Taps, Dies, and Layout Tools

11-14 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. Countersink 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.

Taps and Dies

A tap is used to cut threads on the inside of a hole, while a die is for cutting external threads on round stock. They are made of hard tempered steel and ground to an exact size. There are four types of threads that can be cut with standard taps and dies: National Coarse, National Fine, National Extra Fine, and National Pipe. Hand taps are usually provided in sets of three taps for each diameter and thread series. Each set contains a taper tap, a plug tap, and a bottoming tap. The taps in a set are identical in diameter and cross section and the only difference is the amount of taper. [Figure 11-25] The taper tap is used to begin the tapping process, because it is tapered back for 6 to 7 threads. This tap cuts a complete thread when it is cutting above the taper. It is the only tap needed when tapping holes that extend through thin sections.

The plug tap supplements the taper tap for tapping holes in thick stock. The bottoming tap is not tapered. It is used to cut full threads to the bottom of a blind hole. Dies may be classified as adjustable round split die and plain round split die. The adjustable split die has an adjusting screw that can be tightened so that the die is spread slightly. By adjusting the die, the diameter and fit of the thread can be controlled. [Figure 11-26] Solid dies are not adjustable. Therefore, a variety of thread fits cannot be obtained with this type. There are many types of wrenches for turning taps, as well as turning dies. The T-handle, the adjustable tap wrench, and the diestock for round split dies shown in Figure 11-27 are a few of the more common types. Information on thread sizes, fits, types, and drill speeds are shown in shown in Figure 11-28 through 11-30.

Layout and Measuring Tools

Layout and measuring devices are precision tools. They are carefully machined, accurately marked and, in many cases, are made up of very delicate parts. When using these tools, be careful not to drop, bend, or scratch them. The finished product is no more accurate than the measurements or the layout; therefore, it is very important to understand how to read, use, and care for these tools. Rules Rules are made of steel and are either rigid or flexible. The flexible steel rule bends, but it should not be bent intentionally as it may be broken rather easily. In aircraft work, the unit of measure most commonly used is the inch. The inch may be divided into smaller parts by means of either common or decimal fraction divisions.

