April 2020 – Page 2 – CNC Machining & Turning, Milling, Metal parts

The difference of open-loop, closed-loop and half closed-loop servo systems in CNC lathe machining

(1) NC machining open-loop: this kind of machine has no feedback information from the position sensor. The CNC system processes the part program, transmits the digital instruction to the servo system, and drives the machine to move. The structure of the CNC system is simple, economical and easy to care for, but the speed and precision are low, which is suitable for the small and medium-sized machine tools with low precision requirements, and it is mostly used for the CNC transformation of the old machine tools.

(2) NC machining closed-loop: this kind of machine tool is equipped with a position detection device, which directly measures the displacement of the workpiece; its precision is high, but the system design and adjustment are troublesome, the structure is complex, and the cost is high. It is mainly used in some boring and milling machines, ultra-fine lathes, ultra-fine milling machines, machining centers, etc. with high precision requirements.

(3) NC machining semi closed loop: this kind of NC machine selects the angle measuring element installed on the feed lead screw or the end of the electric motor to measure the rotation angle of the lead screw, to obtain the position feedback information tactfully; it can obtain the more desired accuracy and speed, and numerous NC machine tools choose it, such as the numerical control lathe, NC milling machine and machining center.

12 CNC processing experience summary

CNC machining, also known as CNC machining, refers to the machining with CNC machining tools. Because NC machining is controlled by computer after programming, CNC machining has the advantages of stable machining quality, high machining accuracy, high repetition accuracy, complex profile machining and high machining efficiency. In the actual processing process, human factors and operating experience, to a large extent, will affect the processing quality. Next, let’s take a look at 12 valuable experiences summed up by an old driver with ten years of CNC processing experience

1. How to divide CNC process

The division of CNC machining process can be generally carried out according to the following methods:

The method of tool centralized sequence is to divide the working procedure according to the tool used, and use the same tool CNC to finish all the parts that can be finished. Use the second knife and the third to finish other parts they can finish. In this way, the number of tool changes can be reduced, the empty travel time can be compressed, and the unnecessary positioning error can be reduced. For parts with a lot of CNC machining content, the machining part can be divided into several parts according to its structural characteristics, such as internal shape, shape, curved surface or plane, etc. Generally, plane and positioning surface are processed first, then hole; simple geometry is processed first, then complex geometry is processed; parts with lower precision are processed first, then parts with higher precision requirements are processed. For the parts that are easy to deform in CNC machining, the method of sorting by rough and fine CNC machining needs to calibrate the shape due to the possible deformation after rough machining, so generally speaking, the process should be separated for the parts that need rough and fine machining. To sum up, in the process division, it is necessary to flexibly grasp the structure and process of the parts, the function of the machine tool, the number of CNC machining contents of the parts, the installation times and the production organization of the unit. In addition, it is suggested that the principle of process concentration or process dispersion should be adopted, which should be determined according to the actual situation, but must be reasonable.

2. What principles should be followed in the arrangement of CNC processing sequence

The arrangement of the processing sequence should be considered according to the structure of the part and the condition of the blank, as well as the need of positioning and clamping. The key point is that the rigidity of the workpiece will not be damaged. Generally, the sequence shall be in accordance with the following principles:

The CNC machining of the previous process can not affect the positioning and clamping of the next process, and the machining process of universal machine tool inserted in the middle should also be considered comprehensively. First, process the internal cavity, then process the shape. It is connected by the same positioning and clamping method or the same tool CNC processing process to reduce the number of repeated positioning, tool change and moving of pressing plate. For multiple processes in the same installation, the process with small damage to workpiece rigidity shall be arranged first.

3. What should be paid attention to in the determination of workpiece clamping mode

The following three points should be paid attention to when determining the positioning datum and clamping scheme:

Strive to unify the design, process and programming calculation standards. Reduce the number of clamping as much as possible. After one positioning, all surfaces to be machined can be processed by CNC. Avoid manual adjustment of occupation plan. The fixture shall be smooth, and its positioning and clamping mechanism shall not affect the cutter (such as collision) in CNC machining. In such cases, it can be clamped by means of vise or screw extraction with base plate. 4. How to determine the tool setting point is reasonable? What is the relationship between workpiece coordinate system and programming coordinate system?

