|There are several kinds of hand snips, each of which serves a different
purpose. Straight, curved, hawksbill, and aviation snips are in common
use (figure 12-7). Straight snips are used for cutting straight lines when
the distance is not great enough to use a squaring shear and for cutting
the outside of a curve. The other types are used for cutting the inside
of curves or radii. Snips should never be used to cut heavy sheet metal.
Aviation snips are designed especially for cutting heat treated aluminum alloy and stainless steel. They are also adaptable for enlarging small holes. The blades have small teeth on the cutting edges and are shaped for cutting very small circles and irregular outlines. The handles are the compound leverage type, making it possible to cut material as thick as 0.051 inch. Aviation snips are available in two types, those which cut from right to left and those which cut from left to right.
||Unlike the hacksaw, snips do not remove any material when the cut is
made, but minute fractures often occur along the cut. Therefore, cuts should
be made about one-thirty-second inch from the layout line and finished
by hand filing down to the line.
The common hacksaw has a blade, a frame, and a handle. The handle can be obtained in two styles, pistol grip and straight (figure 12-8).
Hacksaw blades have holes in both ends; they are mounted on pins attached to the frame. When installing a blade in a hacksaw frame, mount the blade with the teeth pointing forward, away from the handle.
Blades are made of high grade tool steel or tungsten steel and are available in sizes from 6 to 16 inches in length. The 10 inch blade is most commonly used. There are two types, the all hard blade and the flexible blade. In flexible blades, only the teeth are hardened.
Selection of the best blade for the job involves finding the right type and pitch. An all hard blade is best for sawing brass, tool steel, cast iron, and heavy cross-section materials. A flexible blade is usually best for sawing hollow shapes and metals having a thin cross section.
The pitch of a blade indicates the number of teeth per inch. Pitches of 14, 18, 24, and 32 teeth per inch are available. A blade with 14 teeth per inch is preferred when cutting machine steel, cold rolled steel, or structural steel. A blade with 18 teeth per inch is preferred for solid stock aluminum, bearing metal, tool steel, and cast iron. Use a blade with 24 teeth per inch when cutting thick walled tubing, pipe, brass, copper, channel, and angle iron. Use a 32 teeth per inch blade for cutting thin walled tubing and sheet metal. When using a hacksaw, observe the following procedures:
1. Select an appropriate saw blade for the job.
2. Assemble the blade in the frame so that the cutting edge of the teeth points away from the handle.
3. Adjust tension of the blade in the frame to prevent the saw from buckling and drifting.
4. Clamp the work in the vise in such a way that will provide as much bearing surface as possible and will engage the greatest number of teeth.
5. Indicate the starting point by nicking the surface with the edge of a file to break any sharp corner that might strip the teeth. This mark will also aid in starting the saw at the proper place.
6. Hold the saw at an angle that will keep at least two teeth in contact with the work at all times. Start the cut with a light, steady, forward stroke just outside the cutting line. At the end of the stroke, relieve the pressure and draw the blade back. (The cut is made on the forward stroke.)
7. After the first few strokes, make each stroke as long as the hacksaw frame will allow. This will prevent the blade from overheating. Apply just enough pressure on the forward stroke to cause each tooth to remove a small amount of metal. The strokes should be long and steady with a speed not more than 40 to 50 strokes per minute.
8. After completing the cut, remove chips from the blade, loosen tension on the blade, and return the hacksaw to its proper place.
A chisel is a hard steel cutting tool which can be used for cutting and chipping any metal softer than the chisel itself. It can be used in restricted areas and for such work as shearing rivets, or splitting seized or damaged nuts from bolts (figure 12-9).
The size of a flat cold chisel is determined by the width of the cutting edge. Lengths will vary, but chisels are seldom under 5 inches or over 8 inches long.
Chisels are usually made of eight sided tool steel bar stock, carefully hardened and tempered. Since the cutting edge is slightly convex, the center portion receives the greatest shock when cutting, and the weaker corners are protected. The cutting angle should be 60° to 70° for general use, such as for cutting wire, strap iron, or small bars and rods.
When using a chisel, hold it firmly in one hand. With the other hand, strike the chisel head squarely with a ball peen hammer.
When cutting square corners or slots, a special cold chisel called a cape chisel should be used. It is like a flat chisel except the cutting edge is very narrow. It has the same cutting angle and is held and used in the same manner as any other chisel.
Rounded or semicircular grooves and corners which have fillets should be cut with a roundnose chisel. This chisel is also used to recenter a drill which has moved away from its intended center.
The diamond point chisel is tapered square at the cutting end, then ground at an angle to provide the sharp diamond point. It is used for cutting B-grooves and inside sharp angles.
Most files are made of high grade tool steels that are hardened and
tempered. Files are manufactured in a variety of shapes and sizes. They
are known either by the cross section, the general shape, or by their particular
use. The cuts of files must be considered when selecting them for various
types of work and materials.
|Files are used to square ends, file rounded comers, remove
burrs and slivers from metal, straighten uneven edges, file holes and slots,
and smooth rough edges.
