Measuring aids.

Measuring aids

Measuring and manufacturing accuracy can be improved by using consistent reference points and devices that reduce repeated marking out and human error.

Datum

A datum is a reference edge, point or surface from which measurements are taken. Measuring related features from the same datum reduces cumulative error.

Templates, jigs and fixtures

• A template provides a repeatable shape or marking profile so identical parts can be produced consistently.
• A jig locates the workpiece and guides the tool, for example guiding a drill to produce repeated holes in the same position.
• A fixture securely holds and locates work during a process but does not guide the cutting tool. Welding fixtures are a common example.

Jigs and fixtures allow repetitive operations to be carried out faster and with less dependence on individual marking-out skill. They are particularly useful in batch and mass production, where consistent parts and interchangeability matter.

A well-designed jig or fixture can also prevent a component being fitted in the wrong orientation, reducing rejects. However, making dedicated tooling takes time and money, so it may not be worthwhile for a one-off product.

Tolerances

A tolerance is the permitted variation from a specified dimension. Tight tolerances are needed where parts must fit, align or move accurately, but unnecessary precision increases manufacturing and inspection cost.

CAD/CAM and CNC

CAD allows geometry and dimensions to be defined precisely, while CAM/CNC can reproduce programmed operations consistently. This does not remove the need for checking: an incorrect datum, tool setting, drawing or program can reproduce the same error across a whole batch.