1. The tool setting point can be set on the part to be machined, but it must be the reference position or the part that has been finished. Sometimes the tool setting point is damaged by CNC machining after the first process, which will cause the second process and the subsequent tool setting point cannot be found. Therefore, when the first process is used for tool setting, it is necessary to set a relative pair at the place that has a relatively fixed dimension relationship with the positioning benchmark Knife position, so that the original tool setting point can be retrieved according to the relative position relationship between them. This relative tool setting position is usually set on the working table or fixture of the machine tool. The selection principles are as follows:

Alignment is easy. Easy to program. The tool setting error is small. It is easy to check during processing.

2. The origin position of the workpiece coordinate system is set by the operator himself. After the workpiece is clamped, it is determined by tool setting, which reflects the distance position relationship between the workpiece and the zero point of the machine tool. Once the workpiece coordinate system is fixed, it is generally not changed. The workpiece coordinate system and the programming coordinate system must be unified, that is, when machining, the workpiece coordinate system and the programming coordinate system are the same.

4. How to choose the cutting route

Tool path refers to the path and direction of the tool relative to the workpiece in the process of NC machining. The reasonable choice of machining route is very important, because it is closely related to the CNC machining accuracy and surface quality of parts. The following points are mainly considered in determining the tool path:

Ensure the machining accuracy of parts. It is convenient for numerical calculation and reduces programming workload. In order to improve the efficiency of CNC machining, it is necessary to find a short CNC machining route and reduce the time of blank cutting. Minimize the number of program segments. To meet the requirements of the surface roughness after CNC machining, the contour should be processed by a continuous cutter. The advance and retreat (cut in and cut out) routes of the tool should also be carefully considered to minimize the tool marks caused by tool stop (elastic deformation caused by sudden change of cutting force) at the contour, and avoid cutting vertically on the contour surface and scratching the workpiece.

How to determine NC machining route in CNC machining

Numerical control skill of numerical control lathe, referred to as numerical control, refers to the skill of using the method of digital control to automatically control the completion of a certain course schedule. It usually controls the direction, viewpoint, speed and other mechanical quantities and the switch quantities related to the flow direction of mechanical energy. The origin of CNC lathe is the origin of machine coordinate system. For a specific machine tool, the origin of the machine tool is constant, which is a physical orientation of the machine tool manufacturer on the machine tool.

The key mission of NC lathe process planning is to determine the detailed processing content, cutting amount, process equipment, positioning device method and tool moving track of the process, so as to prepare for the programming.

The setting of the machining path of CNC lathe is a very important link. The machining path is the moving path of the cutter point related to the workpiece in the machining progress. It not only includes the content of the machining process, but also reflects the organization of the machining sequence. Therefore, the machining path is an important evidence for the preparation of the machining program.

The processing route shall ensure the accuracy and appearance roughness of the workpiece to be processed. Planning the processing route should reduce the time of empty travel and improve the processing power. Simplify the numerical calculation and reduce the program section, and reduce the programming workload. According to the form of workpiece, rigidity, machining allowance and rigidity of machine tool system, the number of cycle machining can be determined.

Correct selection of the down milling or reverse milling method in milling. Generally speaking, CNC lathe uses ball screw, so the leisure time is very small, so the advantages of forward milling are more than reverse milling. CNC lathe machining in CNC lathe has not yet reached the conditions of universal application, usually we should put too much allowance on the blank, the unique is that the allowance structure containing forging and casting hard skin layer is processed on the ordinary lathe. If must use numerical control lathe processing, must pay attention to the vivid organization of the program.

Plastic guide rail of CNC lathe

The plastic guide rail of CNC lathe is a kind of plastic soft belt with the same static and dynamic conflict coefficient on the sliding guide rail matching with the bed guide rail, which is wear-resistant and vibration absorbing. Maybe the plastic guide rail is made by injection molding between the fixed and dynamic guide rails. This kind of plastic guide rail has excellent conflict characteristics, wear resistance and vibration absorption, so it is commonly used in CNC lathe.

The plastic soft belt is based on polytetrafluoroethylene, with bronze powder, molybdenum disulfide, graphite and other fillers added, sintered and made into a soft belt. China has already made a TSF soft belt for guide rail, and a matching DJ adhesive. The process of using the guide rail soft belt is simple, as long as the surface roughness of the guide rail paste surface is semi-finished to ra1.6-3.2um, after cleaning the paste surface, the adhesive is used for bonding, after pressure curing, and then the finish machining can be carried out. Because this kind of guide rail soft belt adopts the bonding method, it is usually called “stick plastic guide rail”.

