improperly sharpened drill can cut oversize by as much as 10%. When you start to drill, the initial penetration should be no more than twice the diameter of the hole before you retract the drill, clear the chips and add coolant with the tip of a small brush. From then on, do not try to drill deeper than the diameter of the drill without clearing the chips and adding coolant. For example:
To drill a 1/8" diameter hole 1" deep:
1st Pass: 2 times diameter or 1/4" 2nd Pass: 1 times diameter or 1/8" 3rd Pass: 1 times diameter or 1/8" Etc.
1/4" 3/8" 1/2" Etc.
You may encounter recommendations exceeding these figures, but they are meant for automatic equipment with pressurized coolant systems.
It is difficult to maintain tolerances of better than +.003"-.000" with a drill. If tolerances closer than these are required, a reamer must be employed. Try to use fractional size reamers whenever possible rather than decimal sizes, because the cost difference can amount to 2 or 3 times higher. (The length of reamers may prevent their use for some operations on machines of this size.)
FIGURE 18-Typical Center Drill
To accurately start holes, center drills must be used. They have a small tip which accurately starts the hole, and then the shaft widens with a 60° cutting face to the final diameter. Care must be taken to employ cutting oil and to clear the drill frequently. If this is not done, the fragile tip may load up and twist off even in soft materials. Center drills are available in a variety of sizes, but for general work we recommend No.1.