Monthly Archive August 2020

Bychengcg

Conventional digital position control in CNC machining

1、 Conventional digital position control

At present, digital AC servo system is widely used in CNC machine tools. The basic composition of the position control system composed of mechanical parts processing is shown in Fig. 1. In the figure, the digital servo drive module and servo motor constitute a high-precision angle closed-loop servo system, whose input is the command pulse given by the CNC system, and the output is the motor angle. Under the closed-loop control with optical encoder as feedback, the rotation angle of motor shaft will strictly follow the command value. Computer gong processing | Dongguan computer gong processing | Shenzhen computer gong processing | mechanical parts processing | Guangzhou computer gong processing is driven by gear pair and screw nut pair, and the angular displacement of motor is converted into the required linear displacement of worktable.

It can be seen from the figure that although the digital servo system with angle as output is a closed-loop system, the position control system composed of it is an open-loop system (or semi closed-loop system) from the perspective of taking the displacement of the machine table as the final controlled quantity. Therefore, the position control accuracy is not only related to the performance of the control system, but also largely depends on the mechanical structure of the machine tool. In this way, the error of information transmission link, the error of mechanical transmission link and the influence of various non-linear factors in the system will make the table displacement deviate from the command value, and the open-loop system can not effectively correct it. Therefore, even if the conventional CNC machine and the machine use high-performance digital servo system, it is difficult to achieve high machining accuracy.

2、 Composition of digital full closed loop position control system

In order to solve the above problems, this paper proposes a digital full closed-loop position control method. Its basic idea is: on the basis of the digital drive of the moving parts of the machine tool, the digital measurement link which directly detects the final displacement of the moving parts is introduced to fully obtain and use the system information to count the motion of each coordinate of the machine tool from the perspective of feedback control The digital closed-loop system consists of various error sources and nonlinear links. This system can not only make the positioning accuracy of the moving parts determined by the measurement accuracy of the detection link, but also effectively dynamically correct the influence of various interference and nonlinear factors on the displacement of the moving parts, so that the actual displacement of the moving parts at any time can be achieved It strictly follows the change of the command value, so as to ensure the high dynamic and steady-state accuracy of the machine tool.

The basic composition of the digital closed-loop position control system is shown in Fig. 2. The system uses grating as the linear displacement detection device to directly obtain the displacement information of the machine tool table. After preprocessing, the two-way displacement pulse signals with phase difference of 90 degrees are obtained. The frequency is directly proportional to the displacement speed of the worktable, and the number is the actual displacement of the worktable divided by the pulse equivalent. The displacement pulse is sent to the reversible counter for counting, and the count value in the counter represents the current actual position of the worktable. The function of the position controller in the system is to control the operation of the whole system according to the difference between the given position value and the position feedback value according to the digital control law designed in advance, so as to ensure that the displacement of the worktable strictly follows the instruction value.

3、 Dynamic structure and controller design of the system

The dynamic structure of the system in Fig. 2 is shown in Fig. 3, which includes three parts: digital position controller, generalized object and feedback channel. The generalized object is composed of zero order holder, pulse generator, digital AC servo system and mechanical moving parts. Among them, the zero order holder plays a bridge style of connecting discrete link (digital controller) and continuous link. The task of pulse generator is to generate command pulse to control the operation of AC servo system according to the control signal given by the position controller, so this link is a proportional link.

AC servo system is a digital driving device in the system. From the macroscopic point of view, there is an integral relationship between the angle θ of servo motor and the command pulse frequency f, but the inertial characteristics between θ and F should be further considered from the microscopic point of view. The function of mechanical moving parts is to convert the rotation angle of the motor into the linear displacement of the worktable. If the influence of transmission error and nonlinear factors is treated as the dynamic disturbance to the system, the link can also be regarded as a proportion link. In this way, the transfer function of the generalized object can be expressed as a feedback channel after proper processing, although it includes detection device, pre-processing, information transfer, reversible counting and many other links, involving more complex information processing process.