Files have three distinguishing features: (1) Their length, measured exclusive of the tang (figure 12-10); (2) their kind or name, which has reference to the relative coarseness of the teeth; and (3) their cut.
Files are usually made in two types of cuts, single cut and double cut. The single cut file has a single row of teeth extending across the face at an angle of 65° to 85° with the length of the file. The size of the cuts depends on the coarseness of the file. The double cut file has two rows of teeth which cross each other. For general work, the angle of the first row is 40° to 45°. The first row is generally referred to as "overcut," and the second row as "upcut"; the upcut is somewhat finer and not so deep as the overcut.
Files - Care and Use
Files and rasps are cataloged in three ways:
Length. Measuring from the tip to the heel of the file. The tang is never included in the length.
Shape. Refers to the physical configuration of the file (circular, rectangular, or triangular or a variation thereof).
Cut. Refers to both the character of the teeth or the coarseness; rough, coarse and bastard for use on heavier classes of work and second cut, smooth and dead smooth for finishing work.
Most Commonly Used Files (see figure 12-11)
Hand files. These are parallel in width and tapered in thickness. They have one safe edge (smooth edge) which permits filing in corners, and on other work where a safe edge is required. Hand files are double cut and used principally for finishing flat surfaces and similar work.
Flat files. These files are slightly tapered toward the point in both width and thickness. They cut on both edges as well as on the sides. They are the most common files in use. Flat files are double cut on both sides and single cut on both edges.
Mill files. These are usually tapered slightly in thickness and in width for about one-third of their length. The teeth are ordinarily single cut. These files are used for drawfiling and to some extent for filing soft metals.
Square files. These files may be tapered or blunt and are double cut. They are used principally for filing slots and key seats, and for surface filing.
Round or rattail files. These are circular in cross section and may be either tapered or blunt and single or double cut. They are used principally for filing circular openings or concave surfaces.
Triangular and Three square files. These files are triangular in cross section. Triangular files are single cut and are used for filing the gullet between saw teeth. Three square files, which are double cut, may be used for filing internal angles, clearing out corners, and filing taps and cutters.
Half round files. These files cut on both the flat and round sides. They may be single or double cut. Their shape permits them to be used where other files would be unsatisfactory.
Lead float files. These are especially designed for use on soft metals. They are single cut and are made in various lengths.
Warding file - Rectangular in section and tapers to narrow point as to width. Used for narrow space filing where other files cannot be used.
Knife file - Knife blade section. Used by tool and die makers on work having acute angles.
Wood file - Same section as flat and half round files. Has coarser teeth and is especially adaptable for use on wood.
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, fibre, rubber, etc. 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.
The following methods are recommended for using files:
1. 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, relieve the pressure during the return stroke.
2. 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 crossfiling with the same file. In drawfiling, 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.
3. 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.
4. 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 will 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 selfclearing 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 will take in caring for his files.
1. Choose the right file for the material and work to be performed.
2. Keep all files racked and separated so they do not bear against each other.
3. Keep the files in a dry place - rust will corrode the teeth points.
4. 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.
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 (figure 12-12). Draw the brush across the file so that the bristles pass down the gullet between the teeth.
There are generally four types of portable drills used in aviation for holding and turning twist drills. 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. In addition a breast plate 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 (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; 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 point, illustrated in figure 12-13. 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 (figure 12-14). The straight shank generally is used in hand, breast, and portable electric drills; the square shank is made to fit into a carpenter's brace. Tapered shanks generally are used in machine shop drill presses.
The metal column forming the core of the drill is the body. The body clearance area lies just back of the margin, 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: (1) By fractions, (2) letters, or (3) numbers. Fractionally, they are classified by sixteenths of an inch (from 1/16 to 3 1/2 in), by thirty-seconds (from 1/32 to 2 1/2 in), or by sixty-fourths (from 1/64 to 1 1/4 in). For a more exact measurement a letter system is used with decimal equivalents: A (0.234 in) to Z (0.413 in). The number system of classification is most accurate: No. 80 (0.0314 in) to No. 1 (0.228 in). Drill sizes and their decimal equivalents are shown in figure 12-15.
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) will be sufficient; however, when drilling soft metals, a cutting angle of 90° may be more efficient.
Typical procedures for sharpening drills (figure 12-16) are as follows:
1. Adjust the grinder tool rest to a convenient height for resting the back of the hand while grinding.
2. 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.
3. Place the hand on the tool rest with the center line of the drill making a 59° angle with the cutting face of the grinding wheel. Lower the shank end of the drill slightly.
4. 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.
5. Check the results of grinding with a gauge to determine whether or not the lips are the same length and at a 59° angle.
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 12-17.
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. 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 tapered holes and recesses.
A countersink is a tool which 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 screwheads. The angle of the standard countersink shown in figure 12-18 is 100°.
Special stop countersinks are available. Stop countersinks (figure 12-18) 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.
When using a countersink, care must be taken not to remove an excessive amount of material since this reduces the strength of flush joints.