Drill Bits Should Not Be Used To Enlarge Holes

“WARNING: TWO FLUTED DRILLS SHOULD NOT BE USED TO ENLARGE PRE-EXISTING HOLES.”
-Triumph Twist Drill

All drill bit manufacturer's support this fact, and it is the reason that they make reamers, core drills, counterbores and multi-flute drills. In their 10 page flier: Technical Information Guide on drilling that Triumph Twist Drill prints, they state the quote above in bold, at the top of page one. Another respected manufacture, Precision Twist Drill states on their website “one of the rules broken most often ... Never use a two flute drill to enlarge or open a hole."
While the practice of “pre-drilling” isn’t uncommon with 118° drill bits (*see below) you will find that most all of the drill bits that we sell are 135° split point and will self-center, and start drilling immediately upon contact with the material. If you must enlarge or "finish" an existing hole, then we suggest any one of these acceptable type of tools:

1.) Three or four flute drills
2.) Three, four, or five flute Counterbores
3.) Three or four flute Core Drills
4.) Reamers - (hundreds of styles and flute patterns)

(All the tools in this list have more than two-flutes and are designed for enlarging and finishing holes exclusively.)

* If you have a 118° drill bit and want to drill a pilot hole, there is one important rule:
“Drill a pilot hole 10% larger than the web thickness (see Figure 1) of the final drill”.
(Not over 10%) The reason for drilling the hole 10% larger than the web thickness is to relieve the thrust generated by the chisel and to stabilize the drill in the cut.

Example: If you want to drill a hole that is 1-1/4" with a 118° drill bit and the web thickness of the bit is .150", you should drill a pilot hole between .155" and .165" (about 5/32")