The raw materials for guide rail injection molding are epoxy resin and molybdenum disulfide as the matrix, plasticizer is added, and the mixture is a two-component plastic with paste shape as one component and curing agent as another component. The Chinese brand is hNT. The injection molding process of the guide rail is simple. After adjusting the mutual position accuracy between the constant guide rail and the movable guide rail, the two-component plastic is injected. After curing, the fixed and movable guide rails are separated into plastic guide rails. The plastic guide rails manufactured in this way are customarily called “injection molding guide rails”

Factors to be considered in the selection of surface machining methods for CNC workpieces

The machining method of CNC workpiece surface depends on the technical requirements of machining surface. However, it should be noted that these technical requirements are not necessarily the requirements of the regulations of the part drawing, and may occasionally be higher than the requirements of the part drawing in some aspects due to technological reasons. For example, the machining requirements for the surface of some CNC machining parts are raised due to the misalignment of datum. Or higher machining requirements may be put forward because it is used as a precision reference.

After understanding the technical requirements of each CNC workpiece surface, we can choose the final processing method that can guarantee the requirements, and we will definitely need several processing steps and processing methods of each step. The selected machining method of CNC machining parts should meet the requirements of parts quality, excellent machining economy and high manufacturing efficiency. For this reason, the following factors should be studied when choosing the processing method:

1. There is an equal boundary between the machining accuracy and the surface roughness of any CNC machining method, but only in a narrow boundary is economic. The machining accuracy of this boundary is economic machining accuracy. Therefore, when choosing the processing method, we should choose the corresponding processing method which can obtain the economic processing precision.

2. The nature of raw materials for CNC machining should be studied.

3. To study the structure and size of CNC parts.

4. To study the production rate and economic requirements. When making a large number of products, we should choose advanced technology with high efficiency. It is even better to fundamentally change the manufacturing mode of blank, which can reduce the workload of machining.

5. To study the existing equipment and technical conditions of the workshop or workshop, we should fully defraud the existing equipment, tap the potential of the enterprise, and develop the enthusiasm and creativity of the workers when choosing the processing mode. But it is also necessary to study and constantly improve the existing processing methods and equipment, select new technology and improve the level of technology.

Precautions in the processing of CNC lathe

Precautions in the processing of CNC lathe. The machining process of CNC lathe is the same as that of ordinary lathe. However, since CNC is a clamping process, all the turning processes are finished continuously and automatically, the following aspects should be paid attention to.

1. Correct selection of cutting parameters

In terms of high efficiency metal cutting, the raw materials to be processed, cutting equipment and cutting conditions are three major factors. These decisions determine machining time, tool life and machining quality. The economic and useful machining method must be the correct choice of cutting conditions.

Three factors of cutting conditions: cutting speed, feed rate and cutting depth directly lead to tool damage. With the increase of cutting speed, the temperature of tool tip will rise, and mechanical, chemical and thermal wear will occur. If the cutting speed increases, the tool life will be reduced by 1 / 2.

The relationship between the feed condition and the wear of the back edge of the tool occurs within a very small boundary. But the feed rate is large, the cutting temperature rises, and the rear edge wear is large. It has less influence on the cutting tool than the cutting speed. Although the influence of cutting depth on the tool is not as high as the cutting speed and feed rate, when the cutting depth is small, the hard layer of the material to be cut will also affect the tool life.

Users should choose the cutting speed according to the raw material, hardness, cutting shape, raw material variety, feed rate, cutting depth, etc. The following table:

The selection of suitable processing conditions is based on these factors. Regular and stable wear and tear is the condition of dream.

However, in the essential work, the choice of tool life is related to tool wear, dimension change, surface quality, cutting noise, machining heat, etc. When confirming the processing conditions, the requirements are discussed according to the nature. As for stainless steel and heat-resistant alloy and other hard to process raw materials, it is advisable to choose a coolant or a rigid blade.

2. Select tools correctly

1) During rough turning, the cutter with high strength and good durability shall be selected to meet the requirements of large back draft and large feed during rough turning.

2) When finishing, the cutting tools with high accuracy and good durability shall be selected to ensure the requirements of machining accuracy.

3) In order to reduce the time of tool change and make the tool setting easier, we should choose the clip knife and clip knife.

CNC lathe

3. Select fixture correctly

1) Although the general fixture is used to clamp the workpiece, the special fixture shall not be used;

2） The positioning datum of parts are overlapped to reduce the positioning deviation.

4. Confirm the processing route

Machining route refers to the moving track and target of the tool relative to the part in the process of NC machining.

