tool magazine of these two kinds of machining centers can only be selected as the hat type and the knife arm type. Generally, we can’t see the bamboo hat type tool magazine on the five axis machining center, and few people will use the hat type tool library on the five axis machining center. The type of tool magazine of five axis high speed machining center is different from that of common machining center. The tool magazine used in high-speed machining center is the tool arm type and the same motion type. Generally, the tool magazine with the same motion is better and the tool change speed is faster. The same is true for the drilling and drilling center. There is no big difference between the arm type magazine and the flying saucer type magazine. The flying saucer type magazine has faster tool changing speed than the clamping arm type one, and the capacity of the general tool magazine is larger than that of the clamping arm type, which can meet the needs of most customers.

3、 Tool magazine brand of five axis machining center

Domestic use of five axis machining center customers, almost will not use domestic tool library, the reason we all understand. Taiwan tool magazine and imported tool magazine are mostly used in domestic five axis machining center. Taiwan’s Dao warehouses, such as Shengjie, shoulun, deta, Okada, etc., are all very good brands with high cost performance. However, the imported tool magazine is not used much. Jieli from the United States, KTC and reichenbacher from Germany are all well-known brands of tool magazine. The quality is good, the precision is good, but the price is very high. How to choose the tool magazine of five axis machining center depends on the customer’s demand and economic ability.

Types and characteristics of diamond tools

As we all know, diamond tool is the most hard tool among all the tools, and its quality is very good. Its tool has high hardness, high wear resistance and good thermal conductivity, which is widely used in machining center tools. The main characteristics of the tool used in the machining center are high efficiency, high stability and long service life. It is indispensable to use the machining center to process the workpiece with high hardness. There are three kinds of materials used in diamond cutting tools, which are pure natural diamond, PCD diamond and CVD diamond.

Natural diamond tools:

Natural diamond has been used as a tool for nearly a hundred years. After fine grinding, the blade of natural diamond tool can be ground very sharp, and can be ground to ultra-thin. After investigation, natural diamond can be finely ground to the edge of 0.002 μ m radius. Using this tool to process workpieces can carry out ultra-thin cutting, and can process the ultimate machining accuracy and smooth surface Said that the surface roughness processed is very low. This kind of natural diamond tool is called irreplaceable ultra precision and ideal tool in the future. Natural diamond tools are easy to use, but they are very expensive. Few enterprises in China are willing to buy such expensive tools.

PCD diamond tools:

PCD diamond material is actually polycrystalline diamond, its English name is polycrystal diamond, PCD diamond is made by high temperature and high pressure synthesis technology. Because of the high price of natural diamond, there are synthetic polycrystalline diamond, its price is only one dozens of natural diamond. However, in terms of performance, polycrystalline diamond can not match, for example, natural diamond can fine grind ultra-thin blade, while polycrystalline diamond can not. In addition, in the machining center, the surface quality of workpiece processed with polycrystalline diamond tool is not as good as that of natural diamond tool. Natural diamond tool can carry out ultra-thin cutting of workpiece, while polycrystalline diamond tool cannot For ultra-thin cutting, polycrystalline diamond tools are suitable for finishing, while natural diamond tools are suitable for ultra-fine machining.

CVD diamond tools:

CVD diamond is also artificially synthesized. It appeared between 1970s and 1980s and was developed by Japan. CVD diamond refers to the synthesis of diamond films on heterogeneous substrates (such as cemented carbide, ceramics, etc.) by chemical vapor deposition (CVD /). The characteristics of CVD diamond are almost the same as that of natural diamond. The tool made of CVD diamond material can be regarded as perfect. It has the advantages of natural diamond and polycrystalline diamond at the same time.

Machining center angle head

The angle head of machining center is a very important part of machining center, but some friends who use machining center do not know enough about it, or even use it very little, so it can not give full play to the powerful performance of machining center.

1、 What is the angle head of machining center

The angle head of machining center is a common accessory head in CNC machining center industry in the market today. It is the only kind of accessory that can complete the side processing without secondary clamping the workpiece, especially for drilling, tapping and milling groove machining in some circular holes and small-sized cavities. It is absolutely essential tool. Because of its special and necessary function, the angle head has been widely recognized in the CNC market.

2、 What is the angle head of machining center

Gantry special heavy angle head BT50 output BT50
Light 90 degree standard angle head — the spindle is BT50 or BT40, and the output is ER40, er32, ER25, ER20, ER16, etc.
Non standard angle head – customized, the general cycle is 1-2 months.
Special heavy cutting angle head for machining center — the spindle is BT50 or BT40, and the output is BT40.
Universal angle head: light universal can rotate 0-180 degrees, heavy universal can rotate 360 degrees.

3、 Application field of machining center angle head

The application of machining center angle head is mainly a specific process. For example: precision workpiece, one-time fixed, need to process multiple faces; hole in hole, milling head or other tools can not probe into the hole to process small hole; machining center can not process inclined hole, inclined slot, etc., such as cylinder, box shell inner hole, when a circle of hole needs to be processed on the periphery of circular workpiece, the fourth axis processing can realize complete automation. The side needs to be processed by various processes, and automatic tool change can be realized by using multiple tools.

