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1、3.5 Clamping of the workpiece,In order to secure the workpiece in the located position whilst under the action of the cutting force, inertial force and gravity force, a clamping device must be incorporated into jigs and fixtures. As the machining quality and production efficiency are directly relate
2、d to the clamping device design and selection, therefore, the following requirements must be ensured in clamping device design, namely: accurate ,rapid, secure and solid clamping, easy and light handling, simple structure and easy manufacturing.,2021/9/18,1,SCUT,2021/9/18,SCUT,2,Concern：,Clamping fo
3、rce,Force magnitude,Clamping direction,Clamping position,Three important aspects,2021/9/18,3,SCUT,3.5.1 The clamping direction,In detail, the following three aspects should be taken into account： （1）When two or more than two surfaces are used as the location datum, if the workpiece is large, the cla
4、mping force should be directed towards each of the location datum faces so as to keep the correct position of the workpiece； If the workpiece is small, as well as the cutting force , the clamping force only needs to be directed towards the main support surface so as to ensure that there is enough co
5、ntact area between the main datum surface and the location elements, and thereafter, to make sure the workpiece is stable and clamped safely.,2021/9/18,4,SCUT,（2）The clamping direction should be easy for setup and to get a low clamping force.,Fig.3-5-1 shows some combined relationships between the c
6、lamping force, gravity and the cutting force,Question: which one needs the least clamping force?,Fig.3-5-1,2021/9/18,5,SCUT,（3）The clamping direction should be chosen to reduce the clamping deformation of the workpiece,As the workpiece may have different stiffnesses in different directions, the clam
7、ping deformation may also be different under different clamping directions. Fig.(a) shows that a three jaw chuck is clamping a thin sleeve workpiece in the radial direction.,2021/9/18,6,SCUT,2021/9/18,7,SCUT,3.5.2 The clamping position,After the clamping direction is decided, the clamping position a
8、nd number will directly affect the stability and deformation of the workpiece. Usually, the following aspects should be taken into account in deciding the clamping position and number：（1）the clamping position should secure the location instead of making the workpiece move or rotate. and close to the
9、 geometrical center of the support surface, and distribute the clamping force uniformly in the contact surface. In Fig. (a) and (b), the clamping force Q would be better replaced by Q1.,2021/9/18,8,SCUT,2021/9/18,9,SCUT,（2）The clamping position should be in the place where the workpiece has a larger
10、 stiffness, and the number of clamps should be sufficient to minimize the clamping deformation. in Fig. (c) and (d), the clamping force Q would be better ced by Q1.（3） The clamping position should be as close as possible to the current machined surface to reduce the cutting torque caused by the cutt
11、ing force, as well as the vibration.,2021/9/18,10,SCUT,3.5.3 The magnitude of clamping force,Ensuring that the workpiece can keep the original location position during machining is not only related to the clamping direction, the clamping position and number, but also more related to the magnitude of
12、 the clamping force. The practical clamping force should be： QK QHere，safe coefficient K ，it is between1.52. ； desired clamping force Q For some very important jigs or fixtures, it is usually achieved by experiment.,2021/9/18,11,SCUT,Appendix.1 -The Commonly used clamping devices,1. Wedge clamping d
13、evice(斜楔夹紧 ）,2021/9/18,13,SCUT,Force transmitting coefficient: ip=Q/P=1/tg2+tg(+1), P is the actual clamping force generated.Application:wedge clamping device is usually used in powered clamping devices to clamp workpieces with small dimensional tolerance.Shortcoming:require a long working distance
14、for the wedge surface, which will make the structure loose, and decrease the speed of clamping velocity.,2021/9/18,14,SCUT,2. Screw clamping device,Screw clamping device is originated from the wedge clamping device, as it works with the same principle of a wedge clamping device in that, the wedge ha
15、s been wrapped around a cylindrical column, so screw down the screw and clamp the workpiece.,2021/9/18,15,SCUT,2021/9/18,16,SCUT,(1) The calculation of screw clamping force,When the workpiece is clamped , shown in Fig.6-40, the following formula is tenable according to the torque balance principle,
16、M=M1+M2 where M- the original torque imposed on the screw M1-the reacted torque imposed on the screw by the screw nut M2-the reacted torque imposed on the screw by the workpiece.,2021/9/18,17,SCUT,自锁条件：,2021/9/18,18,SCUT,Q=R1y while R1x= R1ytg(+1) =Qtg(+1) F2=Qtg2 Therefore,-the screw ascending angl
17、e，generally =24;1-the frictional angle between screw nut and screw；2-the frictional angle between the workpiece and the screw head,2021/9/18,19,SCUT,2021/9/18,SCUT,20,Advantages： simple structure, good machinability, long clamping distance, larger force transmitting coefficient and good lock perform
18、ance。 disadvantage: slow clamping and low efficiency .,Screw clamping device,Standard floating clamping block,The compound clamping device formed by screw and clamping plate,2021/9/18,21,SCUT,2021/9/18,22,SCUT,3.Eccentric cylindrical clamping device,The eccentric clamping device also comes from the
19、wedge clamping device, or in other words, it works with the same principle as the wedge clamping device in that, the wedge is wrapped around a cylindrical disk, and the workpiece can be clamped by rotating the disk.观看动画 . Eccentric clamping device,Advantages and disadvantages:Compared with screw cla
20、mping，the eccentric clamping device has a small clamping distance, small clamping force, and bad lock performance, but it clamps fast, and has a compact structure. Therefore, it is usually used in conditions such as small cutting force and small vibration,the variation of x is 0tg-1(2e/D) 0, accordi
21、ng to the wedge lock condition，the following formula must be satisfied p1+2,1-the frictional angle between the eccentric wheel and the support bearing；2-the frictional angle between the eccentric wheel and the workpiece (or clamping plate),2021/9/18,24,SCUT,2021/9/18,25,SCUT,(2) clamping distance fo
22、r eccentric cylindrical clamping device If are pn is taken as the working surface, shown as Fig. 6-46 (b)， will change between 0 and +900 .Then S=h2-h1=e If arc pm around point P is taken as the working surface，shown as Fig.6-46 (c )，will change between 450 and +450. Then S=h2-h1=2 CO11.4e(3) The cl
23、amping force of eccentric cylindrical wheels,2021/9/18,26,SCUT,4 Centring clamping device,During machining，if the workpiece uses a centre line or a symmetrical surface as the operational datum, in order to ensure =0，a device which can both ensure accurate centering and secure clamping is used, where
24、 the location elements and clamping elements are the same. This kind of device is called a centering clamping device.,Usually used centering clamping devices,Centering clamping by equal movement in different direction,Centering clamping by elastic deformation,2021/9/18,28,SCUT,Appendix.2 The basic r
25、equirement and design steps for jigs or fixtures,5.4.1 The basic requirement for jigs and fixtures the basic requirement for jigs or fixtures involves 4 aspects: (1) they must steadily ensure the machining accuracy for the workpiece； (2) they should raise the productive efficiency of mechanical manu
26、facture； (3) they should have a simple and good structure, good machinability and maneuverability. (4) they should be able to reduce the machining cost of the workpiece.,2021/9/18,29,SCUT,5.4.2 The design steps for jigs or fixtures（1）study the original documents and know the design task；（2 ） Consider and decide the structural scheme, draw the accurate arrangement ；（3）Draw the assembly drawing ；（4）Decide and label the related dimensions and technical specifications ；（5）Draw the engineering design drawings for the parts of the fixture or jigs,2021/9/18,SCUT,30,