Question 5 of 7: CAD, process planning, CAPP, routing sheets and group technology

Question 5: CAD, process planning, CAPP, routing sheets and group technology (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.

Five descriptive parts, each answered in turn. Together they trace one thread: how the design intent created in (a) is turned into manufacturing instructions in (b) and (c), issued to the shop in (d), and made economic by the part families of (e).

(a) Advantages and limitations of CAD

Computer-aided design replaces the drawing board with a geometric database. Its first advantage is productivity in creating and, far more importantly, in changing geometry: an edit that would have meant erasing and re-drawing propagates through views, sections and details automatically, and an associative assembly updates when a component changes. Second, the model is accurate and unambiguous — geometry is held to full machine precision rather than to what can be scaled off paper, and interference between components is found by computation instead of by building a prototype. Third, the same database supports analysis and manufacture without re-entry: finite-element meshes, mass and inertia properties, tolerance stacks, NC tool paths, coordinate-measuring inspection programs and bills of material are all generated from it, which is what makes CAD the front end of CIM rather than a drafting aid. Fourth, it makes standardisation and re-use practical, through libraries of standard parts, features and design rules, and through parametric models that regenerate a whole family from a table of dimensions. Fifth, it improves communication and control: one controlled master model, revision history, concurrent access by design, manufacturing and purchasing, and photorealistic or animated presentation for the customer.

The limitations are real and are usually understated. The capital and skill cost is high — software licences, workstations, data management and, above all, training and the productivity trough while a team learns. The system encourages false confidence: a rendering of a model is not evidence that the part can be made or that the tolerances are achievable, and a designer who has never seen the process can produce geometry that is elegant on screen and expensive on the floor. There is a persistent problem of data exchange and archival longevity, since native formats are proprietary and translation through IGES or STEP can lose features and parametric intent, while a model must remain readable for the twenty or thirty years of a product's liability life. CAD does not itself create design — concept generation, functional decomposition and the choice of principle remain human work, and the ease of modifying an existing model biases teams toward incremental variants of what they already have. Finally, the discipline required to keep a model well structured, so that it can actually be edited later, is often absent, and a badly built parametric model can be harder to change than a paper drawing.

(b) The purposes of process planning, and the role of computers

Process planning is the bridge between the design and the factory: it determines how the designed part will be made. Its purposes are, first, to select the manufacturing processes and their sequence — the raw stock or casting, then the sequence of machining, forming, joining, heat-treatment, finishing and inspection operations that converts it into the finished part while respecting datum and tolerance dependencies. Second, to select the machines, tooling, fixtures and gauges for each operation, balancing capability against availability and cost. Third, to determine the process parameters — cutting speeds, feeds and depths of cut, or their analogues in a forming or welding process — and hence the operation times. Fourth, to establish standard times and hence costs, which feed the estimating, scheduling and cost-accounting systems. Fifth, to specify the inspection plan, saying which characteristics are checked, where in the sequence, and with what equipment. Sixth, to document all of this in the route sheet and operation sheets that are the shop's instructions. Process planning is also the point at which manufacturability is fed back to design: the planner is usually the first person to discover that a tolerance is unattainable or that a feature cannot be reached by any standard tool.

Computers enter this work in several distinct ways. Computer-aided process planning (CAPP) systems generate the route sheet itself, either by retrieving and editing a standard plan or by synthesising one from decision logic. Machinability data systems hold speed and feed recommendations by work material, tool material and operation, replacing the handbook. Cost-estimating and standard-data systems build the time standard from elemental data rather than from time study. Tool and fixture management systems tell the planner what tooling already exists, which is what makes standardisation happen in practice. Group-technology coding and classification systems find the existing plan for a similar part. Simulation and NC verification confirm the plan before metal is cut, and direct links to the CAD model allow features and tolerances to be read rather than re-interpreted. The payoff is consistency as much as speed: two planners working independently on the same part have always produced two different plans, and CAPP removes that variability along with the clerical effort.

(c) The two types of CAPP system

The variant (retrieval) system works by analogy. Parts are classified into families using a group-technology coding scheme, and a standard plan is prepared and stored for each family. When a new part arrives it is coded, the code identifies its family, the standard plan is retrieved, and the planner edits it — deleting operations the new part does not need, adding any it does, and adjusting dimensions and parameters. The system is essentially a disciplined filing and retrieval scheme with an editor. Its virtues are that it is cheap to develop, that it runs on the plant's real accumulated experience, and that it enforces standardisation across a family. Its limits are that it depends completely on the quality of the coding scheme and of the standard plans, that it still requires a skilled planner to do the editing, and that it cannot cope with a part that does not belong to an established family. It suits a plant making many variants of a limited range of part shapes.

