Drilling PDF

CHAPTER 8 DRILLING Round-Hole-Making Methods 8.1 Definition Drilling is a cutting procedure designed to generate holes predominantly with a two-flute tool, the twist drill. When drilling on the drilling mach...

CHAPTER 8 DRILLING Round-Hole-Making Methods 8.1 Definition Drilling is a cutting procedure designed to generate holes predominantly with a two-flute tool, the twist drill. When drilling on the drilling machine, the tool carries out the feed- and the cutting motions. If the hole is machined on a turning- or automatic lathe, then the workpiece performs the cutting motion. 8.2.1. Drilling The most common tool for drilling, a twist drill, is a rod with helical flutes and two or more cutting edges at the end. It is rotated about its axis and fed axially into the work. As it advances, it produces or enlarges a round hole in the workpiece. The chips are carried away from the hole by the flutes in the drill. (When drilling an axial hole with a lathe, the workpiece rotates rather than the drill.) There are other types of drills that may not have helical flutes. Others may have only one cutting edge. The drilling process is very common and is used with a wide variety of machines ranging from the most sophisticated computer-controlled or multiplespindle machines to hand-held electric or crank-driven drills. The most common diameter range for drilled holes is about 1/8 in (3 mm) to 11/2 in (38 mm) although diameters from 0.001 (0.025 mm) to 6 in (150 mm) can be drilled with commercially available special drills. Fig. 7.2 includes an illustration of drilling as performed on turning equipment. Fig. 8.1 shows various types of drill and drilling operations. Fig. 8.2 shows some typical drills. Figure 8.1. Various types of drill and drilling operations. Figure 8.2. A series of drills and, at the bottom of the group, a typical reamer. Chapter 8 1  8.2.2 Counterboring counterboring - enlarges a hole for part of its depth and usually machines a flat bottom in the enlarged portion. The operation is most often performed to provide clearance for a bolt head or multi-diameter part. The rotating cutter is guided by a pilot that fits into the existing hole, so that the counterbored surface is concentric with the original hole. A multi-diameter counterboring tool can produce stepped counterbores. Fig. 8.3 illustrates a counterboring tool in view a) and the counterbored hole it produces in view b). (The tool also can be produce spotfacing, as shown in view c). Figure 8.3. A counterboring/spotfacing Figure 8.4. Reaming is used to improve tool (view a) and a cross-sectional view the accuracy, surface finish and of the counterbored hole it produces straightness of round holes. (view b). View c) shows cross-sections of two slightly different spotfacings produced by the same tool. The purpose of counterboring is to produce a recess of prescribed depth while spotfacing is performed to provide a smooth and perpendicular flat surface for a fastener or other object. 8.2.3 Countersinking Countersinking - is an operation that adds a chamfer at the entry end of a hole. A rotating cutting tool, with the edge set to the angle of chamfer desired, is fed into the hole and removes material at the edge. The tool is centered by the hole; therefore the chamfer is concentric with the hole's axis. The operation is typically used to remove burrs or a sharp edge at the end of a hole, or to provide space for a tapered screw head or other tapered object. 8.2.4 Reaming Reaming - is a secondary machining operation for existing holes. It can provide a more accurate diameter, improved straightness, and a smoother surface finish as it slightly enlarges the hole. A rotating tool, a reamer, is used. The operation can be performed on a drill press or other Chapter 8 2 drilling machine and is sometimes done by hand. Reamers normally remove 0.005 to 0.015 in (0.13 to 0.38 mm) of diameter. Reamers normally float, that is they follow the direction and location of the existing hole, but they can also be guided by bushings to slightly improve the hole's direction or location. The operation is most common with holes from 1/8 to 11/4 in (3 to 32 mm) in diameter but both smaller and larger holes can be reamed. A typical reamer is illustrated in Fig. 8.2 and the reaming operation on a lathe is shown schematically in Chapter 7 and Fig. 8.4. Taper reamers are used for finishing tapered holes. 