November 2020 – Page 4 – CNC Machining & Turning, Milling, Metal parts

Analysis of electrode wire loss

1、 Analysis and measures of electrode wire loss

In NC WEDM, the loss or broken wire of electrode wire seriously affects its continuous automatic operation, especially in high-speed WEDM, due to the repeated use of electrode wire in the processing, with the increase of electrode wire loss, the cutting seam becomes narrower and narrower, which will not only increase the size error of the machining surface; and once the wire breakage occurs in the processing, the processing must be carried out Restart, which not only costs more man hours, but also affects the quality of the machined surface. The causes and measures of electrode wire loss or breakage are as follows:

1) When the electrode wire moves rapidly (8-10 M / s), it will rub with the guide wheel and the conductive block, causing the wear of the guide wheel and the conductive block. When the conductive block is used for a long time, the V-groove bottom of the guide wheel is pulled out of the groove by the electrode wire. When the conductive block is used for a long time, the side contacting the electrode wire is also pulled out of the groove. As a result, the friction resistance of the guide wire is increased and the electrode wire is broken due to hindered movement.

2) When the weight of the cutting material is large, it is easy to deflect and shift at the end of the processing, which will cause the electrode wire to be broken. For this reason, a piece of ground permanent magnetic steel can be added to the plane of the workpiece and the cutting material, so as to keep the cutting material in a fixed position during the cutting process.

3) When the internal stress of workpiece material is too large, the relative equilibrium state of internal stress in workpiece material will be destroyed, which will easily cause the cutting gap to narrow and the electrode wire will be jammed. Therefore, the following measures can be taken:

a. The improvement of heat treatment process is mainly to improve the tempering process in order to reduce the internal stress of the workpiece;

b. Before heat treatment, process holes or slots are added to the cut part to reduce the residual stress released during WEDM.

4) In WEDM, when the workpiece material is eroded and cut into shape, the electrode wire will also be corroded by discharge, that is, the electrode wire will be lost. When the electrode wire is used for a long time, the wire diameter becomes smaller and full of micro discharge pits, the tensile strength decreases, and finally fracture occurs. Generally speaking, when the diameter of the measured wire is 0.03 ~ 0.05mm less than that of the new wire, the new wire should be replaced in time.

5) The electrode wire should be evenly and neatly wound on the wire storage cylinder. The unbalanced inertia will be produced when the inner and outer circles of the silk storage cylinder are different, which will lead to the breakage of the electrode wire. Therefore, the dynamic balance must be corrected.

6) When the electrode wire is folded and threading, the electrode wire is partially broken. At the discount place, the tensile strength and the ability to bear the thermal load decrease, which is easy to fracture. In order to avoid the discount of electrode wire, first of all, the electrode wire with good linearity and high tensile strength should be selected; secondly, careful and standard operation should be carried out during wire feeding and threading.

Principle of electric erosion

Scope of application

Remove taps, drills, reamers, screws, plug gauges and other tools / tools with various diameters broken in the workpiece
It can be processed on workpieces of various sizes and shapes
Processing of various angle positions on workpiece
Machining non precision required hole

Introduction to jx-200a

Power supply: portable power supply

Electrode: use brass rod as electrode

Non destructive machining: remove the broken object without damaging the thread

Working fluid: ordinary water can be used as working medium

Large workpiece: it is very convenient to process the broken tap on the large workpiece

characteristic

Using the principle of electric erosion to remove the broken tool, non-contact processing, no force in the processing, no damage to the workpiece
Portable design, easy to use in various environments
With magnetic base, the processing device can be sucked to the workpiece for processing
The working head can be adjusted at any angle
Automatic feed processing, no occupation of personnel
Single 220 V power supply (or 110V), touch key operation, easy to use
Water can be used as working fluid

Working methods

For general taps, a half diameter electrode is used to remove broken taps. Large taps are processed three times with thin electrode or sheet electrode, or with copper tube electrode. The broken screw is processed with hexagon electrode, and then taken out with hexagon wrench. Other broken tools and cutting tools shall be processed with electrodes of suitable shape and size.