1) It shall be able to meet the requirements of machining accuracy and surface roughness;

2) It is necessary to reduce the processing route and the time of tool empty travel.

5. Correlation between machining route and machining allowance

At present, under the condition that the CNC lathe has not been widely used, it is usually necessary to place too much allowance on the blank, especially the allowance containing forging and casting hard skin layer on the ordinary lathe for processing. If the CNC lathe must be used for machining, it is necessary to pay attention to the sensitive placement of the program.

6. Key points of fixture installation

At present, the connection between hydraulic chuck and hydraulic clamping cylinder is completed by pull rod. The key points of hydraulic chuck clamping are as follows: first, remove the nut on the hydraulic cylinder with the handle, remove the pull tube, and pull it out from the rear end of the main shaft, and then remove the constant screw of the chuck with the handle to remove the chuck.

The polishing edge on the tool refers to a small section of the edge which is parallel to the tool tip milled by the target of the deviation angle of the back edge pair of the tool edge, which is mainly used for the second cutting after the cutting of the tool edge, which is equivalent to the direction burr in the finishing process. The goal is to increase the surface roughness of the workpiece, which is mostly used for the tool of finishing.

CNC processing technology of mobile phone metal shell

CNC machining technology of mobile phone metal shell. In this era of highly valued user experience, more and more manufacturers of mobile phones, laptops and wearable devices start to work on the shell materials. Only from the perspective of mobile phone boundary, mobile phone shell material is also gradually transiting from engineering plastic to metal.

There have been many combination methods in the evolution time of mobile phone shell materials: plastic and metal, glass and metal, all plastic, all glass, etc., but they did not cover the trend of all metal materials. In terms of the essential touch experience, the all metal mobile phone has an outstanding experience in many aspects. It is better than other materials in appearance and feel.

CNC machining

All manufacturers offer all metal shell products

When it comes to all metals, it comes to CNC. At present, all metal shells of 3C products are basically processed by computer numerical control machine tools (CNC). CNC has become a necessary equipment for 3C shell manufacturers because of its advantages of high efficiency, high precision and stable processing quality.

All metal one-piece CNC machining process was initiated by Apple company as early as possible – aluminum plate is die-casting from solid columnar aluminum material. After careful machining, it is cut into the prototype of one-piece fuselage. With the fuselage gradually forming, the keyboard form and various fine structures on the fuselage are milled out. This process includes nine CNC milling processes, after which a well-organized integrated shell can be obtained.

Shell manufacturers need many processes from programming to obtaining molded articles, such as rough machining, semi rough machining, semi finish machining, and finish machining. In many times, the whole process requires more than 10 stations to obtain molded articles. In order to increase the rate of high-quality products, each process needs strict control.

Meizu mx5 obtains shaped articles through 12 processes from right to left

One

Modeling and programming of CNC machining sequence

Before CNC machining, the requirement modeling and programming are the first step. The difficulty of 3D modeling is determined by the product structure. The product modeling with complex structure is more difficult. Programming includes working procedure setting, tool selection, rotation speed setting, interval of each tool feeding and so on. For the rest, the clamping methods of different products are different. Before processing, the clamps should be designed well, and some products with complex structure need special clamps.

Programming includes the processing technology of all products. Although CNC automation is completed in the future, early programming must be operated by experienced staff to prevent repeated trial and error and high cost.

Two

Key processing technology

Using high-speed drilling and tapping center, the aluminum plate is precisely milled into regular three-dimensional volume of certain size, and the next process is planned.

Rough milling: after positioning, rough milling out the inner cavity structure, the positioning column integrated with the fixture, and milling out most of the external redundant raw materials.

Milling the antenna slot: it is difficult to deal with the information problem of the all metal shell. It is necessary to mill out the antenna slot to leave a transmission path for the information and maintain the necessary connection points to ensure the strength and overall sense of the fuselage.

CNC finishing: Finish Milling inner cavity, outer structure, outer surface, side, etc.

Polishing: use high-speed and meticulous CNC machine tool to polish, eliminate knife marks, and plan for subsequent sandblasting.

Sandblasting: it can treat the metal surface into abrasive effect.

Anodizing:

Primary anode: color the mobile phone. Anodizing turns aluminum into gold. At the same time, improve the stability of surface materials

Secondary anode: solid and dense oxide film is formed on the surface of the body, and the wear resistance is further strengthened.

