It will be obvious from these considerations that the more correctly the drill is ground, the longer it will last without regrinding, the greater its amount of feed may be to take an equal depth of cut, and the nearer the diameter of the hole drilled to that of the drill—the most correct results being obtained when the drill will closely fit into the hole it has drilled and will not fall through of its own gravity, a result it is somewhat difficult to attain.
Fig. 1060.
Professor John E. Sweet advocates grinding twist drills as in [Fig. 1060] (which is from The American Machinist), the object being to have a keener cutting edge at the extreme point of the drill.
In a paper on cutting tools read before the British Institution of Mechanical Engineers the following examples of the efficiency of the twist drill are given—
Referring to a 1⁄2 inch twist drill, it is said:
“The time occupied from the starting of each hole in a hammered scrap-iron bar till the drill pierced through it varied from 1 minute 20 seconds to 11⁄2 minutes. The holes drilled were perfectly straight. The speed at which the drill was cutting was nearly 20 feet per minute in its periphery, and the feed was 100 revolutions per inch of depth drilled. The drill was lubricated with soap and water, and went clean through the 23⁄4 inches without being withdrawn, and after it had drilled each hole it felt quite cool to the hand, its temperature being about 75°. It is found that 120 to 130 such holes can be drilled before it is advisable to resharpen the twist drill. This ought to be done immediately the drill exhibits the slightest sign of distress. If carefully examined after this number of holes has been drilled, the prominent cutting parts of the lips which have removed the metal will be found very slightly blunted or rounded to the extent of about 1⁄100th inch, and on this length being carefully ground by the machine off the end of the twist drill, the lips are brought up to perfectly sharp cutting edges again.
“The same sized holes, 1⁄2 inch diameter and 23⁄4 inches deep, have been drilled through the same hammered scrap-iron at the extraordinary speed of 23⁄4 inches deep in 1 minute and 5 seconds, the number of revolutions per inch being 75. An average number of 70 holes can be drilled in this case before the drill requires resharpening. The writer considers this test to be rather too severe, and prefers the former speed.
“In London, upward of 3000 holes were drilled 5⁄8 inch diameter and 3⁄8 inch deep through steel bars by one drill without regrinding it. The cutting speed was in this instance too great for cutting steel, being from 18 to 20 feet per minute, and the result is extraordinary. Many thousands of holes were drilled 1⁄8 inch diameter, through cast iron 7⁄16ths inch deep with straight-shank twist drills gripped by an eccentric chuck in the end of the spindle of a quick-speed drilling machine. The time occupied for each hole was from 9 to 10 seconds only. Again, 1⁄4-inch holes have been drilled through wrought copper 13⁄8 inches thick at the speed of one hole in 10 seconds. With special twist drills, made for piercing hard Bessemer steel, rail holes, 13⁄16ths inch deep and 29⁄32nds inch diameter, have been drilled at the rate of one hole in 1 minute and 20 seconds in an ordinary drilling machine. Had the machine been stiffer and more powerful, better results could have been obtained. A similar twist drill, 29⁄32nds inch in diameter, drilled a hard steel rail 13⁄16ths inch deep in 1 minute, and another in 1 minute 10 seconds. Another drill, 5⁄8 inch diameter, drilled 3⁄4 inch deep in 38 seconds, the cutting speed being 22 feet per minute. This speed of cutting rather distressed the drill; a speed of 16 feet per minute would have been better. The steel rail was specially selected as being one of the hardest of the lot.”