外文翻譯--機械工具的根本原則 英文版【優(yōu)秀】

桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 1 頁 共 35 頁 TC No. 9-524 JOEL B.HUDSON, DENNIS J.REIMER Washington,DC, 29 October 1996 ATTN: ATCL-AO, 801 Lee Avenue, Fort Lee, Virginia 23801-1713. FUNDAMENTALS OF MACHINE TOOLS Chapter 4 DRILLING MACHINES GENERAL INFORMATION PURPOSE This chapter contains basic information pertaining to drilling machines. A drilling machine comes in many shapes and sizes, from small hand-held power drills to bench mounted and finally floor-mounted models. They can perform operations other than drilling, such as countersinking, counter boring, reaming, and tapping large or small holes. Because the drilling machines can perform all of these operations, this chapter will also cover the types of drill bits, took, and shop formulas for setting up each operation. Safety plays a critical part in any operation involving power equipment. This chapter will cover procedures for servicing, maintaining, and setting up the work, proper methods of selecting tools, and work holding devices to get the job done safely without causing damage to the equipment, yourself, or someone nearby. USES A drilling machine, called a drill press, is used to cut holes into or through metal, wood, or other materials (Figure 4-1).Drilling machines use a drilling tool that has cutting edges at its point. This cutting tool is held in the drill press by a chuck or Morse taper and is rotated and fed into the work at variable speeds. Drilling machines may be used to perform other operations. They can perform countersinking, boring, counter boring, spot facing, reaming, and tapping (Figure 4-2). Drill press operators must know how to set up the work, set speed and feed, and provide for coolant to get an acceptable finished product. The size or capacity of the 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 2 頁 共 35 頁 drilling machine is usually determined by the largest piece of stock that can be center-drilled (Figure 4-3). For instance, a 15-inch drilling machine can center-drill a 30-inch-diameter piece of stock. Other ways to determine the size of the drill press are by the largest hole that can be drilled, the distance between the spindle and column, and the vertical distance between the worktable and spindle. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 3 頁 共 35 頁 CHARACTERISTICS All drilling machines have the following construction characteristics (Figure 4-4): a spindle. sleeve or quill. column, head, worktable, and base. The spindle holds the drill or cutting tools and revolves in a fixed position in a sleeve. In most drilling machines, the spindle is vertical and the work is supported on a horizontal table. The sleeve or quill assembly does not revolve but may slide in its bearing in a direction parallel to its axis. When the sleeve carrying the spindle with a cutting tool is lowered, the cutting tool is fed into the work: and when it is moved upward, the cutting tool is withdrawn from the work. Feed pressure applied to the sleeve by hand or power causes the revolving drill to cut its way into the work a few thousandths of an inch per revolution. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 4 頁 共 35 頁 The column of most drill presses is circular and built rugged and solid. The column supports the head and the sleeve or quill assembly. The head of the drill press is composed of the sleeve, spindle, electric motor, and feed mechanism. The head is bolted to the column. The worktable is supported on an arm mounted to the Column. The worktable can be adjusted vertically to accommodate different heights of work. or it may be swung completely out of the way. It may be tilted up to 90 in either direction, to allow for long pieces to be end or angled drilled. The base of the drilling machine supports the entire Machine and when bolted to the floor, provides for Vibration-free operation and best machining accuracy. The top of the base is similar to a worktable and maybe equipped with T-slots for mounting work too large for the table. CARE OF DRILLING MACHINES Lubrication Lubrication is important because of the heat and friction Generated by the moving parts. Follow the manufacturers manual for proper lubrication methods. Clean each machine after use. Clean T-slots. grooves. And dirt from belts and pulleys. Remove chips to avoid damage to moving parts. Wipe all spindles and sleeves free of grit to avoid damaging The precision fit. Put a light coat of oil on all unpainted surfaces to prevent rust. Operate all machines with care to avoid overworking the electric motor. Special Care Operations under adverse conditions require special care. If machines are operated under extremely dusty conditions. Operate at the slowest speeds to avoid rapid abrasive wear on the moving parts and lubricate the machines more often. Under extreme cold conditions, start the machines at a slow speed and allow the parts and lubricants to warm up before increasing the speeds. Metal becomes very brittle in extreme cold. so do not strike the machines with hard tools. Extreme heat may cause the motor to overheat. so use intermittent. Or on and off, operations to keep the motor running cool. TYPES OF DRILLING MACHINES There are two types of drilling machines used by maintenance personnel for repairing and fabricating needed parts: hand-feed or power-feed. Other types of 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 5 頁 共 35 頁 drilling machines, such as the radial drill press. Numerically controlled drilling machine. Multiple spindle drilling machine, gang drilling machine, and turret drill press, are all variations of the basic hand and power-feed drilling machines. They are designed for high-speed production and industrial shops. Drilling depth is controlled by a depth-stop mechanism located on the side of the