Cutting principle
The cutting layer enters the cutting area from the area to be cut, and gradually enters the stressed state from being unstressed. As the cutting area goes deeper, the stress increases sharply. The metal entering the cutting area instantly undergoes elastic deformation, plastic deformation, local micro cracks, and micro cracks. Several processes such as crack propagation and fracture separation
The initial stage of the metal cutting process is very similar to the forward extrusion experiment, which is a process of shearing and sliding.
The cutting layer is squeezed by the rake face to produce sheared chips, which will also flow out along the rake face. The bottom layer will be squeezed and rubbed by the rake face and continue to deform. The cutting edge of the tool is not absolutely sharp, but There is a blunt radius, and a small part of the entire cutting layer thickness will be extruded. After deformation, the machined surface will eventually be formed.
The first deformation zone,
First, elastic deformation occurs. Then, the shear stress reaches the yield limit of the material and reaches the isotangential stress line, which means slip begins, which is the initial slip plane. Slip begins to occur inside the metal, and plastic deformation appears externally. As the tool gradually approaches, the plastic deformation gradually becomes larger, accompanied by deformation strengthening, which reduces the toughness of the material and increases the strength and brittleness. Until the slip is terminated, the main deformation zone consumes a lot of power and generates a large amount of cutting heat, and the slip begins. Line shifting, the final slip line distance is 0.02~0.2mm, the higher the cutting speed, the narrower the width.
The second deformation zone,
The layer to be cut and the parent body of the workpiece are connected as a whole. After passing through the first deformation zone, only the shape changes and they remain a whole. In general, when the bottom of the cutting layer of carbon steel is close to the tool tip, its local compressive stress reaches the strength limit of the material. First, micro-cracks are generated at the tool tip, also known as cracking. Then, because the stress is concentrated and moves rapidly along the shear plane Expands to the surface of the cutting layer, causing fracture and separation from the parent body.
The chips formed after the cutting layer passes through the shear plane are squeezed by the front during the process of flowing out along the front of the tool, causing the bottom of the chip layer to continue to produce a slip deformation area. The deformation in this deformation area is caused by severe friction, so It is also called the friction zone. According to different friction properties, the friction zone can be divided into a bonding zone and a sliding zone.
In the bonding zone, the pressure between the chip and the rake face is very high, up to 2~3GPa. Coupled with the high temperature of several hundred degrees, the plasticity of the material increases and the metal at the bottom of the chip is bonded to the rake face. The phenomenon is similar to that of glue.
When they are bonded, there is no ordinary external friction between them. The metal flow on the bonding surface tends to stagnate. The closer the metal flow speed to the bonding surface, the lower it is. It can be called a retention layer. , the flow of chips is realized by the shear slip that occurs inside the underlying metal.
This phenomenon is called internal friction.
The metal in the stagnant layer undergoes strong plastic deformation, and the deformation amount can be as high as dozens of times that of the first deformation zone. This is from It has been confirmed by observing the fibrosis of the metal grains in the bottom layer of the chip and that the fibrosis direction is almost parallel to the rake face.
Although the thickness of the retention layer is only 1/20 of the nominal chip thickness, the energy consumed accounts for about 1/5 of the total energy consumption. As the distance of the chip from the tool tip increases, the internal friction phenomenon gradually weakens.
When the internal friction is reduced to 0, it enters the sliding zone until it leaves the rake face. In the sliding zone, the pressure between the chip and the rake face gradually decreases, and the temperature also gradually decreases.
The third deformation zone
The transition surface and machined surface of the workpiece are squeezed by the blunt part and back of the cutting edge, and the friction produces an area with slight plastic deformation, and work hardening occurs on the surface.
Deformation coefficient
Workpiece material
Tool rake angle
cutting speed
Cutting thickness: When the cutting thickness increases, the normal stress increases and the friction coefficient also decreases. When the feed rate increases, the chip deformation coefficient decreases.
In essence, the metal in front of the tool tip is a dynamic process, including stress, strain, strain rate, temperature, dynamic behavior of the material at the corresponding temperature, etc. Therefore, this process must not be a single cause, but must be multiple factors. The role and balance are very similar to chemical reactions, showing certain characteristics in a certain state.
keratoderma
High pressure and temperature, friction with rake face
When the workpiece material has high plasticity and low strength, the friction between the chip and the front surface is large, the chip deforms greatly, and it is easy to stick to the tool and produce chip edges.
