CAD, Mechanical Design & Fabrication An overview of prototyping fabrication methods and design workflow BE 107 4/7/2015 Lecture 3

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The Design Workflow

concept

specifications

Prototype

Options – make a huge table of all the possible ideas you can think of to solve each specification independent of the other specifications

Detailed design

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Fabrication possibilities

Combine options

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Iteration cost If each iteration is relatively expensive, pay attention to details sooner in the design process.

v.s.

For example, if placing screw holes in a plate, make sure those screws are available, and that they will interface with the other parts without any problems before making your part.

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Example – a fly arena The wrong way:

Open solidworks, and start designing a fly arena from some idea you have in your head. It is too easy to get lost in details (hole sizes, etc) at this stage, and these decisions will limit your creativity.

The right way:

1. Think of, discuss, and generate a list of the specifications, features, constraints, etc. 2. Create a library of whiteboard type sketches of options that address these specifications independent of the others. Although it may seem that some specs influence others, it is best at this stage to keep an open mind. 3. Decide what fabrication process you will use 4. Depending on the fabrication process, make changes to the whiteboard designs. If the fab process demands it, make a detailed CAD model. 5. Prototype 6. Go back to step 1.

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Intuitive Assembly

Never design two parts that are almost the same, but not quite. Change something about the part to make it clearly different. While retaining the important design difference. Unless there is a good reason not to, use symmetric and mirror reflect able parts.

Prototyping vs Commercial fab

Custom, one-off, prototyping fabrication

Commercial fabrication

Examples: Laser cutter, 3D printing, machining, etc.

Examples: injection molding, folded sheet metal, stereolithography, etc.

Pros: Fast

Pros: cost per part relatively low

Cons: cost per part relatively high

Cons: high start up cost, long lead time (outsourced)

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Prototyping fabrication methods

Method  

Time  

Cost   Material  choices  

Max  size  

Accuracy  

Newport  

minutes  

$$  

Aluminum  

NA  

.02  mm  

80/20  

hours  

$$  

Aluminum  

3  m^3  

.02  -­‐  3  mm  

Laser  cu=er  

minutes   $   minutes  (if   in  house)   $$  

Plas?cs,  wood  

1  cm  x  60  cm^2   0.5  -­‐  1  mm  

2.5  

Anything  

2  cm  x  2  m^2  

0.5  -­‐  1  mm  

2.5  

20  cm^3  

.02  mm  

3   6  

Waterjet  

Tradi?onal  machining   hours  -­‐  days  $$$   Anything   3D  prin?ng  

1-­‐3  days  

$$$   Plas?cs,  metal  powder  20  cm^3  

.5  -­‐  1  mm  

CNC  

1-­‐3  days  

$$$$   Anything  

.02  mm  

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10  cm^3  

Deg  of  Freed.   2-­‐3   3  

3  -­‐  6  

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Fabrication: legos Legos

Newport & Thor Labs, etc. (fancy legos)

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80/20 (legos you chop can cut half)

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Re-usability Whenever possible, try to make parts symmetrical, reversible, and follow standards.

For example: Newport and 80/20 use standard bolts and spacings (1/4-20 in bolts with 1 in spacing). When designing parts, try to use these standard sizes to maximize the inherent compatibility with other parts.

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Traditional machining – lathe Spin the material – radially symmetric parts

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Traditional machining – milling machine Spin the tool, stationary part – 3 degrees of freedom Drill bit

End mill

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Lathe vs Mill

Lathe + Mill

Lathe only

Mill only Floris van Breugel

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Tolerances and Standard sizes Typical maximum accuracy with a lathe or mill is ~ 0.001 in Many applications don’t require that kind of accuracy, though, which can simplify designs and manufacturing, and thus cost less time and money.

Center drill

Drill bit

Reamer

Boring head

Take home: try to design holes, arcs, and radii, to take advantage of standard tool sizes.

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Mill tool constrains

Interior 90 degree angles are impossible on a mill because mill bits are circular

Design slots with tool sizes in mind

Smaller radii are possible, But require more passes

Because physical tools are used to remove material, it is impossible to make a hollow part on a mill or lathe. For complex shapes like this, you will either need 2 pieces, or use a different process (e.g. 3D printing).

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Laser cutting 2D parts, very fast, holes are free, complex shapes are free Materials: limited to primarily plastics like acrylic and delrin. Thin wood, and certain other materials possible too.

Our course lab has a laser cutter, and we will make good use of it! You can also order parts online from, for example: https://www.pololu.com/ Floris van Breugel

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Waterjet Similar to laser cutter, but more material options (including metals), thicknesses, and working space.