spindle. The operator of the machine must use a sense of feel while feeding the cutting tool into the work. The operator must pay attention and be alert. to when the drill breaks through the work, because of the tendency of the drill to grab or snag the work piece, of these machines, operations that require drilling speeds less than 450 revolutions per minute cannot be performed. Reaming, counter boring, and counter-sinking may require slower speeds than drilling and may not be able to be performed for all materials on these machines. Hand-Feed The hand-feed drilling machines (Figure 4-5) are the simplest and most common type of drilling machines in use today. These are light duty machines that are hand-fed by the operator, using a feed handle. so that the operator is able to “feel” the action of the cutting tool as it cuts through the work piece. These drilling machines can be bench or floor-mounted. They are driven by an electric motor that turns a drive belt on a motor pulley that connects to the spindle pulley. Hand-feed machines are essentially high-speed machines and are used on small workplaces that require holes 1/2 inch or smaller. Normally, the head can be moved up and down on the column by loosening the locking bolts. Which allows the drilling machine to drill different heights of work. Power-Feed The power-feed drilling machines (Figure 4-6) are usually larger and heavier than the hand-feed. They are equipped with the ability to feed the cutting tool into the work automatically, at a preset depth of cut per revolution of the spindle, usually in thousandths of an inch per revolution. These machines are used in maintenance shops for medium duty work, or work that uses large drills that require power feeds. The power-feed capability is needed for drills or cutting took that are over 1/2 inch in diameter, because they require more force to cut than that which can be provided by using hand pressure. The speeds available on power-feed machines can vary from about 50 RPM to about 1,800 RPM. The slower speeds allow for special operations, such as counter boring, countersinking, and reaming. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 6 頁 共 35 頁 The sizes of these machines generally range from 17-inch to a 22-inch center-drilling capacity, and are usually floor mounted. They can handle drills up to 2 inches in diameter, which mount into tapered Morse sockets. Larger workplaces are usually clamped directly to the table or base using T-bolts and clamps, while small workplaces are held in a vise. A depth-stop mechanism is located on the head, near the spindle, to aid in drilling to a precise depth. SAFETY PRECAUTIONS DRILLING MACHINE SAFETY Drilling machines are one of the most dangerous hand operated pieces of equipment in the shop area. Following safety procedures during drilling operations will help eliminate accidents, loss of time, and materials. Listed below Keep all loose clothing away from turning tools. Are safety procedures common to most types of drilling a chines found in the machine shop. Make sure that the cutting tools are running straight before starting the operation. Do not support the workplaces by hand. Never make any adjustments while the machine is operating. Never clean away chips with your hand. Use a brush. Never place tools or equipment on the drilling tables. Keep all guards in place while operating. Ease up on the feed as the drill breaks through the work to avoid damaged tools or workplaces. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 7 頁 共 35 頁 Remove all chuck keys and wrenches before operating. Always wear eye protection while operating any drilling machines. TOOLS AND EQUIPMENT TWIST DRILLS Twist drills are the most common cutting tools used with drilling machines. Twist drills are designed to make round holes quickly and accurately in all materials. They are called twist drills mainly because of the helical flutes or grooves that wind around the body from the point to the neck of the drill and appear to be twisted (Figure 4-7). Twist drills are simply constructed but designed very tough to withstand the high torque of turning, the downward pressure on the drill, and the high heat generated by friction. There are two common types of twist drills, high-speed steel drills, and carbide-tipped drills. The most common type used for field and maintenance shop work is the high-speed steel twist drill because of its low cost. Carbide-tipped metal drills are used in production work where the drill must remain sharp for extended periods, such as in a numerically controlled drilling machine. Other types of drills available are: carbide tipped masonry drills, solid carbide drills, TiN coated drills, parabolic drills and split point drills. Twist drills are classified as straight shank or tapered shank (Figure 4-7). Straight shank twist drills are usually l/2-inch or smaller and tit into geared drill chucks, while tapered shank drills are usually for the larger 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 8 頁 共 35 頁 drills that need more strength which is provided by the taper socket chucks. Common twist drill sizes range from 0.0135 (wire gage size No. 80) to 3.500 inches in diameter. Larger holes are cut by special drills that are not considered as twist drills. The standard sizes used in the United States are the wire gage numbered drills, letter drills, fractional drills, and metric drills (See Table 4-1, in Appendix A). Twist drills can also be classified by the diameter and length of the shank and by the length of the fluted portion of the twist drill. Wire gage twist drills and letter twist drills are generally used where other than standard fractional sizes are required, such as drilling holes for tapping. In this case, the drilled hole forms the minor diameter of the thread to be