When drilling two deep holes from opposing sides to create an extra long hole, they may meet only partially. To prevent a flow restriction, the hole depths should be specified so that the holes overlap at least 10 mm at the meeting point. If the deep hole is too close to a surface, there may not be enough metal around it to act as an evenly distributed heat sink, and the coolant is insufficient to remove all the heat generated by drilling. The material will anneal at the side of drilling closer to the surface and cause the drill to wander off in the direction of the surface. Setup of drilling may prevent this from happening by providing an extra heat sink if the surface is flat, e.g, by placing a suitable piece of steel on this surface. A deep hole may also wander off enough and break through into the open, or may weaken the material behind a molding surface or supporting surface. Experience has shown that it is safer to use plates of better grade steel, such as P20 or stainless steels, rather than the cheaper AISI4140 to avoid catastrophic drilling errors by hard spots and resulting wandering drills.