Workholding Devices for Machine Tools PDF

University of the Philippines Diliman Department of Mechanical Engineering Workholding Devices for Machine Tools Lecture 9 Workholding Device ▪ Jigs and Fixtures. ▪ Critical to repeated manufacturing to with high degrees...

University of the Philippines Diliman Department of Mechanical Engineering Workholding Devices for Machine Tools Lecture 9 Workholding Device ▪ Jigs and Fixtures. ▪ Critical to repeated manufacturing to with high degrees of accuracy and precision. ▪ Hold one or multiple parts in one or multiple machine centers to provide stability and repeatable alignment of the part. University of the Philippines Diliman – Department of Mechanical Engineering Conventional Fixture Designs ▪ Two fundamental functions of workholding devices : locating and clamping. Locating - orienting and positioning the part relative to the cutting tool. Clamping - holding the part in its proper orientation with enough force to resist the force of cutting but not deform the part. University of the Philippines Diliman – Department of Mechanical Engineering Example of a workholder FIGURE 25-1 A conventional engine lathe with principal parts named, including the workholder device, the chuck. University of the Philippines Diliman – Department of Mechanical Engineering Example of a workholder FIGURE 25-1 A CNC turning center with two chucks, turrets for cutting tools, and C-axis control for the main spindle. The C-axis control, on the spindle, can stop it in any orientation so the powered tools can operate on the workpiece. University of the Philippines Diliman – Department of Mechanical Engineering Jigs and Fixtures ▪ Jig is a special workholding device that, through built-in features, determines location dimensions that are produced by machining or fastening operations. ▪ Fixture is a special workholding device that holds work during machining or assembly operations and establishes size dimensions ▪ General purpose clamps and chucks are not fixtures or jigs. University of the Philippines Diliman – Department of Mechanical Engineering Location versus Sizing plate showing locating dimensions (a, b, c, d) versus sizing dimensions (e, f, g, h). University of the Philippines Diliman – Department of Mechanical Engineering University of the Philippines Diliman – Department of Mechanical Engineering 25.3 Design Steps ▪The classical design of a workholder (e.g., a drill jig) involves the following steps: 1. Analyze the drawing of the workpiece and determine (visualize) the machining operations required to machine it. Note the critical (size and location) dimensions and tolerances. 2. Determine the orientations of the workpiece in relation to the cutting tools and the movements of the tools and tables. 3. Perform an analysis to estimate the magnitude and direction of the cutting forces 4. Study the standard devices available for workholders and for the clamping functions. Can an off-the-shelf device be modified? What standard elements can be used? University of the Philippines Diliman – Department of Mechanical Engineering Design Steps, cont. 5. Form a mental picture of the workpiece in position in the workholder in the machine tool with the cutting tools performing the required operation(s). 6. Make a three-dimensional sketch of the workpiece in the workholder in its required position to determine the location of all the elements: clamps, locator buttons, bushings, and so on. 7. Make a sketch of the workholder and workpiece in the machine tool to show the orientation of these elements with respect to the cutting tool in the machine tool. University of the Philippines Diliman – Department of Mechanical Engineering 3-2-1 Location Principle The 3-2-1 location principle is used to ensure that every part placed in the device occupies the same position with respect to the cutting tools ▪ The principle is based on first establishing a plane, locating the part on three fixed points. ▪ Then locating the part to a second plane, perpendicular to the first by using two points. ▪ And finally locating the part relative to the first two planes by establishing a third plane perpendicular to the first two planes using a single point. University of the Philippines Diliman – Department of Mechanical Engineering University of the Philippines Diliman – Department of Mechanical Engineering Clamping Considerations ▪Clamping forces ▪ do produce stresses in the part, excess clamping forces can cause distortion ▪ should be in the direction of cutting forces ▪ should be designed such that the cutting forces work against the fixed portion of the clamp, not the movable portion ▪ should be as near in alignment with the cutting forces to minimized torsional moment University of the Philippines Diliman – Department of Mechanical Engineering Clamping Examples University of the Philippines Diliman – Department of Mechanical Engineering Chip Disposal ▪ Jigs and Fixture need to accommodate chip removal ▪ Proper clearances need to be made to ensure chips do build up, increasing heat in the tool. ▪ Chips must also be easy to remove after machining so that they do not interfere with the alignment of the next workpiece. University of the Philippines Diliman – Department of Mechanical Engineering Proper Chip Clearance University of the Philippines Diliman – Department of Mechanical Engineering Unloading and Loading Time ▪ Time to clamp and unclamp a workpiece can reduce the rate of