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Design and Realization Lecture: Mechanics, Physics, and Construction Methods

This lecture covers topics such as bending and stretching, construction methods like molding and welding, and modular systems. It also explores the physics behind these concepts and provides examples of their practical application.

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Design and Realization Lecture: Mechanics, Physics, and Construction Methods

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  1. Design Realization lecture 21 John Canny 11/4/03

  2. Last time • Mechanics • Physics revisited • Electric motor characteristics

  3. Next time • Assignment review/crit. Please bring a graphic of your board design and schematic.

  4. This time • Some more physics: • Bending and stretching • Construction methods: • Molding • Welding • Structural components • Modular systems

  5. y x Bending: moment of inertia again • The 2D moment of inertia about the x-axis is • For a rectangle: b h

  6. M r Bending of a beam • We can make a beam bend by applying a moment (torque) to its end. The radius of curvature satisfies: which isfor a rectangular beam.E is the elastic (Young’s) modulus. y z

  7. L t M r Beam “spring” • The vertical displacement of the tip t is approximately t  L2/2r • The bending momentdue to force F at tip is M=FL • The spring constant atthe tip is k = F/t which is

  8. Beam stretching • On the other hand the spring constant for stretching a rectangular beam is: • Compared with the bendingspring constant which is: • The ratio is h2/6L2. Assuming L >> h, the beam is much stiffer in stretching than in bending.

  9. I-beams and honeycomb • Bending stiffness can be increased by moving material outward (h3 term). • This is the principle of I-beams and honeycomb.

  10. Mass-spring systems • A mass at the end of a spring can vibrate with a natural frequency which is

  11. Example • Vibration in linear actuators: • Natural frequency of the cantilever, ¼” Aluminum, is about 100 Hz (!)

  12. Example • Diagonal bracing: • Bending becomes a streching mode. Natural frequency increases to over 1 kHz

  13. Vibration in plastics • Metals have relatively low loss when deformed. Vibration is often an issue. • Plastics have much higher loss, so vibrational energy dissipates much faster.

  14. Making shapes: sculpting • Molding kits are available from Tap Plastics and others. • Two part epoxy “magic-sculp” provides a clay-like consistency for about two hours, then sets to a hard plastic model. • The model can still be worked when hard by cutting or sanding.

  15. Making shapes: molding • From a plastic, wood, or metal model, a negative mold can be cast from a flexible plastic such as silicone or urethane.

  16. Making shapes: molding • From the negative mold, copies can be cast. The casting materials are typically urethanes or polyester. Can be clear or colored.

  17. Attaching shapes: Gluing • For plastics, glues are available that closely approximate the strength of the original plastic. • This generally gives best results. Excessive strength in a joint can create strain in the material near the joint.

  18. Attaching shapes: Gluing • Many glues are available for wood. At the high end, epoxies are stronger than the wood itself. • Woodworking “yellow glues” provide very good performance as well and have similar strength to the wood being joined. • Dowels or dovetailing can be used to increase the contact surface – but may not necessarily increase strength.

  19. Welding • Welding is joining pieces of similar metal using molten material of the same type. • The most common joining weld is a “fillet weld”

  20. Welding • The size of the weld adds strength. • The shape of the weld relieves stress that can occur at sharp concavities in a part.

  21. Arc Welding • Creates a thick weld using flux/wire which is part of the electrode:

  22. Oxy-Acetylene Welding • Oxygen-Acetylene torch creates heat, flux/wire is fed separately: • Aside: Oxy-acetylene often used for cutting

  23. MIG (Metal Inert Gas) Welding • Aluminum oxidizes instantly in air and requires an inert gas during welding. MIG welding uses an arc surrounding by gas flow (e.g. Argon).

  24. Spot welding • Normally used to join sheet material. • Sheets are placed together and very high current passed through the join.

  25. Truss systems Trussworks Inc.

  26. Truss systems • Come in either aluminum or steel, 2, 3, 4-leg. • Fast-connectscrew connectionsbetween elements.

  27. Modular construction systems • Can be used to build artifacts from linear pieces.

  28. Modular construction systems • “T-slot” systems comprise extruded aluminum beams that are joined using standard fasteners. Examples of “Minitec” framing:

  29. Modular construction systems • Joining Minitec elements:

  30. Modular construction systems • Minitec includes active elements (motorized motion stages):

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