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1、Mechanical Manufacturing Technologycutting processChapter IIIChapter IIIcutting processcutting process The cutting process is the complete process by which a tool removes excess material from the surface of the workpiece,starting with chip formation and continuing until the machined surface is forme
2、d.I.The process of chip formationUnder the action of a cutter,the cutting layer metal undergoes a complex process to become a chip.In the process,the morphology of the cutting layer changes.The root cause of this change is the old deformation of the cutting layer metal under the action of the cuttin
3、g tool.The deformation of the cutting process is accompanied by a series of physical phenomena such as cutting force,cutting heat,cutting temperature,tool wear,accumulation of psoriasis,etc.The deformation of the cutting process is the basis for the study of the cutting process.Figure 1 Shear destru
4、ction of plastic metal materials Three deformation zones of the cutting processThe model shown in Figure 1 illustrates the deformation of the cutting process.The plastic metal material under the action of the tool produces shear-slip deformation along the direction of 45o with the force of action,an
5、d when the deformation reaches a certain limit value,shear-slip destruction occurs along the direction of deformation.If the tool moves continuously,the material above the dotted line will separate from the material below under the action of the tool.The process of metal cutting is essentially simil
6、ar to the process described above.As shown in Figure 2,under the action of the tool,the cutting layer metal undergoes a complex deformation and separates from the workpiece base material to form chips.The deformation resulting from this process can be divided into three zones,i.e.three deformation z
7、ones,namely zone I between the front OAM of the cutting edge,zone II near the front face and zone III near the rear face.Figure 2 Three deformation zones of the cutting process(1)First()deformation zone The first(I)deformation zone is the area between OA and OM shown in Figure 3.Figure 3 Three defor
8、mation zones of the metal cutting processThe metal material undergoes elastic deformation to the left of the OA line.On the OA line,the shear stress of the material reaches the yield strength s,begins to plastic deformation,producing slip,OA is called the beginning slip line.As the tool moves contin
9、uously,the metal on the slip line keeps coming closer to the tool,and the stress and deformation increase gradually,reaching the OM line when the stress and deformation reach the maximum.Above the OM line,the cutting layer metal will flow out along the front face,forming a chip and completing the cu
10、t-off,the OM line is called the final slip line.The first deformation zone is the largest deformation zone during metal cutting deformation,in which the metal will generate a lot of cutting heat and consume most of the power.This area is narrow,with a width of only 0.02 to 0.2mm.(2)Second(II)deforma
11、tion zone1)Deformation of the second(II)deformation zoneThe discharge of the chip along the front of the tool is further impeded by the front,and there is strong squeezing and friction between the tool and the chip interface,causing a secondary deformation of the metal at the bottom of the chip near
12、 the front,which is fibrotic.This area is called the second deformation zone(II).In this area of deformation,the deformation is further exacerbated by the compression and friction of the cutting layer material by the front cutter face,where the material fibers,slows down its flow and even stagnates
13、on the front cutter face.Moreover,the chip and the front surface of the pressure is very high,up to 2 3 GPa,the heat generated by the friction also makes the chip and the surface temperature rise to several hundred degrees of high temperature,the bottom of the chip and the front surface of the tool
14、bonding phenomenon.2)Accumulated psoriasisWhen cutting plastic metal materials,due to the squeezing and violent friction between the bottom surface of the chip and the front surface in the second deformation zone,the flow rate of the bottom layer of the chip is lower than the upper layer,forming a s
15、tall layer.When the friction between the stall layer and the front cutting surface exceeds the chip metal strength limit,metal adhesion of the stall layer accumulates near the cutting edge,forming a chip tumor as shown in Figure 4.Fig.4 LesionsBecause the hardness of the chip is much higher than the
16、 hardness of the workpiece,it can replace the cutting edge to protect the cutting edge and increase the front corner of the tool to reduce cutting deformation and cutting force.Therefore,it is generally accepted that there is some benefit to developing phyllodes during roughing.However,chipoma is un
17、stable,it is sometimes large,sometimes small,sometimes absent,in addition to its tip protruding from the cutting edge and tip,it also makes the cutting depth and cutting thickness constantly change,which affects the machining accuracy and causes the cutting force to fluctuate,and may cause the proce
18、ss system to vibrate,forming grooves on the machined surface,and the shedding chipoma fragments may also be embedded in the machined surface,causing the workpiece surface quality to decline.Therefore,it is important to avoid the formation of phyllodes when finishing.Cutting speed has been shown to h
19、ave the greatest impact on phyllodes,with both high and low speed cutting less prone to phyllodes.The following measures can be used to suppress or eliminate the accumulation of psoriasis.a)Begin by addressing the heat treatment process stage before processing.By heat treatment,the hardness of the p
20、art material is increased and the processing hardening of the material is reduced.b)Adjust the tool angle to increase the front angle,thereby reducing the pressure of the chip on the front face of the tool.c)Reduce the cutting speed,so that the cutting layer and the cutting surface contact surface t
21、emperature is lowered before the tool to avoid the occurrence of bonding phenomenon.Or use a higher cutting speed and increase the cutting temperature,because the temperature is high enough that the phyllodeshia does not occur.d)Replace the cutting fluid with a better lubricating fluid to reduce cut
22、ting friction.(3)Deformation Zone III(III)1)Deformation of the third()deformation zoneThe area of deformation formed by squeezing and rubbing against the back of the tool on the machined surface is called the third deformation zone(III).Since the tool edge cannot be absolutely sharp,the presence of