CNC machining center processing composite materials

There are few and no composite materials processed by machining center, but it is strict to process composite materials with machining center, and there are some precautions:

Now the amount of composite materials has been rising, so the use of machining centers to add composite materials has also been rising. In the past, we used to drill and cut the edge by hand, but now we use machinery to finish it. Using machining center to process composite materials is much faster than manual processing, and the precision is high. However, once there is a problem, there will be serious loss. Therefore, the process, tool selection and processing parameters are particularly important for processing composite materials.

Matters needing attention in selecting cutting tools for machining composite materials

1、 Generally, the size of composite auxiliary materials is relatively large, and its workpiece structure is complex, and its hardness and strength are very high. The cutting force used in the machining process is relatively large, and the heat produced by the processing is difficult to transfer out, which may severely scorch the resin or soften the resin, resulting in severe tool wear. Therefore, the tool used must have carbon fiber, and the cutting speed should be greater than 500 / min, and the feed speed must be small. In the process of edge cutting, carbide hobbing cutter, electroplated diamond grinding wheel, diamond inlaid milling cutter and copper based diamond grain saw blade should be selected.

2、 The cutting effect of the three new types of special milling cutters for machining monolithic cemented carbide composites is better. They all have some common characteristics: high rigidity, small helix angle, even 0 ° angle. The specially designed herringbone blade can effectively reduce the axial cutting force and delamination, and their machining efficiency and effect are very good.

3、 The interlaminar strength of carbon fiber composites is low, and it is easy to produce delamination under the action of cutting force. Therefore, the axial force should be reduced when drilling or trimming. High speed and small feed are required for drilling. The rotating speed is generally 3000 ~ 6000r / min, and the feed rate is 0.01 ~ 0.04mm/r. It is better to select three tips and two edges or two tips and two edges for the drill bit. The sharp tip can cut off the carbon fiber layer first. The two edges play a repair role on the hole wall. The diamond embedded drill head is sharp and wear-resistant. It is a difficult problem to drill the interlayer between composite and titanium alloy. Generally, the integral cemented carbide bit is used to drill according to the cutting parameters of drilling titanium alloy. From the titanium alloy side, the drilling is carried out until the drilling is through. Lubricant is added during drilling to alleviate the burn of composite materials. Boeing company specially developed PCD combined bit for interlayer drilling.

Special attention: the composite chip is powder, which is harmful to human health. High power vacuum cleaner should be used for dust collection, and water cooling can also effectively reduce dust pollution

Advantages and disadvantages of cutting fluid for CNC machining center

The cutting fluid plays a very important role in the machining of workpiece in CNC machining center. Cutting fluid will be used in any material processed by CNC machining center. Therefore, cutting fluid is an indispensable liquid in CNC machining center. The main function of the cutting fluid is lubrication, cooling, anti-corrosion, anti rust, etc. if there is no cutting fluid for processing, the workpiece will be easy to deform and rust under high temperature. Therefore, CNC machining center must use cutting fluid to avoid this undesirable phenomenon.

There are many kinds of cutting fluids. Today, we will talk about several commonly used in the market. These are emulsion, chemical synthetic cutting fluid and cutting oil added with extreme pressure additives. Today, we will introduce these three common cutting fluids.

Advantages and disadvantages of cutting oil with EP additive

Advantages: the general use and cutting speed is not higher than 60m / min, the main cutting speed is not higher than 60m / min, regardless of any material cutting processing. Disadvantages: in high-speed cutting, due to the large amount of heat, oil-based cutting fluid heat dissipation effect is poor, so in high-speed cutting will produce stable and high smoke or fire. Therefore, due to high stability will have a certain impact on the workpiece, such as workpiece deformation, poor machining accuracy and other adverse phenomena.

Advantages and disadvantages of emulsion

Advantages: the biggest advantage of emulsion is strong heat dissipation, can clean the workpiece and high cost performance, which is conducive to the health and safety of operators. Therefore, the general metal processing plant is to choose the use of emulsion processing. In addition to the materials that are particularly difficult to machine, emulsion can be used for almost all light and medium load cutting and most heavy load processing, especially for processing thread grinding, groove hemp cutting and other complex grinding. Disadvantages: the biggest defect of emulsion is that it is easy to make bacteria and mold multiply, and make the effective components in emulsion decompose and stink and deteriorate In general, organic fungicides with low toxicity should be added.

Advantages and disadvantages of chemical synthetic cutting fluid

Advantages: it has the advantages of high cost performance, fast heat dissipation, strong cleaning ability and excellent workpiece visibility. It is very easy to control the size of the workpiece. Its stability and anti-corruption ability are better than emulsion. The disadvantage is that the lubrication ability is not strong, it is easy to suck the adhesion and wear of the moving parts of the machining center. Moreover, the viscous residue left by the chemical synthesis will affect the movement of the parts of the machining center and cause the rust of the overlapping surfaces of the parts.