punch will spread the object and cause it to bind even more.
The transfer punch is usually about 4 inches long. It has a point that tapers, then turns straight for a short distance in order to fit a drill-locating hole in a template. The tip has a point similar to that of a prick punch. As its name implies, the transfer punch is used to transfer the location of holes through the template or pattern to the material.
Wrenches The wrenches most often used in aircraft maintenance are classified as open-end, box-end, socket, adjustable, and special wrenches. The allen wrench, although seldom used, is required on one special type of recessed screw. One of the most widely used metals for making wrenches is chromevanadium steel. Wrenches made of this metal are almost unbreakable.
Solid, nonadjustable wrenches with open parallel jaws on one or both ends are known as open-end wrenches. These wrneches may have their jaws parallel to the handle or at an angle up to 90°;
most are set at an angle of 15°. Basically, the wrenches are designed to fit a nut, bolthead, or other object which makes it possible to exert a turning action.
Box-end wrenches are popular tools because of their usefulness in close quarters. They are called box wrenches since they box, or completely surround, the nut or bolthead. Practically all box-end wrenches are made with 12 points so they can be used in places having as little as 15° swing. In figure 12-4, point A on the illustrated double broached hexagon wrench is nearer the center line of the head and the wrench handle than point B, and also the center line of nut C. If the wrench is inverted and installed on nut C, point A will be centered over side "Y" instead of side "X". The center line of the handle will now be in the dotted line position. It is by reversing (turning the wrench over) the position of the wrench that a 15° arc may be made with the wrench handle.
Although box-end wrenches are ideal to break loose tight nuts or pull tight nuts tighter, time is lost turning the nut off the bolt once the nut is broken loose. Only when there is sufficient clearance to rotate the wrench in a complete circle can this tedious process be avoided.
After a tight nut is broken loose, it can be completely backed off or unscrewed more quickly with an open-end than with a box-end wrench. In this case, a combination wrench is needed, which has a box-end on one end and an open-end wrench of FIGURE 12-4. Box-end wrench use.
the same size on the other. Both the box-end and combination wrenches are shown in figure 12-5.
A socket wrench is made of two parts: (1) The socket, which is placed over the top of a nut or bolthead, and (2) a handle, which is attached to the socket. Many types of handles, extensions, and attachments are available to make it possible to use socket wrenches in almost any location or position. Sockets are made with either fixed or detachable handles. Socket wrenches with fixed handles are usually furnished as an accessory to a machine. They have either a four-, six- or twelvesided recess to fit a nut or bolthead that needs regular adjustment.
Sockets with detachable handles usually come in sets and fit several types of handles, such as the T, ratchet, screwdriver grip, and speed handle. Socket wrench handles have a square lug on one end that fits into a square recess in the socket head. The two parts are held together by a light spring-loaded poppet. Two types of sockets, a set of handles, and an extension bar are shown in figure 12-6.
Box-end The adjustable wrench is a handy utility tool which has smooth jaws and is designed as an openend wrench. One jaw is fixed, but the other may be moved by a thumbscrew or spiral screwworm Open-end adjustment in the handle. The width of the jaws may be varied from 0 to ½ inch or more. The angle of the opening to the handle is 22½° on an adjustable wrench. One adjustable wrench does the work of several open-end wrenches. Although versatile, they are not intended to replace the standard open-end, box-end, or socket wrenches.
When using any adjustable wrench, always exert the pull on the side of the handle attached to the fixed jaw of the wrench.
Special Wrenches The category of special wrenches includes the spanner, torque, and allen wrenches. The hook spanner is for a round nut with a series of notches cut in the outer edge. This wrench has a curved arm with a hook on the end which fits into one of the notches on the nut. The hook is placed in one of these notches with the handle pointing in the direction the nut is to be turned.
Some hook spanner wrenches are adjustable and will fit nuts of various diameters. U-shaped hook spanners have two lugs on the face of the wrench to fit notches cut in the face of the nut or screw plug. End spanners resemble a socket wrench but Box-end wrench have a series of lugs that fit into corresponding notches in a nut or plug. Pin spanners have a pin in place of a lug, and the pin fits into a round hole in the edge of a nut. Face pin spanners are similar to the U-shaped hook spanners except that they have pins instead of lugs.
Combination wrench There are times when definite pressure must be FIGURE 12-5. Box end and combination wrenches. applied to a nut or bolt. In such cases a torque Speed handle Ratchet handle Hinge handle Socket and Socket universal joint combined Extension bar FIGURE 12-6. Socket wrench set.
wrench must be used. The torque wrench is a precision tool consisting of a torque-indicating handle and appropriate adapter or attachments.
