Porauskoko

Porauskoko is a Finnish term for the diameter of a hole produced by drilling. It is a fundamental parameter in machining, manufacturing drawings, and quality control, used to specify the intended size of a drilled hole in metalworking, woodworking, and related fields. The concept covers both metric and imperial sizing, and it is often reported as the nominal hole diameter.

In practice, the porauskoko is shown on drawings as the intended hole size. The actual diameter achieved can differ due to drill geometry, wear, material hardness, and drilling conditions. Holes are typically measured with calipers, bore gauges, or plug gauges to verify whether they fall within tolerance. The nominal size may be accompanied by a tolerance that defines the allowable deviation from the nominal value.

Tolerances and standards: Hole sizes are governed by international standards such as ISO 286, which defines limits and fits. Hole tolerances are commonly designated with a letter and number (for example, H7 in metric systems), indicating the permissible deviation from the nominal size. The choice of porauskoko, together with the corresponding shaft tolerance, determines the type of fit (clearance, transition, or interference) between mating parts.

Relation to other processes: If a drilled hole does not meet the required size, secondary operations such as reaming or boring may be used to adjust the porauskoko to the precise specification. Finishing steps like deburring can also influence the final diameter slightly. Designers choose the porauskoko based on the intended use, such as accommodating fasteners, bushings, or dowel pins, and ensure compatibility with mating components.