production. ▪ Clamp design should minimize the motion needed to remove a part. ▪ Cams latches are faster mechanisms than screw mechanisms. Cycle Time For A Part = Loading and Unloading Time +Machining Time + Delay Time. University of the Philippines Diliman – Department of Mechanical Engineering Types of Jigs ▪ There are several basic forms for jigs, some of the basic types are: ▪ Plate Jig ▪ Channel Jig ▪ Ring Jig ▪ Leaf Jig ▪ Box Jig ▪ Universal Jigs University of the Philippines Diliman – Department of Mechanical Engineering Types of Jigs University of the Philippines Diliman – Department of Mechanical Engineering Types of Jigs University of the Philippines Diliman – Department of Mechanical Engineering Types of Jigs Universal Jig University of the Philippines Diliman – Department of Mechanical Engineering Conventional Fixtures Conventional Fixtures ▪ Vise are general purpose fixtures mounted on subplates and can have their jaws interchanged base on part geometry. ▪ Lathe Chucks are general purpose fixtures for rotational parts University of the Philippines Diliman – Department of Mechanical Engineering Modular Fixturing ▪ Modular Fixtures are similar to conventional fixture, except they are more versatile. ▪ Modular systems use dowel pins and T-slots to provide a rigid, adjustable fixture. ▪ Standard elements are positioned to fit the part needs, such as ▪ Riser blocks Vee blocks ▪ Angle plates Cubes ▪ Box parallels Supports ▪ Locator pins Clamps University of the Philippines Diliman – Department of Mechanical Engineering Modular Fixtures University of the Philippines Diliman – Department of Mechanical Engineering Modular Fixture University of the Philippines Diliman – Department of Mechanical Engineering Setup and Changeover ▪To speed up changeover, master jigs or intermediate jigs can be used. ▪ A Master Jig, is a jig that can be used to make a number of similar parts. ▪ An Intermediate Jig is a jig that is designed hold another jig that can be quickly changed out for each part. University of the Philippines Diliman – Department of Mechanical Engineering Master Jig FIGURE 25-15 Master jig designed for a family of similar components. (a) Part family of rounds plates (six parts, A–F); (b) group jig for drilling, showing adapter and part A. University of the Philippines Diliman – Department of Mechanical Engineering Intermediate Jig FIGURE 25-16 Example of the intermediate jig concept applied to lathe chucks. The actuator is mounted on the lathe and can quickly adapt to three different chuck types. (Courtesy of Sheffer Collet Company.) University of the Philippines Diliman – Department of Mechanical Engineering Clamps FIGURE 25-17 Examples of basic types of clamps used for workholding. The clamp elements come in a wide variety of sizes. University of the Philippines Diliman – Department of Mechanical Engineering Power Actuated Clamps FIGURE 25-18 Examples of power-clamping devices: (a) extending clamp; (b) edge clamp. University of the Philippines Diliman – Department of Mechanical Engineering Other Workholding Devices ▪Other workholding devices include ▪ Assembly jigs ▪ Used to ensure the final assembly meets the location and fit ▪ Magnetic workholders ▪ Limited in holding force, but ensures that there is no distortion of a steel workpiece ▪ Electrostatic workholders ▪ Similar to magnetic chucks, but used on electrically coductive non-ferromagnetic materials, limited clamping force ▪ Vacuum Chucks ▪ Works with any material, initial set up more time consuming. University of the Philippines Diliman – Department of Mechanical Engineering Assembly Jig FIGURE 25-19 Example of large assembly jig for an airplane wing. The body of the wing and flap are held in the correct location with each other and then the flap is mechanically attached. University of the Philippines Diliman – Department of Mechanical Engineering Assembly Jig University of the Philippines Diliman – Department of Mechanical Engineering Welding Jig University of the Philippines Diliman – Department of Mechanical Engineering Magnetic Workholders FIGURE 25-20 Example of magnetic chuck. (Courtesy of O. S. Walker.) University of the Philippines Diliman – Department of Mechanical Engineering Electrostatic Workholders FIGURE 25-21 Principle of electrostatic chuck. University of the Philippines Diliman – Department of Mechanical Engineering Vacuum Chuck FIGURE 25-22 Cutaway view of a vacuum chuck. (Courtesy of Dunham Tool Company, Inc.) University of the Philippines Diliman – Department of Mechanical Engineering T-Slot Table FIGURE 25-23 While the work can be clamped directly to the milling machines table (on the left), the workpiece on the right is located in a fixture mounted in a circular attachment located on the table, so a circular slot can be milled in the workpiece. University of the Philippines Diliman – Department of Mechanical Engineering Economic Justification of Jigs and Fixtures ▪To determine the economic justification of any special tooling, the following factors must be considered: ▪ 1. The cost of the tooling ▪ 2. Interest or profit charges on the tooling cost ▪ 3. The savings resulting from the use of the tooling; can result from reduced cycle times or improved quality or lower-cost labor ▪ 4. The savings in machine cost due to increased productivity ▪ 5. The number of units that will be produced using the tooling University of the Philippines Diliman – Department of Mechanical Engineering