Product cycle and role of CAD/CAM

Why it matters

Every manufactured part passes through two halves of a product cycle: it is designed, and then it is made. CAD and CAM are the computer tools that sit on these two halves, and computer-integrated manufacturing (CIM) is what you get when one shared product database links them to the business functions (orders, scheduling, quality, shipping). Knowing where each tool fits explains why firms invest in it — mostly to shorten lead time, not just to draw faster.

Key ideas

The product cycle. Following Groover's view, the cycle starts with customer demand and a product concept and splits into two processes:

• Design process — (1) synthesis: recognise the need, define the problem, generate concepts and the geometric form; (2) analysis: engineering analysis, design evaluation and optimisation, then design documentation (drawings, bill of materials).
• Manufacturing process — process planning, production scheduling, procurement of tooling and material, NC programming, production, quality control, packaging and shipping to the customer. Business functions (order entry, sales, accounting, purchasing) wrap around both halves.

Where CAD fits (Shigley's design steps). Recognition of need → definition of problem → synthesis → analysis and optimisation → evaluation → presentation. CAD supports four of these directly:

• Geometric modelling — wireframe, surface or solid representation of the part (synthesis).
• Engineering analysis — mass properties, tolerance stack-up, finite-element analysis, kinematic simulation (analysis).
• Design review and evaluation — interference checking, layering, automatic dimensioning, rapid prototypes (evaluation).
• Automated drafting — production of engineering drawings and the bill of materials (presentation).

Where CAM fits. CAM applications are of two kinds:

• Manufacturing planning (the computer supports, offline, the people who plan): computer-aided process planning (CAPP), computer-assisted NC part programming, cost estimating, work standards, production and inventory planning.
• Manufacturing control (the computer is connected to the process): process monitoring and control, quality control, shop-floor control, inventory control and just-in-time delivery.

CAD/CAM integration. The geometry created in CAD is reused downstream rather than redrawn — the NC programmer, the process planner, the CMM inspection program and the fixture designer all work from the same model. This removes transcription errors and, more importantly, shortens the non-productive waiting between steps. CIM extends this one step further to the business functions.

Lead time is the real lever. A part spends most of its time in a batch shop waiting, not being cut. Reducing setup time (offline programming, tool presetting) and non-operation time (better scheduling, fewer re-checks) usually saves far more lead time than speeding up the cutting itself. That is why the quantitative side of this topic is built around manufacturing lead time.

Limits. These relations treat each operation as a single machine with a fixed setup and a fixed per-part cycle; they ignore queuing variation, breakdowns and parallel machines. Use them for comparison and planning, not as a schedule.

Formulas

MLT = n_o · (T_su + Q·T_c + T_no)

• MLT = manufacturing lead time for one batch (h); n_o = number of operations (machines) the batch visits; T_su = setup time per operation (h); Q = batch size (parts); T_c = cycle time per part per operation (h); T_no = non-operation time per operation — waiting, moving, inspection (h). Applies to batch production where every operation has similar times (use averages).

T_p = (T_su + Q·T_c) / Q

• T_p = average production time per part at one machine, setup spread over the batch (h/part).

R_p = 1 / T_p

• R_p = hourly production rate of that machine (parts/h).

Worked examples

Example 1 (standard). A batch of Q = 50 parts visits n_o = 6 machines. At each: T_su = 3 h, T_c = 6 min, T_no = 8 h. Find MLT and the production rate at one machine.

1. Convert: T_c = 6/60 = 0.1 h.
2. MLT = n_o · (T_su + Q·T_c + T_no) = 6 × (3 + 50 × 0.1 + 8) = 6 × 16 = 96 h.
3. T_p = (T_su + Q·T_c) / Q = (3 + 5)/50 = 0.16 h = 9.6 min/part.
4. R_p = 1 / T_p = 1/0.16 = 6.25 parts/h.

Example 2 (GATE level). The same shop adopts integrated CAD/CAM: NC programs are generated offline from the CAD model and tools are preset, cutting setup to 1 h per operation; shared data cuts non-operation time to 5 h per operation. Cycle time is unchanged. Find the new MLT, the percentage reduction in lead time, and the new production rate.

1. New MLT = 6 × (1 + 50 × 0.1 + 5) = 6 × 11 = 66 h.
2. Reduction = (96 − 66)/96 = 30/96 = 31.25 %.
3. New T_p = (1 + 5)/50 = 0.12 h = 7.2 min/part, so R_p = 1/0.12 = 8.33 parts/h.
4. Note that the cutting time Q·T_c = 5 h per operation did not change at all — the whole gain came from setup and waiting time.

Common mistakes

• Treating CAD as "drafting software". Drafting is only the presentation step; modelling, analysis and evaluation are where CAD earns its cost.
• Mixing up the two CAM groups: CAPP and NC part programming are planning (offline); shop-floor control and process monitoring are control (online).
• Using minutes for T_c and hours for T_su in the same MLT formula. Convert everything to one unit first.
• Multiplying T_su by Q. Setup is done once per batch per machine; only the cycle time scales with Q.
• Assuming a faster machine is the main way to cut lead time — in batch shops T_no usually dominates.

For GATE PI

Expect conceptual MCQs on which activities belong to CAD versus CAM (and planning versus control within CAM), the order of the design process, and what CIM adds beyond CAD/CAM. Numerical questions use manufacturing lead time, production time per part and production rate, often comparing a before/after situation. Practise unit conversion and quick percentage changes.

Quick check

1. Name the four CAD functions that map onto the design process.
2. Is computer-aided process planning a CAM planning or CAM control application?
3. Q = 20, n_o = 4, T_su = 2 h, T_c = 15 min, T_no = 6 h. Find MLT.
4. In question 3, what is the production rate at one machine?
5. What does CIM add to CAD/CAM?

Answers: 1. Geometric modelling, engineering analysis, design review and evaluation, automated drafting. 2. Planning. 3. 4 × (2 + 5 + 6) = 52 h. 4. T_p = 7/20 = 0.35 h, so R_p ≈ 2.86 parts/h. 5. Integration of the business functions with design and manufacturing through a shared database.