When the temperature rises to the recrystallization temperature, the material softens and the built-up edge disappears and becomes a tributary layer.
scaly spines
Advanced material separation
Formulation of process route
Selection of positioning datum
coarse datum
The purpose of selecting the rough datum is firstly to ensure that each processed surface has sufficient margin, and secondly to ensure that the size and position of the unprocessed surface meet the requirements of the drawing.
The positioning datum used in the first machining process of the machining process is a rough datum. When selecting a rough datum, first ensure that there is sufficient margin for each processed surface, and secondly ensure that the size and position of the unprocessed surface meet the requirements of the drawing.
When selecting a rough datum, it is very important to have mutual position requirements between the surface that does not need to be processed and the processed surface. This can ensure that the position error between the surface that does not need to be processed and the processed surface is minimized. This is also a machining allowance allocation. process, therefore, this rough benchmark needs to be paid great attention to in the early stage of blank production.
If all the surfaces of the part need to be processed and the blank is relatively accurate, the surface with the smallest machining allowance should be selected as the rough reference
Select the important surface and the surface with the largest processing area as the rough reference
Try to choose a rough surface that is smooth and clean without flash, gates, risers or other defects as a rough reference to ensure accurate positioning and reliable clamping.
The rough datum can only be used once in the same dimension direction. The rough datum is the surface of the blank. The surface is rough and the accuracy is low. Repeated use will cause large positioning errors.
Fine benchmark
The main consideration of precision datum is to reduce positioning errors, ensure processing accuracy and facilitate and accurate clamping.
Choice of processing method
Division of processing stages
The purpose of dividing processing stages
To ensure the processing quality, during rough machining, the machining allowance is large, the cutting amount is large, the clamping force required is large, the cutting force and cutting heat are large.
therefore, the entire process system, especially the workpiece, will produce a lot of stress Deformation, thermal deformation and internal stress of the workpiece.
therefore, a semi-finishing and precision stage is required to gradually reduce the cutting amount and clamping force, reduce the cutting force and cutting heat, gradually correct the workpiece deformation, improve the dimensional accuracy of the workpiece and reduce the surface roughness value, at the same time, mechanical processing is carried out in stages, and there must be a certain time interval between each stage to play a natural aging role in the workpiece. This will help eliminate the internal stress of the workpiece and allow the workpiece to have time to deform. Surface finishing is carried out later in the machining process and is also beneficial to protecting the machined surface.
Defects in the blank can be detected early, and a large margin can be removed during rough processing to detect defects in the blank early.
You can use equipment reasonably. For rough machining, use equipment with high power, strong rigidity and good accuracy. For finishing machining, use equipment with higher machining accuracy but not too much power.
It is convenient to arrange the heat treatment process and insert the corresponding heat treatment process in each stage, so that the mechanics can be achieved and the internal stress and deformation of the workpiece can be gradually removed. Generally, after roughing, there is a stress relief, and after semi-finishing, the surface is strengthened.
Rough machining stage
Rough machining is a processing stage in which a large machining allowance is removed from the blank. Therefore, at this processing stage, it is necessary to efficiently remove most of the machining allowance on the main surface of the part and some surfaces with large machining allowances.
semi-finishing
The task of semi-finishing is to remove the errors left after rough machining of the main surface, prepare for the precision machining of the main surface, and complete the final processing of some surfaces that do not require high precision.
finishing
It is ensured that after parts are processed, all quality requirements must be met for parts that do not require high processing quality. For parts that require extremely high processing quality, their shape accuracy and position accuracy must usually be achieved.
precision finishing, surface polishing
For products with high processing quality requirements, finishing and finishing processing are required. This stage does not remove or remove an extremely thin metal layer from the surface of the workpiece to improve the size and shape accuracy of the processed surface and reduce the surface roughness value. Or methods used to strengthen surfaces, generally not used to correct shape errors and position errors
Centralization and decentralization of processes
Arrangement of process sequence
Details of the process
Processing allowance and process size
Selection of machine tools and process equipment
Machine tool equipment
Process equipment, fixtures, cutting tools, inspection tools
The formulation of cutting amount
According to machining allowance and process system stiffness
When finishing, select the feed according to the machining accuracy and surface roughness requirements of the workpiece.
When rough machining, according to the stiffness and strength of the process system
The stiffness and strength of the process system include machine tools, tool holders, blades, etc.
