Measuring and marking out

Measurement checks a dimension, while marking out transfers dimensions, positions and shapes onto a material before it is cut, drilled or formed. Accurate measuring and marking out reduce errors that can affect fit, function and assembly.

Reference points and marking out

Measurements should be taken from a consistent datum or reference edge where possible. Measuring each new position from the previous mark can allow small errors to accumulate.

The equipment should suit the material and the accuracy required. A rule may be adequate for general dimensions, while Vernier calipers or micrometers are more suitable for precise measurements. Squares, gauges, templates and other marking aids can improve repeatability when positions or shapes must be reproduced.

Tolerances

A tolerance is the permitted variation from a specified dimension. A part specified as 50 ± 0.5 mm is acceptable between 49.5 mm and 50.5 mm. Tight tolerances are important where parts must fit or move together, but unnecessary precision increases manufacturing and inspection cost.

Checking manufactured parts

A go/no-go gauge gives a quick pass or fail result: one side must fit and the other must not. It checks whether a part is within tolerance without giving its exact dimension.

A coordinate measuring machine (CMM) records points on a component and compares them with the required geometry. Non-contact measuring systems can perform similar checks without touching the component.

Sampling

In production, sampling means checking selected products rather than every feature on every item. This saves time, but samples must be taken often enough to detect process problems. Critical dimensions or safety-related features may need more extensive checking.

The required method therefore depends on the material, feature, tolerance, production quantity and consequence of an error.