Basic Knowledge of Machining – YIJIN

There are many ways of machining. Generally speaking, there are cnc turning, CNC milling, and CNC turning and milling. If you want to subdivide it, there are very, very many types. Today, we will first learn about them: part of basic knowledge of machining

Basic Knowledge of Machining: Datum

Parts are composed of several surfaces, and each surface has a certain size and mutual position requirements. The requirement of relative position between parts surfaces includes two aspects: The distance dimensional accuracy and relative position accuracy (such as coaxial, parallelism, perpendicularity and circular runout, etc.) between surfaces are required. The study of the relative position relation between the surfaces of parts is inseparable from the datum, and the position of the surfaces of parts cannot be determined without a definite datum. In its general sense, the datum is the point, line, and surface on the part that is used to determine the position of other points, lines, and surfaces. Datum can be divided into design datum and process datum according to their different roles.

Basic Knowledge of Machining: Datum

1. Design Datum

The datum used to determine other points, lines, and surfaces on the part drawing is called the design datum. In the case of pistons, the design datum refers to the centerline of the piston and the centerline of the pinhole.

2. Process Datum

The datum used by the parts in the process of machining and assembly is called the processed datum. According to different uses, process datum is divided into positioning datum, measuring datum, and assembly datum.

1)Datum Reference: The datum used to make the workpiece occupy the correct position in the machine tool or fixture during processing is called the positioning datum. According to the different positioning components, the most commonly used are the following two categories:

Automatic Centering Positioning: Such as three-jaw chuck positioning.

Positioning Sleeve Positioning: The positioning element is made into a positioning sleeve, such as the positioning of the spigot plate.

Others include positioning in a V-shaped frame, positioning in a semicircular hole, etc.

2)Measuring Datum: The datum used to measure the size and position of machined surfaces during parts inspection is called the measurement datum.

3)Assembly Reference: The datum used to determine the position of parts in components or products during assembly is called the assembly datum.

Basic Knowledge of Machining: Workpiece Installation Methods

In order to process a surface that meets the specified technical requirements on a certain part of the workpiece, the workpiece must occupy a correct position relative to the tool on the machine tool before machining. This process is often referred to as the “positioning” of the workpiece. Due to the role of cutting force, gravity, and so on in the processing, it should also use a certain mechanism to “clamp” the workpiece, so that its determined position remains unchanged. The process of getting the workpiece in the correct position on the machine and clamping the workpiece is called “setup”.

Using the Principle of Workpiece Positioning to Analyze Part Position Tolerance Marking

Using the Principle of Workpiece Positioning to Analyze Part Position Tolerance Marking

Workpiece installation is an important problem in machining, which not only directly affects the machining accuracy, workpiece installation speed, and stability, but also affects the level of productivity. In order to ensure the accuracy of the relative position between the machined surface and its design datum, the workpiece should be installed so that the design datum of the machined surface occupies a correct position with respect to the machine tool. For example, in the process of finishing turned ring grooves, in order to ensure the requirements of the circular run out of the bottom diameter of the ring groove and the axis of the skirt, the workpiece must be installed so that its design datum coincides with the axis of the machine tool spindle.

When machining parts on a variety of different machine tools, there are various installation methods. The installation methods can be classified into three types: direct alignment method(Alignment is to use a scribing tool to check or correct the relevant unmachined surface on the workpiece, or to make the relevant surface and the reference surface in a suitable position.), scribe alignment method and fixture installation method.

YIJIN Hardware engineer

YIJIN Hardware engineer

1)When using the method of direct alignment, the correct position that the workpiece should occupy on the machine tool is obtained through a series of attempts. The specific way is to directly install the workpiece on the machine tool, using a dial indicator or the needle on the needle plate, to visual correction of the correct position of the workpiece, while checking and alignment, until it meets the requirements.

The accuracy and speed of the direct alignment method depend on the accuracy, method, tool, and worker’s skill level. Its disadvantage is that it takes a lot of time, has low productivity, it needs to be operated by experience, and it requires high skills for workers, so it is only used in single-piece and small-batch production. For example, reliance on imitating body alignment is a direct alignment method.