cut, and the major diameter which is cut by tapping corresponds to the common fractional size of the screw. Wire gage twist drills range from the smallest to the largest size； from No 80(0.0135 inch) to No 1 (0.2280 inch). The larger the number, the smaller the diameter of the drill. Letter size twists drills range from A (0.234 inch) to Z (0.413 inch). As the letters progress, the diameters become larger. Fractional drills range from 1/64 to 1 3/4 inches in l/64-inch units； from 1/32 to 2 1/4 inches in 1/32-inch units, and from 1/1 6 to 3 1/2 inches in 1/16-inch units. Metric twist drills are ranged in three ways: miniature set, straight shank, and taper shank. Miniature metric drill sets range from 0.04 mm to 0.99 mm in units of 0.01 mm. Straight shank metric drills range from 0.05 mm to 20.0 mm in units from 0.02 mm to 0.05 mm depending on the size of the drill. Taper shank: drills range in size from 8 mm to 80 mm in units from 0.01 mm to 0.05 mm depending on the size of the drill. The drill gage (Figure 4-8) is used to check the diameter size of a twist drill. The gage consists of a plate having a series of holes. These holes can be numbered, lettered, fractional, or metric-sized twist drills. The cutting end of the drill is placed into the hole to check the size. A micrometer can also be used to check the size of a twist drill by measuring over the margins of the drill (Figure 4-9). The smaller sizes of drills are not usually marked with the drill size or worn drills may have the drill size rubbed off, thus a drill gage or micrometer must be used to check the size. It is important to know the parts of the twist drill for proper identification and sharpening (Figure 4-7). The point is the entire conical shaped end of the drill containing the cutting edges 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 9 頁 共 35 頁 and chisel edge. The body is the part of the drill that is fluted and relieved. The shank is the part that fits into the holding device, whether it is a straight shank or a tapered shank. The chisel edge is the point at which the two lips meet. The 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 10 頁 共 35 頁 chisel edge acts as a chisel when the drill is turning and cuts into the work piece. The chisel edge must always be centered exactly on the drills axis for accurate cutting action. The cutting edge lips cut like knives when fed and rotated into the work piece. The lips are sharp edges formed by grinding the flutes to a conical point. The heel is the conical shaped portion of the point in back of the cutting edge lips. The amount of slope given to the heel in back of the drill lips is called lip clearance. This clearance is necessary to keep the heel from rubbing the bottom of the hole being drilled. Rubbing would prevent the drill from cutting. The flute is the helical groove on the drill. It carries out the chips and admits coolant to the cutting edges. The margin is the narrow surface along the flutes that determines the size of the drill and keeps the drill aligned. The portion of the drill body that is relieved behind the margin is known as the body clearance. The diameter of this part is less than that of the margin and provides clearance so that all of the body does not rub against the side of the hole and cause friction. The body clearance also permits passage of lubricants around the drill. The narrowed end of the tapered shank drill is called the tang. The tang fits the slot in the innermost end of the drill spindle, drill chuck, or other drill holding device and aids in driving the tool. It also prevents the drill from slipping. The web of the drill is the metal section separating the flutes. It runs the length of the body between the flutes. The web gradually increases in thickness toward the shank, increasing the rigidity of the drill. An imaginary line through the center of the drill from end to end is the axis. The drill must rotate evenly about the axis at all times. SPECIAL DRILLS Special drills are needed for some applications that a normal high-speed industrial operations. Other types of special drills general purpose drill cannot accomplish quickly or accurately. Are: left hand drill, Silver and Deming, spotting, slow spiral, Special drills can be twist drill type, straight fluted type, or fast spiral, half round, die, flat, and core drills. The general special fluted. Special drills can be known by the job that they purpose high-speed drill, which is the common twist drill used are designed for, such as aircraft length drills, which have an for most field and maintenance shops, can be reground and extended shank. Special drills are usually 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 11 頁 共 35 頁 used in adapted for most special drilling needs. SHARPENING TWIST DRILLS Twist drills become dull and must be resharpened. The preferred method of resharpening a twist drill is with the drill grinding machine, but this machine is not always available in field and maintenance units, so the offhand method of drill sharpening must be used (Figure 4-10). The off hand method requires that the operator have a knowledge of the drilling geometry (Figure 4-11) and how to change drill angles as needed for any drilling job (see Table 4-2 in Appendix A). Tools needed are a utility or bench grinder with a dressed wheel and a drill point gage (Figure 4-12) or protractor head on the combination square. The drill point gage is set at 59 and adjusted along the steel rule to fit the drill to be sharpened. The cutting lips must be of the same angle, the lip clearance angle must be within a specific degree range, and the cutting lips must be of an equal length. There are several basic characteristics that all twist drills must have to cut properly. The following will cover those characteristics. