Continued from Metal Cutting Tools (Part One)
Most Commonly Used Files
Same section as flat and half-round files. This file has coarser teeth and is especially adaptable for use on wood. [Figure 11-17]
Vixen (Curved-Tooth Files)
Curved-tooth files are especially designed for rapid filing and smooth finish on soft metals and wood. The regular cut is adapted for tough work on cast iron, soft steel, copper, brass, aluminum, wood, slate, marble, fiber, rubber, and so forth. The fine cut gives excellent results on steel, cast iron, phosphor bronze, white brass, and all hard metals. The smooth cut is used where the amount of material to be removed is very slight, but where a superior finish is desired. [Figure 11-17]
The following methods are recommended for using files:
- Crossfiling. Before attempting to use a file, place a handle on the tang of the file. This is essential for proper guiding and safe use. In moving the file endwise across the work (commonly known as crossfiling), grasp the handle so that its end fits into and against the fleshy part of the palm with the thumb lying along the top of the handle in a lengthwise direction. Grasp the end of the file between the thumb and first two fingers. To prevent undue wear of the file, relieve the pressure during the return stroke.
- Drawfiling. A file is sometimes used by grasping it at each end, crosswise to the work, then moving it lengthwise with the work. When done properly, work may be finished somewhat finer than when cross filing with the same file. In draw filing, the teeth of the file produce a shearing effect. To accomplish this shearing effect, the angle at which the file is held with respect to its line of movement varies with different files, depending on the angle at which the teeth are cut. Pressure should be relieved during the backstroke.
- Rounding corners. The method used in filing a rounded surface depends upon its width and the radius of the rounded surface. If the surface is narrow or only a portion of a surface is to be rounded, start the forward stroke of the file with the point of the file inclined downward at approximately a 45° angle. Using a rocking chair motion, finish the stroke with the heel of the file near the curved surface. This method allows use of the full length of the file.
- Removing burred or slivered edges. Practically every cutting operation on sheet metal produces burrs or slivers. These must be removed to avoid personal injury and to prevent scratching and marring of parts to be assembled. Burrs and slivers prevent parts from fitting properly and should always be removed from the work as a matter of habit.
Lathe filing requires that the file be held against the work revolving in the lathe. The file should not be held rigid or stationary but should be stroked constantly with a slight gliding or lateral motion along the work. A standard mill file may be used for this operation, but the long angle lathe file provides a much cleaner shearing and self-clearing action. Use a file with “safe” edges to protect work with shoulders from being marred.
Care of Files
There are several precautions that any good craftsman takes in caring for files.
- Choose the right file for the material and work to be performed.
- Keep all files racked and separated so they do not bear against each other.
- Keep the files in a dry place—rust corrodes the teeth points, dulling the file.
- Keep files clean. Tap the end of the file against the bench after every few strokes to loosen and clear the filings. Use the file card to keep files clean—a dirty file is a dull file. A dirty file can also contaminate different metals when the same file is used on multiple metal surfaces.
Particles of metal collect between the teeth of a file and may make deep scratches in the material being filed. When these particles of metal are lodged too firmly between the teeth and cannot be removed by tapping the edge of the file, remove them with a file card or wire brush. Draw the brush across the file so that the bristles pass down the gullet between the teeth. [Figure 11-18]
The four types of portable drills used in aviation for holding and turning twist drills are the hand drill, breast drill, electric power drill, and pneumatic power drill. Holes 1⁄4 inch in diameter and under can be drilled using a hand drill. This drill is commonly called an “egg beater.” The breast drill is designed to hold larger size twist drills than the hand drill. Also, a breastplate is affixed at the upper end of the drill to permit the use of body weight to increase the cutting power of the drill. Electric and pneumatic power drills are available in various shapes and sizes to satisfy almost any requirement. Pneumatic drills are preferred for use around flammable materials, since sparks from an electric drill are a fire or explosion hazard.
A twist drill is a pointed tool that is rotated to cut holes in material. It is made of a cylindrical hardened steel bar having spiral flutes, or grooves, running the length of the body and a conical point with cutting edges formed by the ends of the flutes.
Twist drills are made of carbon steel or high-speed alloy steel. Carbon steel twist drills are satisfactory for the general run of work and are relatively inexpensive. The more expensive high-speed twist drills are used for the tough materials, such as stainless steels. Twist drills have from one to four spiral flutes. Drills with two flutes are used for most drilling. Whereas those with three or four flutes are used principally to follow smaller drills or to enlarge holes.