2)Marking alignment method is a method of correcting the workpiece according to the line drawn on the blank or semi-finished product with a marking needle on the machine tool so that it can obtain the correct position. Obviously, this method requires one more marking procedure. The drawn line itself has a certain width, and there is ascribing error when scribing, and there is an observation error when correcting the position of the workpiece. Therefore, this method is mostly used for small production batches, low blank accuracy, and rough machining of large workpieces. For example, the determination of the position of the pinhole of the two-stroke product is to use the scribing method of the indexing head to align it.

3)Using fixture installation method: The process equipment used to clamp the workpiece so that it occupies the correct position is called a machine tool fixture. The fixture is an additional device of the machine tool. Its position relative to the tool on the machine tool has been adjusted in advance before the workpiece is installed, so it is not necessary to align the positioning one by one when processing a batch of workpieces, which can ensure the technical requirements of processing. It is an efficient positioning method that saves labor and trouble and is widely used in batch and mass production. Nowadays, piston processing is the use of fixture installation methods.

①After the workpiece is positioned, the operation of keeping the positioning position unchanged during processing is called clamping. The device in the fixture that keeps the workpiece in the same position during processing is called the clamping device.

②The clamping device should meet the following requirements: When clamping, the positioning of the workpiece should not be damaged; After clamping, it should be ensured that the position of the workpiece does not change during processing, and the clamping is accurate, safe, and reliable; The clamping action is fast, the operation is convenient and labor-saving; The structure is simple and the manufacture is easy.

③Precautions when clamping: The size of the clamping force should be appropriate. If it is too large, the workpiece will be deformed. If it is too small, the workpiece will be displaced during the processing and will destroy the positioning of the workpiece.

A welding clamping device for mechanical parts

A welding clamping device for mechanical parts

Basic Knowledge Of Machining: Metal Cutting knowledge

1. Turning Movement and Formed Surface

Turning movement: In the cutting process, in order to remove excess metal, it is necessary to make the workpiece and the tool perform the relative cutting motion. The motion of removing excess metal on the workpiece with a turning tool on a lathe is called turning motion, which can be divided into main motion and feed motion.

The cutting part of a tool

The cutting part of a tool

Main movement: The movement of cutting off the cutting layer on the workpiece directly and turning it into chips to form a new surface of the workpiece is called the main movement. When cutting, the rotational motion of the workpiece is the main motion. Usually, the speed of the main movement is higher, and the cutting power consumed is higher.

Feed movement: The new cutting layer is continuously put into the cutting motion, and the feeding motion is the motion along the surface of the workpiece to be formed, which can be continuous motion or intermittent motion. For example, the movement of the turning tool on the horizontal lathe is continuous, and the feed movement of the workpiece on the planer is intermittent movement.

Workpiece formed surface: In the cutting process, machined surface, machining surface and to be machined surface are formed on the workpiece. A finished surface is a new surface formed by removing excess metal. Machined surface refers to the surface being turned by the cutting edge of the turning tool.

2. The Three Elements of Cutting Amount Refer to the Depth of Cut, the Amount of Feed, and The Cutting Speed

1)Cutting depth: ap=(dw-dm)/2(mm), dw=Unmachined workpiece diameter, dm=Machined workpiece diameter, the depth of cut is also what we usually call the amount of cutting.

Selection of cutting depth: Cutting depth αp should be determined according to machining allowance. When roughing, except for the margin-left for finishing, all roughing margins should be removed as far as possible at one time. This can not only increase the product of cutting depth, feed ƒ, and cutting speed V on the premise of ensuring certain durability, but also reduce the number of cutting trips. When the machining allowance is too large or the process system stiffness is insufficient or the blade strength is insufficient, the cutter should be divided more than two times. At this time, the cutting depth of the first pass should be larger, which can account for 2/3 to 3/4 of the total allowance; In order to obtain smaller surface roughness parameters and higher machining accuracy, the second cutting depth is smaller.

When cutting the casting, forging, or stainless steel with hard skin on the surface of the cutting parts, the cutting depth should exceed the hardness or cold hard layer, so as to avoid cutting edge on the hard skin or cold hard layer.

2)Choice of feed amount: The relative displacement of the workpiece and the tool in the direction of the feed movement when the workpiece or tool rotates once or reciprocates once, in mm. After the cutting depth is selected, the larger feed should be further selected as far as possible. The reasonable value of the feed should be selected to ensure that machine tools and cutting tools will not be damaged due to too much cutting force, the workpiece deflection caused by cutting force will not exceed the allowable value of workpiece precision, and the surface roughness parameter value will not be too large. When roughing, the main limit of feed is cutting force, and in semi-finishing and finishing, the main limit of feed is surface roughness.