2、 Processing sequence: put metal plate into plastic container – fix the processed parts and head – (magnetic base should not be put into water) – brass electrode should be fixed in clamp head, and the electrode diameter should be half of the nominal diameter of tap – connect the cable, connect the wire clamp to the processed part – align the position of electrode and tap, the distance is 1-2 mm, and adjust the stroke finger Position indication – add working fluid into the container, at least 10 mm higher than the surface of the part – turn on the main power switch – select the processing current – press the start key to start processing – finish machining (reaching the set depth), automatically stop processing, and manually operate to make the electrode rise. The tap in the blind hole can be taken out with a driver or tweezers.

Design method of die nozzle

(1) Large gate mold: the runner and gate are on the parting line, and demould together with the product when opening the mold. The design is the simplest, easy to process, and the cost is low. Therefore, many people use the large water gate system for operation.

(2) Fine nozzle mold: the runner and gate are not on the parting line, but directly on the product. Therefore, it is necessary to design an additional set of nozzle parting lines. The design is more complicated and the processing is more difficult. Generally, the fine nozzle system should be selected according to the product requirements.

（3） Hot runner mold: the structure of this kind of mold is roughly the same as that of the fine nozzle. The biggest difference is that the runner is located in one or more hot runner plates and hot pumps with constant temperature. There is no cold material demoulding. The runner and gate are directly on the product, so the runner does not need to be demoulded. This system is also known as no nozzle system, which can save raw materials and is suitable for situations where raw materials are expensive and product requirements are high, It is difficult to design and process, and the mold cost is high.

The so-called 2-plate mold generally refers to the large water mouth system mold, and the 3-plate mold generally refers to the water nozzle system mold. 3-plate die can not be simply considered as one more template than 2-plate mold. According to the product shape and mold design needs, the mold can have more than 3-plate knot

Hot runner system, also known as hot runner system, is mainly composed of hot runner sleeve, hot runner plate and temperature control electric box. Our common hot runner system has two forms: single point hot gate and multi-point hot runner. Single point hot gate is to use a single hot gate sleeve to directly inject the molten plastic into the mold cavity, which is suitable for the plastic mold with single cavity and single gate; the multi-point hot gate is to branch the molten material into each sub Heat Gate sleeve and then into the cavity through the hot runner plate, which is suitable for single cavity multi-point feeding or multi cavity mold

Advantages of hot runner system

(1) No water mouth material, no need for post-processing, so that the entire molding process is fully automated, save working time and improve work efficiency.

(2) The pressure loss is small. The temperature of the hot runner is equal to that of the nozzle of the injection molding machine, which avoids the surface condensation of the raw materials in the runner and reduces the injection pressure loss.

(3) The reuse of nozzle material will degrade the properties of plastic, but the use of hot runner system without nozzle material can reduce the loss of raw materials and reduce the cost of products. In the mold cavity, the temperature and pressure are uniform, the stress of plastic parts is small, and the density is uniform. Under the small injection pressure and short molding time, the injection molding can produce better products than the general injection molding system. For transparent parts, thin parts, large plastic parts or high demand plastic parts, it can show its advantages, and can produce larger products with smaller machines.

(4) The hot nozzle adopts standardized and series design, equipped with various nozzle heads for selection, and has good interchangeability. The electric heating ring with unique design and processing can achieve uniform heating temperature and long service life. The hot runner system is equipped with hot runner plate, temperature controller, etc., with exquisite design, various types, convenient use, stable and reliable quality

Machine tool guide rail error

The guide rail is the datum to determine the relative position relationship of each machine tool component on the machine tool, and it is also the datum of the machine tool movement. The accuracy requirements of lathe guide rail mainly include the following three aspects: straightness in horizontal plane; straightness in vertical plane; parallelism (distortion) of front and rear guide rails.

Straightness error of horizontal lathe guide rail

Influence of straightness error in vertical plane of horizontal lathe guide rail on machining accuracy

In addition to the manufacturing error of the guide rail itself, the uneven wear and installation quality of the guide rail are also important factors causing the error of the guide rail. The wear of guide rail is one of the main reasons for the decline of machine tool precision.

Drive chain error

The relative error between the two ends of the transmission chain refers to the error between the two ends of the transmission chain. It is generally measured by the angle error of the end components of the transmission chain.

The spindle rotation error refers to the variation of the actual rotation axis relative to its average rotation axis at each moment. It can be divided into three basic forms: radial circular run out, axial movement and angle swing.