The fractional divisions for an inch are found by dividing the inch into equal parts: halves ( 1⁄2), quarters ( 1⁄4), eighths (1⁄8), sixteenths (1⁄16), thirty-secondths (1⁄32), and sixty-fourths (1⁄64). The fractions of an inch may be expressed in decimals, called decimal equivalents of an inch. For example, 1⁄8 inch is expressed as 0.0125 (one hundred twenty-five ten- thousandths of an inch). 11-15 Millimeter Fractional Number Decimal Equivalent Millimeter Fractional Number Decimal Equivalent Millimeter Fractional Number Decimal Equivalent Millimeter Fractional Number Decimal Equivalent 0.1 0.0039 — 0.0410 59 2.2 0.0866 — 0.1470 26 0.15 0.0059 1.05 0.0413 2.25 0.0885 3.75 0.1476 0.2 0.0079 — 0.0420 58 — 0.0890 43 — 0.1495 25 0.25 0.0098 — 0.0430 57 2.3 0.0905 3.8 0.1496 0.3 0.0118 1.1 0.0433 2.35 0.0925 — 0.1520 24 — 0.0135 80 1.15 0.0452 — 0.0935 42 3.9 0.1535 0.35 0.0138 — 0.0465 56 2.38 0.0937 ³⁄32 — — 0.1540 23 — 0.0145 79 1.19 0.0469 ³⁄64 — 2.4 0.0945 3.97 0.1562 ⁵⁄32 — 0.39 0.0156 ¹⁄64 — 1.2 0.0472 — 0.0960 41 — 0.1570 22 0.4 0.0157 1.25 0.0492 2.45 0.0964 4.0 0.1575 — 0.0160 78 1.3 0.0512 — 0.0980 40 — 0.1590 21 0.45 0.0177 — 0.0520 55 2.5 0.0984 — 0.1610 20 — 0.0180 77 1.35 0.0531 — 0.0995 39 4.1 0.1614 0.5 0.0197 — 0.0550 54 — 0.1015 38 4.2 0.1654 — 0.0200 76 1.4 0.0551 2.6 0.1024 — 0.1660 19 — 0.0210 75 1.45 0.0570 — 0.1040 37 4.25 0.1673 0.55 0.0217 1.5 0.0591 2.7 0.1063 4.3 0.1693 — 0.0225 74 — 0.0595 53 — 0.1065 36 — 0.1695 18 0.6 0.0236 1.55 0.0610 2.75 0.1082 4.37 0.1719 ¹¹⁄64 — — 0.0240 73 1.59 0.0625 ¹⁄16 — 2.78 0.1094 ⁷⁄64 — — 0.1730 17 — 0.0250 72 1.6 0.0629 — 0.1100 35 4.4 0.1732 0.65 0.0256 — 0.0635 52 2.8 0.1102 — 0.1770 16 — 0.0260 71 1.65 0.0649 — 0.1110 34 4.5 0.1771 — 0.0280 70 1.7 0.0669 — 0.1130 33 — 0.1800 15 0.7 0.0276 — 0.0670 51 2.9 0.1141 4.6 0.1811 — 0.0292 69 1.75 0.0689 — 0.1160 32 — 0.1820 14 0.75 0.0295 — 0.7000 50 3.0 0.1181 4.7 0.1850 13 — 0.0310 68 1.8 0.0709 — 0.1200 31 4.75 0.1870 0.79 0.0312 ¹⁄32 — 1.85 0.0728 3.1 0.1220 4.76 0.1875 ³⁄16 — 0.8 0.0315 — 0.0730 49 3.18 0.1250 ¹⁄8 — 4.8 0.1890 12 — 0.0320 67 1.9 0.0748 3.2 0.1260 — 0.1910 11 — 0.0330 66 — 0.0760 48 3.25 0.1279 4.9 0.1929 0.85 0.0335 1.95 0.0767 — 0.1285 30 — 0.1935 10 — 0.0350 65 1.98 0.0781 ⁵⁄64 — 3.3 0.1299 — 0.1960 9 0.9 0.0354 — 0.0785 47 3.4 0.1338 5.0 0.1968 — 0.0360 64 2.0 0.0787 — 0.1360 29 — 0.1990 8 — 0.0370 63 2.05 0.0807 3.5 0.1378 5.1 0.2008 0.95 0.0374 — 0.0810 46 — 0.1405 28 — 0.2010 7 — 0.0380 62 — 0.0820 45 3.57 0.1406 ⁹⁄64 5.16 0.2031 ¹³⁄64 — — 0.0390 61 2.1 0.0827 3.6 0.1417 — 0.2040 6 1.0 0.0394 2.15 0.0846 — 0.1440 27 5.2 0.2047 — 0.0400 60 — 0.0860 44 3.7 0.1457 — — 0.2055 5 11-16 Millimeter Fractional Number Decimal Equivalent Millimeter Fractional Number Decimal Equivalent Millimeter Fractional Number Decimal Equivalent Millimeter Fractional Decimal Equivalent 5.25 0.2067 7.25 0.2854 9.5 0.3740 16.5 0.6496 5.3 0.2086 7.3 0.2874 9.53 0.3750 ³⁄8 — 16.67 0.6562 ²¹⁄32 — 0.2090 4 — 0.2900 L — 0.3770 V 17.0 0.6693 5.4 0.2126 7.4 0.2913 9.6 0.3780 17.06 0.6719 ⁴³⁄64 — 0.2130 — 0.2950 M 9.7 0.3819 17.46 0.6875 ¹¹⁄16 5.5 0.2165 7.5 0.2953 9.75 0.3838 17.5 0.6890 