The proper tool to use for enlarging a predrilled hole is not a larger two-flute drill, but a core drill.
What we need to make clear in this tip is that the required Speeds and Feeds for enlarging a predrilled hole with either a 3 or 4 flute core drill, are different than a standard 2-flute drill.
The recommended Speed for an application is to run the core drill about 60% of the speed used to drill with a conventional drill.
Example: A 2-flute drill with recommended SFM of 90 SFM in low carbon steel should be run at approximately 60 SFM using a three or four flute core drill.
The feed rate of a core drill, however, must maintain the same chip load per flute as a standard 2-flute drill.
In other words if the feed rate for a standard 2 flute drill was recommended to be .004 per revolution (or .002 per flute), we would run the core drill (depending on if it was three or four flute) at either .006 or .008 per revolution.