Category Archive CNC Technology

1. With the development of science and technology of rotation and temperature rise, the spindle speed of CNC machining of aluminum cavity is higher and higher, and the range of speed change is wider and wider.

Therefore, the requirements of bearing high-speed operation stability are higher and higher. The temperature rise of machine tool spindle bearing is an important factor to limit the bearing speed. Generally, the correct selection of bearing type, worse grade, configuration mode, clearance (preload), lubricant and lubrication mode can improve the high-speed performance of rolling bearing to a certain extent.

1. Service life and bearing capacity for heavy machine tools or heavy cutting machines, the bearing capacity of air filter bearing should be the first.

Because for general machine tools, the life of the spindle assembly refers to the service life to maintain the accuracy of the spindle. Therefore, the bearing accuracy is required to meet the life requirements of the spindle assembly.

1. Stiffness and vibration resistance in order to ensure the machining quality of aluminum cavity CNC machining, the spindle system must have enough stiffness, otherwise there will be a large re mapping error or even vibration.

Anti vibration refers to the ability to resist forced vibration and self-excited vibration. The anti vibration performance of the main shaft depends on the stiffness and damping of the spindle and bearing. The stiffness of the spindle system can be effectively improved by using preloaded rolling bearings.

1. Noise in high-speed grinding machine, the noise of grinding head bearing is an important part of the whole noise, and it is advantageous to select low-noise rolling bearing.

In the fault detection of CNC machining of aluminum cavity, reasoning can be carried out from two aspects: forward and backward. Forward: that is, from the cause of the fault, explore its functional relationship, and check the impact of the cause on the results.

In other words, according to the possible causes of the fault, to see whether it is consistent with the fault phenomenon, to determine the fault point.

By using reasoning method, the object of reasoning analysis is often vague, because in many fault causes or phenomena, we should find out their common characteristics.

So as to determine the fault point, with the deepening of reasoning step by step, we will realize that the personality characteristics of reasoning analysis object is the fault to be found.

Grinding methods can be divided into cylindrical grinding, internal grinding, centerless grinding, surface grinding, form grinding, gear grinding, thread grinding, east-west grinding, general lathe surface grinding, etc. With the development of NC technology, CNC grinding method plays an increasingly important role in grinding.

CNC grinder can process some special parts (such as axis parts with special curve, special-shaped parts with more than three coordinate requirements, mold, etc.) which cannot be processed on general grinder. Therefore, it is used to machining parts in a wide range. In order to maximize the economic benefits of CNC grinding machine, the following processing objects can be given priority.

（1） The parts put into production repeatedly have better benefit

The use of CNC grinding machine process preparation time occupies a higher share. For example, preparation of process analysis, programming, adjustment and trial cutting of the first part of parts.

The sum of these inductive working hours is often dozens to hundreds of times of the working hours of a single part. However, the operation contents of the NC lathe (such as special general lathe fixtures, process documents, programs, etc.) can be saved and used repeatedly. Therefore, when a part is successfully trial produced on a CNC grinder and put into production repeatedly, the production cycle is greatly reduced, and the cost is also less, which can achieve better results Economic benefits.

（2） It is required to guarantee the processing quality and produce the key parts in medium and small batches with high efficiency

Controlled grinding machine can realize high precision, high quality and high efficiency grinding under the control of computer. Compared with special grinding machine, it can save a lot of special process equipment, and has strong flexible manufacturing ability and good economic benefits.

Compared with the general grinding machine, it can remove many artificial disturbing factors in the long process of disorderly processing, and has good accuracy consistency and interchangeability, and high processing efficiency.

（3） The processing batch of parts should be greater than that of general grinder

When NC grinding machine is used to process medium and small batch parts, due to various reasons, the pure cutting time only accounts for 10% – 30% of the actual working hours. When machining on the CNC grinding machine of grinding center, which is a kind of multi process convergence, this share may rise to 70% – 80%, but the preparation adjustment time is often much longer, so the parts batch size will become uneconomical.

（4） The processed parts should fit in with the process characteristics of CNC grinding machine with multi process convergence

When CNC grinding machine is used to process parts, the condition of grinding wheel cutting workpiece is completely the same as that of corresponding non CNC grinding machine, but it can carry out some complex machining with machining accuracy requirements. For example, in the grinding range, the general grinder is mainly used for grinding the end face of cylindrical surface, conical surface or stepped shoulder.

In addition, CNC cylindrical grinder can also grind toroidal surface (including convex and concave surface), as well as the above various forms of disordered combined surface.