However, from the point of view of taking the actual position of the worktable as the input and the position feedback value (the count value in the reversible counter multiplied by the pulse equivalent) as the output, the feedback channel can be regarded as a proportional link in computer gong processing, Dongguan computer gong processing, Shenzhen computer gong processing, mechanical parts processing and Guangzhou computer gong processing, and the transfer function GF (s) = l can be made by proper design. Considering that the generalized object is a third-order system with an integral link, in order to make the closed-loop system composed of it have fast dynamic performance and no steady-state error of tracking slope input, PID controller is used as position controller

Bychengcg

Selection of surface machining methods for CNC machining

1、 Selection of surface processing method selection of surface processing method selection of surface processing method selection of surface processing method:

The selection of surface processing scheme shall be based on the machining accuracy required for each surface of the part

According to the surface roughness and structural characteristics of parts, the corresponding processing methods and schemes are selected.

1. According to the technical requirements of the machined surface, the economical machining precision scheme should be adopted as far as possible.

2. According to the properties of workpiece material and heat treatment, select the corresponding processing method.

For example: quenching steel finishing to use grinding, computer gongs processing, Dongguan computer gongs processing, Shenzhen computer gongs processing, mechanical parts processing, Guangzhou computer gongs processing of non-ferrous metals, in order to avoid blocking the grinding wheel during grinding, adjust the cutting method such as fine turning or fine boring.

Consider the structure, switch and size of the workpiece.

4. Productivity and economy are considered in combination with production type.

5. Consider the existing equipment and technical conditions of the plant (or workshop).

2、 Division of processing stage division of processing stage division of processing stage division of processing stage:

When the machining quality of parts is high, the whole processing process should be divided into the following stages:

1. Rough machining stage rough machining stage rough machining stage rough machining stage

The main task is to remove most of the machining allowance, so how to obtain high productivity should be considered.

2. Semi finishing stage. Computer gong processing, mechanical parts processing, CNC machine tool processing, Dongguan high-speed computer gong processing, complete the secondary surface processing, and prepare for the finishing of the main surface.

3. Finishing stage finishing stage finishing stage finishing stage finishing stage finishing stage. Make the main surface meet the quality requirements specified in the drawing.

Finishing stage finishing stage finishing stage finishing stage finishing stage finishing stage finishing stage. In order to further improve the dimensional accuracy and reduce the surface roughness value, it is necessary to finish the surface with high quality requirements.

3、 Process centralization and decentralization process centralization and dispersion process centralization and dispersion: process centralization and process dispersion are two different principles for determining the number of operations when drawing up a process route.

4、 Arrangement of processing sequence arrangement of work sequence arrangement of work sequence: the machining process route of complex workpiece must go through cutting, heat treatment and auxiliary processes.

Therefore, when drawing up the technological route, the technologists should take the cutting, heat treatment and auxiliary processes into consideration.

5、 Selection of machine tool and process equipment selection of machine tool and process equipment selection of machine tool and process equipment

1. Selection of machine tool equipment selection of machine tool equipment selection of machine tool equipment selection of machine tool equipment.

2. Computer gong processing, mechanical parts processing, CNC machine tool processing, Dongguan high speed computer gong processing, process equipment selection, process equipment selection, process equipment selection.

Bychengcg

The importance of high speed CNC motorized spindle

After the machinists select a certain processing method, the CAD / CAM system can automatically output CNC code, such as computer gong processing, mechanical parts processing, CNC machine tool processing, Dongguan high-speed computer gong processing, which usually refers to G code, and input the code into the controller of CNC machine tool for actual processing operation. The process specification of large-scale computer gongs is one of the process documents which stipulate the machining process and operation method of parts. Under the specific production conditions, the more reasonable technological process and operation method are written into process documents according to the prescribed form, which are used to guide the production after being approved. Machining process specification generally includes the following contents: process route of workpiece processing, specific content of each process and equipment and process equipment used, inspection items and inspection methods of workpiece, cutting amount, time quota, etc. Large computer gongs processing technology is based on the process, changing the shape, size, relative position and nature of the production object to make it a finished product or semi-finished product. It is a detailed description of each process. For example, as mentioned above, rough machining may include blank manufacturing, grinding, etc., and finishing may be divided into turning, fitter, milling machine, etc., and each step is just like this There must be detailed data, such as how much roughness and tolerance should be achieved. The importance of high speed CNC motorized spindle

1. High speed CNC must first check the parameters of the input starting point, and it is strictly forbidden to knock or impact the rotor end of the spindle motor during operation. The operator is required to pay attention to the starting point of z-axis during tool setting to prevent misoperation. It is necessary to avoid the damage to the bearing of spindle motor caused by collision and knife binding during operation. Once there is knife binding phenomenon, stop the machine immediately, so as to prevent the bearing from being damaged by high-speed operation. Then, start running in from low speed, the time is slightly longer than normal running in time, until the motor runs smoothly and normally.

