1. Datum
All parts are composed of several surfaces, and there are certain dimensions and relative position requirements between the surfaces. The relative position requirements between the surfaces of parts include two aspects: the distance dimension accuracy between the surfaces and the relative position accuracy (such as coaxiality, parallelism, verticality and circular runout, etc.). The study of the relative position relationship between the surfaces of parts cannot be separated from the datum. Without a clear datum, the position of the surface of the part cannot be determined. In general, the datum is the point, line, and surface on the part used to determine the position of other points, lines, and surfaces. Datums can be divided into two categories: design datum and process datum according to their different functions.
1. Design datum
The datum used to determine other points, lines, and surfaces on the part drawing is called the design datum. For pistons, the design datum refers to the center line of the piston and the center line of the pin hole.
2. Process datum
The datum used by parts during processing and assembly is called the process datum. According to different uses, the process datum is divided into positioning datum, measurement datum and assembly datum.
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 elements, 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 stop plate positioning.
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 processed 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.
2. Workpiece installation method
In order to machine 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 usually called "positioning" of the workpiece. After the workpiece is positioned, due to the effects of cutting force, gravity, etc. during processing, a certain mechanism should be used to "clamp" the workpiece so that its determined position remains unchanged. The process of making the workpiece occupy the correct position on the machine tool and clamping the workpiece is called "installation".
The quality of workpiece installation is an important issue in mechanical processing. It not only directly affects the processing accuracy, the speed and stability of workpiece installation, but also affects the level of productivity. In order to ensure the relative position accuracy between the processing surface and its design datum, the design datum of the processing surface should occupy a correct position relative to the machine tool when the workpiece is installed. For example, in the process of fine turning the ring groove, in order to ensure the circular runout requirements 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.
There are various installation methods when processing parts on various machine tools. The installation methods can be summarized into three types: direct alignment method, line alignment method and fixture installation method.
1) Direct alignment method When this method is used, the correct position that the workpiece should occupy on the machine tool is obtained through a series of attempts. The specific method is to install the workpiece directly on the machine tool, use a dial indicator or a needle on the needle plate to visually correct the correct position of the workpiece, and calibrate while checking until it meets the requirements.
The positioning accuracy and alignment speed of the direct alignment method depend on the alignment accuracy, alignment method, alignment tools and the technical level of the workers. Its disadvantages are that it takes a lot of time, has low productivity, and must be operated based on experience, and has high requirements for workers' skills, so it is only used in single-piece and small-batch production. For example, alignment that relies on imitating the shape belongs to the direct alignment method.
2) Marking alignment method This method is a method of using a marking needle on a machine tool to align the workpiece according to the line drawn on the blank or semi-finished product to make it obtain the correct position. Obviously, this method requires an additional marking process. The drawn line itself has a certain width, and there is a marking error when marking, and there is also an observation error when correcting the position of the workpiece. Therefore, this method is mostly used for rough processing of small production batches, low blank accuracy, and large workpieces that are not suitable for the use of fixtures. For example, the determination of the position of the pin hole of a two-stroke product is to use the marking method of the dividing head for alignment.
3) Use 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 pre-adjusted before the workpiece is installed. Therefore, when processing a batch of workpieces, it is not necessary to align and position them one by one, and the technical requirements of the processing can be guaranteed. It is both labor-saving and trouble-free. It is an efficient positioning method and is widely used in batch and mass production. Our current piston processing uses the fixture installation method.
①. After the workpiece is positioned, the operation of keeping the positioning position unchanged during the processing is called clamping. The device in the fixture that keeps the positioning position unchanged during the processing is called a clamping device.
②. The clamping device should meet the following requirements: when clamping, the positioning of the workpiece should not be destroyed; after clamping, the position of the workpiece should not change during the processing, and the clamping should be accurate, safe and reliable; the clamping action is fast, the operation is convenient and labor-saving; the structure is simple and easy to manufacture.
③. Precautions when clamping: The clamping force should be appropriate. Too much will cause the workpiece to deform, and too little will cause the workpiece to move during the processing and destroy the positioning of the workpiece.
3. Basic knowledge of metal cutting
1. Turning motion and the surface formed
Turning motion: In the cutting process, in order to remove excess metal, the workpiece and the tool must make relative cutting motion. The motion of using a turning tool to remove excess metal on the workpiece on a lathe is called turning motion, which can be divided into main motion and feed motion.
Main motion: The motion of directly removing the cutting layer on the workpiece and converting it into chips, thereby forming a new surface of the workpiece, is called main motion. During cutting, the rotational motion of the workpiece is the main motion. Usually, the speed of the main motion is higher and the cutting power consumed is larger.
Feed motion: The motion that continuously puts new cutting layers into cutting. Feed motion is the motion along the surface of the workpiece to be formed, which can be continuous motion or intermittent motion. For example, the motion of the turning tool on a horizontal lathe is continuous motion, and the feed motion of the workpiece on a planer is intermittent motion.
