(1) Benchmark
Parts are composed of a number of surfaces, each surface has a certain size and mutual position requirements. The relative position requirements between the surface of the part include two aspects: the distance between the surface dimensional accuracy and relative position accuracy (such as coaxiality, parallelism, verticality and circular runout, etc.) requirements. The study of the relative position relationship between the surface of the part is inseparable from the benchmark, without a clear benchmark can not determine the position of the surface of the part. In its general sense, the benchmark is the point, line, and surface on the part used to determine the position of other points, lines, and surfaces. According to its different role, the benchmark can be divided into two categories: design benchmark and process benchmark.
1. Design basis
The reference used to determine other points, lines and surfaces on the part drawing is called the design reference, and in the case of the piston, the design reference refers to the center line of the piston and the center line of the pin hole.
2. Process standard
The reference used in the processing and assembly of parts is called the process reference. According to different uses, the process reference is divided into positioning reference, measurement reference and assembly reference.
(1) Positioning reference: the reference used to make the workpiece occupy the correct position in the machine tool or fixture during processing, called the positioning reference. 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 the stop plate positioning
Others are positioned in the V-shaped frame, positioned in the semicircular hole, etc.
(2) Measurement benchmark: When parts are inspected, the benchmark used to measure the size and position of the machined surface is called the measurement benchmark.
(3) Assembly benchmark: the benchmark used to determine the position of the part in the component or product during assembly, called the assembly benchmark.
picture
(2) The installation method of the workpiece
In order to produce a surface that meets the specified technical requirements on a certain part of the workpiece, it is necessary to make the workpiece occupy a correct position on the machine tool relative to the tool before machining. This process is often referred to as the "positioning" of the artifact. After the workpiece is positioned, due to the role of cutting force, gravity, etc., in the processing, a certain mechanism should also be used to "clamp" the workpiece so that its determined position remains unchanged. The process of holding the workpiece in the correct position on the machine tool and clamping the workpiece is called "installation".
The quality of the workpiece installation is an important problem in machining, which not only directly affects the machining accuracy, the speed and stability of the workpiece installation, but also affects the level of productivity. In order to ensure the accuracy of the relative position between the machined surface and its design reference, the workpiece should be installed so that the design reference of the machined surface occupies a correct position relative to the machine tool. For example, in order to ensure the circular runout of the bottom diameter of the ring groove and the skirt axis, the workpiece must be installed so that its design reference and the spindle line of the machine tool must coincide.
When machining parts on a variety of different machine tools, there are various installation methods. The installation method can be summarized into three kinds: direct rectification method, line rectification method and fixture installation method.
(1) Direct alignment method When using this method, the correct position of the workpiece on the machine tool is obtained through a series of attempts. The specific way is to install the workpiece directly on the machine tool, use the dial indicator or the needle on the dial dial to correct the correct position of the workpiece by visual inspection, and find the correct position while checking until it meets the requirements.
The positioning accuracy and the speed of the direct alignment method depend on the alignment accuracy, the alignment method, the alignment tool and the technical level of the worker. Its disadvantage is that it takes much time, low productivity, and it has to be operated by experience, and it has high technical requirements for workers, so it is only used in single piece and small batch production. For example, seeking righteousness by hard imitation of form belongs to seeking righteousness directly.
(2) Marking to find the correct method is a method of finding the correct workpiece by using the marking needle on the blank or semi-finished product to make it obtain the correct position. Obviously, this method requires an additional marking process. The line itself has a certain width, and there are scribing errors when scribing, and observation errors when correcting the position of the workpiece, so the method is used for small production batches, low blank accuracy, and large workpieces should not be used in the roughing of the fixture. For example, the location of the two-stroke product pin hole is determined by the marking method of the dividing head.
(3) The fixture installation method: the process equipment used to clamp the workpiece so that it occupies the correct position is called the machine tool fixture. The fixture is an additional device of the machine tool, which has been adjusted in advance before the workpiece is not installed on the machine tool, so in the processing of a batch of workpieces do not have to find positive positioning one by one, it can ensure the technical requirements of the processing, both labor and labor saving, is an efficient positioning method, widely used in batch and mass production. Our current piston processing is the use of fixture installation method.
1) After the workpiece is positioned, the operation that keeps the positioning position unchanged during the processing is called clamping. The device that keeps the positioning position unchanged during the processing of the workpiece in the fixture 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, it should be ensured that the position of the workpiece does not change during the processing, and the clamping is accurate, safe and reliable; Clamping action is fast, easy to operate, labor saving; Simple structure, easy to manufacture.
3) Precautions when clamping: the clamping force size should be appropriate, too large will cause the workpiece deformation, too small will cause the workpiece displacement during processing, destroy the workpiece positioning.
picture
(3) Basic knowledge of metal cutting
1, the turning movement and the surface formed
Turning movement: In the cutting process, in order to remove the excess metal, it is necessary to make the workpiece and the tool relative cutting motion, the movement of the workpiece with the turning tool is called turning movement, which can be divided into the main movement and the feed movement.
Main movement: directly cut the cutting layer on the workpiece, so that it is transformed into chips, so as to form a new surface of the workpiece movement, called the main movement. When cutting, the rotating motion of the workpiece is the main motion. Usually, the speed of the main movement is higher, and the cutting power consumed is larger.
Feed movement: make the new cutting layer continuously into the cutting movement, feed movement is the movement along the workpiece surface to be formed, can be continuous movement, can also be intermittent movement. If the movement of the turning tool on the horizontal lathe is continuous, the feed movement of the workpiece on the shaper is intermittent.