2. High speed CNC should ensure that the motor cooling circulation system of engraving and milling machine works normally before starting up and using. Computer gong processing, mechanical parts processing, CNC machine tool processing, Dongguan high speed computer gong processing, and then turn on the spindle motor. It is strictly prohibited to use the spindle motor without cooling. The working environment temperature should not be higher than 30 ℃. When the ambient temperature is higher than 30 ℃, air conditioner or refrigerator should be used for forced cooling of spindle motor.

3. According to the principle of running from low speed to high speed, the spindle motor should be preheated step by step. When the motor reaches the required speed, the no-load operation is stable, and the temperature is cooled, then the processing can be carried out. Only in this way can the better machining accuracy be ensured.

4. When planning the tool path, it is necessary to ensure that the force of the spindle motor is reasonable, and the engraving process should be carried out according to the cutter principle provided by the technical service personnel. It is necessary to prevent the vertical cutting tool and prohibit the spindle motor from overload operation, so as to ensure the normal service life of the spindle motor.

5. Generally, the spindle motor should be disassembled, cleaned and reassembled for use after running for about one year, so as to ensure the bearing accuracy and prolong the service life. In order to ensure that the spindle motor can work normally and well, the assembly and assembly must be carried out by experienced professional operators, such as computer gong processing, mechanical parts processing, CNC machine tool processing, Dongguan high-speed computer gong processing, clean environment, and the use of appropriate tools. Do not knock at random. For the temporarily unused motorized spindle, the residual water in the water chamber shall be blown out with compressed air, and the sleeve and shaft head shaft hole shall be oiled for rust prevention and placed in a cool and dry place.

6. The water quality of high-speed CNC must be clean, and the cooling water of spindle motor should be pure water. In order to prevent the occurrence of microorganisms and scale, the water should be changed every seven days under normal conditions. When the surrounding environment is higher than 30 ℃ and there is no forced refrigeration measures, the purified water should be replaced once a day at the highest water temperature. In addition, the operator should form the habit of observing the water temperature in the work.

Bychengcg

How to control the accuracy of CNC machining

1. Before processing, it is necessary to determine the processing technology, make clear the content of the process card, especially to determine the shape of the parts to be processed, and know the content of the next processing program.

2. The blank size should be measured before clamping, and the blank placement must be carefully checked to see if it is consistent with the programming.

3. After the rough machining of the blank is completed, maintenance should be carried out to determine whether the workpiece is correctly divided? Computer gong processing | Dongguan high-speed computer gong processing | Dongguan mold processing | Dongguan Mechanical parts processing | CNC processing whether there is dislocation loosening? Is the dimension from the processing part to the reference point meet the drawing requirements? After checking, the shape and scale of rough machining should be measured timely and accurately.

4. Semi finishing or finishing can only be carried out after rough machining and maintenance is completed. After CNC machining, technicians should check the shape and size of the processing parts. Computer gong processing | Dongguan computer gong processing | Shenzhen computer gong processing | mechanical parts processing | Guangzhou computer gong processing mainly check the vertical and inclined processing parts and the base point size marked on the measurement drawing,

When machining with CNC engraving and milling machine, try not to use the maximum cutting capacity of the cutting tool for processing. The cutting tool of each material will have a maximum cutting amount. because

Because this value is only a reference value for users, it can not exceed this value, so in actual operation, try to be less than this value and control it at the value

Two thirds of that would be ideal. In this way, the tool can be used for a long time, and the tool life will be longer.

In addition, CNC engraving and milling machine processing will be

It is divided into two parts: rough machining and finish machining. The first part of rough machining should be faster and better in overall speed and tool selection, and all surplus parts should be cut off as much as possible

In order to save time and improve work efficiency, finishing is to adjust the speed of CNC engraving and milling machine appropriately to ensure the fine processing of products.

