Manufacturing Processes and Production Engineering
This domain tests whether you can select and justify a process route, specify tolerances a real process can hold, and prove capability with numbers rather than opinion. Expect questions that combine process knowledge with cost, tolerance and capability arithmetic.
Process families and selection logic
- Casting (sand, die, investment), bulk and sheet forming (forging, rolling, deep drawing), material removal (turning, milling, drilling, grinding, EDM), joining (arc and laser welding, brazing, adhesive bonding, fasteners), additive manufacturing (LPBF, FDM, SLA) and surface treatment.
- Drivers are material and geometry, achievable tolerance and surface finish, batch size, tooling cost and lead time. Die casting and press tooling is amortised only at high volume; at low volume, CNC from billet or FDM wins even with a higher unit price.
- DFM/DFA: minimise part count, standardise features, avoid deep narrow pockets, provide tool run-out, and design for a single set-up where possible.
Tolerancing, fits and stack-up
- ISO 286 fits: H7/g6 is a close running fit and H7/p6 a light interference (press) fit. Hole-basis is the default because holes are harder to adjust.
- ISO 2768-m covers untoleranced dimensions; form and location are controlled by ISO 1101 GD&T, where maximum-material modifiers add bonus tolerance.
- Stack-up: worst case is arithmetic (sum of tolerances). Root-sum-square is valid only where dimensions are statistically independent and the loss of guaranteed interchangeability is accepted.
Capability and flow
- Cpk = min[(USL − μ)/3σ, (μ − LSL)/3σ]; 1.33 is the usual minimum for a new part and 1.67 where the feature is safety-related or data are short.
- Takt time = available production time ÷ customer demand; OEE = availability × performance × quality.
Worked example
A shaft is specified ∅25.000/24.950 mm. The process runs at μ = 24.975 mm with σ = 0.012 mm. Cpk = min[(25.000 − 24.975)/(3 × 0.012), (24.975 − 24.950)/0.036] = 0.694, far below 1.33 even though the mean is centred. To reach Cpk = 1.33 at the same centring, σ must fall to 0.025/(3 × 1.33) = 0.0063 mm, so the route must change (add finish grinding) or the tolerance must be renegotiated.
Traps that cost marks
- Putting 6σ in the Cpk denominator, or quoting Cp for an obviously off-centre process.
- Reading a saw-tooth X̄ chart as stable: that is tool-wear drift, not a capable process.
- Applying RSS stack-up where processes are correlated or interchangeability is contractual.
- Forgetting that additive parts usually need critical faces post-machined.