Dividers and Pencil Compasses Dividers and pencil compasses have two legs joined at the top by a pivot. They are used to scribe circles and arcs and for transferring measurements from the rule to the work.
Pencil compasses have one leg tapered to a needle point; the other leg has a pencil or pencil Spring inside calipers lead inserted. Dividers have both legs tapered to needle points.
When using pencil compasses or dividers, the Hermaphrodite calipers following procedures are suggested:
1. Inspect the points to make sure they are sharp.
2. To set the dividers or compasses, hold them with the point of one leg in the graduations on the rule. Turn the adjustment nut with the thumb and forefinger;
adjust the dividers or compasses until the point of the other leg rests on the gradua- Firm joint screw tion of the rule which gives the required adjusting inside measurement. calipers
3. To draw an arc or circle with either the pencil compasses or dividers, hold the thumb attachment on the top with the thumb and forefinger. With pressure exerted on both legs, swing the compass in a clockwise direction and draw the desired arc or circle.
The tendency for the legs to slip is avoided by inclining the compasses or dividers in the direction in which they are being rotated. In working on metals, the dividers are used only to scribe arcs or circles that will later be removed by cutting. All other arcs or circles are drawn with pencil compasses to avoid scratching the material.
5. On paper layouts, the pencil compasses are used for describing arcs and circles.
Dividers should be used to transfer critical measurements because they are more accurate than a pencil compass.
Calipers Calipers are used for measuring diameters and distances or for comparing distances and sizes. The three common types of calipers are the inside, the outside, and the hermaphrodite calipers, such as gear-tool calipers. (See figure 12-22.)
Outside calipers are used for measuring outside dimensions, for example, the diameter of a piece of Spring outside calipers Firm joint screw adjusting outside calipers FICURE 12-22. Calipers.
round stock. Inside calipers have outward curved legs for measuring inside diameters, such as diameters of holes, the distance between two surfaces, the width of slots, and other similar jobs.
A hermaphrodite caliper is generally used as a marking gage in layout work. It should not be used for precision measurement.
Micrometer Calipers There are four types of micrometer calipers, each designed for a specific use. The four types are 544
Thread play Thimble cap Micrometer screw Fixed nut adjusting nut Anvil Spindie Measuring faces Barrel Thimble Ratchet stop Clamp ring Frame FIGURE 12-23. Outside micrometer.
commonly called outside micrometer, inside micrometer, depth micrometer, and thread micrometer. Micrometers are available in a variety of sizes, either 0 to ½ inch, 0 to 1 inch, 1 to 2 inch, 2 to 3 inch, 3 to 4 inch, 4 to 5 inch, or 5 to 6 inch sizes.
The outside micrometer (figure 12-23) is used by the mechanic more often than any other type.
It may be used to measure the outside dimensions of shafts, thickness of sheet metal stock, diameter of drills, and for many other applications.
The smallest measurement which can be made with the use of the steel rule is one sixty-fourth of an inch in common fractions, and one one-hundredth of an inch in decimal fractions. To measure more closely than this (in thousandths and ten-thousandths of an inch), a micrometer is used.
If a dimension given in a common fraction is to be measured with the micrometer, the fraction must be converted to its decimal equivalent.
All four types of micrometers are read in the same way. The method of reading an outside micrometer is discussed later in this chapter.
Micrometer Parts The fixed parts of a micrometer (figure 12-23) are the frame, barrel, and anvil. The movable parts of a micrometer are the thimble and spindle. The thimble rotates the spindle which moves in the threaded portion inside the barrel. Turning the thimble provides an opening between the anvil and the end of the spindle where the work is measured.
• The size of the work is indicated by the graduations on the barrel and thimble.
Reading a Micrometer The lines on the barrel marked 1, 2, 3, 4, etc., indicate measurements of tenths, or 0.100 inch,
0.200 inch, 0.300 inch, 0.400 inch, respectively (see figure 12-24).
Each of the sections between the tenths divisions (between 1, 2, 3, 4, etc.) is divided into four parts of 0.025 inch each. One complete revolution of the thimble (from zero on the thimble around to the same zero) moves it one of these divisions (0.025 inch) along the barrel.
The bevel edge of the thimble is divided into 25 equal parts. Each of these parts represents one twenty-fifth of the distance the thimble travels along the barrel in moving from one of the 0.025- inch divisions to another. Thus, each division on the thimble represents one one-thousandth (0.001) of an inch. These divisions are marked for convenience at every five spaces by 0, 5, 10, 15, Barrel Thimble .300 .200→ → 100 / 1/10 of an inch Horizontal line FIGURE 12-24. Micrometer measurements.
and 20. When 25 of these graduations have passed the horizontal line on the barrel, the spindle (baving made one revolution) has moved 0.025 inch.
The micrometer is read by first noting the last visible figure on the horizontal line of the barrel representing tenths of an inch. Add to this the length of barrel between the thimble and the previously noted number. (This is found by 20 multiplying the number of graduations by 0.025 .025 inch.) Add to this the number of divisions on the = .150 -.100 → .025 bevel edge of the thimble that coincides with the line of the graduation. The total of the three (A) .150 in. figures equals the measurement. (Figure 12-25 shows several sample readings.)
