It is, in general, cheaper and more satisfactory to buy twist drills than to attempt their manufacture in the ordinary machine shop; but at times some emergency may call for a special size or length of drill which it will be necessary to make.

For the smaller sizes, it is best to use commercial drill rod. For drills larger than 1/2-inch diameter, select larger stock and turn it to the desired size. In the case of the latter drills, if true holes of the size of the drill are required, it is advisable to turn them .010 to .015 inch larger than finish size, and grind to size after hardening. A projection, Fig. 36, containing the center, should be left on the cutting end of the drill until the grinding has been done. After cutting the flutes and grinding the drill, the projection may be ground off and the cutting lips ground to the proper shape, as shown in Fig. 37. When making drills of the smaller sizes from drill rod, the blanks may be cut and pointed to the proper angle on the cutting end; this may be done in the lathe, the blank being held in a chuck. The proper angle is 59 degrees from one side of the blank. When milling the flutes of a twist drill on a universal milling machine, the shank of the drill, if straight, may be held in a chuck or collet of the right size, and, if very long, may be allowed to pass through the spiral head.

Blank for Twist Drill.

Fig. 36. Blank for Twist Drill.

Finished Twist Drill Courtesy of Union Twist Drill Company, Athol, Massachusetts.

Fig. 37. Finished Twist Drill Courtesy of Union Twist Drill Company, Athol, Massachusetts.

Milling Flutes

The accompanying explanation and table are taken from the Brown and Sharpe Manufacturing Company's book, "Construction and Use of Milling Machines", and are intended to use with the cutters manufactured by them for making the flutes in twist drills.

The cutter is placed on the arbor directly over the center of the drill, and the bed is set at the angle of the spiral, as given in Table II.

Table II. Data For Cutting Twist Drills

Diameter of drill (in)

Thick ness of cutter (in)

Pitch

(in)

Gear on

Worm

First

Gear on

Stud

Second

Gear on

Stud

Gear on

Screw

Angle of

Spiral

1/14

.06

.67

24

86

24

100

16°

20'

1/2

.08

1.12

24

86

40

100

19o

20'

1/14

.11

1.67

24

64

32

72.

19°

25'

1/4

.15

1.94

32

64

28

72

21°

5/16

.19

2.92

24

64

56

72

20°

1

.23

3.24

40

48

28

72

21°

7/16

.27

3.89

56

48

24

72

20°

10'

1/2

.31

4.17

40

72

48

64

20°

30'

9/14

.35

4.86

40

64

56

72

20°

3/4

.39

5.33

48

40

32

72

20°

12'

11/15

.44

6.12

56

40

28

64

19°

30'

3/4

.50

6.48

56

48

40

72

20°

11/16

.66

7.29

56

48

40

64

19°

20'

1

.62

7.62

64

48

32

56

19°

50'

15/16

.70

8.33

48

32

40

72

19°

30'

l

.77

8.95

86

48

28

56

19°

20'

The depth of groove in a twist drill diminishes as it approaches the shank, in order to obtain increased strength at the place where the drill is otherwise generally broken. The variation in depth depends on the desired strength or the use of the drill. To obtain the necessary variation of depth, the spindle of the spiral head is elevated somewhat, depending on the length of the flute to be cut; when less than 2 inches in length, the angle should be 1/2 degree; 5 inches and over in length, 1 degree. Usually this will be found satisfactory, but for extremely long drills the elevation must exceed these amounts. The outer end of the drill must be supported as shown in Fig. 38; and when small, should be pressed down firmly until the cutter has passed over the end.

Fig. 38. Supporting Twist Drill for Grinding.

It is somewhat better to use left-handed cutters, so that the cut may begin at the shank end, in order to lessen the tendency to lift the drill blank from the rest. When large drills are held by the centers, the head should be depressed in order to decrease the depth of the groove as it approaches the shank.

Backing Off Rear Of Lip

Another very important operation on the twist drill is that of hacking off the rear of the lip, to give it the necessary clearance. In Fig. 39 the bed is turned to about 1/2 degree, as for cutting a right-hand spiral; but as the angle depends on several conditions, it will be necessary to determine what the effect will be under different circumstances. A study of the figure will be sufficient for this by assuming the effect of different angles, mills, and the pitches of spirals. The object of placing the bed at an angle is to cause the mill F to cut into the lip at C' and just touch the surface at E'. The line R being parallel to the face of the mill, the angular deviation of the bed in comparison with the side of the drill is clearly shown at A.

Backing Off Rear Of Lip 30034Bucking Off' a Twist Drill.

Fig. 39. "Bucking Off' a Twist Drill.

While the drill has a positive traversing and relative movement, the edge of the mill at C must always touch the lip a given distance from the front edge, this being the vanishing point; the other surface, forming the real diameter of the drill, is beyond the reach of the cutter, and is left to guide and steady it while in use. The point E, as shown in the enlarged view, Fig. 39, shows where the cutting commences, and its increase until it reaches a maximum depth at C, where it may be increased or diminished according to the angle employed in the operation, the line of cutter action being represented by II.

Before backing off, the surface of the smaller drills in particular should be oxidized by heating until it assumes some distinct color to show clearly the action of the mill on the lip of the drill, for, when satisfactory, a uniform streak of oxidized surface, from the front edge of the lip back, is left untouched by the mill, as represented in the cut at E.

If the drills are to be ground without being centered, pointed projections with a 60-degree angle may be made on the ends, as shown in Fig. 39; these projections may be run in female centers in the grinding machine. In grinding, if the drills are tapered back about .003 inch in 6 inches, it will be found that the clearance thus obtained will cause them to run much better.