Basic common sense of machining, don’t do it if you don’t understand it!

1. Benchmark

Parts comprise several surfaces, each with a specific size and mutual position requirements. The relative position requirements between the surfaces of the parts include two aspects: the distance dimensional accuracy between the surfaces and the relative position accuracy (such as coaxiality, parallelism, perpendicularity and circular runout, etc.) requirements. The study of the relative positional relationship between the surfaces of the parts is inseparable from the datum, and the position of the part surface cannot be determined without a clear 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. According to their different functions, benchmarks can be divided into two categories: design benchmarks and process benchmarks. 

1. Design basis

The datum used to determine other points, lines and surfaces on the part drawing is called the design datum. For the piston, the design datum refers to the centerline of the piston and the centerline of the pin hole.

2. Process benchmark

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

1) Positioning datum: 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 and 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 stop plate.
Others include positioning in a V-shaped frame, positioning in a semicircular hole, etc.

2) Measurement datum: The datum used to measure the size and position of the machined surface during part inspection is called the measurement datum.

3) Assembly datum: The datum used to determine the position of the part in the component or product during assembly is called the assembly datum.

Second, the installation method of the workpiece

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 "positioning" of the workpiece. After the workpiece is positioned, due to the action of cutting force, gravity, etc. during processing, a certain mechanism should be used to "clamp" the workpiece so that the determined position remains unchanged. The process of getting the workpiece in the correct position on the machine and clamping the workpiece is called "setup".

The quality of workpiece installation is an important issue in machining. It not only directly affects the machining accuracy, the speed and stability of workpiece installation, but also affects the level of productivity. In order to ensure the relative positional accuracy 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 relative to the machine tool. For example, in the process of finishing ring grooves, in order to ensure the requirements of the circular runout 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, scribe alignment method and fixture installation method.

1) Direct alignment method When using this method, the correct position that the workpiece should occupy on the machine tool is obtained through a series of attempts. The specific method is to use the dial indicator or the scribing needle on the scribing plate to correct the correct position of the workpiece by visual inspection after the workpiece is directly mounted on the machine tool, until it meets the requirements.
The positioning accuracy and the speed of the direct alignment method depend on the alignment accuracy, alignment method, alignment tools and the technical level of the workers. Its disadvantage is that it takes a lot of time, low productivity, and 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) Scribing alignment method This method is to use a scribing needle on the machine tool to align the workpiece according to the line drawn on the blank or semi-finished product, so that it can obtain the correct position. Obviously, this method requires one more scribing process. The drawn line itself has a certain width, and there is a scribing 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 large workpieces. It is not suitable to use fixtures. in rough machining. For example, the position of the pin hole of the two-stroke product is determined by using the marking method of the indexing head.

3) Using the fixture installation method: the process equipment used to clamp the workpiece and make it occupy the correct position is called the 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. Our current piston processing is the fixture installation method used.

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

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

③. Precautions when clamping: 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 damage the positioning of the workpiece.

3. Basic knowledge of metal cutting

1. Turning movement and formed surface

Turning motion: In the cutting process, in order to remove excess metal, it is necessary to make the workpiece and the tool perform 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. give exercise.

Main movement: The cutting layer on the workpiece is directly cut off to convert it into chips, thereby forming the movement of the new surface of the workpiece, which 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 movement of making the new cutting layer continuously put into cutting, the feed movement is the movement along the surface of the workpiece to be formed, which can be continuous movement or intermittent movement. 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.
Surfaces formed on the workpiece: During the cutting process, machined surfaces, machined surfaces, and surfaces to be machined are formed on the workpiece. Finished surface refers to a new surface that has been removed from excess metal. The surface to be machined refers to the surface from which the metal layer is to be cut. The machined surface refers to the surface that the cutting edge of the turning tool is turning.
2. The three elements of cutting amount refer to cutting depth, feed rate and cutting speed.
1) Cutting depth: ap=(dw-dm)/2(mm) dw=diameter of unmachined workpiece dm=diameter of machined workpiece, the cutting depth is what we usually call the amount of cutting.
Selection of cutting depth: The cutting depth αp should be determined according to the machining allowance. When roughing, in addition to leaving the finishing allowance, all the roughing allowance should be removed in one pass as far as possible. This can not only make the product of cutting depth, feed rate ƒ and cutting speed V large under the premise of ensuring a certain degree of durability, but also reduce the number of passes. When the machining allowance is too large or the rigidity of the process system is insufficient or the strength of the blade is insufficient, it should be divided into more than two passes. 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; and the cutting depth of the second pass should be smaller, so that the finishing process can be obtained. Smaller surface roughness parameter value and higher machining accuracy.
When the surface of the cutting parts is hard-skinned castings, forgings or stainless steel and other severe chilled materials, the depth of cut should exceed the hardness or chilled layer to avoid cutting edges from cutting on the hard or chilled layer.
2) Selection of feed amount: the relative displacement of the workpiece and the tool in the direction of the feed movement every time the workpiece or tool rotates or reciprocates once, the unit is mm. After the cutting depth is selected, a larger feed should be selected as far as possible. The selection of a reasonable value of the feed should ensure that the machine tool and the tool will not be damaged due to too much cutting force, the deflection of the workpiece caused by the cutting force will not exceed the allowable value of the workpiece accuracy, 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 certain point on the cutting edge of the tool relative to the surface to be machined in the main movement direction, the unit is m/min. When the depth of cut αp and the feed rate ƒ are selected, the maximum cutting speed is selected on these basis, and the development direction of cutting processing is high-speed cutting. stamping part

Fourth, the mechanical concept of roughness
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 process system. Due to different processing methods and workpiece materials, the depth, density, shape and texture of marks left on the machined 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. aluminum casting part
Roughness representation
After the surface of the part is processed, it looks smooth, but it is uneven after magnification. 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 function of the surface of the part is different, and the required surface roughness parameter value is also different. The surface roughness code (symbol) should be marked on the part drawing to describe the surface characteristics that must be achieved after the surface is completed. There are 3 types of surface roughness height parameters:
1. Contour arithmetic mean deviation Ra
The arithmetic mean of the absolute value of the distance between points on the contour line in the measurement direction (Y direction) and the reference line within the sampling length.
2. Ten-point height Rz of microscopic unevenness
Refers to the sum of the average of the 5 largest profile peak heights and the 5 largest profile valley depths within the sampling length.
3. The maximum height of the contour Ry
The distance between the line of the highest peak and the line of the lowest valley of the profile within the sampling length.
At present, Ra. is mainly used in the general machinery manufacturing industry.
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4. Roughness Representation Method

5. The effect of roughness on the performance of parts

The surface quality of the workpiece after processing directly affects its physical, chemical, and mechanical properties. The working performance, reliability and 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. CNC machining aluminum part

6. 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, 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 caused by thermal deformation.
Lubrication: The cutting fluid can penetrate between 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 it can reduce the friction between the tool chip and the workpiece , to reduce the cutting force and cutting heat, reduce the wear of the tool 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) Type: There are two types 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. Cutting fluid is mainly used to cool the tool and workpiece, improve tool life and reduce thermal deformation. The emulsion contains more water, and the lubrication and rust prevention functions are 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.