Plastic shell molding using injection molding technology, only need to open mold can be mass produced, yield is very high. While the CNC machining yield of all metal fuselage is determined by the complexity of the shell structure and the accuracy requirements. It is difficult to reach the high yield of plastic shell. In order to average high cost, most manufacturing enterprises will choose to purchase a large number of CNC equipment to reduce the comprehensive cost and reach the production rate in the face of value war.

Application of fixture

The fixture is a process device used to fasten the workpiece quickly and maintain its precise relative position. That is to say, fixture is an indispensable part of machining. With the development of technology to the goal of high speed, high efficiency, thoughtfulness, integration, intelligence and environmental protection, fixture technology is developing towards the goal of high precision, high efficiency, module, combination, generality and economy.

Fixture is also called fixture. In a broad sense, any process in the process, which is used to install workpiece quickly, simply and safely, can be called fixture. For example, welding fixture, inspection fixture, equipped fixture, machine tool fixture, etc. Often referred to as fixture.

When processing the workpiece, in order to make the surface of the workpiece reach the technical requirements such as the size, geometric form and the mutual position accuracy with other surfaces in the drawing regulations, the workpiece must be installed (positioned) and clamped (clamped) before processing.

The use of jigs is conducive to ensuring the processing accuracy, safe and stable production of the workpiece; to increasing the production rate and reducing the cost; to improving the working conditions of the workers and ensuring the safe production; to enlarging the process boundary and completing the “one machine and multi use”.

The fixture is usually composed of positioning elements (to confirm the exact position of workpiece in the fixture), clamping device and indexing device, so that the workpiece can be processed in several positions in one installation, including two types of rotary indexing device and linear moving indexing device, connecting elements and clamp details (fixture base), etc.

In order to improve the production efficiency and product quality, in the middle and back section of the production, the fixture is often used for performance test or auxiliary equipment (can be equipped with constant appearance and height, etc.). The essence is widely used in the manufacturing plant, because the right and wrong mark is customized, so there is only something unexpected but not impossible

Maintenance rules of grinding machine for precision parts processing equipment

Grinding machine is one of the basic equipment used in precision parts processing, its shape will also have a positive impact on the processing quality, so if you want to improve the precision of precision parts processing, you must carry out regular inspection and care for the grinding machine, so that it will always maintain a good shape.

First of all, we need to know that as a precision parts processing equipment, the grinder should be specially assigned to be in charge of maintenance and application. When its homework is completed, it is necessary to clean all parts of the machine, especially the sliding parts, and then apply oil to maintain excellent smoothness. At the same time, it is also necessary to eliminate the grinding debris at all parts of the grinder, so as to avoid adverse effects on the processing process. Some parts are still subject to rust prevention treatment,

Before using the grinder, don’t forget to correct the grinding wheel average, and choose the grinding wheel carefully according to the material and hardness of the work materials, which is one of the key factors affecting the processing efficiency of precision parts. Don’t forget to pay attention to the rotation target of the grinder spindle, prevent the use of air to clean the work materials and machinery, and keep the dust collection pipeline clean, otherwise it will lead to burning.

Secondly, we pay attention to the suction cup of the grinding machine for precision parts processing equipment, whether it is the permanent magnetic casting suction cup or the electromagnetic suction cup, and the disk surface is the basis of whether the grinding precision of the working object can be dream, so we must properly and precisely care for and maintain it.

If the precision of the work object is poor or the disk surface is damaged, the disk surface must be grinded again. Only when the precision of the disk surface of the suction cup of the grinder meets the requirements, can the precision of the precision parts be ensured. In addition, there is a smooth system of conditioning, but it is nothing more than the replacement of smooth oil and wash filter.

CNC machining hardware parts suitable for processing

1. For the content of hardware parts suitable for NC machining, the following sequence research can be carried out during selection:

(1) The content that cannot be processed by general machine tool shall be the priority;

(2) General machine tools are difficult to process and their quality is difficult to guarantee;

(3) General machine tools have low machining efficiency and high manual operation intensity, which can be selected when there is abundant machining strength in CNC machine tools.

2. Hardware parts not suitable for NC machining

(1) It takes a long time to adjust. If the first fine datum is located and processed with the rough datum of the blank, the content of special tooling coordination is required;

(2) Processing location distribution, need to install and configure the origin repeatedly. At this time, it is very difficult to select CNC machining, and the effect is not obvious, so general machine tools can be placed for supplementary machining;

(3) The appearance of the profile machined according to certain manufacturing documents (such as sample plate, etc.). The key reason is that it is difficult to obtain data, easy to conflict with checking credentials, which increases the difficulty of programming.