Conventional twist drills are designed to penetrate the workpiece at the
center of the point, known as the chisel, and cut over the entire length of the
cutting lips. Once the outside diameter penetrates the face of the part, the
finish size is established. However, when using a conventional drill to
enlarge a hole, the drill point will hit the pre-drilled hole in the middle of
the cutting lips. Because of the positive rake created by the helix of the
drill, this can cause the cutting lips to chip, creating a groove or worn spot
across the land on the end of the point. In addition, the drill
tends to chatter or seize in the hole, effecting hole size, hole shape, and
finish in the part.
Reamers on the other hand, are intended to improve finish and control size. They follow the existing hole and are designed to remove up to a 1/16” of material on diameter depending upon size. Because of their multiple and shallow flutes, they do not have the chip carrying capability to remove large amounts of material necessary to enlarge holes. Chips will pack in the flutes, preventing cutting fluids from reaching the front of the reamer. This will cause excessive heat and wear in the chamfer area. In extreme cases the chamfer teeth will chip or break.
The ideal tools for enlarging holes are core drills. They have a 118°
included angle point that does not cut to center and can open up a pre-drilled
hole from as small as 60% of the tools diameter. They usually come
in three or four flutes with straight or tapered shanks. Four flutes have
less chip space and are recommended for shallower holes, but will produce the
best finish and size. Three flutes provide more chip space allowing them
to be used in deep holes where a larger volume of chips are created.
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