The surface formed on the workpiece: in the cutting process, the machined surface, the machined surface and the surface to be machined are formed on the workpiece. Finished surface refers to the new surface formed by removing excess metal. The surface to be machined is the surface on which the metal layer is to be cut. Machining surface refers to the surface on which the cutting edge of the turning tool is being turned.
2, the three elements of cutting parameters refer to the depth of cutting, feed and cutting speed.
(1) Cutting depth: ap=(DW-DM)/2(mm) dw= diameter of unmachined workpiece dm= diameter of machined workpiece, cutting depth is what we usually call the amount of cutting tool.
The choice of cutting depth: the cutting depth αp should be determined according to the processing allowance. When roughing, in addition to leaving the margin of finishing, all roughing margin should be cut out as much as possible at one time. This can not only make the product of cutting depth, feed rate and cutting speed V large under the premise of ensuring a certain durability, but also reduce the number of tool trips. In the case of excessive machining allowance or insufficient rigidity of the process system or insufficient blade strength, it should be divided into two or more cutting tools. At this time, the cutting depth of the first cutting tool should be larger, which can account for 2/3 to 3/4 of the total margin; The cutting depth of the second cutting tool is smaller, so that the finishing process can obtain smaller surface roughness parameter values and higher machining accuracy.
When cutting parts with hard skin on the surface of the casting, forging or stainless steel and other cold hard materials, the cutting depth should exceed the hardness or cold hard layer to avoid cutting edge on the hard skin or cold hard layer.
(2) The selection of feed amount: each rotation or reciprocation of the workpiece or tool, the relative displacement of the workpiece and the tool in the direction of feed movement, the unit is mm. After the cutting depth is selected, a larger feed should be further selected as far as possible. The selection of the reasonable value of the feed quantity should ensure that the machine tool and the tool will not be damaged by too much cutting force, the workpiece deflection 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 when semi-finishing and finishing, the main limit of feed is surface roughness.
(3) The selection of cutting speed: when cutting, a certain point on the cutting edge of the tool is relative to the instantaneous speed of the surface to be machined in the main direction of movement, the unit is m/min. When the cutting depth αp and feed rate are selected, the maximum cutting speed is selected on the basis of these, and the development direction of cutting is high-speed cutting.
picture
(4) The concept of roughness mechanics
In mechanics, roughness refers to the micro-geometric characteristics of small spacing and peaks and valleys on the machined surface. It is one of the problems of interchangeability research. Surface roughness is generally formed by the machining method used and other factors, such as the friction between the tool and the surface of the part during the machining process, the plastic deformation of the surface layer metal when the chip is separated, and the high-frequency vibration in the process system. Due to the different processing methods and workpiece materials, the depth, density, shape and texture of the traces left by the machined surface are different. Surface roughness is closely related to the mating 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
The surface of the part looks smooth after processing, but it is uneven when viewed in magnification. Surface roughness refers to the micro-geometric shape features composed of small spacing and small peaks and valleys on the surface of the machined parts, which are generally formed by the processing method taken and/or other factors. The required surface roughness parameter values are also different due to the different function of the surface of the part. The surface roughness symbol number should be marked on the part drawing to indicate the surface characteristics to be achieved after the surface is completed. There are 3 kinds 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 along the measurement direction (Y direction) and the reference line over the sampling length.
2. Micro unevenness 10 o 'clock height Rz
Refers to the sum of the average of the 5 maximum contour peak heights and the average of the 5 maximum contour valley depths within the sampling length.
3, the maximum height of the outline is Ry
The distance between the highest peak line and the lowest bottom line of the outline within the sampling length.
At present, Ra is mainly used in the general machinery manufacturing industry.
picture
4. Roughness representation method
picture
5. The impact 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, and the working performance, reliability and life of the product largely depend on the surface quality of the main parts. In general, the surface quality requirements of important or critical parts are higher than those of ordinary parts, because good surface quality parts will greatly improve their wear resistance, corrosion resistance and fatigue damage resistance.
6. Cutting fluid
(1) The role of cutting fluid
Cooling effect: cutting heat takes away a lot of cutting heat, improve heat dissipation conditions, reduce the temperature of the tool and the workpiece, thereby extending the service life of the tool, can prevent the workpiece due to thermal deformation caused by dimensional error.
Lubrication: The cutting fluid can penetrate between the workpiece and the tool, so that the tiny gap between the chip and the tool to form a thin layer of adsorption film, reduce the coefficient of friction, so that the friction between the tool chip and the workpiece can be reduced, the cutting force and cutting heat can be reduced, 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 Hank holes, the chips are easy to plug in the chip tank, affecting the surface roughness of the workpiece and the service life of the tool. The use of cutting fluid can quickly wash away the chips, is the smooth cutting.
(2) Type: There are two categories of commonly used cutting fluids
Emulsion: mainly plays a cooling role, emulsion is the emulsified oil with 15 to 20 times the water dilution, this kind of cutting fluid specific heat, small viscosity, good fluidity, can absorb a lot of heat, the use of this kind of cutting fluid is mainly to cool the tool and workpiece, improve tool life, 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, the specific heat of this kind of cutting fluid is small, the viscosity is large, the fluidity is poor, mainly plays the role of lubrication, commonly used is the lower viscosity of mineral oil, such as oil, light diesel, kerosene and so on.