Yearly Archive September 25, 2020

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.

CNC system is a numerical control system which uses computer to control machining function. According to the control program stored in computer memory, the CNC system of large-scale CNC machining performs some or all numerical control functions, and is equipped with interface circuit and servo drive device. CNC system consists of NC program, input device, output device, CNC device, PLC, spindle drive device and feed (servo) drive device (including detection device).

The core of CNC system is CNC device. Because of the use of computer, the system has software function, and PLC is used to replace the traditional logic control device of machine tools, which makes the system more compact, flexible, universal, reliable, easy to realize complex CNC functions, easy to use and maintain, and has the function of connecting with the upper computer and remote communication.