Surface formed on the workpiece: During the cutting process, the workpiece forms a machined surface, a machined surface, and a surface to be machined. The machined surface refers to the new surface formed by turning away excess metal. The surface to be machined refers to the surface where the metal layer is about to be cut off. The machining surface refers to the surface that the cutting edge of the turning tool is turning.
2. The three elements of cutting parameters 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, and cutting depth is what we usually call cutting depth.
Selection of cutting depth: The cutting depth αp should be determined according to the machining allowance. During rough machining, in addition to leaving the allowance for finishing, all rough machining allowances should be removed in one pass as much as possible. This can not only make the product of cutting depth, feed rate ƒ, and cutting speed V large while ensuring a certain durability, but also reduce the number of passes. In the case of excessive machining allowance, insufficient rigidity of the process system, or insufficient strength of the blade, the pass should be divided into two or more 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 obtain a smaller surface roughness parameter value and higher machining accuracy.
When cutting castings, forgings or stainless steel with a hardened surface, the cutting depth should exceed the hardness or hardened layer to avoid cutting the cutting edge on the hardened layer.
2) Selection of feed rate: The relative displacement of the workpiece and the tool in the feed motion direction for each rotation or reciprocation of the workpiece or tool, in mm. After the cutting depth is selected, a larger feed rate should be selected as much as possible. The selection of a reasonable value of the feed rate should ensure that the machine tool and the tool are not damaged due to excessive cutting force, the deflection of the workpiece caused by the cutting force does not exceed the value allowed by the workpiece accuracy, and the surface roughness parameter value is not too large. During rough machining, the main limiting factor of the feed rate is the cutting force, while during semi-finishing and finishing, the main limiting factor of the feed rate is the 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 to be machined in the main motion direction, in m/min,. When the cutting depth αp and feed rate ƒ are selected, the maximum cutting speed is selected on this basis. The development direction of cutting processing is high-speed cutting processing.
IV. Roughness mechanical concept
In mechanics, roughness refers to the microscopic geometric shape characteristics composed of small spacing and peaks and valleys on the machined surface. It is one of the issues in interchangeability research. Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the part surface during processing, the plastic deformation of the surface metal during chip separation, and the high-frequency vibration in the process system. Due to the differences in processing methods and workpiece materials, the depth, density, shape and texture of the 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.
Roughness representation method
After processing, the surface of the part looks very smooth, but it is uneven when magnified. Surface roughness refers to the microscopic geometric features composed of small spacing and tiny peaks and valleys on the surface of the processed parts, which are generally formed by the processing methods and (or) other factors. The functions of the part surface are different, and the required surface roughness parameter values are also different. The surface roughness code (symbol) should be marked on the part drawing to illustrate the surface characteristics that must be achieved after the surface is completed. There are three surface roughness height parameters:
1. Arithmetic mean deviation Ra of the contour
The arithmetic mean of the absolute value of the distance between the point on the contour line along the measurement direction (Y direction) and the reference line within the sampling length.
2. Ten-point height of micro-roughness Rz
Refers to the average value of the five largest contour peak heights and the average value of the five largest contour valley depths within the sampling length.
3. Maximum contour height Ry
The distance between the highest peak top line and the lowest valley bottom line of the contour within the sampling length.
At present, Ra is mainly used in general machinery manufacturing industry.
4. Roughness representation method
5. The influence 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 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 key parts are higher than those of ordinary parts. This is because parts with good surface quality will greatly improve their wear resistance, corrosion resistance and fatigue resistance.
6. Cutting fluid
1) The role of cutting fluid
Cooling effect: Cutting heat can take away a large amount of cutting heat, improve heat dissipation conditions, reduce the temperature of the tool and workpiece, thereby extending the service life of the tool and preventing dimensional errors caused by thermal deformation of the workpiece.
Lubricating effect: The cutting fluid can penetrate between the workpiece and the tool, forming a thin adsorption film in the tiny gap between the chip and the tool, reducing the friction coefficient, thereby reducing the friction between the tool chip and the workpiece, reducing the cutting force and cutting heat, reducing the wear of the tool and improving the surface quality of the workpiece. Lubrication is especially important for finishing.
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, the chips are easy to clog in the chip groove, affecting the surface roughness of the workpiece and the service life of the tool. Using cutting fluid can quickly wash away the chips, so that cutting can proceed smoothly.
2) Types: There are two major types of commonly used cutting fluids
Emulsion: It mainly plays a cooling role. Emulsion is made by diluting emulsified oil with 15 to 20 times of water. This type of cutting fluid has a large specific heat, low viscosity, good fluidity, and can absorb a large amount of heat. The main purpose of using this type of cutting fluid is to cool the tool and workpiece, increase the tool life, and reduce thermal deformation. The emulsion contains more water, and the lubrication and anti-rust functions are poor.
Cutting oil: The main component of cutting oil is mineral oil. This type of cutting fluid has a small specific heat, a large viscosity, and poor fluidity. It mainly plays a lubricating role. Commonly used are mineral oils with low viscosity, such as engine oil, light diesel oil, kerosene, etc.
