Question 1 of 7: Process Planning in Discrete Parts Manufacturing

Question 1: Process Planning in Discrete Parts Manufacturing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — the role of process planning. Process planning is the engineering function that converts a released design into a manufacturing method. Its input is the design database for a discrete part — the model or drawing, the dimensional and geometric tolerances, the material specification, the surface-finish and heat-treat callouts, and the forecast quantity. Its output is the route sheet: the ordered list of operations, the work centre or machine assigned to each, the tooling, fixtures and gauges required, the cutting or forming parameters, the setup and run times that become the standard, and the inspection points at which conformance is demonstrated. Every downstream system feeds off that route sheet. Material requirements planning explodes the bill of material against it; capacity planning loads work centres from its standard times; cost estimating and quoting price the job from it; numerical-control programming and fixture design begin from the operation it names; purchasing uses it to decide the stock form and size to buy, and to settle make-or-buy for each feature. In a discrete parts industry — where a plant may hold thousands of part numbers, each routed through a different subset of the same machine population — the process plan is the only document that ties a part number to a physical sequence of resources, and it is therefore the hinge on which the whole production-control system turns.

Process planning also carries the firm's manufacturing knowledge. A planner who specifies "rough turn, stress relieve, finish turn between centres, cylindrical grind" rather than "turn to size" is encoding hard-won experience about how a long slender part behaves. Because that knowledge is embedded in routings rather than in individuals, process planning is where a company's process capability is formally recorded and where continuous improvement is captured when a better method is found.

Part (b) — the effect on quality and cost. Process planning determines quality before a single part is cut, because it chooses the processes whose inherent capability must cover the design tolerance. A bore toleranced at ±0.01 mm cannot be held by drilling; it demands boring or reaming followed by honing, and the planner who does not recognise this has guaranteed a scrap rate that no amount of operator care will remove. Three further planning decisions drive variation directly. The datum and fixturing strategy decides which surfaces locate the part at each operation, and a change of datum between operations introduces a tolerance stack that adds to the process variation. The operation sequence decides whether errors accumulate or are removed — machining a critical feature after heat treatment removes distortion from the tolerance chain, machining it before adds distortion to it. The number of setups matters because each re-location of the part is an independent opportunity for misalignment, so a plan that completes a part in one setup on a machining centre is inherently more capable than one that visits four machines.

Cost is affected even more strongly, because process planning fixes most of the manufacturing cost after design has already committed the majority of the product cost. The stock form and size chosen set the material utilisation and therefore the largest single cost element in most machined parts. The machine assigned sets the burden rate charged per hour, and putting a simple operation on an expensive multi-axis machine because it happened to be free wastes the difference for the life of the part. The cutting parameters trade cycle time against tool life; the setup strategy trades setup cost against work-in-process, because a plan that requires large batches to amortise a long setup forces inventory that the plant then has to finance and store. Finally, the placement of inspection decides where a defect is caught: detecting an out-of-tolerance bore at the machine costs the value added so far, while detecting it at final assembly costs the value of a complete unit, and shipping it costs a warranty claim. The planner therefore controls both the internal and the external failure cost of the part.

Part (c) — selecting a process planning system. The choice between manual planning, a variant computer-aided process planning (CAPP) system that retrieves and edits a standard plan for a part family, and a generative system that synthesises a plan from the part model and a rule base, rests on the following factors.

Part mix and repeat business. Variant planning pays only where parts fall into stable families and most new parts resemble old ones; it depends on a group-technology coding scheme, and the cost of coding the existing part population is a real project cost. A job shop that sees genuinely novel geometry every week gets little from retrieval and needs either generative logic or skilled planners.

Volume of planning work. The number of new and revised part numbers per year, multiplied by the planner-hours each consumes, sets the size of the prize. A firm releasing thirty new parts a year cannot recover the cost of a generative system.

Integration. The system must read the CAD model — ideally recognising manufacturing features rather than requiring re-entry of the geometry — and must write routings and standard times directly into the ERP or MRP system, and NC programs into the machine post-processors. Neutral standards such as ISO 10303 (STEP) protect the investment against a change of CAD vendor. A planning system that produces paper the ERP system cannot consume simply moves the transcription work rather than removing it.

Fidelity of the resource database. Generative logic is only as good as the machine, tool, fixture and material data it reasons over, and that database must be maintained as the plant changes. Ask who will own it.

Consistency, capture and control. A major benefit is that all planners produce the same plan for the same part, embedding best practice and removing the variability between an experienced and a novice planner. Against that, the system must support revision control, approval routing and traceability, because process plans are quality records under ISO 9001 and are subject to audit.

Cost estimating and quoting. If the system produces reliable standard times, it also produces quotations, and for a make-to-order firm that capability is often worth more than the planning saving itself.

Implementation cost, skills and vendor support. Licence cost is usually the smallest term; coding the part population, populating the resource database, training planners and running the two systems in parallel dominate. The firm should evaluate total cost of ownership over five years against the planner-hours saved, the scrap and rework reduction from more consistent plans, and the quoting improvement, and should insist on a pilot over one real part family before committing.