7a. Design Rules for High-Speed Automatic Assembly

In high-speed automatic assembly, the time taken to complete an assembly does not primarily dictate the assembly cost.
Assembly cost outweighs time considerations in automatic assembly.
Automated systems possess limitations in feature discrimination compared to human vision, often necessitating vision systems.
Re-designing for automation yields more significant savings than simply implementing automated technology.
Thoroughness at the design stage is crucial.
If a part is suitable for automatic handling, it is generally suitable for automatic assembly.
Design for automation requires careful attention to designing parts that facilitate automatic feeding and orienting.
Assembly costs in automatic assembly are associated with:
- Cost of all equipment.
- Cost of feeding and orienting.
- Cost of automatic insertion.
- Number of operators and technicians.
- Assembly rate.

Minimize the number of parts.
Ensure that:
- The product has a suitable base part for building the assembly.
- The base part has features that enable stable location in a horizontal plane.
- The product can be built up in layers (assembled from above).
Provide chamfers or tapers to guide and position parts correctly.
Avoid expensive and time-consuming fastening operations like screw fastening and soldering.
Avoid projections, holes, or slots that cause tangling when parts are placed in bulk in the feeder.
Attempt to make parts symmetrical.
If symmetry is not achievable, incorporate asymmetrical features for orienting the parts.
Use self-centering screws whenever possible to prevent nesting or tangling.

Eliminating a part eliminates a complete station on an assembly machine, including:
- Part feeder
- Specific workhead
- Associated portion of the transfer device

Design features in the base part to aid placement in the work carrier to ensure horizontal placement (e.g., using an edge).
Facilitate base part placement in the horizontal plane using tapered dowel pins mounted in the work carrier.
Use tapered pegs to provide guidance.

Allow for assembly in a sandwich or layer fashion.
Each part is placed on top of the previous one.
Gravity can assist in feeding and placing parts.
Place workheads and feeding devices above the assembly station for accessibility in the event of a fault (e.g., defective part feeding).
If assembly from above is not feasible, divide the assembly into sub-assemblies.
- Example: Power Plug
  - Problem: Difficult to position and drive the two cord grip screws from below.
  - Solution: Treat the two cord screws, cover screw, and plug base as a sub-assembly dealt with before the main machine assembly.

Automation can be facilitated by introducing guides and chamfers.
Sharp corners should be removed to guide the part into its correct position during assembly, leading to:
- Less control required by the placement device.
- Potential elimination of the placement device.

The most versatile parts feeder is the vibratory bowl feeder.
Three basic design principles:
- Feeding devices tend to be part specific.
- Avoid designing parts that will tangle, nest, or shingle.
- Make the parts symmetrical.
- If parts cannot be symmetrical, provide asymmetrical features for orienting the parts.

Part a: A part that would be difficult to handle.
Part b: A redesigned part which could be fed and oriented in a vibratory bowl feeder at a high rate.

The slightly asymmetrical threaded stud would not present significant problems in manual handling and insertion.
For automatic handling, an expensive vision system would be needed to recognize its orientation.
If the part were made symmetrical, automatic handling would be simple.

Deliberately add asymmetrical features for the purpose of orienting.
Examples:
- No feature sufficiently significant for orientation.
- When correctly oriented, will hang from rail.
- Triangular shape of part makes automatic hole orientation difficult.
- Nonfunctional shoulder permits proper orientation to be established in a vibratory feeder and maintained.