The principal parts of a twist drill are the shank, the body, and the heel. [Figure 11-19]
The drill shank is the end that fits into the chuck of a hand or power drill. The two shank shapes most commonly used in hand drills are the straight shank and the square or bit stock shank. The straight shank generally is used in hand, breast, and portable electric or pneumatic drills. The square shank is made to fit into a carpenter’s brace. Tapered shanks generally are used in machine shop drill presses. [Figure 11-20]
The metal column forming the core of the drill is the body. The body clearance area lies just back of the margin. It is slightly smaller in diameter than the margin to reduce the friction between the drill and the sides of the hole. The angle at which the drill point is ground is the lip clearance angle. On standard drills used to cut steel and cast iron, the angle should be 59° from the axis of the drill. For faster drilling of soft materials, sharper angles are used.
The diameter of a twist drill may be given in one of three ways: by fractions, letters, or numbers. Fractionally, they are classified by sixteenths of an inch (from 1⁄16 to 31⁄2 inches), by thirty-secondths (from 1⁄32 to 21⁄2 inches), or by sixty-fourths (from 1⁄64 to 11⁄4 inches). For a more exact measurement, a letter system is used with decimal equivalents: A (0.234 inch) to Z (0.413 inch). The number system of classification is most accurate: No. 80 (0.0314 inch) to No. 1 (0.228 inch). Drill sizes and their decimal equivalents are shown in Figure 11-21.The twist drill should be sharpened at the first sign of dullness. For most drilling, a twist drill with a cutting angle of 118° (59° on either side of center) is sufficient. However, when drilling soft metals, a cutting angle of 90° may be more efficient.
Typical procedures for sharpening drills are as follows: [Figure 11-22]
- Adjust the grinder tool rest to a convenient height for resting the back of the hand while grinding.
- Hold the drill between the thumb and index finger of the right or left hand. Grasp the body of the drill near the shank with the other hand.
- Place the hand on the tool rest with the centerline of the drill making a 59° angle with the cutting face of the grinding wheel. Lower the shank end of the drill slightly.
- Slowly place the cutting edge of the drill against the grinding wheel. Gradually lower the shank of the drill as you twist the drill in a clockwise direction. Maintain pressure against the grinding surface only until you reach the heel of the drill.
- Check the results of grinding with a gauge to determine whether or not the lips are the same length and at a 59° angle.
Alternatively, there are commercially available twist drill grinders available, as well as attachments for bench grinders that ensure consistent, even sharpening of twist drills.
Reamers are used to smooth and enlarge holes to exact size. Hand reamers have square end shanks so that they can be turned with a tap wrench or similar handle. The various types of reamers are illustrated in Figure 11-23.
A hole that is to be reamed to exact size must be drilled about 0.003 to 0.007 inch undersize. A cut that removes more than 0.007 inch places too much load on the reamer and should not be attempted.
Reamers are made of either carbon tool steel or high-speed steel. The cutting blades of a high-speed steel reamer lose their original keenness sooner than those of a carbon steel reamer; however, after the first super keenness is gone, they are still serviceable. The high-speed reamer usually lasts much longer than the carbon steel type.
Reamer blades are hardened to the point of being brittle and must be handled carefully to avoid chipping them. When reaming a hole, rotate the reamer in the cutting direction only. Do not back a reamer out of a hole by rotating it opposite the cutting direction. Turn the reamer steadily and evenly to prevent chattering, or marking and scoring of the hole walls.
Reamers are available in any standard size. The straight fluted reamer is less expensive than the spiral fluted reamer, but the spiral type has less tendency to chatter. Both types are tapered for a short distance back of the end to aid in starting. Bottoming reamers have no taper and are used to complete the reaming of blind holes.
For general use, an expansion reamer is the most practical. This type is furnished in standard sizes from 1⁄4 inch to 1 inch, increasing in diameter by 1⁄32-inch increments.
Taper reamers, both hand and machine operated, are used to smooth and true taper holes and recesses.
A countersink is a tool that cuts a cone-shaped depression around the hole to allow a rivet or screw to set flush with the surface of the material. Countersinks are made with various angles to correspond to the various angles of the countersunk rivet and screw heads. The angle of the standard countersink shown in Figure 11-24 is 100°.
Special stop countersinks are available. Stop countersinks are adjustable to any desired depth, and the cutters are interchangeable so that holes of various countersunk angles may be made. Some stop countersinks have a micrometer set arrangement (in increments of 0.001 inch) for adjusting the cutting depths. [Figure 11-24]
When using a countersink, care must be taken not to remove an excessive amount of material, since this reduces the strength of flush joints.