What design does in production
A factory cannot build an idea; it needs drawings and a list of parts. Design is the step that turns a need into exact information: shape, sizes, material, tolerances, surface finish and how parts fit together.
Design also decides what process is used (casting, forging, machining, welding) and how many parts there are. So design sits between the customer and the workshop.
Design fixes quality, cost and safety
Three things matter in production: Quality, Cost and Delivery time (QCD). All three are mostly set at the design stage. It is often said that about 70% of a product cost is decided by design. A bad drawing cannot be rescued by a good machine.
Design also decides safety (sharp corners, enough strength) and the effect on the environment (material choice, ease of recycling, energy used in making and use).
Design for manufacture and assembly
Design for manufacture (DFM) means drawing parts that are easy to make: simple shapes, tolerances as loose as the job allows, common materials and standard sizes. Design for assembly (DFA) means fewer parts, parts that fit only one way, and no extra tools.
Rules of thumb: reduce the number of parts; use standard parts such as bolts and bearings; do not ask for a tight tolerance unless needed; make the part easy to hold in a machine.
Feedback loop: design, test, use, redesign
Design is not one-time. A prototype (first sample) is made and tested. Problems found go back to design, and the drawing is changed. After sale, complaints and repair records show weak points. This loop of design, make, test and use makes the next product better. It also links to the design process and to standards (see those lessons).
Try it: build a better paper box
Make a box from one sheet of paper with no glue, using folds only (one part, easy to assemble). Then make a box from four separate strips and tape. Which was faster? Which is stronger? In the 3D, move the slider and note how time and cost change with the number of parts.
Key formulas and definitions
- QCD: Quality, Cost, Delivery. Design strongly affects all three.
- DFM: design for manufacture (easy to make).
- DFA: design for assembly (few parts, easy to join).
- Chain: Need, Design, Make, Test, Use, then feedback to Design.
Worked examples
1. In the 3D model, assembly time is 3 minutes per part. A product has 8 parts. How long to assemble? How long if redesigned with 5 parts?
8 × 3 = 24 min. With 5 parts: 5 × 3 = 15 min. Saving 9 min per product.
2. In the 3D, cost = 20 + 8 × (number of parts) units. Find the cost for 6 parts and for 4 parts. What is the percent saving?
6 parts: 20 + 48 = 68. 4 parts: 20 + 32 = 52. Saving = 16, which is 16 ÷ 68 = 23.5% (about 24%).
3. A product costs 1000 units. If design fixes about 70% of the cost, how much is fixed at the drawing board?
0.70 × 1000 = 700 units.
4. A shop makes 500 units a day. Redesign saves 9 minutes of assembly per unit. How many hours are saved per day?
500 × 9 = 4500 minutes. 4500 ÷ 60 = 75 hours saved per day.
Common mistakes
- Thinking design is only "how it looks". Design also fixes size, material, process, cost and safety.
- Asking for very tight tolerances everywhere. This raises cost a lot and is usually not needed.
- Treating production as the place to fix mistakes. It is much cheaper to fix a drawing than a finished batch.
- Believing fewer parts is always better. Too few parts can make repair or recycling hard; the designer balances both.