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 12 頁 共 35 頁 PRECHECK Before sharpening a twist drill, the operator must check the condition of the drill for chipped and cracked lips or edges that must be ground off during the sharpening process. The operator must also check the references for the proper lip angle and lip clearance angle for the material to be drilled. After setting up the bench grinder for offhand drill sharpening, the operator assumes a comfortable stance in front of the grinding wheel to sharpen the twist drill. The suggested method is to grind the lip angle first, then concentrate on grinding the lip clearance angle, which will then determine the lip length. The usual lip angle is an included angle of 118(59 x 2) (Figure 4-13), which is the lip angle of general purpose drills. Use the drill point gage frequently to check lip angle and lip length. When grinding, do not allow the drill to become overheated. Overheating will cause the drill edges to become blue which is an indication that the drills temper has been lost. The blue area must be ground completely away to reestablish the drills temper. If a drill becomes too hot during sharpening, the lips can crack when dipped into cold water or coolant. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 13 頁 共 35 頁 DRILL POINT When grinding the lip angle, use the drill point gage and grind one lip perfectly straight and at the required angle (usually 590). Then flip the drill over and grind the other lip. Once the angle is established, then the lip clearance angle and lip length can be ground. If both lips are not straight and of the same angle, then the chisel edge (Figure 4-14) will not be established. It is it important to have a sharp and centered chisel edge or the drill will not rotate exactly on its center and the hole will be oversized. If the drill point is too steep, the drill will require more power and cut slowly. When the angles of the cutting lips are different, then the drill will only have one lip cutting as it revolves. The hole will be oversized and the drill will wear very rapidly. When both the angles and the length of the angles are incorrect, then excessive wear is put on both the drill and machine, which will result in poor workmanship (Figure 4-15). 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 14 頁 共 35 頁 CLEARANCE ANGLE When grinding the lip clearance angle, (Figure 4-13), relief must be given to both cutting edges allowing them to enter into the work piece to do the cutting. General purpose drills have a clearance of 8 to 12. The chisel edge of a correctly ground drill should be at an angle of about 45 with the line of the cutting edges. The angle of the chisel edge to the lips is a guide to the clearance (Figure 4-16). Too much clearance will cause the drill to break down because of insufficient support of the lip, and there will not be enough lip thickness to carry away the generated heat. Too little clearance will result in the drill having little or no cutting edges, and the increased pressure required to feed it into the hole will cause the drill to break. By looking straight onto the cutting tip of the drill, the operator can see if the chisel edge is correct. If the chisel edge is correct at 45 to the lips, then it is an indication that the lip clearance angle is correct. An incorrect chisel edge is usually 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 15 頁 共 35 頁 produced by holding the drill at an incorrect angle to the wheel (Figure 4-17) when grinding. A good guide is to hold to the ground, and make slight adjustments. RAKE ANGLE The angle between the flute and the axis the drill parallel of the drill that forms the cutting edge is known as the rake angle (Figure 4-18). 30 being the most common. Drills used on armor plate or other very hard materials need a reduced rake angle to increase the support behind the cutting edge. Soft materials, like brass and bronze, also use a reduced rake angle to prevent the drill from grabbing. The rake angle partially governs the tightness with which the chips curl and the amount of space they occupy. If the rake angle is too small, the lips may be too thin and break under the strain of drilling. Too large of a rake angle makes the drill chatter and vibrate excessively. The web of a drill is made thicker toward the shank to strengthen the tool. In smaller size drills, the difference is not noticeable, but in larger drills, when the point is ground back by repeated sharpening, the thickness of the web becomes greater and the chisel edge of the drill becomes wider. This causes the chisel edge to scrape on the bottom of the hole and requires excessive pressure to be applied to the drill. This can be corrected by thinning the web (Figure 4-19). The point is ground thinner on a thin grinding wheel with a rounded face to fit into the flute. An equal amount of metal should be ground from each flute. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 16 頁 共 35 頁 DRILL GRINDING MACHINES Drill grinding machines (Figure 4-20) make the accurate grinding of all types and sizes of drills an easy job. Comparatively little skill is required to sharpen drills with these machines while following the operating instructions. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 17 頁 共 35 頁 They are particularly valuable when a large number of the same general type of drills is to be sharpened. Two basic designs for the bench-type drill grinding machines are available. Both perform the same operations but use different drill holding devices. The capacity of these machines is stated in the horsepower of the electric motor and the sizes of drills which can be accommodated by the drill holding devices. SINGLE WHEEL FIXTURE One kind of bench-type drill grinding machine consists of an electric motor, a grinding abrasive wheel attached to the motor shaft, and fixtures to hold and position all types of twist drills for drill grinding. A web thinning drill grinding attachment, drill holder assembly, and swinging arm hold the drill in a fixed position for each 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 18 頁 共 35 頁 grinding operation and permit the cutting edge lips to be ground symmetrically at the correct angle and with the correct clearance to ensure long life and efficient cutting. Collets and bushings are supplied with the drill grinding machine to hold a wide range of different sized drills. The grinding machine has a diamond set in the wheel dressing arm to dress the grinding wheel as necessary. DOUBLE WHEEL SWING ARM Another kind of bench type drill grinding machine is equipped with two grinding abrasive wheels, one at each end of the motor shaft. One wheel is beveled for thinning the web of the drill at the point. The other wheel is used for lip grinding. The grinder includes a wheel holder assembly for mounting the drill and providing a means for bringing the drill into contact with the grinding wheel at the correct angle and feed to obtain proper clearance angles. A thinning drill point rest is mounted forward of the beveled grinding abrasive wheel to rest and guide the drill during web thinning operations. A wheel dresser is provided to dress the grinding wheel as necessary. OTHER TYPES OF CUTTERS Drilling machines use cutters that are not drills, to produce special holes. Below are listed the most common types. COUNTERSINKS Countersinks (Figure 4-21) are special angled cutters used to countersink holes for flathead screws so they are flush with the surface when mounted. The most common countersinks are cone shaped with angles of 82. Cone angles of 60, produce special holes. Below are listed the most common 90,100,110, and 120 are for special needs. COUNTER BORES Counter bores (Figure 4-21) are special cutters that use a countersink holes for flathead screws so they are flush with pilot to guide the surface when mounted. The most common countersinks Common uses are for enlarging a hole to make a bolt head fit flush with the surface. COMBINED COUNTERSINK AND CENTER 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 19 頁 共 35 頁 DRILL This special drilling tool (Figure 4-21) is used to start holes accurately. These tools are mainly used to center drill and countersink the end of round stock in a lathe machine. REAMERS Reamers (Figure 4-21) are cutting tools that are used to enlarge a drilled hole by a few thousandths of an inch for a precise fit. BORING TOOLS Boring tools (Figure 4-21) are not usually considered with drilling, but they can be used to bore a hole using the power feed drilling machines. These tools consist of an arbor with a tool bit attached that cuts a preset sized hole according to the distance that the tool bit protrudes from the arbor. DRILL HOLDING DEVICES The revolving vertical spindle of the drilling machine holds and drives the cutting tool. In order to use various sizes and shapes of drills in various machines three types of drill holding devices, which fit the spindle of the drilling machines, are used: the geared drill chuck, the drill sleeve, and the drill socket (Figure 4-22). The larger drilling machines have a spindle that has a standard Morse taper at the bottom end. There are three types of drill holding devices: the geared drill chuck, the drill sleeve, and the drill socket. GEARED DRILL CHUCKS Drills with straight shanks are held in geared drill chucks which have three adjustable jaws to clamp onto the drill. Smaller size drills are made with straight shanks because of the extra cost of providing these size if tapered. Geared drill chucks come in various sizes, with the 3/8 or 1/2-inch capacity chuck being the most common. The shank of the chuck is set into the spindle of the drilling machine by inserting the chucks shank into the spindles internal taper and seating the shank into the taper with a light blow with a soft hammer. Both the internal and external taper surfaces must be clean and free of chips for the shank to seat and lock properly. The drill is locked into the chuck by using the chuck key to simultaneously tighten the three chuck jaws. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 20 頁 共 35 頁 DRILL SOCKETS AND DRILL SLEEVES Morse taper shank drills come in several sizes, thus, adapters must be used for mounting them into various drilling machine spindles. Drill sleeves and drill sockets are designed to add to or subtract from the Morse taper for fitting a drill into the chuck spindle. For example, it is common for a 3/4 inch twist drill to have a Morse taper of size #2, #3, or #4. It is also common for a drilling machine spindle to have a Morse taper of size #3 or #4, and it can be adapted for many other Morse taper sizes, depending on the size of the drill. A drill too small for the machine spindle may be fitted into a socket or sleeve which has a taper hole of the proper size to hold the drill and a taper shank of the proper size to fit the drill spindle. Sometimes, more than one socket or sleeve is needed to build up the shank to tit into the drilling machine spindle. Sockets and sleeves may be obtained in a number of different sizes and hole shank taper combinations. Sockets, sleeves, and taper shank drills are mounted into the aligning slots of the spindle and lightly tapped with a soft hammer to seat in place. