Jigs and Fixtures Jig Fixture 1. It is a work holding device that 1. It is a work holding device that holds, supports and locates the holds, supports and locates the workpiece and guides the cutting workpiece tool for a specific operation operation but does not guide the 2. A jig's provide and primary purpose repeatability, is to accuracy, interchangeability manufacturing of products in the for a specific cutting tool 2. The main purpose of a fixture is to locate and in some cases hold a workpiece during either a machining operation or some other industrial process 3. A device that does both functions (holding the 3. Fixtures are specific tools used particularly in milling work and guiding a tool) is machine, shapers and slotting called a jig 4. The jigs are special tools particularly reaming, in drilling, tapping boring operation 5. Lighter in construction and machine 4. Heavier in construction Conventional process-without Jig & Fixture marking on the blank showing the location of the hole and its axis on the blank punch the centre at the desired location and prick punch the periphery of the hole to be made in the disc mount the blank in a drilling vice using parallel block, a small Vee block etc. to provide support and clamp the blank firmly position the vice along with the marked blank to bring the hole axis in alignment with the drill axis by either adjusting the vise position w.r.t. the fixed drill axis or moving the drilling machine table and then locking the table position or moving the radial arm and the drilling head, if it is a radial drilling machine after fixing the blank, vise and the table, alignment is checked again if error, like eccentricity, is found to occur then readjustment of location of the hole – axis is to be done before and even after starting drilling drilling is accomplished The basic purposes of developing and using suitable jigs and fixtures for batch production in machine shops are : 1. to eliminate marking, punching, positioning, alignments etc. 2. easy, quick and consistently accurate locating, supporting and clamping the blank in alignment of the cutting tool 3. guidance to the cutting tool like drill, reamer etc. 4. increase in productivity and maintain product quality consistently 5. to reduce operator’s labour and skill – requirement 6. to reduce measurement and its cost 7. enhancing technological capacity of the machine tools 8. reduction of overall machining cost and also increase in interchangeability The following factors are essentially considered during design, fabrication and assembly of jigs and fixtures 1. easy, quick and consistently accurate locating of the blank/workpiece in the jig or fixture in reference to the cutting tool 2. providing strong, rigid and stable support to the blank 3. tool guidance for slender cutting tools like drills and reamers 4. easy and quick loading and unloading the job to and from the jig or fixture 5. use of minimum number of parts for making the jig or fixture & use of standard parts 6. reasonable amount of flexibility or adjustability- job - dimensions. 7. prevention of jamming of chips, i.e. wide chips-space and easy chip disposal 8. easy, quick and accurate indexing system if required 9. easy and safe handling 10.easy and quick removal and replacement of small parts 11. manufacturability i.e. ease of manufacture 12.durability and maintainability 13.service life and overall expenses For accurate machining, the workpiece is to be placed and held in correct position and orientation in the fixture (or jig) which is again appropriately located and fixed with respect to the cutting tool and the machine tool. It has to be assured that the blank, once fixed or clamped, does not move at all Some basic principles or rules need to be followed while planning for locating blanks in fixtures, such as 1. One or more surfaces (preferably machined) and / or drilled / bored hole(s) are to be taken for reference 2. The reference surfaces should be significant and important feature(s) based on which most of the dimensions are laid down 3. Locating should be easy, quick and accurate 4. In case of locating by pin, the pins and their mounting and contact points should be strong, rigid and hard 5. A minimum of three point must be used to locate a horizontal flat surface 6. The locating pins should be as far as possible 7. Vee block and cones should be used for self-locating solid and hollow cylindrical jobs Locating by flat surfaces Locating by holes Supporting – principles and methods A workpiece has to be properly placed in the jig or fixture not only for desired positioning and orientation but also on strong and rigid support such that the blank does not elastically deflect or deform under the actions of the clamping forces, cutting forces and even its own weight Basic principles or rules to be followed while designing or planning for supporting 1. supporting should be provided at least at three points 2. supporting elements and system have to be enough strong and rigid to prevent deformation due to clamping and cutting forces 3. supporting should keep the blank in stable condition under the forces 4. for supporting large flat area proper recess is to be provided round or cylindrical workpieces should be supported (along with locating) on strong vee block of suitable size 5. heavy workpieces with pre-machined bottom surface should be supported on wide flat areas, otherwise on flat ended strong pins or plugs. 6. if more than three pins are required for supporting large workpieces then the additional supporting pins are to be spring loaded or adjustable Clamping 1: Template Jig: This is the simplest type of jig; It is simply a plate made to the shape and size of the work piece; with the require number of holes made it. It is placed on the work piece and the hole will be made by the drill; which will be guided through the holes in the template plate should be hardened to avoid its frequent replacement This type of jig is suitable if only a few part are to be made 2: Plate Type Jig: This is an improvement of the template type of jig. In place of simple holes, drill bushes are provided in the plate to guide the drill. The work piece can be clamped to the plate and holes can be drilled. The plate jig are employed to drill holes in large parts, maintaining accurate spacing with each other. 3:Open Type Jig: In this jig the top of the jig is open; the work piece is placed on the top. 4:Channel jig; The channel jig is a simple type of jig having channel like cross section. The component is fitted within the channel is located and clamped by locating the knob. The tool is guided through the drill bush. 5: Leaf Jig: It is also a sort of open type jig , in which the top plate is arrange to swing about a fulcrum point , so that it is completely clears the jig for easy loading and unloading of the work piece. The drill bushes are fitted into the plates , which is also known as leaf , latch or lid. When the holes are to drill more than one plane of the work piece , the jig has to be provided with equal number of bush plates.