It measures the amount of turning or twisting force applied to a nut, bolt, or screw.
The three most commonly used torque wrenches are the deflecting beam, dial-indicating, and micrometer-setting types. When using the deflecting beam and the dial-indicating torque wrenches, the torque is read visually on a dial or scale mounted on the handle of the wrench. The micrometer-setting torque wrench is preset to the desired torque. When this torque is reached, a sharp impulse or breakaway is noticed by the operator.
Before each use, the torque wrench should be visually inspected for damage. If a bent pointer, cracked or broken glass (dial type), or signs of rough handling are found, the wrench must be tested. Torque wrenches must be tested at periodic intervals to ensure accuracy.
Most headless setscrews are the allen type and must be installed and removed with an allen wrench. Allen wrenches are six-sided bars in the shape of an I. They range in size from 64 to 2 inch and fit into a hexagonal recess in the setscrew.
METAL CUTTING TOOLS Hand Snips 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.
Straight hand snips Circle snips Hawks-bill snips Trojan snips Aviation snips FIGURE 12-7. Snips.
Hacksaws 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 FIGURE 12-8. Hacksaws.
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:
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.
Chisels 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).
60°-70°
1. Select an appropriate saw blade for the job.
2. Assemble the blade in the frame so that Flat cold the cutting edge of the teeth points away chisel Convex 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 Single bevel as possible and will engage the greatest point 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. Double This mark will also aid in starting the saw bevel point 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, Round nose 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 Diamond allow. This will prevent the blade from point 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 FIGURE 12-9. Chisels.
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.
Files 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 corners, 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.
Tang - Length Heel Face Edge Point FIGURE 12-10. Hand file.
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, Half-round files. These files cut on both the fat 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.
Hand-taper width, parallel thickness.
Mill-taper width, parallel thickness.
Pillar-taper thickness, parallel width.
Wurding-much taper width, parallel thickness.
Square, round and three square-taper.
Half round-taper.
Knife-taper.
Vixen-parallel edges and sides.
FIGURE 12-11. Types of files.
The following methods are recommended for using files:
I. 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 inclinded 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 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 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 cor-
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 Card Pneumatic drills are preferred for use around flammable materials, since sparks from an electric drill are a fire or explosion hazard.
Twist Drills 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 highspeed alloy steel. Carbon steel twist drills are satisfactory for the general run of work and are relatively inexpensive. The more expensive highspeed 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.
Lip or cutting edge Lip clearance (125°-135°) Brush Slot for pick . Flute Pick FIGURE 12-12. File Card. Land Drills There are generally four types of portable drills used in aviation for holding and turning twist Heel angle drills. Holes ¼ inch in diameter and under can be (12°-15° ) drilled using a hand drill. This drill is commonly called an "egg beater." The breast drill is designed Lip angle (normally 59°) to hold larger size twist drills than the hand drill.
In addition a breast plate is affixed at the upper Lips or cutting edges end of the drill to permit the use of body weight to increase the cutting power of the drill. Electrie and pneumatic power drills are available in various shapes and sizes to satisfy almost any requirement. FIGURE 12-13. Twist drill.
- Point
- Margin
- Flute Straight shank Size stamped here Taper shank Square shank (used in brace) FIGURE 12-14. Drills.
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 6 to 3½ in.), by thirty-seconds (from ⅔2 to 2/½ in.), or by sixtyfourths (from 64 to 1¾ 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 gage to determine whether or not the lips are the same length and at a 59° angle.
Reamers 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.
Milli- Meteı ............. Frac- Num tional ber ........ Frac- tional Num ber Milli- Meter Equiv. tional ber Dec. Frac-Num Milli- Dec. Frac- Num bes 22 .2677 .15 50 .2716 .2 21 .2720 .25 20 .2756 .3 49 .2770 J 80 .2795 .35 48 19 .2811 79 .2812 9/32 .39 1/64 5/64 .2835 47 18 .2854 78 11/64 .2874 .45 17 .2900 77 46 .5 45 16 .2950 76 75 15 19/64 .55 44 74 14 .3020 IN 13 43 3/16 12 42 11 5/16 3/32 10 69 41 .75 68 40 8 P .79 1/32 39 7 13/64 21/64 6 .85 37 Q 5 .9 36 R 7/64 .95 35 11/32 62 3 34 33 1/32 S 32 2 T 31 23/64
1.1 ı/8
1.15 .19 3/64 30 .95 15/64 U B 29 3/8 55 78 28 54 9/64 D 27 1/4 W 26 F 1/16 25 .3937 G 52 24 .4040 Y
1.65 17/64
1.7 23 .4130 51 5/32 H .4134 FIGURE 12-15. Drill sizes.
