When a drill bit is marked with a specific size, such as 3 mm, it typically refers to the nominal or theoretical diameter of the hole it will create. However, the actual hole diameter may be slightly larger due to several factors, including the design of the drill bit, the material being drilled, and the drilling process itself.
Drill bits are often manufactured to have a slightly larger diameter than the nominal size. This is known as the "oversize" or "clearance" allowance. The purpose of this oversize is to ensure that the drilled hole is not undersized and meets the desired minimum diameter.
The amount of oversize can vary depending on the type of drill bit, its quality, and the intended application. It's common for the oversize to be around 0.1 to 0.3 mm larger than the nominal size. For example, a 3 mm drill bit might create a hole with a diameter of approximately 3.1 to 3.3 mm.
It's worth noting that the specific oversize allowance can differ between manufacturers and even between different drill bits from the same manufacturer. Therefore, if you require precise hole dimensions, it's recommended to consult the manufacturer's specifications or perform tests to determine the actual hole size created by a particular drill bit.