機械制造技術(shù)基礎(chǔ)課件-新第2章課件(2)

1、 The forces acting on the tool and the workpiece are called cutting forces. 2.1.2 Cutting forces 2.1.2.1 Sources of cutting forces, resultant forces as well components of cutting forces 2.1.2.2 Measurement of cutting force and establishment of empirical formulas 2.1.2.3 Specific cutting force and sp

2、ecific cutting power 2.1.2.4 Influence of various factors on cutting force 2.1.2.1 Sources of cutting forces, resultant forces as well as components of cutting forces The forces acting on the tool come from two sources: resistance to metal elastic and plastic deformation in the deformation regions;

3、frictions between tool and chip and between tool and machined surface. (a) free orthogonal cutting Fig.2.21 Resultant forces and components of cutting forces Resistance Fny anti- plastic deformation on the rake face Friction force Ffy tangential to the rake face The resultant force Fr Resistance Fna

4、 anti- elastic and plastic deformation normal to the flank face friction force Ffa tangential to the flank face Fig.2.21 Resultant forces and components of cutting forces （b）oblique cutting. In order to measure, calculate cutting forces and analyze the effects of cutting forces, usually the force Fr

5、 is decomposed into three components of forces: Main cutting force Fz Radial thrust force Fy Axial thrust force Fx 2.1.2.1 Sources of cutting forces, resultant forces as well as components of cutting forces （c） oblique cutting From Fig 2.21(b) there is the relation: where, Fxy is the component of Fr

6、 in the tool reference plane, Fy= Fxycosr Fx= Fxysinr 22222 xyzxyzr FFFFFF (2.11) 2.1.2.1 Sources of cutting forces, resultant forces as well as components of cutting forces Fig.2.22 strain transducer- dynamometers l一transducer ；2一circuitry；3一strain gauge；4一recording 2.1.2.2 Measurement of cutting f

7、orce and establishment of empirical formulas (1) The principle of dynamometers A single factor method and a multi-factor method can be used to establish empirical formula for cutting force. The regular forms of empirical formula of cutting force are as follows: effect coefficients, their values are

8、related to experiment conditions; effect indices of back engagement of the cutting edge ap on the cutting force; effect indices of feed f on the cutting force; correction coefficients on the cutting force due to the difference between calculation and practice (2) Establishment of empirical formulas

9、for turning z z F z F z F yx pFz KfaCF y y F y F y F y x pFy KfaCF x x F x F x F yx pFx KfaCF (N) (2.12) (N) (2.13) (N) (2.14) zyX FFF CCC, z y x F F F xxx, z y X F F F yyy, zyx FFF KKK, 2.1.2.2 Measurement of cutting force and establishment of empirical formulas Take the building of the empirical f

10、ormula of main cutting force Fz as example Fig,2.25 Straight line in the dual-logarithm charts (a) pFz ；（b） fFz (a)(b) With a series of experimental values for ap-Fz and f-Fz, we can connect these points in the dual-logarithm charts and obtain two straight lines shown in Fig 2.25. Cutting depth feed

11、 2.1.2.2 Measurement of cutting force and establishment of empirical formulas The equations for two lines are: lgFz1gCap十xFz lgap 1gFz1gCf十yFzlgf The above equations can be changed into the following forms: Combining equations (a) and (b) we get the empirical formula for the main cutting force the s

12、lope of straight line ap-Fz , f-Fz. the intercept of straight line ap-Fz , f-Fz the evaluation from the combination of equations (a), (b) Fz y fz fCF z F p x paz aCF (a) (b) z F z F z yx pFz faCF (c) zz FF yx, fa CC p , z F C 2.1.2.2 Measurement of cutting force and establishment of empirical formul

13、as (3) The principle of piezo-electric transducer Fig 2.26 is a picture of a piezo-electric transducer for measuring forces in three orthogonal directions Fig.2.26 piezo-electric transducer 2.1.2.2 Measurement of cutting force and establishment of empirical formulas 2.1.2.3 Unit cutting force and cu

14、tting power (1) Unit cutting force Unit cutting force p means the main cutting force required in the removal of a unit area of cutting layer. It is calculated with the following equation: The formula shows that p is not influenced by back engagement of the cutting edge ap, because when ap changes cu

15、tting force Fz, and cutting lay area AD also change in the same proportion. Although AD is increased with the increase of f, Fz increases slightly. It is more straightforward to utilize the unit cutting force p for the calculation of the main cutting force Fz. 2 1 /mmN f C fa faC A F p Z F z z F z F

16、 z y F p yx pF D Z (2.15) （2）Cutting power Cutting power means the power expended in the cutting region. Usually the power expended by the main motion is calculated as. where Fz the main cutting force (N); vc the cutting speed of the main motion (m/min). The power PE required by the machine motor is

17、: PE=Pm/ kW （2.17） where machine transmission efficiency. kW vF P cz m 60 10 3 (2.16) 2.1.2.3 Unit cutting force and cutting power Unit cutting power Ps means the power expended in a unit time for the removal of a unit volume metal Zw. Ps=Pm/Zw kW/（ ） (2.18) The following relation can be developed :

18、 Table 2.1 lists some experimental values of unit cutting force p and unit cutting power Ps. In the experiment a carbide tool was used and feed f=0.3mm/r was fixed. If feed f is changed, the values of p and Ps in the table should be multiplied by the correctional coefficients Kfp、Kfps which are list

