core drill - side and front view
Core drills are designed to enlarge previously drilled, cored, or punched holes 75% of the tool diameter. They have a 30 degree relieved chamfer nose suitable for drilling rough cores. Three and four flute core drills have better hole sizing than two flute drills and produce a very good surface finish, although not to the standard produced by a reamer.

Note: Core drills cannot drill an original hole. Also, core drilling should be executed at approximately the same drilling speeds as would be employed in using a 2 flute drill. Feeds should be intermediate between those recommended for a drill and for a reamer for similar circumstances.
3 FLUTE
Three Flute Core Drills have more chip clearing than four flute core drills. They are used for deep holes and for holes requiring substantial enlarging.
3 Flute - Straight Shank page 88
3 Flute - Morse Taper Shank page 89
4 FLUTE
Four Flute Core Drills have a better surface finish than three flute core drills and can be run at higher metal removal rates. They are used for holes requiring light to medium enlarging. Most users find them easier to re-sharpen and measure.
4 Flute - Straight Shank page 88
4 Flute - Morse Taper Shank page 89