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 21 頁 共 35 頁 DRILL DRIFTS Drill drifts are flat, tapered keys with one rounded edge that are designed to fit into a spindle chucks slot to force a tapered shank drill loose. The rounded top of the small end of the drill drift is designed to face upward while inserting the drift into the slot. There are two types of drill drifts, the standard type and the safety type (Figure 4-23). The standard drift must be inserted into the chucks slot and then struck with a soft hammer to jar the taper shank drill loose. The drill will fall quickly if not held by the hand and could break or cause injury. The safety drill drift has a sliding hammer weight on the drift itself to allow for a free hand to stay constantly on the drill as it comes loose. WORK HOLDING AND DRILLING DEVICES Work holding devices are used to hold the work steady for an accurate hole to be drilled, and so a safe drilling operation can be accomplished. Drilling support devices are used to keep the work piece above the worktable or vise surface and to keep the work piece aligned for drilling. Some devices are fairly simple and are used for drilling operations that do not require a perfect hole. Other devices are very intricate and designed for more accurate drilling. Many work holding devices are used with one another to produce the most stable work setup for drilling. MACHINE TABLE VISES A machine table vise is equipped with jaws which clamp against the work piece, holding it secure. The vise can be bolted to the drilling table or the tail can be swung around to lay against the column to hold itself steady. Below are listed many types of special purpose machine table vises available to machine operators. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 22 頁 共 35 頁 The standard machine table vise is the simplest of all vises. It is equipped with two precision ground jaws for holding onto the work and a lead screw to tighten the one movable jaw to the work (Figure 4-24). The swivel vise is a machine vise that has an adjustable base that can swivel through 360 on a horizontal plane(Figure 4-24). The angle vise is very similar to the table vise. Except this vise can be tilted to 90. To be perpendicular to the work table (Figure 4-24).Many other vises are available. They include the compound vise. Universal vise, magnetic vise, and contour vise. STEP BLOCKS These holding devices are built like stairs to allow for height adjustments in mounting drilling jobs and are used with strap clamps and long T-slot bolts (Figure 4-25). 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 23 頁 共 35 頁 CLAMPS Clamps are small, portable vises or plates which bear against the work piece and holding devices to steady the job. Clamps are made in numerous shapes to meet various work holding needs. Common types of clamps are the C-clamp, the parallel clamp, the machine strap clamp, the bent-tail machine clamp, the U-clamp, and the finger machine clamp(Figure 4-25). V-BLOCKS V-blocks are precision made blocks with special slots made to anchor clamps that hold workplaces. The V-slot of the block is designed to hold round workplaces. The V-block and clamp set is usually used to hold and drill round stock. ANGLE PLATES Angle plates are made in a 90 angle with slots and bolt holes for securing work to the table or to other work holding devices (Figure 4-25). T-SLOT BOLTS These specially made bolts have a T-shaped head that is designed to slide into the T-slots of the drilling machines worktable. A heavy duty washer and nut are used with the T-bolt to secure the work. JIGS Drill jigs are devices designed for production drilling jobs.The workplaces are 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 24 頁 共 35 頁 clamped into the jig so that the holes will be drilled in the same location on each piece. The jig may guide the drill through a steel bushing to locate the holes accurately. DRILLING SUPPORT DEVICES These devices are important to keep the work piece parallel while being supported above the worktable or vise surface and to keep the drill from cutting into the holding device or worktable. The following two devices are the most common used. Blocks are used with clamps to aid in securing and supporting the work. These blocks are usually precision ground of hard steel for long life. Parallels are precision ground rectangular bars are used to keep the work piece parallel with the worktable when the Work piece must be raised above the worktable surface, such as when drilling completely through a work piece(Figure 4-26). Parallels come in matched sets and can be solid or adjustable as needed. CUTTING FLUIDS Cutting fluids, lubricants, and coolants are used in drilling work to lubricate the chip being formed for easier removal, to help dissipate the high heat caused by friction, to wash away the chips, to improve the finish, and to permit greater cutting speeds for best efficiency. In drilling work, the cutting fluid can be sprayed, dripped, or machine pumped onto the work and cutting too! to cool the action and provide for maximum tool life. Drilling, reaming, and tapping of various materials can be improved by using the proper cutting fluids (see Table 4-3 in Appendix A). Cutting- 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 25 頁 共 35 頁 fluids can be produced from animal, vegetable, or mineral oils. Some cutting fluids are very versatile and can be used for any operation, while other cutting fluids are specially designed for only one particular metal. LAYING OUT AND MOUNTING WORK LAYING OUT WORK Laying out work for drilling consists of locating and extreme accuracy, then laying out may be a simple operation, such as scribing two intersecting lines and center punching for drilling (Figure 4-27). For a precise layout, to within a few thousandths of an inch, precision layout procedures must mark the exact centers of the holes to be drilled. The be followed. Precision tools, such as a surface plate, surface accuracy of the finished work piece depends, in most part, on gage, calipers, and sharp scribes must be used. The work piece the accuracy of the layout. If the work does not require should be cleaned and debarred before applying layout dye. LAYING OUT HOLE CENTERS The position of the center of the hole to be drilled is marked by scribing two or more lines which intersect at the whole center. This intersecting point is then marked lightly with a prick punch and hammer. Check to see that the punch mark is exactly at the center of the intersection； use a magnifying glass if necessary. Use a pair of dividers, set to the radius of the hole to be drilled, to scribe a circle on the work piece. The prick punch is then used to mark small indentations, known as “witness marks,” on the circumference (Figure 4-27). This completes marking the circle. If a check is needed, have another circle scribed outside of the original circle, which can be checked for alignment after drilling (Figure 4-27). 