（5） Balance of inductive processing capability of parts

As a single CNC grinding machine, it is difficult to complete all the processing contents of a part, and it needs to transfer and cooperate with the processing procedures of other equipment, so it has the requirements of production rhythm and workshop production capacity balance.

Therefore, it is necessary to give full play to the processing characteristics of CNC grinding machine, and reasonably organize matching balance process on other processing equipment.

（6） Some special parts processing considerations

Although the processing batch of some parts is very small, ordinary lathes have disordered shapes, high quality and good interchangeability, which can not meet the above requirements on non CNC grinding machines, and can only be organized to be processed on CNC grinding machines, such as parabola, cycloid cam and special reflector mirror

CNC lathe is a kind of high-precision and high-efficiency automatic machine tool. The use of CNC lathe can improve the processing efficiency and create more value. The emergence of CNC lathe makes enterprises get rid of backward processing technology.

The processing technology of CNC lathe is similar to that of ordinary lathe. However, since CNC lathe is a kind of clamping device, which can complete all turning process continuously and automatically, attention should be paid to the following aspects.

For efficient metal cutting, machining materials, cutting tools and cutting conditions are the three main factors. These factors determine the processing time, tool life and machining quality. An economical and effective machining method must be a reasonable choice of cutting conditions.

Three factors of cutting conditions: cutting speed, feed and cutting depth directly lead to tool damage. With the increase of cutting speed, the tool tip temperature increases, resulting in mechanical, chemical and thermal wear. When the cutting speed is increased by 20%, the tool life is reduced by 1 / 2.

The relationship between feed condition and tool wear is very small. However, when feeding, the cutting temperature increases and the wear occurs. It has less effect on the tool than on the cutting speed. Although the influence of cutting depth on cutting speed and feed rate is not as great as that of cutting speed and feed rate, the hardened layer produced by cutting material will also affect the tool life when cutting at small cutting depth.

The user should select the cutting speed according to the material, hardness, cutting state, material type, feed rate and cutting depth.

According to these factors, select the most suitable processing conditions. The ideal state is to wear regularly and stably to reach the service life.

But in practice, the choice of tool life is related to tool wear, size change, surface quality, cutting noise, processing heat and so on. When determining the processing conditions, it is necessary to study according to the actual situation. For refractory materials such as stainless steel and heat resistant alloy, coolant or hard blade can be used.

(1) Point control CNC lathe processing robot. The CNC lathe robot of CNC equipment can only control the moving parts of the lathe from one position to another accurately, that is to say, only the value of controlling the end point of coordinate travel does not have any reduction in the moving process, as for the speed and path between the relevant two points depends on the productivity.

In order to locate accurately on the basis of as high productivity as possible, the movement between the two related points is first to move quickly to the new position, and then slow down the 1-3 stage, let it slowly locate the point, so as to ensure its positioning accuracy.

(2) Point line control CNC lathe processing robot. This kind of NC lathe robot processing, not only to control the position between the two related points, but also to control the movement speed and route between the two related points.

Its route usually consists of straight lines parallel to each axis. The difference between the control and the point of NC lathe robot is that when the moving parts of the machine tool move, it can be processed along an axis direction (generally it can be cut at 45 ° oblique line, but not any slope linear cutting). The auxiliary function is the position control of the NC lathe robot, such as improving the spindle speed control, feed processing cycle, tool selection, and other functions.

(3) Contour control CNC lathe processing robot. The control device of this kind of CNC lathe can control two or more coordinate axes continuously at the same time.

In the process of machining, we should not only control the starting point and the end point, but also control the speed and position of each point in the whole processing process, so that the CNC lathe robot can process complex shaped parts according to the drawing requirements. Its auxiliary functions are also relatively complete.

CNC machining is to use the automatic control system to install digital program flow commands to control the automatic start stop, direction change and speed change of CNC machine tool bearings. NC blade can be selected, the drilling amount and moving track of NC blade can be changed, and all kinds of auxiliary posture needed for lifetime machining can be carried out. CNC machining can greatly reduce the number of work clothes, and produce parts with complex processing methods, only need to change the process flow. CNC machining is relatively stable and will not cause machining error. What are the reasons that affect the accuracy of CNC machining? Let’s take a look at it

From the long-term data analysis and practical operation of parts processing, we can see the machining accuracy error caused by CNC machine tool positioning. The positioning of machine tools has a great influence on the machining accuracy of CNC machine tools. From the perspective of structure, the machining error of CNC machine tools is mainly caused by the accuracy grade, and the feed system of machine tool is the key stage affecting the accuracy level. The feed system of CNC machine tool is generally composed of mechanical transmission system and electrical control system. In the overall design, the accuracy level is related to the mechanical transmission system. In the closed-loop control system, CNC machine tools can generally avoid the position error of key components (such as ball screw and other components) according to the positioning inspection equipment. However, for the open-loop system, because of many factors and complex conditions, it is impossible to carry out positioning detection, so the machining accuracy of CNC machine tools is greatly affected.