The main reasons for the radial rotation error of the main shaft are: the coaxiality error of several sections of the spindle journal, various errors of the bearing itself, the coaxiality error between the bearings, and the winding degree of the main shaft. However, their influence on the radial rotation accuracy of the spindle varies with different machining methods.

The main reason for the axial movement is the Perpendicularity Error of the spindle shoulder end face and bearing bearing end face to the spindle rotation axis.

Different machining methods lead to different machining errors caused by spindle rotation error. When machining outer circle and inner hole on lathe, the radial rotation error of spindle can cause roundness and cylindricity error of workpiece, but it has no direct effect on the end face of workpiece. The axial rotation error of the spindle has little effect on the machining of outer circle and inner hole, but has great influence on the perpendicularity and flatness of the machined end face. In the process of thread turning, the rotation error of the main axis can make the lead of the processed thread produce periodic error.

The rotation accuracy of machine tool spindle can be improved by properly improving the manufacturing accuracy of spindle and box, selecting high-precision bearings, improving the assembly accuracy of spindle components, balancing high-speed spindle components, and preloading rolling bearings.

Factors of mould water mark

What are the causes of water mark and how to improve it? For example, U-shaped parts

Water mark is also called water line, which is the melting line caused by the combination of two strands of plastic injection flow.

The reasons are: the nozzle design position is not correct or the nozzle design is not good. Poor die exhaust, etc

During injection molding, the mold temperature is too low, the material temperature is too low, and the pressure is too small.

improve:

In the structure, the bone position is added at the place where the water line is easy to be generated. The short side of the U-shaped part should be designed to be consistent with the flow direction of the nozzle.
Improve the nozzle.
Improve beer and plastic.

Please list the operation process of mobile phone assembly

Mobile phone assembly process:

The auxiliary material is usually beer plastic factory first installed on the rubber shell, PCB is generally a whole board.

PCB to a shell: button assembly on case a – PCB board – Case B (screw) – battery cover – Test – package

PCB to B shell: fix the PCB on the B shell and limit the position – install the button on the upper limit of the a-shell – screw AB shell – install the battery cover – Test – package

Please draw the size chain of the whole mobile phone

Take the straight plate machine as an example, there is no decoration on the surface, and the thickness is based on the battery: the figure will not be drawn, but the thickness distribution will be discussed. A shell glue thickness 1.0 + LCD foam 0.30 + PCB board thickness (whole board, not to mention the thickness) + battery and battery cover clearance 0.15 + battery cover thickness 1.0

P + R keyboard matching profile

Take P + R + steel plate button as an example: the figure will not be drawn. Let’s talk about the thickness distribution.

The distance between dome sheet and conductive base is 0.05 + the height of conductive base is 0.30 + the thickness of silica gel body is 0.30 + the thickness of steel sheet is 0.20 + the distance between steel sheet and a shell is 0.05 + the adhesive thickness of a shell is 1.0 + the key cap is generally 0.50 higher than the surface of a shell

What should be paid attention to in the design and assembly of steel keys

Attention should be paid to the design of steel keys

The steel sheet should not be too thick, about 0.20, or the handle is too poor.
The steel sheet cannot transmit light, and the light can only pass through silica gel.
The steel sheet is required to be positioned, and the steel sheet is fixed on the a shell on the long bending wall.
The steel sheet shall be grounded.

What should be paid attention to in the design and assembly of PC keys

Attention should be paid to the design of PC key:

The PC chip should not be too thick, about 0.40, or it will feel too bad. It can’t be too thin, or it’s very soft, causing poor hand feeling.
The light transmission of PC is not limited, and radium carving can be done at the light transmission place.
If the surface of PC chip is to be cut, the slot width shall not be less than 0.80, and the sharp corner shall be rounded (r0.30).
The assembly is generally connected with PCB by pasting double-sided adhesive tape on the back of silica gel, or long positioning column on a shell, and positioning hole is opened on silica gel to limit and assemble on a shell.

What should be paid attention to in the design and assembly of PMMA keys;

The design requirement is the same as that of PC. Generally, the surface of PMMA sheet should be hardened.

What should be paid attention to in the design of metal shell

When the metal shell is disassembled, it is generally 0.05mm lower than the large surface and 0.05mm lower in the Z direction.

