DM308 Part 3 Jig Fixture Design PDF

DM308 Production Techniques 2 Jig and Fixture Design Xichun Luo [email protected] Room JW7.04j; Tel: 0141 574 5280 Contents, learning outcomes and assessment NC introduction (Not be assessed in exam) Basic concept of jig and fixture Principles of locating and clamping Application cases To unde...

DM308 Production Techniques 2 Jig and Fixture Design Xichun Luo [email protected] Room JW7.04j; Tel: 0141 574 5280 Contents, learning outcomes and assessment NC introduction (Not be assessed in exam) Basic concept of jig and fixture Principles of locating and clamping Application cases To understand the basic principles of locating and clamping for jig and fixture 12 multiple-choice questions (12 marks) References Jig and Fixture Design, by Edward G Hoffman, Delmar Cengage Learning, ISBN:978-1-4018-1107-5 Advanced Fixture Design for FMS, by A. Y. C. Nee, Jones, Springer-Verlag, ISBN: 354019908X Jigs and Fixtures Jigs and fixtures are essentially locating and clamping devices for workpieces during machining operations. Typically jigs are used in drilling operations and fixtures in milling, turning, grinding and similar operations. A jig will have a means of locating and clamping a workpiece plus a means of guiding the cutting tool during the cutting process. It is also likely to move in at least one plane to ensure the cutting tool is able to freely follow the tool guide Drill Jigs A Drill Jig A Compound Angle Drill Jig A fixture will have a means of locating and clamping a workpiece plus it may have a means of setting the cutting tool before the cutting process begins. It is often clamped to a machine tool table or a chuck. A Milling Fixture A Milling Fixture for Multiple Parts WORKPIECE LOCATION Simple confinement of a workpiece does not ensure location. For example clamping a part in a lathe chuck will constrain the part but the part could still adopt an infinite number of positions. Unless location points are added that will ensure the workpiece, or identical workpieces, can only be held in one specific position then the workpiece cannot be said to be located. For this reason we need to recognise the six degrees of freedom that a free body has in space and consider ways by which each of these can be removed to ensure precise location of the workpiece in position and orientation each time it is worked on in the jig or fixture. The Six Degrees of Freedom Pitch Yaw Roll 6 DOF - 3T & 3R 1 DOF - 1T 3 DOF - 2T & 1R 0 DOF Location Requirements 1)The six degrees of freedom and the criteria for their restriction 2)The relative merits of available points for location 3)The condition of the locating surface 4)The shape of the part 5)The surfaces to which registration is required The test of a locating device will be how it satisfies the above requirements. Selecting the Position of Locating Points Clark Maxwell’s Principle (1876) This defines how to select the most suitable bearing points for location. “If one of the bearings by which a three legged part is located were removed, the direction in which the corresponding point of the instrument would be left free to move by other bearings must be as nearly as possible normal to the tangent plane at the bearing.” However there is no need to memorise this! A more practical manifestation of this Principle can be found in the: 3-2-1 Principle of Location The 3-2-1 Principle of Location “To locate a part fully, place and hold it against three points in a base plane, two points in a vertical plane, and one point in a plane square with the first two.” 4 Plate to be located 5 1 3 2 6 Y X Z Points 1, 2, and 3 eliminate three degrees of freedom, and the addition of points 4 and 5 eliminates two more and point 6 makes the location complete thus allowing no degrees of freedom. Thus 6 points give a positive location to the part. Classwork Two methods of machining a workpiece, which one is better? The Condition of the Locating Surface When choosing the best form of location for a surface, the surface condition must be considered. Basically there are three types of surface: 1)Finished surfaces which have dependable geometric truth and linear tolerances. 2)Semi-finished surfaces that have been machined but are subject to deviations from the truth. 3)Rough surfaces which are unmachined and show wide variations. A finished surface within certain limits may be located against a fully contacting surface rather than locating points, i.e. in the previous example the plate may be located against a flat surface rather than points 1, 2, and 3. These surfaces may be located on four points rather than three for reasons of stability although this can introduce a new difficulty – consider the problems with unsteady four legged restaurant or café tables. When using the four points compared to three if one pad wears out, or a piece of grit is left on a pad, the workpiece will rock as it will not sit flat, and if clamped in such a position the part may warp. 4 5 1 3 2 6 Flat surface instead of points 1,2,&3 A rough surface can only be located on a three point location. If extra location points were used this would result in serious rocking of the part due to its unevenness of surface. If a rough surface is brought to bear against a continuous surface, then it will be found that it is only the high spots of the rough surface that will be in contact. Thus a relieved location should be used such that only the outer edges bear, allowing the part to settle in a much more stable position. Diamond turning space mirror James Webb Space Telescope (www.jwst.nasa.gov) Cranfield Nanocentre Machining setup The world most accurate space optics of this kind was obtained : Form error < 20 nm Surface roughness (Rq): < 5 nm Slicer mirror 21 Spatial positioning error of individual mirrors: < 20 um DM308 Production Techniques 2 Jig and Fixture Design Xichun Luo [email protected] Room JW7.04j; Tel: 0141 574 5280 Design should always carry allowances for swarf that is not cleared. Workpiece Fixture Poor Better Plane Location and Swarf Consideration Drill Workpiece Support Clearance hole for drill Swarf clearance but no support for workpiece Redesign of support to prevent distortion due to drilling force Chucks are also locating devices. 4 Jaw chuck for irregular shaped workpieces 3 Jaw chuck for circular, triangular, or related work shapes Workpiece Vee not fixed may be adjustable or spring loaded Fixed Vee Drill Bush Obtaining external concentric location using Vee Blocks Work piece A redundant location is said to exist when two locators are attempting to constrain one degree of freedom from two location points. This must be avoided. In the illustration below the purpose of pin 2 is to prevent rotation about pin 1 but the result of this design means that both pins are attempting to constrain the workpiece along X-X. Not practical as both workpiece and location system would have to be perfect. Redundant location can happen in other designs, e.g. two fixed Vee locators. N.B. All pins should have 45o chamfers top and bottom. ‘A’ Avoiding redundant location ‘Diamond Pin Locator’ The second locator must be shaped as shown so that it will only influence the workpiece along A - A ‘A’ ‘A’ ‘A’ Max width of locator slightly smaller than diameter of location hole. Another Example of Redundant Location Incorrect Correct Clearance Redundant Location Each location hole and mating peg diameter would have to be ‘perfect’.. Only one diameter used for location. Principles of Clamping The purpose of clamping is to bring a workpiece against the points and surfaces which locate it and hold it there securely against all disturbing forces. To fulfil this a clamp must direct and maintain a force. Considerations Direct clamping pressure to supported and/or rigid positions of the work so as not to distort the workpiece. Facilitate easy and rapid loading and unloading. Maintain required workpiece location. Provide minimum hazards to operator, workpiece, fixture, and cutting tool before, during, and after the work cycle. If possible allow its incorporation as an integral part of the machine. Position of Clamps As one of the basic duties of a clamp is to hold the workpiece securely against the points with which the workpiece is located, the position of these location points must strongly influence the clamp position. A B C The locating points are A, B, C. The component of the clamping force normal to the plane should pass through the shaded area. F1 F2 The force F1 creates an unopposed moment and should be avoided. The correct application of the force, F2 is acting between the locators. F3 The force F3 must be within the zone shown for stability. FH The extension of the line of action of the single horizontal clamping force, FH must pass between the outside locating points.