3)Selection of cutting speed: During cutting, the instantaneous speed of a point on the cutting edge of the tool relative to the surface is to be machined in the main direction of motion, in m/min. When cutting depth αp and feed ƒ are selected and the maximum cutting speed is selected on the basis of these, the development direction of cutting is high-speed cutting.

Hole machining of necessary knowledge points for cutting tools

Hole machining of necessary knowledge points for cutting tools

Roughness Mechanical Concept

In mechanics, roughness refers to the microscopic geometrical properties consisting of small spacings and peaks and valleys on a machined surface. It is one of the problems of interchangeability research. Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the surface of the part during processing, the plastic deformation of the surface metal when the chips are separated, and the high-frequency vibration in the processing system. Due to the different processing methods and workpiece materials, the depth, density, shape, and texture of the traces left on the processed surface are different. Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration, and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products.

1. Roughness Representation

After processing, the surface of the parts looks smooth, but under magnification, it is uneven. Surface roughness refers to the micro-geometric features composed of small distances and tiny peaks and valleys on the surface of the processed part, which are generally formed by the processing method and (or) other factors. The required surface roughness parameters are also different for different functions of parts. The surface roughness code shall be marked on the part drawing to indicate the surface characteristics to be achieved after completion of the surface. There are 3 types of surface roughness height parameters:

  • The arithmetic means deviation Ra of the contour is the arithmetic mean of the absolute value of the distance between the points on the contour along the measurement direction (Y direction) and the reference line within the sampling length.
  • The ten-point height Rz of micro-roughness refers to the sum of the average value of the 5 largest profile peak heights and the 5 largest profile valley depths within the sampling length.
  • The maximum height Ry of the contour is the distance between the top line of the highest peak and the bottom line of the lowest valley within the sampling length. At present, Ra is mainly used in the general machinery manufacturing industry.

2. The Effect of Roughness on the Performance of Parts

The surface quality of the workpiece after processing directly affects the physical, chemical, and mechanical properties of the workpiece. The working performance, reliability, and service life of the product depend to a large extent on the surface quality of the main parts. Generally speaking, the surface quality requirements of important or critical parts are higher than ordinary parts, because parts with good surface quality will greatly improve their wear resistance, corrosion resistance, and fatigue damage resistance.

3. Cutting Fluid

1)The Role of Cutting Fluid

Cooling Effect: The cutting heat can take away a large amount of cutting heat, improve the heat dissipation conditions, and reduce the temperature of the tool and the workpiece, thereby prolonging the service life of the tool and preventing the dimensional error of the workpiece due to thermal deformation.

Lubrication Function: The cutting fluid can penetrate the workpiece and the tool so that a thin layer of adsorption film is formed in the tiny gap between the chip and the tool, which reduces the friction coefficient, so can reduce the friction between the tool chip and the workpiece. Reduce cutting force and cutting heat, reduce tool wear and improve the surface quality of the workpiece, for finishing, lubrication is especially important.

Cleaning Effect: The tiny chips generated during the cleaning process are easy to adhere to the workpiece and the tool, especially when drilling deep holes and reaming holes, the chips are easily blocked in the chip flute, which affects the surface roughness of the workpiece and the service life of the tool. The use of cutting fluid can quickly wash away the chips so that the cutting can be carried out smoothly.

2)Category: There are two main categories of commonly used cutting fluids

Emulsion: It mainly plays a cooling role. The emulsion is made by diluting the emulsified oil with 15~20 times of water. This kind of cutting fluid has large specific heat, low viscosity, and good fluidity, and can absorb a lot of heat. It is to cool the tool and workpiece, improve tool life and reduce thermal deformation. There is more water in the emulsion, and the lubrication and rust prevention function is poor.

Cutting Oil: The main component of cutting oil is mineral oil. This kind of cutting fluid has small specific heat, high viscosity, and poor fluidity. It mainly plays a lubricating role. Mineral oils with low viscosity are commonly used, such as motor oil, light diesel oil, kerosene, etc.

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