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The jig and fixture design procedure

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Jigs and Fixtures: The jig and fixture design procedure,

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The jig and fixture design procedure

  1. 1. The Fixture Design Procedure Prof. Ms. Amruta A. Rane Assistant Professor, DJSCE, University of Mumbai, Mumbai.
  2. 2. Steps in Fixture Design Procedure 1. Locating 2. Clamping 3. Supporting 4. Applying cutter guides 5. Drawing the fixture outline as an envelop that combines all the previously drawn elements. In practice, one operates continuously with the part and hence the part outline should be drawn either in thin lines or in coloured lines.
  3. 3. Locating and Degree of Freedom • The forces acting (weight, clamping force, cutting forces) are not taken into consideration with respect to their magnitude. • Only direction of the forces are considered to ensure that part is located in a position of static stability.
  4. 4. The Six Degrees of Freedom • Figure shows a body that is free in space. • A body in this condition had six degrees of freedom, three of these are freedoms of translation and three are freedoms of rotation.
  5. 5. Locating and Degree of Freedom • Consider a rectangular block (casting for forging), with unmachined rough sides. • Locate the bottom surface on three points not in a straight line. • Assume hold-down forces acting on the block such that it can not be lifted off. • These three points prevent the motion in the vertical direction and rotation around a longitudinal and cross-wise axis. • The block is deprived of three degrees of freedom.
  6. 6. Locating and Degree of Freedom • Two locating points are added against one of the vertical sides. • Not in same vertical line. • Hold-down force is added. • This prevents the motion in crosswise direction and also rotation about vertical axis. • Thus deprives the block of two degrees of freedom.
  7. 7. Locating and Degree of Freedom • One locating point is added on a vertical wall. • Hold-down force against the end • This eliminates the sixth and last degree of freedom i.e. motion along the length.
  8. 8. Locating and Degree of Freedom • The addition of the forth point at the bottom surface would theoretically make the system redundant.
  9. 9. Locating and Degree of Freedom • In actual fixture design the very first step is to deprive the part of its six degrees of freedom by applying six individual locating points.
  10. 10. Clamping Elements • Hold-down forces in above example are clamping forces. • In practice they are applied by using bolts, straps, cams, etc. • The number of clamping elements used are not necessarily equal to the number of locating points.
  11. 11. Clamping Elements • One vertical clamp over the centre of the black would be sufficient to take care of three locating points. • One clamp acting on the corner and directed along the diagonal would take care of all six locating points. • Clamping force must be applied as directly as possible without causing any elastic deformation.
  12. 12. Support • Some support is provided by locating points. • The locators only provide sufficient support to ensure the geometrical stability of the part. • The locating points are not sufficient to absorb all the acting loads without causing elastic deformation of the part. • Support – Sufficient in number and strength to absorb all acting loads. • Should not interfere with locating points. • Should not interfere with loading and unloading. • Shouldn’t produce geometrical redundancy.
  13. 13. Cutter Guidance • To provide guidance for the cutter. • By providing the drill bushings in drill jigs • By providing the tool guides to position the tool prior to start of the cut in fixtures.
  14. 14. Completing the body • To draw the jig or fixture body that carries all the individual elements. • Has enough strength and rigidity to hold them in proper places under load. Jig or fixture body must accommodate following conditions: • Must accommodate the part • Have clearance for loading and unloading • Clearance for chips • Should have feet or some other supporting surface to place it on the machine table. • Locating elements for aligning it with machine table, machine spindle
  15. 15. The Six Point Location Principle • For processing the body, it is required to restrain all the degree of freedom (DOF). • Six pads showing a system of location and clamping that produces the effect necessary for complete constrain. Six-point Location
  16. 16. 3-2-1 Principle • It represents the minimum requirement for the locating elements. • The locators along with the clamps which hold the part in place, provides equilibrium for all the forces. • Do not necessarily guarantee stability during machining. • Stability is satisfactory if three rest buttons are widely spaced and resultant cutting force hits within the triangular area between the buttons. • It resultant cutting force hits outside the triangular area then it generates a moment which tends to tilt or overturn the part. Six-point Location
  17. 17. 4-2-1 Principle • By addition of the fourth locator in the base, the shape of the supporting area can be changed from a triangle to a rectangle. • For the rough castings one of the four base locators may be adjustable. • If the surface is machined then all the locators may be fixed.
  18. 18. References 1. Jig and Fixture Design Manual, Erik K. Henrikson, Industrail Press. 2. Jigs and Fixture, P.H. Joshi, THM. 3. An introduction to jig and tool Design, M.H.A. – Kempster, III Ed.Pub ELBS.
  19. 19. Thank You!

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