Design Process

Design is the creative process of converting a need into a workable, manufacturable and economical product; synthesis means combining known elements, principles and materials into a new arrangement that meets the need. The process is iterative: results of a later step often send the designer back to an earlier one.

 Recognition of need
        |
 Definition of problem
        |
 Gathering information  <-----+
        |                     |
 Conceptualization            | feedback
        |                     | (iteration)
 Evaluation of alternatives --+
        |
 Detailed design / analysis
        |
 Communication (drawings, report)

Steps of the process

1. Recognition of need. The designer becomes aware of a problem or opportunity, for example the need for a low-cost grain mill for small farms. The need is often vague at first.
2. Definition of the problem. The need is stated precisely: what the machine must do, inputs and outputs, size and cost limits, operating environment, life and safety requirements.
3. Gathering relevant information. Existing products, patents, standards and codes, material data, available manufacturing facilities, user opinion and market data are collected.
4. Functional requirements. The functions are listed in measurable form (for example power 2 kW, speed 300 rpm, mass below 40 kg, price below a limit). These become the specification.
5. Conceptualization. Several possible solutions are generated by brainstorming, analogy and morphological charts. Rough sketches and simple calculations are made for each concept.
6. Evaluation of alternatives. Concepts are compared against the specification using a decision matrix, cost estimates and feasibility checks. The best concept is selected or combined concepts are formed. If none is satisfactory, the designer returns to step 1, 2 or 5.
7. Detailed design and analysis. The chosen concept is sized: loads, stresses, deflections, materials, fits and tolerances, standard parts. Prototypes may be tested.
8. Communication. The design is presented as assembly and part drawings, bill of materials, calculation sheets, manufacturing and assembly instructions, and a report. A design that is not communicated cannot be made.

The cost of changes rises sharply in later steps, so most effort should go into correct problem definition and concept evaluation.

A weighted decision matrix multiplies the rating of each alternative on every criterion by the weight of that criterion, and the totals are compared.

(a) Ranking

Weights: cost 5, reliability 4, maintenance 3, compactness 2 (sum = 14). Maximum possible score = 10 x 14 = 140.

Score of A $= 5(7) + 4(8) + 3(9) + 2(6) = 35 + 32 + 27 + 12 = 106$

Score of B $= 5(8) + 4(6) + 3(7) + 2(7) = 40 + 24 + 21 + 14 = 99$

Score of C $= 5(9) + 4(5) + 3(6) + 2(7) = 45 + 20 + 18 + 14 = 97$

Score of D $= 5(6) + 4(8) + 3(5) + 2(9) = 30 + 32 + 15 + 18 = 95$

Ranking: A > B > C > D.

Answer: Alternative A (V-belt drive) is the best choice with a score of 106 out of 140.

(b) Limitations

• The ratings and weights are subjective; a small change in weights can change the ranking (here B, C and D are within 4 points).
• The method treats the criteria as independent and adds them linearly, so a very poor rating on one critical criterion (for example safety) can be hidden by high ratings elsewhere. Important constraints must be checked separately as pass/fail.