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Sheet metal bending

Bend sheet metal up to 1/8-1/4 inch. Carefully consider bend order, and bending radius.

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3D printing

Layer by layer deposition, or phase transformation

Extruded, 2+ materials

Layer by layer sintering of resin, powdered metals, or plastics. Single material. Floris van Breugel

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Examples Order online, for example, through Quickparts: http://www.3dsystems.com/quickparts

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Continous resin “sintering”

https://www.youtube.com/watch?v=74BjdHDJeE0 Floris van Breugel

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Active research – what may be possible in the near future Cornell Creative Machines Evan Malone & Hod Lipson Fab@Home project (2006-2009) Thermoplastic and elastomer structures and flexures Conductive wiring Elastomer strain gauges Zinc-air batteries Artificial muscles (actuated elastomers) Electromechanical relays Polymer transistors Inductors and electromagnets Living tissue (cartilage) Food

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Active research – what may be possible in the near future

Biomedical: 3d printed cells, cartilage, etc.

Robotics: 3D printed components

3D printed food Floris van Breugel

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Digital (voxel) printing

Hiller & Lipson, 2009

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CNC – computer numerical control

https://www.youtube.com/watch?v=MwZCuTlSKeY Floris van Breugel

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Prototyping fabrication methods

Method  

Time  

Cost   Material  choices  

Max  size  

Accuracy  

Newport  

minutes  

$$  

Aluminum  

NA  

.02  mm  

80/20  

hours  

$$  

Aluminum  

3  m^3  

.02  -­‐  3  mm  

Laser  cu=er  

minutes  

$  

Plas?cs,  wood  

1  cm  x  60  cm^2   0.5  -­‐  1  mm  

2.5  

Waterjet  

hours  -­‐  days  $$  

Anything  

2  cm  x  2  m^2  

0.5  -­‐  1  mm  

2.5  

20  cm^3  

.02  mm  

3   6  

Tradi?onal  machining   hours  -­‐  days  $$$   Anything   3D  prin?ng  

1-­‐3  days  

$$$   Plas?cs,  metal  powder  20  cm^3  

.5  -­‐  1  mm  

CNC  

1-­‐3  days  

$$$$   Anything  

.02  mm  

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10  cm^3  

Deg  of  Freed.   2-­‐3   3  

3  -­‐  6  

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Laser cutting 2D parts, very fast, holes are free, complex shapes are free Materials: limited to primarily plastics like acrylic and delrin. Thin wood, and certain other materials possible too.

Our course lab has a laser cutter, and we will make good use of it! You can also order parts online from, for example: https://www.pololu.com/ Floris van Breugel

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Etching By carefully controlling the settings it is possible to etch 3D shapes into parts.

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Laser cutting pitfalls Laser beam Focus Material

“kerf ” Resulting part:

Instead of:

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Laser cutting tricks See demo pieces for examples of 2D – 3D shapes Layered design

Acrylic weld

Thermo-pressed threaded brass inserts

Clip-together designs

Holes are (practically) free with laser cutters and waterjets. If you suspect you may want a hole, add one. Floris van Breugel

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Minimum design requirements

No design required

3D CAD models required (e.g. SolidWorks)

2D computer drawing required 3D CAD models optional

Multiple view drawings required 3D CAD models recommended

3D CAD models required (e.g. SolidWorks) Floris van Breugel

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SolidWorks Demo: geometric relations and careful dimensioning 1.  Make a simple part similar to the fly bowl parts (extruded part) 2.  Rescale the part to demonstrate how clever dimensioning can make life easier 3.  Show how you can put multiple parts together in an assembly 4.  Show how to make a drawing of the part

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Design automation Python CAD tools: PyCAD, PythonOCC, FreeCAD, OpenSCAD / Py2SCAD, Blender

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Access to fabrication techniques The ME shop has access to many of the tools discussed here, including: -  -  -  -  - 

Laser cutter Waterjet 3D printer (ABS plastic) CNC machines Mills, lathes

For details, see: http://me71.caltech.edu/index.php?title=ME71_Shop Several shops on campus provide machining services, including the Aero and ChemE shops (for a price, of course) There are online resources for ordering parts, e.g.: 3D printing: http://www.3dsystems.com/quickparts Laser cutting: https://www.pololu.com/ CNC: http://www.emachineshop.com/ Floris van Breugel

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Evolutionary design

http://creativemachines.cornell.edu/videos

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