And it is a kind of application technology constrained by its own economic strength. Computer gong processing, Dongguan high-speed computer gong processing, Dongguan mold processing, Dongguan Mechanical parts processing, CNC machining center selection, because the price is far more expensive than ordinary machine tools, so it is subject to more constraints, and whether the machine tool selection is reasonable or not is more prominent. Correct selection of machining center is the key to make good use of machining center and make it play an effective role.

Bychengcg

Development of precision casting

Compared with foreign casting machine tool industry, computer gong processing, Dongguan high-speed computer gong processing, Dongguan mold processing, Dongguan Mechanical parts processing, CNC processing, China’s casting machine tool industry is obviously backward in the production process level, which makes it has obvious deficiencies in the technical level and running speed of the core operating parts, the accuracy of finished products and the reliability of machine tools. It can be seen that the development of China’s foundry machine tool industry must start to actively seek new direction and concept.

Revolution becomes a new driving force in the field of casting machine tools

In order to meet the needs of industrial structure adjustment and low-carbon economy development and realize the scientific development of precision casting industry, the following work should be focused on:

1. To cultivate strategic emerging industries and modern production and service industries, so as to adapt to the transformation requirements of downstream areas. Computer gong processing, Dongguan computer gong processing, Shenzhen computer gong processing, mechanical parts processing, Guangzhou computer gong processing provide support for the stable and rapid development of China’s equipment production industry. Dongguan CNC processing provides the basis for the establishment of a modern casting industry system.

2. The network system of education and training in the field of precision casting has been constructed comprehensively. Various forms of school running can effectively cover the whole field of foundry education and training.

3. To help enterprises effectively use WTO and other international rules to deal with trade disputes such as anti-dumping and strive for the right of discourse in the international trade system.

4. Adhering to the principle of “go out, please come in”. On the basis of well running the high-level forum on China’s precision casting industry and the high-level forum on casting industry in mainland China, high-speed computer gongs processing in Dongguan, mold processing in Dongguan, mechanical parts processing in Dongguan, CNC processing, etc., will further improve the high-level forum of Asian casting industry and the high-level forum of BRICs casting industry, and actively participate in the world foundry industry Layer forum.

5. Substantive participation in international standardization work. During the 12th Five Year Plan period, we will submit several draft International Foundry specifications to the international standardization organization

Bychengcg

Summary of ISO knowledge in CNC machining

Summary of ISO knowledge

ISO9000 has several main characteristics, which can be summarized as “one essence and one center, two basic points; three characteristics, computer gong processing | Dongguan computer gong processing | Shenzhen computer gong processing | mechanical parts processing | Guangzhou computer gong processing | 4 general affairs and 4 products, 5 modules, 6 documents and 8 principles”. I simplify it to “112 344 568”

① One essence: speaking, writing and doing are consistent;

② One center: customer centered

③ Two basic points: customer satisfaction and continuous improvement

④ Three characteristics: suitability, sufficiency and effectiveness.

⑤ Four things: everyone is responsible for everything, there are rules to follow for everything, there is evidence for everything, and there is someone to supervise everything.

⑥ Four products: service, software, hardware and process materials.

⑦ Five modules: (1 total process, 4 major processes)

Quality management system; management responsibility; resource management; product realization; measurement, analysis and improvement

⑧ Six documents: the six procedure documents clearly put forward by ISO9000:2000 standard must be formulated

Document control procedures, quality record management procedures, internal audit procedures, computer gong processing | Dongguan computer gongs processing | Shenzhen computer gongs processing | mechanical parts processing | Guangzhou computer gongs processing | unqualified products control procedures, corrective measures control procedures, preventive measures control procedures.

⑨ Eight principles: customer centered, leadership role, CNC computer gong processing in Dongguan

Staff participation, process method, system management, continuous improvement, fact based, mutually beneficial relationship with suppliers.

10. Seven techniques of IE

① Program analysis;

② Time analysis;

③ Movement analysis

④ Pipeline analysis;

⑤ Dynamic analysis of crops;

⑥ Material analysis;

⑦ Environmental analysis

11. Types of common surface treatments:

① Grinding (polishing)

② Electroplating

③ Spraying / baking

④ Heat treatment

⑤ PVD (vacuum coating)

⑥ Sandblasting

⑦ Vehicle pattern

⑧ Scratch (wire drawing)

⑨ Oxidation (coloring)

⑩ Matte

Making black

(12) trimming

The flower of the flower`

12. Ten steps to RoHS compliance

The first step is to determine the correlation between enterprise products and ROHS directive. (if it is determined that the products produced by the enterprise are within the jurisdiction of RoHS, then go to the second step

Step 2: set up a company wide “compliance” team within the enterprise.

