Chamfering and Countersinking Tapping

348 Section 4 • Drill Press CHAMFERING AND COUNTERSINKING A countersink is used to perform countersinking or chamfering operations on existing holes. Speeds for countersinking are normally about 25% of those used when drilling the same material. Feeds for countersink• ing are usually light, just as when counterboring or spot- facing, because of the large amount of surface contact between the tool and the workpiece. Feed depth for a countersink depends on the angle of the tool, the existing hole size, and the desired coun• tersink, or chamfer, diameter. When using a 90-degree countersink, feed depth can be found using the formula D-d —-—, where D = desired countersink or chamfer FIGURE 4.3.25 A countersinking operation and proper posi• diameter to be produced on the existing hole, and d = tion of a table stop to prevent vise movement. existing hole diameter. Figure 4.3.24 illustrates this con• ©Cengage Learning 2012 cept and shows variations of this formula for different countersink angles. A countersink will center itself in an existing hole, ing operation and proper position of a table stop to pre• so the workpiece or vise is normally allowed to float on vent vise movement. the drill press table instead of being tightly clamped. A positive stop is clamped to the worktable to prevent the vise or workpiece from rotating. After mounting the TAPPING tool in the spindle, a depth reference location can be The drill press can also be used to properly align taps, set and used for setting feed depth of the tool. Set the eliminating the need for using a square as when hand tap• depth stop so the tool is within about 1/32" of touching ping. If the workpiece has been moved between the drill• the edge of the existing hole. Turn on the spindle and ing and tapping operation, the hole can be realigned with adjust the stop until the countersink touches the edge of the spindle using a center point mounted in the spindle, the existing hole. Then adjust the depth stop to achieve as shown in Figure 4.3.26. Clamp the vise or workpiece the required depth. Figure 4.3.25 shows a countersink- securely to the table before tapping to prevent the work from moving out of position. This center point fits in a hole in the top of the tap or in the back of a tap wrench. The tap can be placed at the top of the drilled hole and the quill lowered so Countersink Included Formula for 'F' the tip of the tap center is in the hole in the tap or tap Angle (Amount of Feed) wrench. Some tap centers have spring-loaded tips. These (D-d" types of tap centers supply even pressure on the tap while 60 F = X1.732 I 2 , it is being turned into the hole. When using a spring- loaded center, bring the center into contact with the tap or wrench by lowering the quill until the spring is com• 82 F = X1.15 pressed. Lock the quill in postion with the quill lock. (See Figure 4.3.27.) If using a solid tap center, one hand (D-d" must apply light pressure to the quill feed handle to keep 90 F = I 2 ,) the tap aligned while turning the tap with the other hand, as shown in Figure 4.3.28. When tapping, advance the fD-d) 100 F = X 0.839 tap about one half to one full turn at a time. The tap < 2 J should then be backed out by half a turn before continu• ing. This helps break the chip being formed as it cuts. FIGURE 4.3.24 Calculating depth for a desired countersink Not backing the tap up will often cause the tap to bind in diameter. ©Cengage Learning 2012 the hole and break. .

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