Straight reamer (Hand) Spiral taper reamer Taper reamer (Power) (Hand) Straight reamer (Power) Expansion reamer Adjustable reamer FIGURE 12-17. Reamers.
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 (D) (E) 12°-15° most practical. This type is furnished in standard sizes from ¼ inch to l inch, increasing in diameter by ⅓2-inch increments.
Taper reamers, both hand- and machineoperated, are used to smooth and true tapered holes and recesses.
Countersink A countersink is a tool which cuts a coneshaped depression around the hole to allow a rivet or screw to set flush with the surface of the material. Countersinks are made with various FIGURE 12-16. Drill sharpening procedures. angles to correspond to the various angles of the 541
Cutter shaft Cutter Pilot Body
• Lock nut 100° Stop Chip opening Standard countersink Stop countersink Fiber collar FIGURE 12-18. Countersinks.
countersunk rivet and screwheads. The angle of the 7/18 - - 9/16 standard countersink shown in figure 12-18 is 100°. 5/167 • 11/16 Special stop countersinks are available. Stop 3/16 - - 13/16 countersinks (figure 12-18) are adjustable to any desired depth, and the cutters are interchangeable 1/16 - 15/16 so that holes of various countersunk angles may be made. Some stop countersinks have a micrometer set arrangement (in increments of 0.001 inch) 2 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.
1/8.
LAYOUT AND MEASURING TOOLS • 7/8 3/8 3/4 Layout and measuring devices are precision tools. They are carefully machined, accurately 1/2 5/8 marked and, in many cases, are made up of very 3/64 5/64 delicate parts. When using these tools, be careful not to drop, bend, or scratch them. The finished product will be no more accurate than the 1/64 - 7/64 measurements or the layout; therefore, it is very important to understand how to read, use, and care for these tools.
32 Rules Rules are made of steel and are either rigid or flexible. The flexible steel rule will bend, but it 1/32- • 6/32 should not be bent intentionally as it may be broken rather easily. 2/32 - 5/32 In aircraft work the unit of measure most commonly used is the inch. The inch may be 3/32 4/32 divided into smaller parts by means of either common or decimal fraction divisions. The frac- FIGURE 12-19. Rules.
Scriber Level Protractor head
• Center head Stock head FIGURE 12-20. Combination set.
tional divisions for an inch are found by dividing the inch into equal parts--halves (½), quarters (¼), eighths (⅛), sixteenths (%), thirty-seconds
32), and sixty-fourths (⅙4) -as shown in figure
12-17.
The fractions of an inch may be expressed in decimals, called decimal equivalents of an inch;
for example, ⅛ inch is expressed as 0.0125 (one hundred twenty-five ten-thousandths of an inch).
Rules are manufactured in two basic styles, those divided or marked in common fractions (figure 12-19) and those divided or marked in decimals or divisions of one one-hundredth of an inch. A rule may be used either as a measuring tool or as a straightedge.
Combination Sets The combination set (figure 12-20), as its name implies, is a tool that has several uses. It can be used for the same purposes as an ordinary trisquare, but it differs from the tri-square in that the head slides along the blade and can be clamped at any desired place. Combined with the square or stock head are a level and scriber. The head slides in a central groove on the blade or scale, which can be used separately as a rule.
The spirit level in the stock head makes it convenient to square a piece of material with a surface and at the same time tell whether one or the other is plumb or level. The head can be used alone as a simple level.
The combination of square head and blade can also be used as a marking gage to scribe lines at a 45° angle, as a depth gage, or as a, height gage.
A convenient scriber is held frictionally in the head by a small brass bushing.
The center head is used to find the center of shafts or other cylindrical work. The protractor head can be used to check angles and also may be set at any desired angle to draw lines.
Scriber The scriber is designed to serve the aviation mechanic in the same way a pencil or pen serves a writer. In general, it is used to scribe or mark lines on metal surfaces. The scriber (figure 12-21) is made of tool steel, 4 to 12 inches long, and has two needle-pointed ends. One end is bent at a 90° angle for reaching and marking through holes.
Before using a scriber always inspect the points for sharpness. Be sure the straightedge is flat on the metal and in position for scribing. Tilt the scriber slightly in the direction toward which it will be moved, holding it like a pencil. Keep the scriber's point close to the guiding edge of the straightedge. The scribed line should be heavy enough to be visible, but no deeper than necessary to serve its purpose.
FIGURE 12-21. Scriber.