Step 3: establish RoHS compliance statement.

Step 4: establish the implementation plan of RoHS compliance within the enterprise.

The fifth step: evaluate the relevance between the enterprise’s supply chain and ROHS directive.

Bychengcg

CNC machining design basis

Design datum: the datum used to determine the position of other points, lines and planes on the part drawing, which is called design datum.

Process datum: the datum used in the process of machining and assembling parts, which is called process datum. According to different uses, process datum can be divided into assembly datum, measuring datum and locating datum.

(1) Assembly datum: the datum used to determine the position of parts in parts or products during assembly, which is called assembly datum.

(2) Measurement datum: the datum used to check the size and position of machined surface, which is called measuring datum.

(3) Positioning datum: the datum used for workpiece positioning during machining, which is called positioning datum. Computer gong processing, Dongguan high speed computer gong processing, Dongguan mold processing, Dongguan Mechanical parts processing, CNC machining as the positioning reference surface (or line, point), in the first process, only the rough surface can be selected. This positioning surface is called the rough datum. In the following processes, the machined surface can be used as the positioning reference, and the positioning surface is called the precise reference.

In the process of turning a blank into a finished product, the total thickness of the metal layer cut off on a machining surface is called the total machining allowance of the surface. The thickness of the metal layer cut by each process is called the machining allowance between processes. For the rotating surface such as outer circle and hole, the machining allowance is considered from the diameter, so it is called symmetrical allowance (i.e. bilateral allowance), that is, the actual thickness of the metal layer cut off is half of the machining allowance on the diameter. The machining allowance of the plane is the unilateral allowance, which is equal to the actual thickness of the metal layer removed. The purpose of leaving machining allowance on the workpiece is to remove the machining errors and surface defects left by the previous process, such as the surface cold and hard layer, air hole and sand inclusion layer on the casting surface, oxide scale, decarburization layer and surface crack on the forging surface, internal stress layer and surface roughness after cutting. The precision and surface roughness of the workpiece are improved. The size of machining allowance has great influence on machining quality and production efficiency. If the machining allowance is too large, it not only increases the labor of machining, reduces the productivity, but also increases the consumption of materials, tools and power, and increases the processing cost. If the machining allowance is too small, it can not eliminate all kinds of defects and errors in the previous process, and can not compensate for the clamping error in this process, resulting in waste products. The selection principle is to make the allowance as small as possible on the premise of ensuring the quality. Generally speaking, the more finish machining, the smaller the process allowance.

Applicability:

In the process of turning a blank into a finished product, the total thickness of the metal layer cut off on a machining surface is called the total machining allowance of the surface. The thickness of the metal layer cut by each process is called the machining allowance between processes. For the rotating surface such as outer circle and hole, the machining allowance is considered from the diameter, so it is called symmetrical allowance (i.e. bilateral allowance), that is, the actual thickness of the metal layer cut off is half of the machining allowance on the diameter. The machining allowance of the plane is the unilateral allowance, which is equal to the actual thickness of the metal layer removed. The purpose of leaving machining allowance on the workpiece is to remove the machining errors and surface defects left by the previous process, such as the surface cold and hard layer, air hole and sand inclusion layer on the casting surface, oxide scale, decarburization layer and surface crack on the forging surface, internal stress layer and surface roughness after cutting. Computer gong processing, Dongguan high speed computer gong processing, Dongguan mold processing, Dongguan Mechanical parts processing, CNC processing, so as to improve the accuracy and surface roughness of the workpiece. The size of machining allowance has great influence on machining quality and production efficiency. If the machining allowance is too large, it not only increases the labor of machining, reduces the productivity, but also increases the consumption of materials, tools and power, and increases the processing cost. If the machining allowance is too small, it can not eliminate all kinds of defects and errors in the previous process, and can not compensate for the clamping error in this process, resulting in waste products. The selection principle is to ensure the quality of the premise

Bychengcg

CNC machining method

It is the primary cause of deformation that the bearing is damaged most by using brute force and striking bearing directly with hammer.

If the equipment is not in place, the equipment is wrong or not installed in the bearing position, the bearing clearance is too small. The inner and outer rings are not in the same center of rotation, forming different concentricity.

