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SPEEDS AND FEEDS FOR DEEP-HOLE DRILLING
The planning of drilling operations often fails to take into account the depth of holes to be drilled and its influence on permissible rates of feeds and speeds.
If the hole to be drilled equals or exceeds three times the diameter of the drill, the operation falls in the deep-hole drilling class, and some adjustment of feeds and speeds is necessary in order to secure maximum efficiency. In considering the depth of the hole, the effect of bushings must be taken into account. A drill bushing set very close to the work becomes in effect an extension of the hole itself, and on holes from 2 to 3 diameters deep complicates the problem of chip disposal.
In its final analysis it is entirely a matter of conducting away the heat generated and of preventing its accumulation. It is an evident fact that if less heat is conducted away than is generated, the drill will eventually overheat and be destroyed.
While the nature and amount of coolant applied to a given drilling operation will affect the depth of hole that can safely be drilled at ordinary feeds and speeds, the above-mentioned depth of three diameters is a good point at which to begin the stepping-down. This applies particularly to speed.
The table below will serve as a guide to the proper reductions as the depth of hole increases:
SPEEDS
DEPTH OF HOLE ----------------------------REDUCTION OF SPEED, PER CENT
3 TIMES DRILL DIAMETER ---------------------10
4 TIMES DRILL DIAMETER ---------------------20
5 TIMES DRILL DIAMETER ---------------------30
6 TO 8 TIMES DRILL DIAMETER ---------------35 TO 40
FEEDS
DEPTH OF HOLE --------------------------------REDUCTION OF FEED, PER CENT
3 TO 4 TIMES DRILL DIAMETER -------------------10
5 TO 8 TIMES DRILL DIAMETER -------------------20
If excellent cooling conditions are present, the above rates may be increased; but if there is little or no provision for cooling, it may be necessary to step down even lower than the figures given.
It should be kept in mind that a somewhat lower rate, constantly maintained, will show more production per day than a high rate with long and frequent interruptions. In the latter case, too, the tool costs will be much higher.