5.56 0.2187 ¹⁄32 — 7.54 0.2968 ¹⁹⁄64 — 9.8 0.3858 17.86 0.7031 ⁴⁵⁄64 5.6 0.2205 7.6 0.2992 — 0.3860 W 18.0 0.7087 — 0.2210 2 — 0.3020 N 9.9 0.3898 18.26 0.7187 ²³⁄32 5.7 0.2244 7.7 0.3031 9.92 0.3906 ²⁵⁄64 — 18.5 0.7283 5.75 0.2263 7.75 0.3051 10.0 0.3937 18.65 0.7344 ⁴⁷⁄64 — 0.2280 1 7.8 0.3071 0.3970 X 19.0 0.7480 5.8 0.2283 7.9 0.3110 — 0.4040 Y 19.05 0.7500 ³⁄4 5.9 0.2323 7.94 0.3125 ⁵⁄16 — 10.32 0.4062 ¹³⁄32 — 19.45 0.7656 ⁴⁹⁄64 — 0.2340 A 8.0 0.3150 — 0.4130 Z 19.5 0.7677 5.95 0.2344 ¹⁵⁄64 — — 0.3160 O 10.5 0.4134 19.84 0.7812 ²⁵⁄32 6.0 0.2362 8.1 0.3189 10.72 0.4219 ²⁷⁄64 20.0 0.7874 — 0.2380 B 8.2 0.3228 11.0 0.4330 20.24 0.7969 ⁵¹⁄64 6.1 0.2401 — 0.3230 P 11.11 0.4375 ⁷⁄16 20.5 0.8071 — 0.2420 C 8.25 0.3248 11.5 0.4528 20.64 0.8125 ¹³⁄16 6.2 0.2441 8.3 0.3268 11.51 0.4531 ²⁹⁄64 21.0 0.8268 6.25 0.2460 D 8.33 0.3281 ²¹⁄64 — 11.91 0.4687 ¹⁵⁄32 21.03 0.8281 ⁵³⁄64 6.3 0.2480 8.4 0.3307 12.0 0.4724 21.43 0.8437 ²⁷⁄32 6.35 0.2500 ¹⁄4 E — 0.3320 Q 12.30 0.4843 ³¹⁄64 21.5 0.8465 6.4 0.2520 8.5 0.3346 12.5 0.4921 21.83 0.8594 ⁵⁵⁄64 6.5 0.2559 8.6 0.3386 12.7 0.5000 ¹⁄2 22.0 0.8661 — 0.2570 F — 0.3390 R 13.0 0.5118 22.23 0.8750 ⁷⁄8 6.6 0.2598 8.7 0.3425 13.10 0.5156 ³³⁄64 22.5 0.8858 — 0.2610 G 8.73 0.3437 ¹¹⁄32 — 13.49 0.5312 ¹⁷⁄32 22.62 0.8906 ⁵⁷⁄64 6.7 0.2638 8.75 0.3445 13.5 0.5315 23.0 0.9055 6.75 0.2657 ¹⁷⁄64 — 8.8 0.3465 13.89 0.5469 ³⁵⁄64 23.02 0.9062 ²⁹⁄32 6.75 0.2657 — 0.3480 S 14.0 0.5512 23.42 0.9219 ⁵⁹⁄64 — 0.2660 H 8.9 0.3504 14.29 0.5625 ⁹⁄16 23.5 0.9252 6.8 0.2677 9.0 0.3543 14.5 0.5709 23.81 0.9375 ¹⁵⁄16 6.9 0.2716 — 0.3580 T 14.68 0.5781 ³⁷⁄64 24.0 0.9449 — 0.2720 I 9.1 0.3583 15.0 0.5906 24.21 0.9531 ⁶¹⁄64 7.0 0.2756 9.13 0.3594 ²³⁄64 — 15.08 0.5937 ¹⁹⁄32 24.5 0.9646 — 0.2770 J 9.2 0.3622 15.48 0.6094 ³⁹⁄32 24.61 0.9687 ³¹⁄32 7.1 0.2795 9.25 0.3641 15.5 0.6102 25.0 0.9843 — 0.2811 K 9.3 0.3661 15.88 0.6250 ⁵⁄8 25.03 0.9844 ⁶³⁄64 7.14 0.2812 ⁹⁄32 — — 0.3680 U 16.0 0.6299 25.4 1.0000 1 7.2 0.2835 9.4 0.3701 16.27 0.6406 ⁴¹⁄64 11-17 1 2 3 4 5 12°-15° 59°59° 59° Adjustable reamer Expansion reamer Spiral taper reamer (power)Straight reamer (power) Straight reamer (hand) Taper reamer (hand) Rules are manufactured in two basic styles — those divided or marked in common fractions 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.

[Figure 11-31] Combination Sets The combination set, as its name implies, is a tool that has several uses. It can be used for the same purposes as an ordinary tri-square, 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. [Figure 11-32] 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 gauge to scribe lines at a 45° angle, as a depth gauge, or as a height gauge. A convenient scriber is held frictionally in the head by a small brass bushing. 11-18 Adjusting screw Adjustable round split die Plain round split die Plug Bottoming Taper 100° Cutter shaft Cutter Pilot Body Lock nut Stop Chip opening Fiber collarStop countersinkStandard countersink Profile view Top view 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 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. [Figure 11-33] 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.