Suggestion: select appropriate or professional bearing equipment, and use special instrument to test the equipment.

According to investigation, poor lubrication is one of the primary causes of bearing premature failure. The main reasons include: not filling lubricant or lubricating oil in time; not filling lubricant or lubricating oil in place; improper selection of lubricant or lubricating oil; incorrect lubrication method, etc. Or to consult and then use CNC computer gong processing equipment correctly

Suggestion: choose the right lubricant or lubricating oil, and use the correct lubricating filling method.

Pollution can also lead to premature failure of bearings. Pollution refers to dust and metal chips entering the bearing. The main reasons include: open the bearing package too early before use, causing pollution; the working environment of the equipment is not clean, causing pollution; the working environment of the bearing is not clean, and the working medium is polluted.

Stainless steel part 2

Suggestions: it is better not to unpack the bearing before use; keep the equipment environment clean and clean the bearing to be used; strengthen the sealing equipment of the bearing.

Fatigue damage is a common way of bearing damage. The common causes of fatigue damage may be: Bearing long-term overload operation; not timely repair; improper repair; equipment aging, etc.

Suggestion: select the appropriate bearing type and replace the tired bearing in time.

The processing center and CNC computer gong processing center are rarely introduced since the establishment of the station, because C computer gongs processing center in the market is relatively small, the user purchase center is also very few, generally choose the vertical machining center more users. The reason is that the vertical machining center occupies a small area and has high cost performance; while the computer gong processing center occupies a large area and its price is more expensive than that of the vertical machining center

Bychengcg

What parts are suitable for processing?

It is not possible to carry out fine processing for all materials. Some materials are too hard, and if they cross the hardness of the machined parts, the parts may be broken. Therefore, these materials are not suitable for fine machining, unless they are parts made of special data or laser cutting. So what are the requirements for the information of fine parts processing? Let’s understand it together.

The data about CNC lathe processing in Dongguan can be divided into two categories, metal data and non-metallic data.

For metal data, the hardness of stainless steel is the largest, followed by cast iron, followed by copper, and finally aluminum.

The processing of ceramics and plastics is attributed to the processing of non-metallic materials.

1. The first is the requirement for the hardness of the data. For some occasions, the higher the hardness of the data, the better. It is only limited to the hardness requirements of the machined parts. The processed data should not be too hard, and it is impossible to process if it is harder than the machine parts.

2. Secondly, the software and hardware of the data should be moderate, at least one level lower than the hardness of the machine parts. Together, we should also see what the function of the processed devices is, and select reasonable materials for the parts.

In a word, there are still some requirements for data in fine machining. Not all data are suitable for processing. For example, if the data are too soft or too hard, the former is not necessary to be processed, while the latter cannot be processed.

Stainless steel parts

Therefore, we must pay attention to the density of the data before processing. If the density is too high, it is equivalent to the hardness. If the hardness crosses the hardness of the machine part (lathe tool), it will not be processed. It will not only damage the parts, but also pose risks, such as turning tool flying out to hurt people. Therefore, generally speaking, regarding the mechanical processing, the data material should be lower than the hardness of the machine tool, so that the ability can be processed.

The automatic numerical control lathe is a kind of machine tool which uses the numerical control technology, the numerical control lathe processing completes the action according to the program which has been programmed in advance. It consists of program carrier, input equipment, CNC unit, servo system, position feedback system and mechanical parts of machine tool. CNC lathe parts processing basic knowledge, parts processing steps

In NC lathe parts processing parts generally go through the following steps:

1. According to the drawing and process plan of the processed parts, compile the program sheet with the specified code and program pattern, and record it on the carrier; do not want to work at the bottom,

2. Input the program on the program carrier into CNC unit through the input device;

3. After the input program is processed by CNC unit, the signal is announced to the servo system of each coordinate of the machine tool;

4. According to the signal announced by CNC unit, the servo system drives the moving parts of the machine tool and controls the auxiliary operation;

5. Through the mechanical parts of the machine tool to drive the relative movement of the cutter and the workpiece, the required workpiece is processed;

6. Detect the movement of CNC lathe and feed back to CNC unit through feedback equipment to reduce machining error. Of course, there is no detection and feedback system for the open-loop CNC lathe.