The influence of CNC machining position error on machining accuracy. Position error refers to the transformation or offset level of the specific surface layer, center line or symmetry face of the machined part relative to its ideal position, such as flatness, symmetry, symmetry, etc. Position error in NC machining generally refers to over-current protection error. The key reason for the position error is the machining error caused by the gap and elastic deformation caused by the transmission system in the whole machining process of machine tool parts, and the position error caused by the sliding friction force that the special tool head needs to get rid of in the production process. In the open-loop system, the position precision is greatly affected, while in the closed-loop control system, the key to the position precision is the offset test.

The machining accuracy error caused by geometric figure error of CNC machining. In the processing of CNC machine tools, because of the influence of external factors such as external force, heat generated during processing, the geometric precision of the machine tool is affected, and the geometric shape deformation of parts processed on the machine tool will cause geometric error. According to scientific research, there are two reasons for the geometric error of CNC machine tools: internal factors and external factors. The internal factors causing geometric errors of machine tools refer to the geometric errors caused by the machine itself, such as the flatness of the machine tool table, the flatness and parallelism of the guide rail of the machine tool, and the geometric precision of the special tools and fixtures of the machine tool. The key of external factors refers to the geometric error caused by environmental factors and thermal deformation in the production process. For example, the geometric error caused by thermal deformation and deformation of NC blade or parts in the whole drilling process will affect the machining accuracy of machine tools and parts.

As the machining center is a large and medium-sized CNC machine tool, the machine tool factory needs to disassemble the machine tool into several parts when it is delivered. After the machine tool is in place, it needs to be re connected and installed. Therefore, after the machine tool is in place, it is necessary to repeatedly adjust the level of the main bed of the machine tool by adjusting the foot bolts and pad iron used in the fixed foundation of the machine tool, so that the parallelism of each axis in the full stroke is within the allowable range. And in order to ensure the stability and reliability of the machine tool in the power cutting, it is also necessary to pay attention to make all the pad iron in the pad state. Considering that the cement foundation is not stable enough, it is generally required to fine adjust the machine tool level after several months to half a year, so as to ensure the accuracy stability of the machine tool in K period.

The main parts of the machining center need to be adjusted, such as the relative position of manipulator, spindle and tool magazine, as well as the relative position of pallet and exchange table, so as to ensure the accuracy, stability and reliability of tool change and worktable exchange. The adjustment methods and requirements are as follows.

(1) Adjust the relative position of manipulator, spindle and tool magazine. Use gz8yozo or c30yozo program to make the machine automatically run to the tool change position, and then use it to change the tool step by step in the dynamic mode, and check whether the actions of grasping, loading and drawing are accurate and appropriate. If there is any error, you can adjust the stroke of the manipulator or move the position of the manipulator support or tool magazine. If necessary, you can also change the setting / (parameter setting /) of the coordinate value of the tool change base. After adjustment, tighten the adjusting screws, and then change the tool several times. It is better to use several toolholders which are close to the maximum allowable weight for repeated tool changing experiments, so as to achieve accurate action, no impact and no knife drop.

(2) Adjust the change of supporting plate spoon: I is the relative position of the table. For double or multiple worktables, the relative position of the pallet and the exchange worktable must be carefully adjusted to ensure the stability and reliability of the automatic exchange of the worktable. During the adjustment, the worktable should be equipped with more than 50% of the rated load for the automatic exchange operation of the worktable.

The so-called boring is to enlarge or refine the original hole on the workpiece. It is characterized by correcting the eccentricity of the lower hole, obtaining the accurate position of the hole, and achieving high-precision roundness, cylindricity and surface finish. Therefore, boring as a high-precision machining method is often used in the final process. For example, the bearing holes of various machines and the processing of boxes and covers of various engines.