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 26 頁 共 35 頁 Center-Punching the Layout When all scribing is finished, enlarge the prick punch mark with a center punch to aid the center drilling process. Enlarging the mark with a center punch allows the center drill point to enter the work piece easier and cut smoother. Layout of Multiple Holes When more than one hole must be drilled, lay out the holes along a common reference line, then put in the intersecting lines and scribe the circles. Throughout the layout process, avoid making the layout lines too heavy. Use lines as thin as possible, and avoid any scratches or other marks on the surface to be drilled. MOUNTING WORKPIECES Before attempting to use a drilling machine, some provision must be made for holding the work piece rigidly and securely in place. The work piece should always be firmly fastened to the table or base to produce holes that are located accurately. Use work holding devices to hold the work piece (Figures 4-24 and 4-25). The two best methods to mount workplaces are explained below. Vise Mounting Most hand-feed drilling machines have no means of clamping or bolting workplaces to the table or base. The work piece must be secured tightly in a machine table vise and swung around so that the tail of the vise contacts the column of the drill press. The hole must be centered by hand so that the center drill point is directly over the center punched mark. Other larger drilling machines have slotted tables and bases so that the work and work holding devices can be bolted or clamped firmly. All work should be securely clamped or set against a stop for all drilling to avoid letting the drill grab and damage the work piece or injure the machine operator. Table or Base Mounting When a work piece is table or base mounted (Figure 4-28), the strap clamps must be as parallel to the table or base as possible. All bolts and strap clamps should be as short as possible for rigidity and to provide for drilling clearance(Figure 4-29).Parallel bars should be set close together to keep from bending the work. Washers and nuts 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 27 頁 共 35 頁 should be in excellent condition. The slots and ways of the table, base, or vise must be free of all dirt and chips. All work holding devices should be free of burrs and wiped clean of oil and grease. Work holding devices should be the right size for the job. Devices that are too big or too small for the job are dangerous and must be avoided. GENERAL DRILLING OPERATIONS THE DRILLING PROCESS After a work piece is laid out and properly mounted, the drilling process can begin. The drilling process, or complete operation, involves selecting the proper twist drill or cutter for the job, properly installing the drill into the machine spindle, setting the speed and feed, starting the hole on center, and drilling the hole to specifications 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 28 頁 共 35 頁 within the prescribed tolerance. Tolerance is the allowable deviation from standard size. The drilling process must have some provisions for tolerance because of the over sizing that naturally occurs in drilling. Drilled holes are always slightly oversized, or slightly larger than the diameter of the drills original designation. For instance, a l/4-inch twist drill will produce a hole that may be several thousandths of an inch larger than l/4-inch. Over-sizing is due to several factors that affect the drilling process: the actual size of the twist drill, the accuracy of the drill point, the accuracy of the machine chuck and sleeve, the accuracy and rigidity of the drilling machine spindle, the rigidity of the entire drilling machine, and the rigidity of the work piece and setup. Field and maintenance shop drilling operations allow for some tolerance, but over-sizing must be kept to the minimum by the machine operator. Selecting the Drill Selecting the proper twist drill means getting the right tool for the job (see Table 4-2 in Appendix A). The material to be drilled, the size of that material, and the size of the drilled hole must all be considered when selecting the drill. Also, the drill must have the proper lip angles and lip clearances for the job. The drill must be clean and free of any burrs or chips. The shank of the drill must also be clean and free of burrs to fit into the chuck. Most drills wear on the outer edges and on the chisel point, so these areas must be checked, and resharpened if needed, before drilling can begin. If the twist drill appears to be excessively worn, replace it. Installing the Drill Before installing the drill into the drilling machine spindle, clean the spindle socket and drill shank of all dirt, chips, and burrs. Use a small tile inside the socket to remove any tough burrs. Slip the tang of the drill or geared drill chuck into the sleeve and align the tang into the keyway slot (Figure 4-30). Tap the end of the drill lightly with a soft hammer to seat firmly. Another method used to seat the drill into the sleeve is to place a block of wood on the machine table and force the drill down onto the block. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 29 頁 共 35 頁 Selecting Drill Speed Speed refers to the revolutions per minute (RPM) of the drilling machine spindle. For drilling, the spindle should rotate at a set speed that is selected for the material being drilled. Correct speeds are essential for satisfactory drilling. The speed at which a drill turns and cuts is called the peripheral speed. Peripheral speed is the speed of a drill at its circumference expressed in surface feet per minute (SFPM). This speed is related to the distance a drill would travel if rolled on its side. For example, a peripheral speed of 30 feet per minute means the drill would roll 30 feet in 1 minute if rolled on its side. It has been determined through experience and experiment that various metals machine best at certain speeds； this best speed for any given metal is what is known as its cutting speed (CS) (see Table 4-2) in Appendix A. If the cutting speed of a material is known, then a simple formula can be used to find the recommended RPM of the twist drill. The slower of the two recommended speeds is used for the following formulas due to the varying conditions that may exist, such as the rigidity of the setup, the size of the drilling machine, and the quality of finish. RPM = CSx4 Where RPM = drill speed in revolutions per minute. CS = Recommended cutting speed in surface feet per minute. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 30 頁 共 35 頁 4 = A constant in all calculations for RPM (except metric). D = The diameter of the drill itself. For example, if a 1/2-inch (0.500-inch) twist drill is to cut aluminum, the formula would be setup as follows: RPM = 200 X 4 = 800 = 1600 RPM Thus, the drilling machine would be set up to drill as close to 1,600 RPM as possible. It is best to use the machine speed that is closest to the recommended RPM. metric system of measurement, a different used to find RPM: RPM = CS (m) x 320 D (mm) When using the formula must be Where RPM = Drill speed in revolutions per minute. CS = Recommended cutting speed in surface meters per minute. 320 = A constant for all metric RPM calculations. D = Diameter of the twist drill in millimeters. For example, if a 15-mm twist drill is to cut medium-carbon steel, with a recommended cutting speed of 21.4 meters per minute, the formula would be set up as follows: RPM= 21.4 x320 = 6848 RPM = 21.4 x320 = 6.848 = 456.533 RPM Round this RPM up or down to the nearest machine speed. The speeds on these tables are just recommendations and can be adjusted lower if needed, or to higher speeds if conditions permit. SELECTING DRILL FEED Feed is the distance a drill travels into the work piece during each revolution of the spindle. It is expressed in thousandths of an inch or in millimeters. Hand-feed drilling machines have the feed regulated by the hand pressure of the operator； thus, the skill of the operator will determine the best feeds for drilling. Power feed drilling machines have the ability to feed the drill into the work at a preset depth of cut per spindle revolution, so the best feeding rate can be determined (see Table 4-4 in Appendix A). The selection of the best feed depends upon the size of the drill, the material to be drilled, and the condition of the drilling machine. Feed should increase as the size 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 31 頁 共 35 頁 of the drill increases. After starting the drill into the work piece by hand, a lever on the power-feed drilling machine can be activated, which will then feed the drill into the work until stopped or disengaged. Too much feed will cause the drill to split； too little feed will cause chatter, dull the drill, and possibly harden the work piece so it becomes more difficult to drill. Drills 1/2 inch or smaller can generally be hand-fed, while the larger drills require more downward torque and should be power fed. ALIGNING AND STARTING HOLES To start a twist drill into the work piece, the point of the drill must be aligned with the center-punched mark on the work piece. Some drilling operations may not require a precise alignment of the drill to the work, so alignment can be done by lining up the drill by hand and eye alone. If a greater precision in centering alignment is required, than more preparation is needed before starting to drill. STARTING HOLES WITH CENTER DRILL The best method to align and start a hole is to use the combination countersink and drill, known as the center drill (Figure 4-31). Set the drilling machine speed for the diameter of the tip of the center drill, start the machine, and gently lower the center drill into contact with the work, using hand and eye coordination. The revolving center drill will find the center punched mark on the work piece and properly align the hole for drilling. The depth of the center-drilled hole should be no deeper than two third the length of the tapered portion of the center drill. DRAWING A DRILL BACK ON CENTER Often, the drill will not be on center, sometimes due to a poorly made center-punched mark or a hard spot on the metal. To draw the twist drill back to the position desired (Figure 4-31), a sharp chisel is used to make one or more nicks or grooves on the side toward which the drill is to be drawn. The chisel marks will draw the drill over because of the tendency of the drill to follow the line of least resistance. After the chisel mark is made, the drill is again hand-fed into the work and checked for being on center. This operation must be completed before the drill point has enlarged the hole to full diameter or the surface of the work piece will be marred by a double hole impression. 桂林電子科技大學(xué) 外文翻譯（原 文 ） 用紙 第 32 頁 共 35 頁 DRILLING After the d