The generative system creates the plan from first principles. Part geometry, material and tolerances — ideally read directly from the CAD model as recognised features — are fed to a body of decision logic (rule bases, decision tables, expert-system inference, process-capability data and machinability databases) that selects each process, sequences the operations, chooses tooling and computes parameters and times without reference to any stored plan. Its advantages are consistency, the ability to plan a genuinely new part, straightforward re-planning when a machine is unavailable, and true integration with CAD and with the NC programming that follows. Its costs are the difficulty and expense of capturing the manufacturing logic in the first place, the dependence on reliable automatic feature recognition, and the fact that fully generative planning remains restricted in practice to well-bounded process domains (rotational machining, sheet-metal fabrication, printed-circuit assembly). Most commercial systems are semi-generative hybrids: generative logic proposes the plan and a planner confirms it.

(d) The routing sheet: features and necessity

The routing sheet (route sheet, or in some plants the traveller) is the document that carries the process plan to the shop floor. Its header identifies the part number, revision, description, material specification and the quantity or lot size, together with the plan's own revision level, the planner's name and the date — a routing is a controlled document, and knowing which revision of the routing made which lot is a traceability requirement. The body is a sequence of numbered operations, one line each, giving the operation number and description, the work centre or machine at which it is done, the tooling, fixtures and gauges required, the set-up time and the run time per piece (from which the shop computes load and cost), and the process parameters where they matter. Inspection operations appear in the sequence with the characteristics to be checked and the sample size, and outside-processing operations name the vendor and the expected turnaround. Many routings carry a sketch or a reference to the operation sheet that details a complex operation, and space for the operator to sign off quantities completed and scrapped.

The routing sheet is necessary for reasons that go well beyond telling the machinist what to do next. It is the authoritative statement of the sequence, which matters because process sequence determines whether tolerances can be held; departing from it is a documented deviation, not an operator's choice. It is the input to production planning and control: capacity requirements planning explodes the routing's set-up and run times against the schedule to compute the load on each work centre, and shop-floor control tracks the job against the routing's operations. It is the basis of cost accounting, since standard cost is built from the routing's times and the work-centre rates, and variances are reported against them. It provides traceability when a defect is found, because the routing plus the completed traveller records which machines and which operators touched the lot. And it is the vehicle for continuous improvement — a change to a method is not real until it is on the routing.

(e) Group technology: what, why and the advantages

Group technology is a manufacturing philosophy that identifies and exploits the similarity of parts. Parts are grouped into families whose members share geometry, size, material or, more usefully, a common sequence of processing operations; the production equipment is then grouped into cells dedicated to producing one or more families. Families are identified in three ways: by visual inspection, by classification and coding (Opitz, MICLASS, KK-3 and similar schemes assign each part a digit string describing shape, dimensions, material and tolerance), or by production-flow analysis, which clusters the machine–part incidence matrix taken from existing routings.

Group technology was developed to break a specific deadlock. Manufacturing has traditionally been organised at two extremes: the process (functional) layout of the job shop, which is flexible but produces long, tangled material flows, large queues, long lead times and enormous work in process; and the product (line) layout, which is efficient but requires the volume of mass production to justify dedicated equipment. Most manufacturing — the estimate has long been that around three-quarters of all machined parts are made in lots of fifty or fewer — falls between the two, and was condemned to job-shop economics. The insight behind group technology is that although each part may be low volume, families of similar parts are collectively high volume; organising by family therefore buys much of the efficiency of flow production without the volume of a single part. A second driver was the sheer waste of duplicated engineering: without a retrieval scheme, firms repeatedly designed and planned parts they had already made.

The advantages follow directly. In design: retrieval of existing designs prevents duplication, standardisation of features and tolerances reduces the variety of tooling, and design retrieval is what makes variant CAPP possible at all. In process planning: standard plans per family, consistent methods, and far less planning effort per new part. In manufacturing: because all members of a family use the same set-up family, set-up times fall sharply, which in turn makes small lots economic; material handling collapses because the cell contains all the machines the family needs; throughput time and work in process fall by large factors; scheduling is simplified from a job-shop routing problem to a sequencing problem within a cell; and quality improves because a small dedicated team owns the part and feedback is immediate. In management: the cell is a natural unit of accountability, which supports team working and simplifies cost collection. The costs are the disruption and expense of rearranging the plant, the possibility of duplicated equipment across cells, the loss of some routing flexibility when a cell machine fails, and the effort of coding a large part population in the first place.