Lubrication of Drills

When drilling tough metals, such as steel and wrought and malleable iron, heat is generated by the bending or changing of the form of the metal being removed and by friction caused by the chips moving over the lips of the drill. The heating is similar to the heating of a piece of wire bent quickly back and forth. As there is danger of heating the drill to a temperature that will draw the temper and soften the drill, plenty of lard oil, or a mixture of potash and water, should be used. This is not so much for lubrication as to conduct away the heat.

Copper is the most difficult to drill of all the common metals on account of its extreme toughness; then, too, copper heats to a higher temperature on account of its low specific heat. Brass does not require the use of oil, and cast iron is usually drilled dry.

Fig. 68. Oil Tube Drill Courtesy of Cleveland Twist Drill Company, Cleveland, Ohio

Fig. 68. Oil Tube Drill Courtesy of Cleveland Twist Drill Company, Cleveland, Ohio.

As the heat is produced at the point of the drill, it is desirable, particularly in the case of deep holes, that the oil be applied directly at the drill point. For this purpose, oil-tube drills, such as shown in Fig. 68, are used. The oil is supplied under pressure, and not only removes the heat but also carries away the chips.

Speed of Drills, The speed at which drills should be rotated depends both on the diameter of the drill and on the material operated upon. No absolute rule can be given for any one metal or diameter of drill because of the variation in hardness and tenacity of the material and the condition of the cutting edges of the drill. The operator must exercise his own judgment.

Table I, giving the speed of drills in revolutions per minute, is based on a peripheral speed of 30 feet a minute for mild steel, 35 feet per minute for cast iron, and 60 feet per minute for brass, using carbon tool steel drills.

The rate of feed also depends on the drill diameter and on the material. The Cleveland Twist Drill Company gives, as a maximum, one inch of feed for 95 to 125 revolutions.

Table I. Speed Of Drills

Diameter of Drill.

Soft Steel, or Wrought Iron

(r, p. m.)

Cast Iron (r. p. m.)

Brass (r. p. m.)

1/16

1824

2128

3648

1/8

912

1064

1824

3/16

608

710

1216

1/4

456

532

912

5/16

365

425

730

3/8

304

355

608

7/16

260

304

520

1/2

228

226

456

9/16

203

236

405

5/8

182

213

365

11/16

166

194

332

3/4

152

177

304

13/16

140

164

280

7/8

130

152

260

15/16

122

142

243

1

114

133

228

1 1/16

108

125

215

l 1/8

102

118

203

1 3/16

96

112

192

1 1/4

91

106

182

1 5/16

87

101

174

1 3/8

83

97

165

1 7/16

80

93

159

l l/2

76

89

152

1 9/16

73

85

145

1 5/8

70

82

140

1 11/16

68

79

135

1 3/4

65

76

130

1 13/16

63

73

125

1 7/8

60

71

122

1 15/16

59

69

118

2

57

67

114