CAD Models of Movable Clamps in Fixture Design

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Bethany Sparks
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1 CAD Models of Movable Clamps in Fixture Design Wilson (Zhongming) Liang Purdue University Fort Wayne Abstract Students in the tool design class are taught to develop CAD models whose clamps are movable. With such models, students gain much better knowledge of the kinematics and functionality of the clamp as well as its design considerations such as the opening configuration of a clamp should provide sufficient space for loading and unloading of the workpiece. In addition, student gain better knowledge of use of the CAD system for general mechanical design. Introduction The tool and fixture design course, offered in each semester with an enrollment of twenty, is one of the several design courses in the curriculum of mechanical engineering technology. This three-hour course includes two hour lecture and one hour lab. The lecture part covers the basic principles and methods in fixture design 1,2 and in computer-aided tool and fixture design 3. The lab part teaches the basic design practice and skills. Since tool designers in industry typically create their design on CAD systems with importing standard component CAD models available from tooling companies, all the labs and the term project of the course are performed on CAD, with tooling components downloaded from the website of CarrLane 4 or transferred from the CD set of Jergens 5. As the solid modeling CAD course in the curriculum, a prerequisite of this course, teaches Solid Edge 6 by Siemens, we continue to use Solid Edge for fixture design, though it can be well preformed on other CAD platforms as well 7. Clamp CAD models as provided by tooling companies have rigid connections between links in the holding configurations of the clamps. Normal design practice is to insert these clamp models directly into fixture assembly models and hence only the closing configuration of the fixture is seen. To give students better knowledge how a fixture functions, in the fall semester of 2008, the labs of the course had the students convert the rigid clamp models into movable clamp models. Several learning benefits were observed with this method: In fixture design: students were able to visualize movement of the clamps and better examine validity of the entire design as the clamp opens and closes. In mechanisms theories: students were able to gain better knowledge of the principles of the clamping mechanisms and understand geometric constraints on the links.

In CAD: students gained knowledge and skills in CAD modeling The main steps to covert a right clamp into a movable clamp are Free each movable link by removing the ground constraint Determine the

cam clamp, an edge clamp, and a screw-strap clamp with a spherical washer in a modular fixture.

2 In CAD: students gained knowledge and skills in CAD modeling The main steps to covert a right clamp into a movable clamp are Free each movable link by removing the ground constraint Determine the types of geometric constraints between links Apply the geometric constraints between links by applying proper CAD assembly relations Four labs will be presented in the following: a toggle clamp, a cam clamp, an edge clamp, and a screw-strap clamp with a spherical washer in a modular fixture. Animation, which will be shown in the conference presentation, is not shown in this paper because their file sizes are large. Instead, the ideas of the teaching method will be illustrated via figures of different configurations of the clamps. Since specific commands of CAD vary with CAD systems, only the main ideas will be outlined. Movable Toggle Clamp Clamping arm Pivot 1 Pivot 2 Pivot 3 Figure 1 Closed Toggle Clamp Figure 2 Opened Toggle Clamp The original CAD model of a toggle clamp is the closed configuration as shown in Figure 1, where each of the four links is ground and each of the pivot pins is ground. To make the clamp movable, ground relations were removed from all elements except for the ground base link; revolute relations were applied at the pivots and mate relations were applied at each pair of contact faces. After it the clamp can be opened as shown in Figure 2. With the movable CAD model, students could see how the links of the mechanism move, which was learned in the past only with real clamps.

3 Very importantly, this CAD model was well used in learning self-locking. When the middle pivot 2 is above the line of the left pivot 1 and the right pivot 3, the mechanism is not self locking: it can be driven open by moving the clamping arm up, which represents reaction force from the work-piece (not shown) below the clamping arm. Hence students were able to better understand a limitation of the toggle clamp: the height variation of the work-pieces must be small. Cam Clamp Work-piece Strap Cam actuator Washers Heel screw Figure 3 Cam Clamp Closed Figure 4 Cam Clamp Open (The compression spring under the strap is modeled in a different lab) The original CAD model of the cam is rigid in the closed position, as shown in Figure 3. Ground relations on the cam, on the strap, on the washers were replaced by new geometric constraints simulating true physical relations so that the cam became movable as shown in Figure 4. When the cam is turned, the strap swings about the heel screw. The washer between the cam and the strap is tilted by the strap. The washer below the strap remains co-axial with the stud as forced by the compression spring under it (the spring as a self-adjustable part is modeled in a separate lab.) Many students started with having some difficulty with these kinematic relations of mechanisms and learned them in going through the lab.

Edge Clamp Work-piece Clamp jaw Clamp screw Clamp washer Figure 5 Edge Clamp Shorter Work Piece Clamp screw Clamp washer Figure 6 Edge Clamp Longer Work Piece The original edge clamp CAD model has

4 Edge Clamp Work-piece Clamp jaw Clamp screw Clamp washer Figure 5 Edge Clamp Shorter Work Piece Clamp screw Clamp washer Figure 6 Edge Clamp Longer Work Piece The original edge clamp CAD model has one rigid position of the clamp screw with respect to the clamp jaw and therefore is completely correct only for one dimension of the work-piece from the locating surface to the clamp jaw surface. Proper relations were applied so that the cone of the screw and the cone of the washer remained in contact with the side of the inner cones of the clamp jaw that is toward the teeth of the jaw, which is in contact with work-piece. Screw-Strap Clamp with Spherical Washer in a Modular Fixture

different lab) The strap, the heel, and the stud in this clamp were modeled in similar ways to those in the cam clamp discussed in the above.

5 This is a lab for learning the concepts of modular fixturing. Modular plate V-block Strap Heel screw Work-piece Spherical nut Spherical washer Figure 7 Screw Strap Clamp Down Figure 8 Screw Strap Clamp Up (The compression spring under the strap is modeled in a different lab) The strap, the heel, and the stud in this clamp were modeled in similar ways to those in the cam clamp discussed in the above. An important feature of this model is the spherical nut and washer: the center of the external spherical surface of the nut is made to be connected with the center of the internal spherical surface of the washer. As shown in Figures 7 and 8, when the strap tilts about the heel, the spherical washer not only rotates but also shifts, which explains why the tilt of the strap has limits. As said before, the modeling of the compression spring under the strap as a self-adjustable part was in a separate lab. Conclusions Making CAD clamp models movable was very beneficial for learning of fixture elements as well as of general mechanisms. This method is also useful in real design for examining and predicting the behavior of a fixture. These labs, however, require good preparation and interest of students in using CAD for design because they are more time-consuming. References: 1. Edward Hoffman, Jig and Fixture Design, 5 th edition, Delmar Learning, Fundamentals of Tool Design, 5 th edition, SME, Yiming (Kevin) Rong, Samuel Hwang and Zhikun Hou, Advanced Computer-Aided Fixture Design, Elsevier Inc., May CarrLane, Standard Components for Tooling and Production, Dec 2007

6 5. Jergens, Standard Components with the Highest Standards, Solid Edge with Synchronous Technology, Siemens, AutoCAD Inventor Data Import & Fixture Design, data-import-fixture-design.aspx Biographical Information: ZHONGMING (WILSON) LIANG is associate professor of mechanical engineering technology. He received his graduate education at Huazhong (Central China) University of Science and Technology, City University of New York, and Stevens Institute of Technology in New Jersey. His main experience is in machine elements design and computer-aided design and manufacturing.

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