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Jeremy Fielding - Engineering Principles for Makers
• Part One: The Problem
• Part Two: Material Properties
Part One: The Problem
The "DR. FARM" Method:
Define The Problem
• Be as specific as possible without contraining yourself to any particular solution.
Grass Analogy: Do we need a better lawn mower or a way of shortening grass? The former presupposes a particular solution, whereas the latter is more open to various possibilities.
• List real constraints (e.g.: space, weight, materials, tolerances, manufacturability, durability, mobility, etc.).
• Keep in mind that there might already be a pre-existing solution!
• How does the machine actually solve the problem? What solution does it provide?
Appearance (or Geometry)
• The shape of the device determines how it handles the stress applied to it.
Water Analogy: The stress moves from particular points to the floor like water flowing through the device.
• Demolish the idea or prototype. Find every possible point of failure.
• Work backwards. Assume it has failed and "reverse diagnose" why.
• Build a scale model (3D and/or actual).
• Go back and remove the flaws from the model before making the final structure. Make sure nothing is forgotten. For example, do we have to remove, reconfigure, or reinforce any sections?
• Design of Machinery (2nd Edition) by Robert L. Norton
• Shigley's Mechanical Engineering Design by Richard Budynas & Keith Nisbett
• Design of Weldments by Omer W. Blodgett
• The Elements of Mechanical Design by James Skakoon [Notes]
Part Two: Material Properties
Which material should you choose for your project, steel or wood?
Principles To Consider:
1. Shape is just as important as material.
If something fails, consider both! Deflection (i.e.: bending) can be prevented by using the same amount of material in different ways. For example, we can cut wood into an "I-beam" shape.
"Area Moment of Inertia" is generally a measure of how stiff a particular geometry will be when a load is placed on it.
2. Strength is not rigidity.
A material could be strong, but not rigid, or vice versa. For example, rubber is strong but not rigid, whereas pretzel sticks are rigid but not strong. Some materials are brittle (such as glass), and will break with very little bending.
"Young's Modulus of Elasticity" is generally a measure of how flexibile a material is. It is usually measured in "Gigapascals". The higher the number, the stiffer the material.
Steel has a particular Young's Modulus. Therefore, if a piece of steel bends more than you want, getting a higher grade of steel won't help. Different types of wood may have different measures for Young's Modulus though.
3. Design with triangles.
Triangles act to reinforce. Using more material does not necessarily mean that a structure is stronger. How is it put together?
4. Consider all loads.
Think about all of the loads that will impact your structure. For example, a dresser may be able to hold weight on top of it, but will it also stay standing if someone leans against it? Put the greatest amount of depth in the direction where force is greatest.
5. Avoid overhung loads.
A load is "overhung" when it is applied to a point that is unsupported. Add a triangle, shorten the length, or change the design to be fully supported.
• Why Are I-Beams Shaped Like An "I"? by Real Engineering