Compared with other machining, boring is a difficult process. It only needs to adjust one blade / (or blade holder /) to process micron sized holes such as H7 and H6. With the popularization of machining center, the boring process only needs programming and button operation. Because of this, it is necessary to have simpler, more convenient and more precise cutting tools to ensure the quality of products. This paper mainly analyzes the boring process of machining center from the perspective of tool technology.

1. Characteristics of boring process in machining center

2.1 tool rotation

Different from lathe processing, because of tool rotation in machining center, it is impossible to master the situation of tool tip in time to adjust the feed rate. It is impossible to change the machining diameter just by adjusting the NC button like the NC lathe. This has become a big obstacle to fully automated processing. It is also because the machining center does not have automatic diameter adjustment function (except for those with U-axis function), the boring cutter must have fine adjustment mechanism or automatic compensation function, especially in fine boring, sometimes it must be adjusted at micron level according to tolerance requirements.

In addition, due to the continuous change of chip flow direction during the boring of machining center, it is much more difficult to cool the tool tip, workpiece and chip discharge than when machining with a lathe. Especially when the longitudinal machining center is used for rough boring of steel blind hole, this problem has not been completely solved.

2.2 chatter

Chattering is the most common problem in boring. The main causes of chatter on machining center are as follows:

① Rigidity of tool system: including the rigidity of toolholder, boring bar, boring head and middle connecting part. Because of the cantilever machining, the rigidity of the tool system is particularly important when machining small holes, deep holes and hard workpieces.

② Balance of tool system: relative to the rotating axis of the tool system, if there is an unbalanced mass of the tool itself, the chatter will occur due to the action of unbalanced centrifugal force during rotation. Especially in high speed machining, the dynamic balance of the tool has a great influence.

③ Clamping rigidity of workpiece itself or workpiece: for example, some small and thin parts cannot be fully fixed with reasonable fixture due to insufficient rigidity or workpiece shape.

④ The geometry of edge /: the cutting resistance is different with different rake angle, escape angle, nose radius and chip breaking groove shape.

Tool setting point is the reference point used to determine the position of workpiece coordinate system in the machine coordinate system after the workpiece is aligned and clamped on the machine tool.

The tool setting point can be selected on the workpiece or clamping positioning element, but there must be an accurate, reasonable and simple position corresponding relationship between the tool setting point and the workpiece coordinate point, so as to facilitate the calculation of the position of the workpiece coordinate point on the machine tool / (the machine coordinate of the workpiece coordinate point). The tool setting point should coincide with the workpiece coordinate point.

Tool change point: the machining center has a tool magazine and an automatic tool change device, which can automatically change tool according to the needs of the program. The tool change point should be in the position where there is no collision and interference among workpiece, fixture, tool and machine tool during tool change. The tool change point of machining center is usually fixed.

Tool setting method:

Tool setting in horizontal direction / (x, y coordinates)

(1) Lever dial indicator tool setting: the tool setting point is the center of the cylindrical hole;

(2) Tool setting with edge finder: round hole or reference edge

(3) The tool is set by touching or trial cutting.

Tool setting in Z direction / (Z coordinate /):

(1) On machine tool setting: use Z direction setter to set tool.

(2) Tool presetting outside the machine and tool setting on the machine.

(3) External tool setting instrument: measure the diameter, length, edge shape and angle of the cutter.

(4) Tool setting in multi position machining of horizontal machining center

The non-traditional machine tools widely used in production mainly include EDM and WEDM. Therefore, this paper mainly describes the safety operation procedures of these two kinds of special machining machine tools.

1. Safe operation procedures for EDM machine tools

(1) Before starting the machine, carefully read the operation manual of the machine tool. If you are not familiar with the operation of the machine tool, you can avoid safety accidents.

(2) Before machining, pay attention to check the discharge gap, that is, a certain distance must be kept between the tool and the workpiece to form the discharge gap. It is generally 0.01-0.1mm. ,

(3) The clamping and calibration of the upper electrode must ensure that the feed direction of the tool electrode is perpendicular to the worktable plane.

Ensure that the output voltage pulse waveform of the pulse power supply on the workpiece and tool electrode added in the liquid medium is half

(5) There should be enough pulse discharge energy to ensure the metal melting or gasification in the discharge part.

(6) The discharge must be carried out in a liquid medium with constant insulation properties.

(7) During the operation, pay attention to check the filter element of the working fluid system. If the filter element is blocked, the working fluid should be replaced in time, which can automatically maintain a certain cleanliness.

(8) For the flammable type of working fluid in Sichuan, attention should be paid to fire prevention when using towel.

