15.7 Computer Integrated Manufacturing: CAD/CAM & Integration Tools
Key Takeaways
- Basic concepts of CAD/CAM and their integration tools form a named bullet of the CIL Mechanical Paper-II syllabus.
- Computer integrated manufacturing is the use of a common database to link design, process planning, production control and inspection so that data is entered once and reused throughout.
- Geometric modelling progresses from wireframe through surface to solid modelling, with solid models alone carrying unambiguous volume and mass properties.
- Group technology classifies parts into families by shape or process similarity, which enables cellular manufacture and variant process planning.
What CIM Actually Means
Computer Integrated Manufacturing is not a machine or a software package. It is the use of a common database so that information created once — a designer's geometric model — is reused by every downstream function without re-entry, re-drawing or re-interpretation.
The traditional alternative is a chain of islands: the designer draws, a planner interprets the drawing and writes a route sheet, a programmer re-creates the geometry to produce a toolpath, and an inspector re-creates it again to write an inspection plan. Each re-creation is slow and introduces error. CIM eliminates the re-creation.
The CIM wheel
The conventional representation places a common database at the centre, surrounded by:
| Function | Abbreviation |
|---|---|
| Computer aided design | CAD |
| Computer aided process planning | CAPP |
| Computer aided manufacturing | CAM |
| Computer aided quality / inspection | CAQ / CAI |
| Computer aided engineering (analysis) | CAE |
| Manufacturing resource planning | MRP II |
| Shop floor control and automation | — |
CIM is therefore an integration philosophy, and its measure is how much data flows between these functions without human re-keying.
Computer Aided Design
Geometric modelling
The three levels form a clear hierarchy, and the distinction between them is a standard examination point.
| Model | Represents | Capability | Limitation |
|---|---|---|---|
| Wireframe | Edges and vertices only | Simplest, smallest data | Ambiguous — a given wireframe can represent more than one solid; no surface or volume data |
| Surface | Faces as well as edges | Complex sculptured shapes; NC toolpaths; shading | Does not know inside from outside; no mass properties |
| Solid | Complete unambiguous volume | Volume, mass, centre of gravity, moments of inertia; interference checking; automatic sectioning | Largest data; heaviest computation |
Only a solid model carries the topological information needed to state unambiguously which side of a face is material. That is why mass properties, finite element meshing and interference detection all require solid models.
Solid modelling approaches
| Approach | Method |
|---|---|
| Constructive Solid Geometry (CSG) | Combines primitives — cube, cylinder, sphere, cone, torus — using Boolean union, difference and intersection. Compact; the build history is retained. |
| Boundary Representation (B-rep) | Defines the solid by its bounding faces, edges and vertices with explicit topology. Fast to display; handles complex shapes. |
Most commercial systems are hybrid. Modern practice adds parametric, feature-based modelling, in which the model is built from engineering features — hole, pocket, fillet, rib — governed by dimensional parameters and constraints. Changing a parameter regenerates the model, and design intent is captured rather than merely geometry.
Data exchange
Because different CAD systems store geometry differently, neutral formats are needed:
| Standard | Nature |
|---|---|
| IGES | Initial Graphics Exchange Specification; older; wireframe and surfaces |
| STEP (ISO 10303) | Standard for the Exchange of Product model data; current; carries solids, tolerances and product data |
| DXF | Drawing Exchange Format; chiefly 2D |
| STL | Triangulated surface tessellation; the standard input for additive manufacturing |
| Parasolid, ACIS | Widely licensed geometric modelling kernels |
STEP is the strategically important one, because it can carry not only geometry but tolerances, materials and product structure — which is what genuine integration requires.
Computer Aided Engineering
CAE analyses the model before anything is made:
- Finite element analysis for stress, deflection, thermal and modal behaviour.
- Computational fluid dynamics for flow and heat transfer.
- Kinematic and dynamic simulation of mechanisms.
- Process simulation — mould filling and solidification in casting, metal flow in forming.
Computer Aided Manufacturing
CAM converts the geometric model into machine instructions. The chain is:
- Import the CAD model.
- Define the stock and the setup orientation.
- Select operations — facing, pocketing, contouring, drilling — and assign tools.
- Set cutting parameters: speed, feed, depth of cut, stepover.