Metal requires grounding, grounding generally with conductive foam, conductive cloth, spring, etc.

When the metal parts are buckled, the amount of fastening should not be too large, generally about 0.30.

When screw holes are made on metal parts, bottom holes shall be made first and then mechanical tapping shall be carried out.

The influence of process selection on ESD test?

Generally speaking, if there are hardware on the surface, poor grounding will affect ESD test.

If there are electroplated decorative parts on the surface, ESD test will be affected.

mechanical polishing

1、 Mechanical polishing on the surface of aluminum products:

Mechanical polishing process is: rough grinding, fine grinding, polishing, polishing, sandblasting, brushing or rolling, etc., according to the roughness of the surface to take different procedures.

2、 Chemical oil removal: formula and process conditions of chemical oil removal liquid:

Formula: sodium hydroxide 30-50g / L, industrial detergent 0.5-1ml / L, water 70-125g.
Process conditions: temperature: 50-60 ℃, time: 1-2min
Rinse with water after degreasing.
Chemical removal of oxide film: acid pickling is carried out to neutralize the residual alkali solution on the surface of the product, and the natural oxide film is removed to expose the aluminum and aluminum alloy matrix of the product. For the production of aluminum alloy containing silicon, the mixed solution must be used for acid pickling to remove the dark silicon floating ash on the surface.

Formula of pickling solution:

Concentrated nitrate solution 200 ~ 270ml / L

Temperature: room temperature time: 1-3min

The formula of pickling solution for removing oxide film and silica fume on the surface of aluminum alloy products containing silicon is as follows

3 volume of concentrated nitric acid and 1 volume of concentrated hydrofluoric acid.

Temperature: room temperature time: 5-15min

After chemical pickling, aluminum and aluminum alloy parts must be cleaned immediately with flowing warm water and cold water to remove residual acid, and then immersed in water for chemical polishing.

CNC cutting machine

At present, in the normal work of the CNC cutting machine used in the factory, the operator can successfully complete the cutting work by using the breakpoint continuous cutting function of the cutting machine. The method is as follows: when the operator wants to stop the work and save the breakpoint of the cutting site in the process of cutting, press the s key to stop when running to the idle section, and then press the Enter key to exit the cutting operation. At this time, the breakpoint data is memorized by the system. At this time, the operator should remember the coordinate values in X and Y directions, and then turn to the idle path operation to make the cutter move in the opposite direction until the coordinate values in X and Y directions are the same as the coordinate values memorized above, and the positive and negative signs are different (this position is the starting point of the program and also the origin of the steel plate). Next time when starting the machine, enter the letter C at the corner of the steel plate to continue to complete the last interrupted cutting operation.

According to the above operation method, the operator must first control the cutter to return to the starting point of the program (or the origin of the steel plate) in order to continue cutting in the next work. Obviously, if the cutting machine breaks down suddenly in normal operation (such as power failure, servo system failure, control computer crash, etc.), the operator cannot make it return to the starting point of the program (or the origin of the steel plate) because it has no power-off memory function.

In this case, our general practice is: after the cutting machine is repaired and restored to normal, the remaining steel plate is aligned, and then a new program is compiled by the programmer for cutting. In this way, some parts at the breakpoint are not cut completely, which makes them unable to be used, resulting in the waste of raw materials, increasing the production cost, and also bringing repetitive labor to the programmer, which leads to the reduction of production efficiency.

In the production practice, we successfully solve the problem of returning to the program starting point (or steel plate origin) after the cutting machine failure occurs by setting the zero point artificially. The specific operation method is as follows: first, operate the cutting machine to make the longitudinal sliding seat contact the measuring rod slowly, and compress the spring to make it deform properly. At this time, set the dial indicator head to “0”; then use the same method to set the dial indicator head of another zero setting device to “0”. No matter how many X and Y coordinates are displayed by the cutting machine, press Z key to make them all return to zero. This position is the zero point set by human for the cutting machine. Then operate the cutting machine according to the position of the steel plate to set the starting point of the program (or the origin of the steel plate). After setting, remember the coordinates of X and Y directions at this time. This coordinate value is the relative position between the zero point of the cutting machine and the starting point of the program (or the origin of the steel plate). When the fault occurs, first remove the fault, after the cutting machine returns to normal, use the above method to make it zero, and then use the coordinate value remembered to find the starting point of the program (or the origin of the steel plate). In this way, the breakpoint continuous cutting can be realized smoothly until the whole steel plate is cut.

