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Lesson 47 of 78 · Manufacturing & Reliability

Process Selection & Design for Manufacture

MIT Design and Manufacturing II joins process physics, equipment and control, manufacturing systems, and design for manufacture 1. A process is not just a way to make shape; it sets variation, rate, surface integrity, residual stress, material yield, tooling, labor, and inspection.

A matrix compares machining, fabrication, casting, molding, and additive manufacturing across volume, tooling, geometry, rate, and tolerance
Process choice couples geometry, material, volume, rate, quality, flexibility, and the cost of proving the result. Credit: StudyCorner original diagram · CC BY 4.0 · Source

Screen by capability, then compare economics

Ask:

  • Can the process make the material, size, wall, feature, tolerance, and surface?
  • At expected volume, what are tooling, setup, cycle, labor, yield, and secondary operations?
  • How will the characteristic be measured and controlled?
  • What design change makes the process easier without harming function?

Machining offers flexibility and precision with material removal. Sheet fabrication exploits bending and joining. Casting and molding can consolidate geometry but add tooling, flow, shrinkage, draft, and defect concerns. Additive processes permit otherwise inaccessible geometry but introduce orientation, support, surface, anisotropy, and qualification issues.

Break-even example

Process A uses no dedicated tool, costs $42 per part, and needs $500 setup. Process B needs a $30,000 tool but costs $8 per part. Break-even volume satisfies

\[ 500+42N=30{,}000+8N, \]

so \(N\approx868\) parts. Include financing, tool maintenance, scrap, design change risk, and capacity before treating this as a decision.

Design for assembly

Reduce unnecessary part count; create one obvious orientation; use chamfers and lead-ins; separate location from clamping; provide tool access; avoid loose hardware over equipment; and design connectors so wrong mating is difficult. A part-consolidation move is good only if it does not make service, replacement, or process control worse.

Process plan

Choose a motor mount. Produce a route sheet from raw stock to finished and inspected part. For every step state the datum, machine or tool, characteristic created, expected variation, and inspection. Then redesign the part for a second volume scenario. The lesson is the changing design-process pair, not a universal winning process.

Source trail

References

  1. 1
    Design and Manufacturing II. MIT OpenCourseWare. verifiedModern manufacturing organized around process physics, equipment and control, manufacturing systems, and design for manufacture. Cited at: four course emphases.
Further reading
  • Design and Manufacturing I. MIT OpenCourseWare. verifiedProject-centered course on electromechanical design, machine elements, experiments, drawings, modeling, fabrication, and technical review.

Check your understanding

  1. Why can an expensive tool reduce unit cost?
  2. Which redesign usually improves assembly robustness?