Computer-aided process planning

Why it matters

A process plan is the bridge between a drawing and the shop floor: it says which raw material to use, which operations to perform in which order, on which machines, with which tools, fixtures, speeds, feeds and inspection steps. Done by hand, the plan depends on one planner's experience, two planners give two different plans for the same part, and planning becomes a bottleneck. Computer-aided process planning (CAPP) makes plans faster, consistent and directly usable by scheduling, costing and NC programming.

Key ideas

What a process plan contains. The route sheet (operation sheet) lists, for each operation: operation number and description, machine or work centre, tooling and fixtures, cutting conditions, setup and cycle time standards, and inspection requirements. Process planning also covers interpreting the drawing, choosing the stock, sequencing operations under precedence constraints (datum faces first, roughing before finishing, holes after the faces they are drilled in), and make-or-buy decisions.

Variant (retrieval) CAPP. Built on group technology:

1. Code the existing parts (for example with a GT code) and group them into families.
2. For each family, store a standard process plan (composite plan) in the database, indexed by a family matrix.
3. For a new part: code it, retrieve the family's standard plan, and let the planner edit it for the differences. Variant systems are relatively easy to set up and match the company's existing practice, but they still need an experienced planner, they reproduce the old plans' habits, and they cannot plan a part with no family.

Generative CAPP. Creates a plan from the part description using stored manufacturing logic, with no standard plan to start from:

• Part description — GT code, a feature list, or (best) machining features extracted from the CAD solid model: holes, slots, pockets, steps, threads, with dimensions, tolerances and surface finish.
• Decision logic — decision tables, decision trees, rules in an expert system, or optimisation, which select the process for each feature (for example: tolerance IT7 hole, Ø 20 → drill, then ream), sequence the operations, and choose machines, tools and conditions from a manufacturing database. Generative systems give consistent plans and can handle new parts, but building and maintaining the logic is hard, so fully generative systems are usually limited to particular part classes. Many commercial systems are semi-generative (hybrid): retrieve a plan, then apply rules automatically.

CAPP in the CIM chain. CAPP sits between CAD (geometry, tolerances, features) and CAM (NC programs), and feeds production planning (MRP routings, capacity planning), cost estimating and shop-floor control. Feature recognition from the CAD model and machinability databases are the main enabling technologies.

Numbers in process planning. Even in a computer system, the planner's decisions rest on machining times and costs: the system estimates the time of each operation from the cutting conditions and compares alternative routes or machines by cost per part for the batch size in question.

Formulas

N = 1000 · v / (π · D)

• N spindle speed (rev/min); v cutting speed (m/min); D diameter (mm).

T_m = L / (f · N) = π · D · L / (1000 · v · f)

• T_m machining time for one turning pass (min); L length of cut including approach (mm); f feed (mm/rev).

C_batch = C_r · (T_su + Q · t_p)

• C_batch cost of one operation for a batch (₹); C_r machine-plus-operator rate (₹/h, or ₹/min if times are in min); T_su setup time; Q batch size; t_p time per part (machining + handling). Use consistent time units.

Q* = (C_rA·T_suA − C_rB·T_suB) / (C_rB·t_pB − C_rA·t_pA)

• Break-even batch size between route A (higher setup cost, lower per-part cost) and route B. Above Q*, route A is cheaper.

Worked examples

Example 1 (standard). A CAPP system plans a finishing pass on a 50 mm diameter shaft, 200 mm long, at v = 100 m/min and f = 0.25 mm/rev. Estimate the machining time.

1. N = 1000 · v / (π · D) = 1000 × 100 / (π × 50) = 636.6 rev/min.
2. T_m = L / (f · N) = 200 / (0.25 × 636.6) = 200 / 159.2 = 1.26 min.

Example 2 (GATE level). For one operation the planner can use route A — a CNC lathe at ₹1200/h, setup 1.5 h, 1.5 min per part — or route B — a conventional lathe at ₹600/h, setup 0.5 h, 5 min per part. Find the break-even batch size, and the cheaper route and cost per part for a batch of 200.

1. Route A cost: 1200 × 1.5 = ₹1800 setup + (1200/60) × 1.5 = ₹30 per part → C_A = 1800 + 30Q.
2. Route B cost: 600 × 0.5 = ₹300 setup + (600/60) × 5 = ₹50 per part → C_B = 300 + 50Q.
3. Break-even: 1800 + 30Q = 300 + 50Q → Q* = 1500/20 = 75 parts.
4. For Q = 200: C_A = 1800 + 6000 = ₹7800 (₹39.0 per part); C_B = 300 + 10 000 = ₹10 300 (₹51.5 per part). Route A (CNC) is cheaper, at ₹39 per part.
5. A generative CAPP system makes exactly this comparison automatically for each batch size, which is why its plans can change when the order quantity changes.

Common mistakes

• Saying variant CAPP "needs no GT". It is built on part families and codes.
• Saying generative CAPP needs no human input at all — the logic and the manufacturing database are built and maintained by experts, and plans are still reviewed.
• Forgetting handling time or setup time when comparing routes, or comparing per-part costs without spreading the setup over the batch.
• Mixing ₹/h rates with times in minutes.
• Sequencing a finishing operation before the datum faces it depends on are machined.

For GATE PI

Expect MCQs comparing variant and generative CAPP (what each needs, which uses standard plans or decision logic, which suits new parts), and on what a route sheet contains and where CAPP sits in CIM. Numerical questions use machining-time estimates and cost comparison between alternative machines or routes, including break-even batch size. Practise setting up cost lines and solving for the crossover.

Quick check

1. Which CAPP approach retrieves and edits a standard plan for a part family?
2. What does a generative system use instead of standard plans?
3. Find the turning time for D = 40 mm, L = 150 mm, v = 120 m/min, f = 0.2 mm/rev.
4. Route A: ₹900/h, 2 h setup, 2 min/part. Route B: ₹450/h, 0.5 h setup, 6 min/part. Find the break-even batch size.
5. Name two inputs a generative CAPP system needs about the part.

Answers: 1. Variant. 2. Decision logic (decision tables, trees, rules) and a manufacturing database. 3. π × 40 × 150/(1000 × 120 × 0.2) = 0.785 min. 4. (1800 − 225)/(45 − 30) = 105 parts. 5. Features with dimensions, tolerances and surface finish; material (also batch size).