Micro system EDM machining technology

In the field of microsystem technology, product development needs the common development of a variety of micro technologies. For example, it requires the joint efforts of micro mechanical parts, micro optical parts and micro electronic components. Micro digital cameras need the cooperation of the above micro technologies. The increase of product demand in the field of micro technology also leads to the increase of demand for micro machinery products, which puts forward higher requirements for the production and processing of mechanical products. Therefore, the production technology of micro products, the production and manufacturing of micro structural parts and the research and development of micro components are new areas that need to be strengthened urgently.

Because of the limited machinability, high precision and small manufacturing tolerance, the machining of high hardness micro parts is a very difficult task. In the industrial mass production, the main production process of micro structural parts is rolling, micro die casting, hot stamping and so on. In addition to grinding and laser cutting, EDM is also a feasible processing technology for micro structure parts with high wear resistance. Due to the corrosion characteristics of the electrode in micro machining system, EDM is widely used in micro parts. These two processing methods have nothing to do with the properties of the workpiece material, are not affected by the workpiece material, and almost do not produce cutting force, so they can produce very small structural parts.

The concept of micro EDM corrosion machining (abbreviated as μ – EDM) refers to the production and processing of micro workpieces and micro structural parts by means of EDM. It is an application of EDM in the field of micro machining in mechanical machining. In micro EDM, the contour size of the workpiece is obviously less than 1 mm. The difference between micro EDM and traditional EDM is first of all very small electrode. In addition, it is the electrical parameters and production process parameters suitable for micro workpiece and micro structural parts. Due to the small volume and size of the micro workpiece, all processing parameters can not exceed the limit of the thermal load of the workpiece. This means: to reduce the discharge energy, reduce the current density.

In addition to the standard motor with adjustable current, the modern EDM equipment is equipped with a relaxation oscillator, which can be used for micro EDM and surface precision machining. According to the discharge capacitance and distributed capacitance of the electrical conduction distance, the capacitive micro oscillator can generate very small discharge energy. 1 μ J / each discharge process; the minimum operating current is 0.1A; the shortest discharge duration is 50ns; and the discharge frequency reaches 10MHz. The minimum distance of spark can reach 15 μ M. At present, the minimum structure size that can be achieved by micro EDM is about 20 μ M. Japanese researchers have successfully machined holes with a diameter of 5 μ m on a steel sheet with a thickness of 10 μ M. When machining this kind of hole, they fully control the discharge capacitance of electrical wire and fixture system in the process of electric spark corrosion, making it less than 10PF. In micro EDM corrosion machining, the machine tool is only allowed to have very small discharge capacitance, such as using ceramic clamps and shielded electrical wires to reduce the discharge capacitance of the machine tool system; capacitors and circuit boards should also be strictly separated from the processing electrode in space.

Tool path generation method

The following is a brief introduction to the tool path generation method. NC tool path generation method for lines, surfaces and volumes

The technology started from 2D drawing, experienced the development stage of 3D wireframe, surface and solid modeling, and has been to the parametric feature modeling. NC machining is mainly driven by points and lines, such as hole processing, contour processing, plane area processing and so on. This kind of processing requires high level of operator and complex interaction. In the development stage of surface and solid modeling, solid based machining appears. The machining object of solid machining is an entity (generally represented by CSG and B-rep), which is obtained by some basic voxels through set operation (Union, intersection and difference operation). Solid machining can be used not only for rough machining and semi finishing machining of parts, but also for the research and development of feature-based NC programming system, which is the basis of feature-based machining.

Feature based NC tool path generation method

Parametric feature modeling has a certain period of development, but the research on Feature-based tool path generation method is just beginning. Feature processing makes NC programmers not to operate the low-level geometric information, but to directly carry out NC programming for the features that meet the habits of engineering and technical personnel, which greatly improves the programming efficiency.