23、a blunt radius makes it impossible to completely cut the nominal cutting thickness in the cutting layer parameters,a small part will be squeezed onto the machined surface,friction with the back of the tool surface,and further pop,plastic deformation,thus affecting the quality of the machined surface
24、.Chip types and factors affecting chip deformationFigure 5 Chip typea)strip chips b)extruded chips c)unit chips d)chipped chipsChip control is all about controlling the type,flow,curl and break of chips.Chip control is of great significance for the normal,smooth and safe cutting process.In some case
25、s,chip control is a decisive factor in whether the process can proceed.Chip control is particularly important in CNC machining and automated manufacturing processes.1、Type of chipA chip is the result of a complex series of deformations in the metal cutting process.According to the deformation charac
26、teristics of the cutting process metal and the degree of deformation,the chips can be divided into four categories,as shown in Figure 5(a).1)Strip chipsIn the processing of plastic metal materials,if the shear slip deformation of the cutting layer of metal does not reach the shear damage limit of th
27、e material,the chip is a continuous band,which is the band chip.Its inner surface is smoother due to the extruded friction with the front cutter surface,and the outer surface can be seen as a hairy streak on the shear surface.At this time,the cutting force fluctuates little and the machined surface
28、quality is good.It is the most common flaky shape.2)Lumpy shavings(extruded shavings)In the case of low cutting speed and large cutting thickness,cutting steel and cutting brass and other materials,the outer surface of the chip local to the limit of shear damage,cracking is nodular,but the shape is
29、still band,which is nodular chip.3)Granular chips(unit chips)When cutting steel and lead materials with very low cutting speed and high cutting thickness,the shear deformation completely reaches the materials damage limit,and the cut chips fracture into homogeneous particles,or granular chips.This i
30、s when the cutting force fluctuates the most.4)Crushing chipsWhen cutting brittle metal materials such as cast iron,the cutting layer metal is brittle under the bending stress without obvious plastic deformation,and irregular fine-grained chips are obtained.At this point the machined surface is of p
31、oor quality and the cutting process is not smooth.Obviously,the chip type is determined by the material characteristics and the degree of deformation,processing the same plastic material,using different processing conditions,such as in the formation of nodular chip conditions,further reduce the fron
32、t angle,increase the cutting thickness,you can get granular chip;conversely,such as increasing the front angle,improve the cutting speed,reduce the cutting thickness,you can get strip chip.In production,chip conversion conditions are often used to obtain more favorable chip types.Strip chips are the
33、 better type in terms of smoothness of the machining process,guaranteed machining accuracy and surface quality.Strip chips also come in different shapes.As shown in Figure 6.The continuous,long strips of chips are inconvenient to handle and can easily become entangled in the workpiece or tool,interf
34、ering with the cutting process and even injuring people.Therefore,a C-shaped chip is a better shape on a CNC machine.However,its high frequency of breakage can affect the smoothness of the cutting process.Therefore,it is better to be a spiral chip when finishing the car,the formation process is smoo
35、th and easy to clean.In the heavy lathe with a large cutting depth,large feed rate when turning steel parts,usually make the chip curl into a bar shape,top break on the surface of the workpiece,and fall by self-weight.On the automatic line,the pagoda shaped chips dont tangle and are easy to clean up
36、 and are in a better chip shape.Turning cast iron,brittle brass and other brittle materials,chip crumbling,splashing,easy to injure people,and abrasion of the sliding surface of the machine tool,should try to make the chip even into a short spiral roll.Figure 6 Shape of the chip 2、Chips flow directi
37、on,curl1)The flow of chipsAs shown in Figure 7,in right-angle free cutting(right-angle cutting refers to the main cutting edge of the cutting edge of the tool inclination angle s=0 when the cutting,the main cutting edge and cutting speed direction into a right angle,so also known as orthogonal cutti
38、ng.When a straight cutting edge participates in the cutting process(a condition called free cutting),the chips flow out in an orthogonal plane.In right angle non-free cutting,due to the influence of the tip arc radius and cutting edge,the chip outflow direction and the orthogonal plane to form a chi
39、p angle,and the main deflection angle kr and the sub cutting edge working length;oblique angle cutting,the chip flow direction is affected by the edge inclination angle s,the chip angle is equal to the edge inclination angle s.Figure 8 is a diagram of the influence of s on chip flow direction.The be
40、vel of the chip flute also affects chip flow and chip shape.The external inclined groove shape makes the chips flow out in the direction of the workpiece,while the internal inclined groove shape makes the chips flow out in the direction of the back of the workpiece.Figure 7 Flow direction of chips F
41、igure 8 Effect of s on chip flow2)Curling of chips The curl of the chip is due to plastic deformation and frictional deformation during the cutting process,additional deformation as the chip flows out.Adequate additional deformation of the chip can be achieved by means of chip chutes(breakers),tabs,
42、additional stops and other obstacles on the front cutter face.The use of chip groove can reliably promote chip curling,chip in the flow through the chip groove,by the external force F action to produce a moment M to make the chip curling,as shown in Figure 9.The curl of the chip is due to plastic de
43、formation and frictional deformation during the cutting process,additional deformation as the chip flows out.Adequate additional deformation of the chip can be achieved by means of chip chutes(breakers),tabs,additional stops and other obstacles on the front cutter face.The use of chip groove can reliably promote chip curling,chip in the flow through the chip groove,by the external force F action to produce a moment M to make the chip curling,as shown in Figure 9.Figure 9 Curl of cuttings.Thank you