(9) To be civilized and strict, we must clean the working site and cut off the power supply of the system before leaving.

1. Safety operation rules of WEDM

Because WEDM is developed on the basis of EDM, it uses wire electrode / (copper or copper wire, etc.) to cut the upper part by spark discharge. Therefore, the safety operation regulations of WEDM machine tool are roughly the same as those of EDM plus machine. In addition, attention should be paid to the following aspects in operation.

(1) In order to reduce the pulling range of wire electrode and improve the machining accuracy, it is necessary to ensure that the wire electrode has a certain pre tightening force during wire winding.

(2) Check that the working fluid system is equipped with a resin cartridge to ensure that the working fluid can move freely and maintain a certain resistivity.

(3) In the process of EDM, it is necessary to make the working fluid fully surround the electrode wire to prevent the wire breaking phenomenon caused by the heat generated by the electrode wire passing through the large pulse current.

(4) Strengthen the regular inspection of the mechanical device of the machine tool

(5) To achieve civilized production, after the end of processing operation, cut off the power supply of the system before leaving. Mouth lubrication. Must clean the workplace, wipe clean machine tools, and cut off the power supply before leaving

According to the motion track, the guide rail can be divided into linear motion guide rail and circular motion guide rail; according to the working nature, it can be divided into main motion guide rail, feed motion guide rail and adjustment guide rail; according to the friction property of contact surface, it can be divided into sliding guide rail, rolling guide rail and hydrostatic guide rail.

1. Sliding guide rail

The sliding guide has the advantages of simple structure, convenient manufacturing, good stiffness and high vibration resistance. The traditional cast iron cast iron and cast iron quenched steel guide rail has the disadvantages of large static friction coefficient, dynamic friction coefficient changes with speed, and the friction loss is large. It is easy to crawl at low speed / (1-60 mm / min /), which reduces the positioning accuracy of moving parts. Therefore, it is not used in other CNC machine tools except economical CNC machine tools. At present, most CNC machine tools use plastic guide, that is, a layer of plastic film soft belt composed of plastic and other chemical materials is pasted on the friction surface of moving guide rail.

There are two kinds of guide rail Plastics: PTFE soft belt and epoxy wear-resistant rail coating. The characteristics of plastic sliding guide are good friction characteristics, good wear resistance, smooth movement, good vibration damping and good technology.

1. Rolling guide

Rolling guide is to place ball, roller, needle and other rolling elements between the guide surfaces, so that the sliding friction between the guide surfaces becomes rolling friction. Compared with sliding guide, rolling guide has the following advantages: 1) high sensitivity, and little difference between dynamic friction coefficient and static friction coefficient, so it moves stably, and crawling phenomenon is not easy to appear when moving at low speed; (2) high positioning accuracy, repetitive positioning accuracy can reach 0.2 um; 8) low friction resistance, light movement, small wear, good accuracy retention and long service life. However, the anti vibration performance of rolling guide is poor, which requires high protection.

The rolling guide rail is especially suitable for the working parts of machine tools requiring uniform movement, sensitive motion and high positioning accuracy. This is the reason why rolling guide is widely used in CNC machine tools.

1. Hydrostatic guide

Hydrostatic guideway can be divided into hydrostatic guideway and aerostatic guideway.

There is an oil chamber between the two guide working faces of hydrostatic guideway. After the lubricating oil with certain pressure is injected, the hydrostatic oil film can be formed, which makes the working surface of the guide rail in pure liquid friction, without wear, and has good accuracy retention. At the same time, the friction coefficient is also very low, which greatly reduces the driving power rate; at low speed In addition, the oil has the function of absorbing vibration and good vibration resistance. Its disadvantage is that the structure is complex, there must be an oil supply system, and the cleanliness of the oil is required to be high. Hydrostatic guideway is widely used in machine tools. The hydrostatic guideway of CNC turret punch press can be divided into open type and closed type. Figure 2.25 shows the working principle of the open hydrostatic guideway.

The aerostatic guideway can form a static pressure air film after passing a certain pressure between the working faces of the two guide rails, so that the two guide surfaces of the CNC punch press can be evenly separated to obtain high-precision movement. At the same time, the friction coefficient is small, and it is not easy to cause heating deformation, but it will change with the air pressure fluctuation, and the bearing capacity is small, which is often used in occasions with small load 。 In addition, we must pay attention to the dust on the guide rail surface, because the dust falling into the air guide surface will cause damage to the guide rail.