W. In their research, r.mail and a.j.mcleod proposed a feature-based NC code generation subsystem. The working principle of this system is that each machining process of a part can be regarded as the sum of the machining of the shape feature group of the part. Then the whole shape feature or shape feature group is processed separately, then the part is finished. The NC code of each shape feature or shape feature group can be generated automatically. The developed system is only suitable for 2.5D parts processing.

Lee and Chang developed a tool path generation system for convex free-form surfaces by using virtual boundary method. The working principle of this system is that a minimum rectangle block is embedded in the convex free-form surface, so that the convex free-form surface feature is converted into a concave feature. The combination of the smallest rectangle and the final product model constitutes an indirect product model called virtual model. The tool path generation method is divided into three steps: (1) cutting polyhedron feature; (2) cutting freeform surface feature; (3) cutting intersection feature.

The basis of feature processing is solid processing, which can also be considered as higher-level entity processing. However, feature processing is different from solid processing, which has its own limitations. There are several differences between feature processing and solid machining

In concept, feature is the functional element of a part, which conforms to the operating habits of engineering and technical personnel and is well known by engineering and technical personnel; entity is a low-level geometric object, a geometric body obtained through a series of Boolean operations, without any functional semantic information; entity processing is often a one-time processing of the whole part (entity). But in fact, a part is not likely to be processed with only one knife at a time. It often needs to go through a series of steps such as rough machining, semi finishing and finishing. Different parts of parts are generally processed with different cutters; sometimes a part needs to be turned and milled. Therefore, solid machining is mainly used for rough machining and semi finishing of parts. Feature processing solves the above problems in essence, and feature processing has more intelligence. Certain fixed processing methods can be specified for specific features, especially those already specified in the step standard. If we have developed a specific processing method for all the standard features, we can imagine the convenience of machining those parts which are made up of standard features. If CAPP system can provide corresponding process characteristics, NCP system can greatly reduce interactive input and have more intelligence. However, these solid machining can not be realized; feature processing is helpful to lay a good foundation for the realization of CAD, CAPP, CIMS and even Concurrent Engineering (CE), while entity machining is powerless to these

High precision characteristics of computer gongs

From the development of precision machining to ultra precision machining, is the development direction of the world’s industrial powers. Its precision ranges from micron level to submicron level and even nanometer level (< 10nm).

At present, under the requirement of high machining precision, the machining accuracy of ordinary computer gongs has been increased from ± 10 μ m to ± 5 μ m; the machining accuracy of precision machining center has been increased from ± 3 ~ 5 μ m to ± 1 ~ 1.5 μ m, or even higher; the ultra precision machining accuracy has entered the nano level (0.001 μ m), the spindle rotation accuracy is required to reach 0.01 ~ 0.05 μ m, the roundness is 0.1 μ m, and the surface is machined Roughness Ra = 0.003 μ m, etc. These machine tools generally adopt vector control variable frequency drive motorized spindle (motor and spindle integration), the spindle radial runout is less than 2 μ m, the axial movement is less than 1 μ m, and the shafting imbalance reaches g0.4 level.

There are mainly two types of feed drive of high-speed and high-precision machining machine tools: “rotary servo motor plus precision high-speed ball screw” and “direct drive of linear motor”. In addition, the new parallel machine tool is also easy to realize high-speed feed.

Due to its mature technology and wide application, ball screw not only has a high precision (ISO 3408 grade 1), but also has a relatively low cost to realize high-speed, so it is still used by many high-speed machine tools. The current high-speed machining machine driven by ball screw has a maximum moving speed of 90m / min and an acceleration of 1.5g.

Ball screw belongs to mechanical transmission. In the transmission process, there are inevitably elastic deformation, friction and reverse clearance, which will cause motion lag and other nonlinear errors. In order to eliminate the influence of these errors on machining accuracy, linear motor direct drive was applied to machine tools in 1993. Because of the “zero transmission” without intermediate link, it has small inertia and rigid system High speed and acceleration can be achieved with high speed and fast response, and the stroke length is not limited theoretically. The positioning accuracy can also reach a higher level under the action of high-precision position feedback system. It is an ideal driving mode for high-speed and high-precision machining machine tools, especially for medium and large-scale machine tools. At present, the maximum moving speed of high-speed and high-precision machining machine tools using linear motor has reached 208 M / min and the acceleration is 2G, and there is still room for development.