8.2.5 Boring Boring - is an operation that enlarges and improves the accuracy of an existing hole. Either the work or the cutting tool rotates about the center axis of the hole. The single point tool describes a circle, removing material from the surface of the existing hole as it advances, enlarging the hole, normally increasing the precision of any of a number of factors. They are: its location, diameter, direction, cylindricity, and finish. When this operation is performed on a boring machine, the workpiece is stationary and the cutting tool rotates; when performed on a lathe, the workpiece rotates. On a lathe, the operation can then be considered to be internal turning. The tool spindle and the workpiece holder must be rigid enough to provide the desired accuracy in the bored hole. The operation is performed on holes from about 1/4 in (6 mm) in diameter and larger but is more common on larger holes, especially those too large to be drilled accurately, and for the machining of cast or forged large holes. Fig. 8.5 illustrates the process, and Fig. 7.2 shows it as one of a series of lathe operations. a b Figure 8.5. Boring operation. a) Boring operations slightly enlarge and improve the precision of an existing hole. b) A horizontal boring mill. This machine can perform boring, milling, and drilling operations. Jig boring - is performed on jig boring machines, which are vertical boring machines of very high accuracy. The table movement is extremely accurate and the spindle and spindle bearings are very precisely made. The machines are mainly used for making jigs, gages, dies, and fixtures, especially where accurate layout and hole location are essential. Horizontal boring mills - are basically large horizontal milling machines capable of performing boring, milling, and other machining operations on large and often complex parts. These units are sometimes called, horizontal boring and milling machines. The table can move in x and y directions. (Some machines have a table that also swivels.) The headstock that holds the spindle can be raised or lowered. The tool-holding spindle can move inward or outward. These machines are used in the machining of large components that have horizontal holes requiring the precision that boring provides. The machines normally include an end support column, opposite the spindle, for Chapter 8 3 long boring bars. Tolerances with the machines can be as low as one or two ten thousandths of an inch (0.003 to 0.005 mm). Fig. 8.5 b illustrates a horizontal boring mill. Vertical boring mills - are machines with a horizontal table rotating on a vertical axis, and a precision tool head (often two tool heads) capable of movements up and down and side to side (in and out radially). There may be more than one cross slide with tool-holding capability. These machines can be considered to be large lathes turned on end. They are especially suited to boring and other operations on parts too large for a conventional lathe. Workpieces are typically round and heavy with large diameters and shorter lengths. The workpiece is clamped to the rotating table, which can be as large as 40 ft. (12 m) in diameter. Both boring and facing are possible. There is no spindle for milling cutters; all cutting is by single point tools. Fig. 8.6 illustrates a typical vertical boring mill. Figure 8.6. A vertical boring mill. (vertical boring and turning machine.). 8.2.6 Gun drilling Gun drilling - is shown in Fig. 8.7. A rotating single-flute drill, normally carbide-tipped, is guided by a bushing at the start of the drilled hole and is self-guided thereafter by a bearing surface opposite the cutting edge, A hole through the whole length of the drill provides a means for oil coolant to flow at high pressure to the cutting edge and to flush chips from the hole. Deep, straight, holes are possible with the process which was originally developed for manufacture of gun barrels. Hole depths of over 250 times diameter are possible. Figure 8.7. A typical gun drill, viewed from the cutting end. Chapter 8 4  8.3 Drilling tools 8.3.1. Twist drill The twist drill (Figure 8.8) consists of the body with the drill point and the shank. Figure 8.8. Elements of the twist drill. While the drill point performs the actual metal removal, the body with the flute is engineered to remove the chips. The shank has to hold the drill in the drill spindle of the drilling machine. 8.4 Drilling machines Drilling machines are used for drilling holes, tapping, reaming and other general-purpose, small-diameter boring operations. Drilling machines are generally vertical, the most common type of which is a drill press. Its major components are shown in fig. 8.9. The workpiece is placed on an adjustable table, either by clamping it directly into the slots and holes on the table or by using a vice, which in turn can be clamped to the table. The workpiece should br properly clambed, for safety and accuracy, because the drilling torque can be high enough to rotate present rates. The drill is lowered manually by hand wheel or by power feed at preset rates. Manual feeding requires some skill in judging the appropriate feed rate. In order to maintain proper cutting speeds at the cutting edges of drills, the spindle speed on drilling machines has to adjustable to accommodate different sizes of drills. Ajustment are made by means of pulleys, gear boxes, or variable-speed motors. Drill presses are usually designated by the largest worpiece diameter that can be accommodated on the table. Size typically range from 150 mm to 1250 mm. Chapter 8 5  Figure 8.9. Schematic illustration of the components of a vertical drill press. Chapter 8 6