1、JIGS AND FIXTURESINTRLDUCTIONIt has already been stated that the workpiece must be located relative to the cutting tool , and be secured in that position .After the workpiece has been markec out ,it is still necessary to position it with respect to the machine morement,and to clamp it in that posito
2、n before machining is started.When several identical sorkpieces are to prlduced the need to mark out each part is eliminatde by the use of jigs and fixtures,but if a casting or forging is involved ,a trial oorkpiece is markec out ,to ensure that the workpiece can be produced from it ,and to ensure t
3、hat ribs, cores, etc. have not become misplaced.Jigs amd fixtures are alike in that they both incorporate devices to ensure that the workpiece is correctly bocated amd clamped, but they differ in that jigs incorpoorate means of tool guiding during the actual cutting operationj, and fixture do not .
4、In practice, the only cutting tools that can be guided while actually cutting are drills, reamers, and similar cutters; and to figs are associated with drilling operations, and fixtures with all other operations. Fixtures may incorporate means of setting the cutting tools relative to the location sy
5、stem.The advantage of jigs and fixtures can be summarized as follows:1) marking out and other measuring and setting out methods are eliminated;2) unskilled worked may proceed confidently and quickly in the knowledge that the workpiece can be positionde correctly, and the tools guided or set;3) The a
6、ssembly of parts is facilitated,since all components will be identical within small limits , and reyingand filling of work is eliminated;4) the parts will be intrechangeable ,and if the product is sold over a wide area, the problem of spare parts will be simplified;Bolt holes often have 1.5mm of eve
7、n 3.0mm clearance for the bolt , and the reader may doubt the necessity of making precision jigs for such work. It must be rememgered that the jigs, once made, will be used on many components, and the estra cost of an accurately made jig is spread orer a large output, Furthermore,it is surprising ho
8、w small errors accumulate in s mechanism during its assembly, When a clearance is specified, it is beteer to ensure its observance, rather than to allow careless marking our and machining to encroach upon it.1) The location of workpeece.Fig,10.1 represents a body that is completely freeing space, in
9、 this condition it has six degrees of freedon. Consider these freedons with respect to the three mutually perpendicular axes XX, YY, and ZZ. The body can move along any of these axes; it therefore has three freedoms of translation. It can also rotate about any of the three axes; it bherefore has thr
10、ee freedoms of rotation. The total number of freedoms is six. When work is located, as many of these freedoms as possible must be eliminated ,to ensure that the operation is performed with the required accuracy. Accuracy is ensured by machining suitable location features as early as possible, and us
11、ing them for all location unless other considerations mean that other location features must be used. If it is necessary, the new location features must be machined as a result of lication from the former location features.2) The clampoing of the workpiece, The clamping system must be such that the
12、workpiece is held against the cutting forces, and the clamping forces must not be so great as to cause the workpiece to become distorted or damaged. The workpiece must be supported beneath the point of clamping, to ensure that the forces are taken by the main frame of the fig or fixture, and on to t
13、he machine table andbed. When jigs and fixtures are designed, the claming system is designed to ensure that the correct clamping force is applied, and that the clamps can be oprated quickly but with asfety.DEFINTION OF A DRILL JIG A drill jig is a device for ensuring that a hole to be drilled, tappe
14、d, or reamed in a workpiece will be machined in the proper place, Basically it consists of clamping device to hold the part in position under hardened-steel bushings through which the drill passes during the drilling operation、The drill is guided by the bushings. If the workpiece is held by the jig,
15、 and the jig is arranged so that the workpiece can be quickly inserted and as quickly removed after the machining ooperation is performed.Jigs make it possible to drill, ream, and tap holes at much greater speeds and with greater accuracy than when the boles are produced by conventional hand methods
16、. Another advantage is that skilled workers are not required when jigs are used. Responsibility for the accturacy of hole location is taken from the operator and given to the jig The term jig should be used only for devices employed while drilling, reaming, tapping holes, It is not fastened to machine on which it is used and my be moved around on the table of the drilling machine to bring each bushing directly under the drill. Jigs physically limit and control th