- Generate the toolpath as cutter location data.
- Verify by simulation, checking for gouges, collisions and uncut material.
- Post-process into G-code for the specific machine controller.
The post-processor is essential and often overlooked: cutter location data is machine-independent, and the post-processor translates it into the particular dialect of G and M codes that the target controller understands. Different controllers require different post-processors from the same toolpath.
Computer Aided Process Planning
Process planning decides the sequence of operations, machines, tools and fixtures. Two approaches exist:
| Approach | Method |
|---|---|
| Variant (retrieval) | Retrieves a standard plan for the part family and edits it. Requires group technology coding. Simple and fast, but limited to existing families. |
| Generative | Creates the plan from the part geometry using decision logic and manufacturing rules. Flexible, handles new parts, but far harder to implement. |
Group Technology
Group technology identifies and exploits similarities among parts by classifying them into families according to design attributes (shape, size, material, tolerance) or manufacturing attributes (operations, machines, tooling).
Parts are assigned a code, structured as:
| Code structure | Character |
|---|---|
| Monocode (hierarchical) | Each digit's meaning depends on the previous ones; compact, deep information |
| Polycode (chain) | Each digit has a fixed independent meaning; easy to interpret and search |
| Hybrid | Combination; used by most practical systems such as Opitz |
The benefits follow directly: design retrieval avoids redesigning a part that already exists; variant process planning becomes possible; tooling and fixtures are standardised across a family; and machines can be rearranged into cells.
Cellular manufacture
A manufacturing cell groups the dissimilar machines needed to complete a part family in one place, replacing the traditional process layout in which all lathes sit together and all mills sit together.
| Layout | Character |
|---|---|
| Process (functional) | Machines grouped by type; flexible; long travel distances; high work in progress; long lead times |
| Product (line) | Machines in sequence for one product; efficient; inflexible |
| Cellular (group) | Machines grouped by part family; combines much of the flexibility of process layout with much of the efficiency of line layout |
Cellular layout typically cuts material handling, work in progress and throughput time dramatically, which is why it underpins lean manufacturing.
Flexible Manufacturing Systems
An FMS links several CNC machines with an automated material handling system — conveyors, rail-guided vehicles or robots — under central computer control, so that a variety of parts can be produced in random order with minimal changeover.
The classic positioning is by volume and variety:
| System | Volume | Variety |
|---|---|---|
| Stand-alone CNC | Low | High |
| FMS | Medium | Medium |
| Transfer line / dedicated automation | High | Low |
FMS fills the gap in the middle, where neither manual flexibility nor dedicated automation is economic. Its weaknesses are very high capital cost and considerable complexity in scheduling and reliability — a single handling failure can stop the whole system.
Enabling Technologies
| Technology | Role in CIM |
|---|---|
| Automated Guided Vehicles (AGV) | Material movement between cells |
| Automated Storage and Retrieval (AS/RS) | Buffer stock under computer control |
| Industrial robots | Loading, unloading, welding, assembly, painting |
| Bar code and RFID | Automatic part identification and tracking |
| Programmable Logic Controllers | Machine and cell level control |
| SCADA and MES | Supervisory control and manufacturing execution |
| Additive manufacturing | Direct production from the CAD model with no tooling |
| Digital twin | Live simulation model mirroring the physical asset |
Benefits and Barriers
Benefits. Shorter lead times; fewer errors from re-entry of data; consistent quality; reduced work in progress; better utilisation of capital equipment; rapid response to design change; and reliable traceability.
Barriers. Very high capital cost; the difficulty of integrating legacy equipment with no digital interface; incompatible data formats between vendors; the requirement for a skilled workforce; organisational resistance where departments guard their own data; and the risk that a single integrated system creates a single point of failure.
In a coal-sector engineering workshop the practical entry point is rarely a full FMS. It is more usually CAD plus CAM for the machine shop, a group-technology coding scheme for the spares catalogue so that existing designs can be retrieved rather than redrawn, and a maintenance management system linked to the same part database.
Which geometric model type is inherently ambiguous, in that a single model may represent more than one physical solid?
In constructive solid geometry, complex solids are built by:
Group technology contributes to computer integrated manufacturing chiefly by:
The role of the post-processor in a CAM system is to: