CNC, CAD/CAM and Computer Integrated Manufacturing

Modern factories link design and production by computer. This lesson covers CNC, part programming, and the main ideas of computer integrated manufacturing (CIM).

NC and CNC

• NC: machine motions controlled by coded instructions. CNC: a computer inside the controller stores and edits programs. DNC: several machines fed from one computer.
• Point-to-point control (drilling) versus contouring (milling curves), which needs interpolation.
• Open-loop systems use stepper motors with no feedback; closed-loop systems use servo motors with encoders.
• Resolution of a stepper drive: with a lead screw of pitch p and s steps per revolution, BLU (basic length unit) = p/s.

Part programming

APT (Automatically Programmed Tools) is a high-level part programming language.

CAD/CAM and CIM

• CAD: geometric modelling (wireframe, surface, solid: CSG and B-rep). Transformations use homogeneous matrices.
• CAM: generating tool paths from the CAD model.
• Group technology: grouping parts into families by similar shape or process (Opitz coding), so that cellular manufacturing can be used.
• FMS: a group of CNC machines linked by automated material handling under computer control; mid-volume, mid-variety production.
• CAPP: computer-aided process planning (variant and generative).

Additive manufacturing

Parts are built layer by layer from a CAD model: stereolithography (SLA), fused deposition modelling (FDM), selective laser sintering (SLS). Useful for prototypes and complex internal shapes.

Jigs and fixtures

• A jig holds the work and guides the tool (drill bushes). A fixture holds the work only.
• 3-2-1 principle: 6 locating points (3 on a base plane, 2 on a side face, 1 on an end face) stop the part moving towards the locators. The clamps then press the part against them, so the part cannot move in the opposite directions either.

Practice