Jigs and Fixtures
MIDHUN K. G RISHIKESH KAUSHIK SHYAM KUMAR SUSHILDEEP KUMAR
AGENDAIntroduction Research Objectives Methodology Taxonomy Welding Jigs & Fixtures Design Consideration Advantages Conclusion
DEFINITIONFIXTURE: a device used to hold, position and/or align a work piece for an operation or process. Shipyard uses include assembly, subassembly, foundation and part fabrication. JIG: a device used to guide a tool; a template. Shipyard uses include pin tables and other devices for assembly of foundations, bulkheads, assemblies and parts. Jigs are often incorporated into fixtures.
Jigs and fixtures are production tools used to accurately manufacture , duplicate and interchangeable parts Jigs and fixtures are specially designed so that large numbers of components can be machined or assembled identically, and to ensure interchange ability of components. jigs and fixtures position components accurately and hold components rigid and prevent movement during working in order to impart greater productivity and part accuracy
DIFFERENCE BETWEEN JIGS AND FIXTURESJIGS: Jigs are used to hold the work piece and also to guide the tool while performing the operation. Jigs are mostly used for drilling operation FIXTURES: Fixtures are used only to hold the work piece and it wont guide the tool. Fixtures are mostly used for milling operation
ELEMENT OF JIGS AND FIXTUREJigs and fixture consist of three elements : Locating element Clamping element Tool guiding and setting element
CONTDuLOCATING ELEMENT : These position the work piece accurately wrt. the tool guiding or setting element in the fixture CLAMPING ELEMENT : These hold the work piece securely in the located position during operation. TOOL GUIDING AND SETTING ELEMENT : These guiding or setting the tools in the correct position with respect to the work piece . Drill bushes guide the drill accurately to the work piece. Milling fixture use setting for correct positioning of milling cutters with respect to the work piece.
RESEARCH OBJECTIVESIncreased 1st time quality = Lower Costs accuracy fairness gap minimization Increased Productivity = Lower Costs less man-hours man increase throughput lessen skilled labor requirement
METHODOLOGY1. Survey and Classify 2. Preliminary Design New System of Jigs or Fixtures 3. Economic Analysis 4. Final Design New System of Jigs or Fixtures 5. Documentation and Dissemination
WHY COMPLETE A TAXONOMY ?It involves a relatively exhaustive search Relationships between different jigs and fixtures are highlighted Through understanding Plan for Improvement
JIGS & FIXTURESPortable Fixtures Stationary Fixtures Guiding Jigs Restraining Jigs
PORTABLE FIXTURESPortable Platen Shaft Fixture w/ Rollers
This platform can be relocated by truck.
It is metal and creates a non permanent workspace.
SHAFT SUPPORT WITH ROLLERS
This fixture supports the shaft in position and occasionally rotates to prevent sagging of the shaft.
Cradle Metal Columns Mocks
This metal table made of a grid framework allows slag to fall through the spaces.
CRADLELarge wooden cradles are used to support and maintain the shape of the hull during the erection stage.
METAL COLUMNSRound Pipe supports, fixed in height provide adequate room for a crawler to drive under.
Wooden chocks and wedges are used where necessary to level the unit.
THREADED ADJUSTABLE RUDDER MOCKSupport for the Rudder unit has been constructed to match the unit shape. It is used only once for a single hull design.
This fixture holds the rudder while bearings are installed during assembly of the rudder.
This mock resembles a mold of the inner bottom shell plating. The shape is maintained while stiffeners are welded.
GUIDING JIGSPin Jigs Pipe Jigs Wireways
PIN JIGSUnique shaped panels can be formed over the Pin Jigs.
Threaded pipes are positioned in a grid and pins are used to adjust their heights.
PIPE SUPPORTSPipe supports are used to hold irregular shaped units in position while being outfitted.
They may also be used during unit erection.
These trees help to guide ductwork or gas lines so that access can be maintained during construction.
RESTRAINING JIGSCable and Stay Chains Clamps Pipe Supports
CABLE & STAY
Cable and stays are used to maintain vertical alignment prior to welding.
This cable and stay is adjustable via the use of a ratcheting come along.
Chains are used in the Pipe shop to restrain or fasten pipes to the work table.
Clamps are used to hold the work piece in fixed position for the intended work to be performed.
ADJUSTABLE PIPE SUPPORTSPipe support on wheels help make the work piece movement easier than use of a crane and hold the work piece in position.
DOWN SELECT PROCESSTranslate Discussions to Customer Requirements Establish Production Needs Translate to Required Attributes
Participants Planning Mold Loft Pipe Shop Machine Shop Unit Assembly Accuracy Control Ship Erection Package Units Maintenance
PIN JIG CONCERNSRust and Corrosion Desire Manual Adjustment Must Manage Horizontal Component Jig Must Not Distort Work Piece under Load
WELDING JIGS & FIXTURES
WHY ???To minimize distortion caused by heat of welding. To permit welding in a more convenient position. To increase welding efficiency and productivity. To minimize fit-up problems. fit-
WELDING JIGSWelding jigs are specialized devices which enable the components being welded to be easily and rapidly set up and held. held. QUALITY REQUIRED: REQUIRED: Rigid and strong since it has to stand contractional stresses without deforming. Simple to operate, yet it must be accurate. Designed such that it is not possible to put the work in it the wrong way. Faced with wear-resistant material to stand wearcontinual wear.
WELDING FIXTURESA welding fixture serves the same purpose as a welding jig; but, in addition, it permits the changing of the position of the work during the actual welding, so as to place the welds in a plane convenient to the operator at all times This increases welding speed. Fixture Characteristics Supporting Clamping Grounding and Imparting movement (usually referred to as travel or traveling) to name but a few
WELDING FIXTURE CLASSIFICATIONThose that act on the work being welded: clampthe parts together while simultaneously supporting them, the objective being to effect a weld in the flat position. position. hold the work stationary in which case the welding equipment moves or is traveled over it or may be mechanized and move the work under the stationary welding equipment. Includes rotating positioners, power turning rolls, lathes, and mill type setups etc.
Those that act on the welding equipment:provide only stationary support or both support and travel to the welding equipment. include the beam with carriage, carriage, the self-propelled tractor, the spud welding unit selftractor, and a variety of boom type welding manipulators. manipulators.
FIXTURE SELECTIONIn selecting fixtures for mechanized welding, the engineer must evaluate considerations such as Production volume Joint geometry Movability of the welding head.
DESIGN OF GOOD WELDING JIGS AND FIXTURES The fixture should be strong and light but rigid enough to ensure accurate alignment. alignment. Whenever possible, a fixture should be positioners, enabling all positioners, welds to be brought to a convenient welding position. position. The fixture should permit quick and easy positioning (by one hand, if possible). Thus, balancing of the fixture may be necessary. Design should be as simple and inexpensive as possible; accuracy and elaboration should be no greater than required. Only essential dimensions should be controlled in a fixture. Appearance should be disregarded. The use of light alloys for moving parts reduces weight. Air or revolving, electric motors should be used for revolving, and air or hydraulic rams for tilting the fixture assembly.
CONTDu should be built around the work and should locate and clamp components in position so that assembly, tacking, and welding may be carried out in one fixture. should permit freedom of movement in one plane to avoid residual stress in the completed weldment. Design should permit heat dissipation to release, rather than bind, the assembly being welded. bind, Joints must be readily accessible for welding. By slots or other means, the fixture should readily present seams on the reverse side of the object Fixtures should be kept cool enough to handle air, water, fins or insulated handles can be used. Parts should be prebent in the fixture, if necessary, for final accuracy.
CONTDu Clamps must operate quickly. Screws and moving parts quickly. should be protected against weld spatter. spatter. Either integral or separate copper backing bars should be used in cases of poor fit or for light plate. Use of nuts and bolts, wrenches, C-clamps, wedges and bolts, wrenches, clamps, hammers and hand screws should be minimum. minimum. Methods of grounding the work are important considerations in fixture design, since they affect arc action, quality of weld, and speed at which welds are produced. The ground must make good contact with the work through the medium of copper, copper graphite brushes, sliding shoes etc.
CONTDu A good fixture design avoids the problem of arc blow by taking following measures: (a) All steel other than being welded should be kept .at arc. least 25 mm from the arc. (b) Clapping fingers may be made from a non-magnetic nonmaterial. (c) The ground connection should be made directly with the work and not with the backing bar. The point of connection should be as far from the arc as possible. The ground can be split to offset arc blow. (d) When possible, the fixture should be built from low normalicarbon steel, large masses of steel should be normalisteel, zed to minimize residual magnetism. (e) When horn type fixtures are used, the welding should be toward the closed end.
CONTDu The fixture should be designed for ease of accessibility to the welding head when procedure adjustments and servicing are necessary. Accessibility is also necessary in the positioning of welding controls. The design of fixture should be such that the weldment can be easily and quickly removed from the fixture.
