Attendance
Rube Goldberg Demos!
Lessons learned? Can you identify simple machines (levers, pulleys, inclined plane, screws, wheels) used in your machine?
Student page reminder: You MUST create a page. This is where you will post all class presentation info, pictures, video, etc.
Cool Thing of the Week: DIY CNC Plasma Cutter
CAD software tryouts?
Design of Non-Permanent Joints
Non-permanent joints: fasteners, screws, nuts
- Practical, quick, lots of options
- Disassemble-ability is important for prototyping and learning
Permanent joints: welding, rivets
- Hard to teach welding w/o equipment
- You can’t really take apart these things without a helluva lot of effort
General/Metal
Screws/Bolts
- Identified by major diameter and threads per inch, followed by length
- ¼-20 x 5/8” has ¼” diameter, 20 threads per inch, and is 5/8 inch long
- UNC vs. UNF
- Screws smaller than ¼” use numbers as designators
- Ex. A 2-56 screw has a major diameter of .086” and 56 threads per inch
- Metric screws are called out with an M, their diameter, their pitch, then length
References:
Socket Head Cap Screws (SHCS): Standard SHCS is screw of choice for high tensile load application
Other types of screws McMaster
Nuts - Whenever possible design through holes so you can use nuts instead of tapping!
*In general, the screw should extend a couple threads past the end of the bolt
Washers
- Avoid marring base material
- Spread screw fastening force over larger area
- Spacer to avoid stressing the inside corner of the screw head
- Indicate preload has been reached
Construction using tapped holes Tap Drill Chart
Taps
- Some screws are self tapping
- Some materials don’t need to be tapped (wood, some plastics), you can drive a screw right into them
- For all others, proper technique will save you a lot of time and broken taps!
- Types of Taps
Tapping tools and techniques
- Get yourself a tap and a tap handle
- Make sure you are perpendicular to the material
- Advance at ½ turn forward, ¼ turn back
- Tapping fluid should be used
Dowel Pins - Screws should NOT be used for alignment
Other pins - Clevis, Hitch, Cotter
Retaining Rings
Minimum Constraint Design
MinCD
- No binding
- No looseness
- No stresses from assembly
- Easier disassembly
- Loose tolerances
- Semi-skilled labor
- Cost savings
- Ex: tripods, tricycles, trailers, tandem shafts
Position of body defined by 6 degrees of freedom. If more constraints exist than are needed, system has redundant constraints (RedCD)
Why RedCD? Stability
Overconstrained is RedCD with no benefits
Ex: 3 bearings on one shaft, Chairs and tables, Pipe flanges
Manufacturing tolerances
- ¼” is never ¼”
- Different industries use different standard tolerances
- Matching features/alignment
Moscone Center Case Study