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JIGS AND FIXTURES
INTRLDUCTION
It 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 positon 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 that 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 . 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 system.
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 assembly of parts is facilitated,since all components will be identical within small limits , and ‘reying’and 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 even 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 how 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 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 three 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 using 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 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 the 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, tapped, 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, 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. 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 the path of the cutting tool.
If the operation includes machining operations like millling, planning, shaping, turning, etc.the term fixture should be used, A fixture holds the work during machining operations but dses not contain special arrangements for guiding the cutting tool, as drill figs do.
TYPICAL JIGS AND FIXTRRES
Typical drill jig, Figure 10.2 illustrates a drilling jig for drilling jig for drilling four holes in the flange of a workpiece that has been completed escept for the four holes. The workpiece has an accurately machined bore, and is located form the bore and the end face, from a cylindrical post. There is no need to control the rotational position bout the axis of the bore, because up to the tine when the holes are drilled, it is symmetrical about that axis The four bushes used to control the drill are held in the drill plate, which, with the hand nut, is used to clamp the workpiece against the base of the fixture, Typical milling fixture, figure 10.3 illustrates a simple milling fixture for milling the slot in the otherwise completed workpiece shown, The workpiece is located from two lf the four hole in its base, and from the underside of the base, The workpiece is clamped in position, and cutter is located against the setting block, which provides setting or cutter position and depth of cut. The fixture must be positioned relative to the machine table, this is done by engaging the two tenons at be bottom of the foxture in the slot in the machine table. The fixture is secured to the machine table with T---bolts. Also engaging in the slots in the table.
中文資料
鉆模與夾具
介紹
工件必須位于相對于刀具安全的位置上。然后在工件上必須劃線，移動到需要加工的位置上，并且在加工前夾緊在這個位置。
當(dāng)許多特殊的工件進(jìn)行加工前，需要通過鉆模和夾具對每部分進(jìn)行劃線，但如果是鑄件，為了保證工件能被加工出來，必須對工件做出劃線，以保證加強(qiáng)肋、型芯不發(fā)生變形或錯位。
鉆模和夾具要結(jié)合起來才能保證工件正確的定位和夾緊。鉆模是意味著加工過程中刀軌不同，夾具就不是這樣。在實(shí)際中，只是刀具是鉆頭，絞刀和類似刀具才能進(jìn)行導(dǎo)向。因此，鉆模用于鉆加工，夾具用于所有其它加工操作中。夾具用于刀具裝置相對于定位的結(jié)合。
鉆模和夾具的優(yōu)點(diǎn)能分成以下幾方面：
1） 減少和消除了一些測量和安裝的工序。
2） 非專業(yè)人員也可以準(zhǔn)確快速的掌握刀具安裝。
3） 裝配部分的工件更加方便，所有工件也很少拆裝。
4） 零件將實(shí)現(xiàn)互換，如果工件是固定在一個領(lǐng)域的話，零件將實(shí)現(xiàn)單一化。
螺栓孔經(jīng)常有1.5mm到3mm的均勻螺距，讀者可能會懷疑用鉆模加工它的精確性。必須記住鉆模是一次制造而成，用于許多元件精確制造鉆模的費(fèi)用是額外大的，此外，令人驚奇的是小小的錯誤聚焦在它裝配的時候。粗心的劃線和機(jī)器碰它。
1） 工件位置。表明了物體在空間的自由度，有三個自由度分別是XX、YY、ZZ物體能夠沿著這些軸進(jìn)行軸向移動；還有三個自由度是角度方向的。它們可以繞坐標(biāo)軸轉(zhuǎn)動。自由度的個數(shù)是六個。當(dāng)工作固定后，它的自由度可能被消除，以保證工件正確操作的需要。要盡可能的利用夾具保證工件的加工位置。將它應(yīng)用到所有加工位置，除非用到其它夾具來保證加工位置時才換它。如果有必要的話，新的位置夾具，必須被加工到以前的加工位置才行。
2） 工件夾緊。夾緊裝置必須能夠抵抗切削力，夾緊力必須大于工件變形力或破壞力。夾緊點(diǎn)必須在工件下面，以保證這個力被鉆模或夾具體框架在機(jī)床上所能承受。當(dāng)鉆模和夾具被設(shè)計(jì)時，夾緊裝置被設(shè)計(jì)或能夠準(zhǔn)確提供夾緊力并且夾緊裝置能夠快速安全地操作。
鉆模的定義
鉆模是一個能夠保證孔在工件被鉆、擴(kuò)、鉸的裝置。在夾具裝置夾緊工件的位置，在鉆模中位于淬火鋼軸承的下面。如果工件結(jié)構(gòu)相似，鉆?？赡鼙粖A緊在工件上。在大多數(shù)情況下，工件通過鉆模夾緊，利用鉆模是為了在加工過程中，實(shí)現(xiàn)工件被快速插入和快速移開。
鉆模可能是為了最快速度實(shí)現(xiàn)鉆、擴(kuò)、鉸孔，并且實(shí)現(xiàn)比人工加工孔更準(zhǔn)確。另一個優(yōu)點(diǎn)是在使用鉆模不使工人造成傷害。孔定位的準(zhǔn)確。不靠操作者保證，而是靠鉆模來保證鉆模應(yīng)該被 只是用來鉆、擴(kuò)、鉸孔。它如果在機(jī)器上不能固定的話，可能會繞著鉆床移動而直接帶動鉆頭下面的鉆模套。鉆模直接限制和控制刀具的路徑。
如果加工中包括銑、成型加工、車等操作的話。應(yīng)該使用夾具。夾具在加工過程中不包括特殊導(dǎo)向校正的話，用鉆模也行。
典型的鉆模和夾具
典型的鉆模中，在一個已加工表面中加工四個孔。在這個工件上有一個準(zhǔn)確的加工中心孔，并且位于中心孔和最終表面。沒有必要去控制圍繞孔腔軸線的圓周位置。因?yàn)橹钡礁骺妆患庸r，它都是相對于該軸心對稱的。這四個襯套被用于控制鉆模被 夾緊在鉆模盤，用手動螺母去夾緊工件，使其緊貼在夾具上。
典型的夾具是一個簡單的在工件表面加工槽的銑操作。工作被固定在基面下四個孔中的兩個。工件被夾緊在這個位置上，刀具被緊貼在刀具安裝或刀具定位切削嘗試的安裝塊上。夾具必須相對于機(jī)器的工件定位，這是通過把夾具底部的兩個楔頭做入機(jī)器工作臺上開槽來達(dá)到的。夾具對于T型螺栓加工槽來說是安全的。