TYPES OF JIGS AND FIXTURESA C-clamp is good enough for most welding jobs. It can be quickly and cheaply made in the shop. A simple, but effective V rest for supporting short bars or pieces of pipe for tacking or welding on the bench Rotating fixtures for making circumferential welds or for building up rollers, rings, valves, drilling bits and axles are very valuable, as they speed up the job and make for neater welds with less waste. A few clamps for sheet or plate joints To turn over very large pieces of work for positioning in welding, it is often useful to build adapters which bolt or clamp to the work and contain the trunnions. A roller bed for welding pressure vessels, containers and other cylindrical objects.
DESIGN PRINCIPLEThe 3-2-1 Principle of Jig Fixture Design 3For a fixture designer, the major portion of design time is spent deciding how to locate the work piece in the fixture. You know that any free body has a total of twelve degrees of freedom as below: 6 translational degrees of freedom: +X, -X, +Y, -Y, +Z, -Z 6 rotational motion(3 clockwise & 3 anticlockwise) about respective axes. It must required to fix all the 12 degrees of freedom except the three transitional degrees of freedom (-X, -Y (and -Z) in order to locate the work piece in the fixture. So, 9 degrees of freedom of the work piece need to be fixed.
BUT, HOW?Rest the work piece on three non-collinear points of the nonbottom surface (XY), and you will be able to fix the +Z, CROT- ACROT- CROTCROT-X, ACROT-X, CROT-Y and ACROT-Y degrees of ACROTfreedom. Now, rest the work piece at two points of side surface (XZ), and you will be able to fix the +Y and ACROT-Z ACROTdegrees of freedom. Now, rest the work piece at one point of the adjacent surface (YZ), and you will be able to fix the +X and CROTCROT-Z degrees of freedom. So, you can successfully fixate 9 required degrees of freedom by using the 3-2-1 principle of fixture design. 3-
DESIGN CONSIDERATION FOR JIGS AND FIXTUREDesign consideration for jigs and fixture depends upon following three factors. Tooling cost Tooling detail and Tooling operation These design consideration are independent of each other.
TOOLING COSTSThe total cost of any jig or fixture is frequently the major area of consideration many work holder designs. Permanent fixtures have distinct advantages in the production of high-volume and high-precision parts. highhighIt reduce machine setup time, machine cycle time and the level of operator skill required to produce satisfactory quality output GeneralGeneral-purpose work holders are more expensive than temporary tools in most cases, but their utility and flexibility often allow these work holders to be regarded as a capital cost to be amortized over a period of time without regard to actual usage.
CONTDuModular fixturing is typically a capital investment to be amortized over a set lifespan, with an average cost assigned to usage for each anticipated job. Modular fixturing is typically a capital investment to be amortized over a set lifespan, with an average cost assigned to usage for each anticipated job. GeneralGeneral-purpose tools are reused extensively, but still incur some costs for maintenance and storage. Modular fixtures will be disassembled, and the components maintained, stored, and reused frequently.
TOOLING DETAILSIt is the overall construction characteristics and special features incorporated into the jig or fixture. depends upon the type and complexity of the individual tooling elements, the extent of secondary machining and finishing operations on the tool, the tooltool-design process, and the amount of detail in the work holder drawings. Permanent work holders are designed and built to last longer than temporary work holders. Permanent work holders contain different commercial tooling components based on expected tool usage.
CONTDu. Permanent jigs intended for a high-volume drilling highoperation. The secondary operations normally associated with tooling include hardening, grinding, and similar operations to finish the work holder permanent work holders are hardened and ground to assure their accuracy over a long production run.
TOOLING OPERATIONSThe performance of any work holder is critical to the complete usefulness of the tool. he performance of a permanent, modular, or generalgeneral-purpose work holder is considered, several factors about the machine tools must be known. These factors include the type, size, and number of machine tools needed for the intended operations. Figure 1-6 shows a typical temporary work 1holder for drilling and milling operations on the same part
ADVANTAGES OF JIGS & FIXTURES cost savings precise and accurate increase productivity Interchangeability less skill requirement Time saving Alignment of a part or work piece. Securing the part or work piece. Duplication of a process.
CONTDu Maintain or improve part quality. Reduce cost of production. Improve overall safety to the part, the operator, and the machine.
CONCLUSIONBoth the jigs and the fixtures are used to reduce the nonproductive time of any mass production process. The 3-2-1 method is the fundamental principle 3for all types of fixture design. There are varieties of jigs & fixtures are used in shipyards for different purposes It should be standardized at part and assembly level to reduce tooling and manufacturing costs.