19、ed in Table 2.2. 13. smm )./(1010 1000 1363 smmkWp fva fvpa Z P P cp cp w m s (2.19) （3）Unit cutting power 2.1.2.3 Unit cutting force and cutting power Table 2.1 Unit cutting forces and cutting powers for various work materials cut by a carbide turning tool Workpiece material Conditions of experimen

20、t Unit cutting force unit cutting power NameMark State Hardnes s （HB） Cutting tool geometry Range of cutting paramet ers P（N/ ）PkW/( ) Structural carbon steel and structural alloy steel A3 Hot rolled Or normalized 134 137 015 kr75 s0 br10 with breaker on rake face ap1 5mm f0.1 0.5mm/r vc 90 105m/m i

21、n 18841884x 4518719621962x 40Cr21219621962x Stainless steel 45 normalized 22923052305x 40Cr28523052305x Stainless steel 1CR18Ni 9Ti Hardened and tempered 170 179 020with breaker on rake face 24532453x Grey cast iron HT20 40 Annealed 170 with breaker on rake face ap2 10mm f0.1 0.5mm/r vc 70 80m/mi n

22、11181118x Forgeable cast iron KT306Annealed170 with breaker on rake face 13441344x 2 mm 13. smm 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 Table 2.2 Correctional coefficients of feed on Unit cutting force and Unit cutting power f0.10.150.20.250.30.350.40.450. 50.6 Kfp.Kfps1.181.111.061.031.00.970.960.9

23、40.930.9 2.1.2.3 Unit cutting force and cutting power 2.1.2.4 Influence of various factors on cutting force There are some main factors influencing the cutting force: workpiece material, cutting parameters, cutting tool and so on. （1）Influence of workpiece material Workpiece materials influence the

24、cutting force through their shear yield strength, plastic deformation, chip-tool friction etc. From Table 2.1 we may know the effect of various workpiece materials on the cutting force. Back engagement of the cutting edge and feed The increase of back engagement of the cutting edge ap and feed f mak

25、es the width bD and the thickness hD of the cutting layer increase respectively, the cross-sectional area of the cutting layer AD is increased, so that the deformation resistance and friction is increased, which results in the increase of cutting force. (2) Influence of cutting parameters Cutting sp

26、eed Cutting speed influences the cutting process by the formation of build-up edge and friction, just as the similar way as that of cutting deformation. 2.1.2.4 Influence of various factors on cutting force Within a low cutting speed range (520m/min), with the increase of vc the cutting deformation

27、is decreased,and then the main cutting force Fz is decreased ; when cutting speed is in the medium range (about 20m/min), cutting deformation was decreased to its minimum,and then Fz got the minimum value; with the increase of vc ,the cutting deformation is increased then Fz is increased ; When vc i

28、s over 35m/min, cutting deformation is decreased again, then Fz is decreased gradually and then tends to a stable value. workpiece material: normalized steel 45, shown in Fig 2.30 Fig. 2.30 Influence of cutting speed on cutting force 2.1.2.4 Influence of various factors on cutting force Table 2.3 Co

29、rrection coefficient KvFz for main cutting force Fz as cutting speed changes Vc（m/min） workpiece 5075100125150175200 Steel 45 Steel 40Cr 1.051.021.000.980.960.950.94 Rake angle As the value of the rake angle is increased, the cutting force decreases due to the decrease of cutting deformation. The ra

30、ke angle influences Fz, Fy and Fx in different way . Table 2.4 lists the correction coefficient for cutting force in the turning of steel 45 (r=75) as rake angle changes. （3） Influence of the tool geometry 2.1.2.4 Influence of various factors on cutting force Work material 0 Correction coefficient 1

31、0010152030 Steel 45 K0Fz 1.281.181.051.000.890.85 K0Fy 1.411.231.081.000.790.73 K0Fx 2.151.701.241.000.500.30 Gray cast iron K0Fz 1.371.211.051.000.950.84 K0Fy 1.471.301.091.000.950.85 K0Fx 2.441.831.221.000.730.37 Table 2.4 Correction coefficient K0F for cutting force as rake angle changes 2.1.2.4

32、Influence of various factors on cutting force With the change of tool cutting edge angle r, the cutting force changes either. Fz will decrease with the increase of r, and it reaches a minimum value as the value of r is 60 75because the cutting layer thickness hD increases as r increases, the deforma

33、tion coefficient h is down. Fig.2.32 Influence of cutting edge angle r on main cutting force Fz Tool cutting edge angle 2.1.2.4 Influence of various factors on cutting force Table 2.5 Values of correction coefficient of cutting force in turning asr changes Work material kr Correction coefficient 304

34、5607590 Steel 45 K0Fz 1.101.051.001.001.05 K0Fy 2.001.601.251.000.85 K0Fx 0.650.800.901.001.15 Gray cast iron HT2040 K0Fz 1.101.001.001.001.00 K0Fy 2.801.801.171.000.70 K0Fx 2.801.801.171.000.70 2.1.2.4 Influence of various factors on cutting force Cutting edge inclination s The cutting edge inclina

35、tion s has a smaller influence on the main cutting force, but has obvious influences on the radial thrust force Fy and axial thrust force Fx. As s increases, the working length of the major cutting edge is increased, which causes severer friction. But on the normal section, the tools nose radius r i

36、s decreased, and obtained sharper edge decreases the cutting deformation . The combined result is that Fz changes a little . The influence of s on Fy and Fx is as shown in Fig.2.23. When the value of s changes from positive to negative, the effective direction of normal pressure Fn is changed. The radial thrust force Fy is increased and axial thrust force Fx is decreased. Usually, for each 1change of s, the radial thrust force Fy w