Jigs and Fixture Essay
Jigs and Fixture
Jigs r provided with tool guiding elements such as drill bushes.These direct the tool to the correct position on the w/p. Jigs are rarely clamped on the m/c table bcz it is necessary to move the jig on the table to align the various bushes in the jig with the m/c spindle. Fixture hold the w/p securely in the correct position with respect to the m/c during operation. There is sometimes a provision in the fixture for setting the tool with respect to the w/p, but the tool is not guided as in a jig. Fixture are often clamped to the m/c table. Elements of J&F(Locating,Clamping,Tool Guiding & Setting Elements ) Adv of J&F(Productivity,Interchageability,Skill Reduction,Cost Reduction) Limits & Fits Diff b/w high and low limit , is called tolerance(unilateral,bilateral).
Classification of Fits (Running,Push,Press Fit,Force Fit) Cost Estimation(Matl,M/cing,Heat Treatment, Assembling Cost) Principles of Location (1 Location must be related to the dimensional rqmts stated on the components drawing 2) It is prefable to use a more accurately m/chined surface than a less accurate surface Location, 3) The w/p should be prevented from moving along and rotating around X,Y and Zaxes 4) Location sys should facilitate easy and quick loading and unloading of the w/p and aim at motion economy 5) Redundant Locators must be avoided 6) Location sys should positevely prevent wrong loading of the w/p by foolproofing. Locating Methods of Plane Surfaces 1 A reasonably flat suface can be located by 3 pins of equal height having spherical surfaces at the location points 2 A rough, uneven or tapered plane should be loacted by 3 adjustable location pins having spherical ends 3 Additional adjustable supports are neccesary to prevent vibrations or distortion of the w/p during m/c operation. The force for adjusting the supports should be kept minimum so that the w/p doesnot get dislocated or lifted from the location pins 4) A machined surface can be located by pads having flat surface.
5 There should be ample clearance for burr or dirt to ensure proper seating of the workpiece surfaces. 6 A cube can be prevented from linear movement and rotation around axes X,Y,Z by six location Pads. Profile 1 can be located approx by aligning it with a slightly bigger sighting plate 2 Locating pins can be used to locate a profile or cylindrical w/p 3 Variations in w/p sizes ffrom batch to batch can be taken care of by using eccentric locators whose eccentricity can be set to suit the batch. 4 W/p with little variation can be located precisely with nesting plates with suitable provision for unloading or ejection Cylinder Spigots used for locating bores should have ample lead for easy entry and their length should be short to prevent jamming of the w/p 2 Long locators for fragile w/p should be relieved at the centre 3 Location Posts which are also used for clamping , should be retained by a nut or a grub screw 4 When two location pins are used,the less important one should be made diamond shaped .
The important full pin should be longer than the diamond pin in order to facilitate easy loading of the w/p. 5 Rough cored holes and bosses are located by conical locators, which often have integral clamping arrangement and drill bush 6 Fixed V blocks are used to locate approx the outside surface of a cylinder. 7 For precise location, an adjustable guided V block is necessary.The V Block can be adjusted by a screw or a cam. It can be with drawn quickly by using a swinging eyebolt. 8 V blocks should be positioned in such a way that the varition in the w/p would not affect the location for the operation. For drilling central holes, the centre line of V should be vertical. 9 Clearance grooves should be provided at flash line of cast, forged and moulded w/p .
Principles of Clamping(Position, strength , prdouctivity,operator fatique, workpiece variation) Types of Clamps 1 Screw , 2 Strap (Retractable strap,swinging strap, edge , pivoted, hinged,swinging latch 3 Quick action (cam,bayonet, quarter turn screw, toggle) 4 Multiple Double, stacking, power clamping pnumatic,hydraulic,magnetic 5 Non Conventional Clamping (adhesive, Fusion- cast of low melting point bismut alloy) Indexing is a process of quick, accurate location of a w/p or fixture in a no of specific positions. Indexing involves periodic linear or rotary movement of the indexed part precisely in each position.
Rqmt of a good drill jig: 1 Quick & accurate location of the w/p 2 Easy loading and unloading of the w/p and prevention of wrong loading 3 Prevention of bending or movement of the w/p during drilling 4 Ample chip clearance with facilities for swarf removal and cleaning 5 Light weight to minimise operator fatique due to repeated handling 6 Prevention of loss of loose parts by chaining them to the jig body 7 Clearance for overshoot of the drill Drill Bushes( Press Fit, Renewable, Slip, Threaded, Special) Various types of Jigs(Plate jigs and channel jigs with w/p pots, Angle Plate jigs, Turn Over Jigs, Leaf or latch jigs, Box Jigs, Trunion , Sandwich and pump jigs, Jigs for multispindle m/cs Milling Fixtures Essentials (Strength, Thrust, Cutter Setting, Machine tennons, Rigid Clamping, Motion economy,swarf disposal)
Facing Fix , Sloting Fixture Boring Fix – Chucks(Self centring 3 jaw , Independent 4 jaw, combination, power operated, special jaws, soft jaws) Face Plate, Collets( Push out, pull in, dead length) Mandrels( Tapered, Axial, Expanding, Threaded) Fixture Module Design Concept. A standard self contained unit. It is jargon for standardised, easily connectable, replaceable, sub assembled unit like a timer or acontractor.Modular construction assembles the commercially available modules in an arrangement suitable for the rqd applications. Modular Construction is like universal setup but not built on m/c Modular Fixture base can have t Slots or no of reamed holes. Modular tooling eliminates nned for manufacture procurement of standard parts neccesary accesories like fasteners, springs, washers, etc. Inspection Devices Standard Gauges(Go not go, Calliper , snap), Special Gauges(Key way and slot checking, Flush surface gauge for tapered hole, snap gauge for checking centre dist)
Reciever gauges for assembly W/p marking and setting gauge Universal Jig- T nut –stud as T Bolt,Universal Clamp,edge calmp, Face plate Universal Fixture Transfer Machines(in Line tranfer machines (Walking beam, poered roller conveyor, chain drive), Rotary Indexing table m/c (Rack and pinion, ratchet and pawl, Geneva ) Tranfer Devices – Conveyor system(Roller , Wheel, Chute, Belt, Chain, Magnetic, Bucket ) Die is a specialized tool used in manufacturing industries to cut or shape material using a press. (Die block, Punch plate ,Blank punch, Pierce punch,Stripper plate, Pilot, Guide / Back gage / Finger stop, Setting (Stop) Block , Shank) . General Die Design(Strip layout and selection of tooling, Economies of the strip- acc to tool shape also, tonnage calculation and selection of the press, Die shoe size, Method of Parts Ejection)
Progressive Die Design(Washers and Round Blanks- 19.5, 30 , Feed=Blank dia. +Sp/tan 19.5 and strip width Pilots and pilot Holes, Skipping of stations, Nesting and Locating) Deep Drawing(Blank Holding- clearance 5%, pressure- minimum force necessary to prevent wrinkling, Drawing Ratios-Blank dia/throat dia of die. Punch load increases with blank dia in linear manner ,Die profile radius-sharper the die radius the greater is the max punch load bcz of the inreased process work due to plastic bending under tension, Punch profile radius- t-1.5 t 0.5t over 3.25mm sheetMore generous the punch radii , the more gradual is the rise of punch load and longer punch travel, but the max punch load is almost unaffected., Radial clearance-1.2t 30% for general purpose. Ironing-10%, Drawing speed can affect the yield stress of the material and efficiency of the lubricant, lubrication.
University/College: University of Arkansas System
Type of paper: Thesis/Dissertation Chapter
Date: 17 November 2016
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