Vernier Scale Some micrometers are equipped with a vernier 001 scale which makes it possible to read directly the fraction of a division that may be indicated on the thimble scale. Typical examples of the vernier scale as it applies to the micrometer are shown in figure 12-26.
.025 = .151 All three scales on a micrometer are not fully
- .100 - .025 visible without turning the micrometer; but the examples shown in figure 12-26 are drawn as (B) .151 in though the barrel and thimble of the micrometer were laid out flat so that all three scales can be seen at the same time. The barrel scale is the lower horizontal scale; the thimble scale is vertical on the right; and the long horizontal lines (0 through 9 and 0) make up the vernier scale.
09670990414 (C) .160 in. 20 20 Barrel scale
0.2947
0.2944 20 15 (D) .175 in.
0.2153 0.1005 FIGURE 12-25. Reading a micrometer. FIGURE 12-26. Vernier scale readings.
In reading a micrometer, an excellent way to remember the relative scale values is to remember that the 0.025-inch barrel scale graduations are established by the lead screw (40 threads per inch). Next, the thimble graduations divide the
0.025 inch into 25 parts, each equal to 0.001 inch;
then the vernier graduations divide the 0.001 inch into 10 equal parts, each equal to 0.0001 inch.
Remembering the values of the various scale Adjusting screw graduations, the barrel scale reading is noted. The thimble scale reading is added to it; then the A Adjustable round split die vernier scale reading is added to get the final reading. The vernier scale line to be read is always the one aligned exactly with any thimble graduation.
In the first example in figure 12-26, the barrel reads 0.275 inch and the thimble reads more than
0.019 inch. The number 1 graduation on the thimble is alined exactly with the number 4 graduation on the vernier scale. Thus, the final reading is 0.2944 inch.
In the second example in figure 12-26, the barrel reads 0.275 inch, and the thimble reads more than 0.019 inch and less than 0.020 inch. On the vernier scale, the number 7 graduation coincides with a line on the thimble. This means that the thimble reading would be 0.0197 inch. Adding this to the barrel reading of 0.275 inch gives a total measurement of 0.2947 inch.
The third and fourth examples in figure 12-26 are additional readings that would require use of the vernier scale for accurate readings to tenthousandths of an inch.
Using a Micrometer The micrometer must be handled carefully. If it is dropped, its accuracy may be permanently affected. Continually sliding work between the Taper B Plain round split die FIGURE 12-28. Types of dies.
anvil and spindle may wear the surfaces. If the spindle is tightened too much, the frame may be sprung permanently and inaccurate readings will result.
To measure a piece of work with the micrometer, hold the frame of the micrometer in the palm of the hand with the little finger or third finger, whichever is more convenient. This allows the thumb and forefinger to be free to revolve the thimble for adjustment.
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. They are: 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; the only difference is the amount of taper (see figure 12-27).
Plug 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 Bottoming sections. The plug tap supplements the taper tap FIGURE 12-27. Hand taps. 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 (see figure 12-28).
The adjustable-split die has an adjusting screw Tap wrenchs. 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.
Diestock 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 12-29 are a few of the more common types.
Information on thread sizes, fits, types, etc., is FIGURE 12-29. Diestock and tap wrenches. shown in figs. 12-30, 12-31 and 12-32.
NATIONAL COARSE THREAD SERIES NATIONAL FINE THREAD SERIES MEDIUM FIT. CLASS 3 (NC) MEDIUM FIT. CLASS 3 (NF) Tap Drill Tap Drill Dia. of Dia. of Size hody Pref'd Nearest Size body Pref'd Nearest and for Body dia. of stand'd and for Body dia. of stand'd Threads thread hole Drill Size threads thread Drill hole Drill Size 0-80 .060 52 .0472 3/64 1-64 .073 47 .0575 #53 1-72 .073 47 0591 #53 2-56 .086 42 .0682 #51 2-64 .086 42 0700 #50 3-48 _099 37 .078 5/64 3-56 .099 37 0810 #46 4-40 .112 31 0866 #44 4-48 .112 31 0911 #42 5-40 125 29 0995 #39 5-14 .125 25 .1024 #38 6-32 138 27 1063 #36 6-40 .138 27 113 #33 8-32 .164 18 1324 #29 8-36 .164 18 136 #29 10-24 .190 10 .1472 #26 10-32 .190 10 159 #21 12-24 .216 2 .1732 #17 12-28 .216 2 .180 #15 1/4-20 .250 • 1/4 .1990 #8 1/4-28 .250 F .213 #З 5/16-18 .3125 5/16 .2559 #F 5/16-24 .3125 5/16 .2703 3/8-16 375 3/8 .3110 5/16" 3/8-24 375 3/8 .332 7/16-14 .4375 7/16 .3642 7/16-20 .4375 7/16 .386 1/2-13 .500 1/2 .4219 27/64" 1/2-20 .500 1/2 449 7/16" 9/16-12 .5625 9/16 .4776 31/64" 9/16-18 .5625 9/16 ,506 1/2" 5/8-11 .625 5/8 .5315 17/32" 5/8-18 625 5/8 .568 9/16" 3/4-10 750 3/4 .6480 41/64" 3/4-16 .750 3/4 .6688 11/16" 7/8-9 .875 7/8 .7307 49/64" 7/8-14 .875 7/8 .7822 51/64" 1-8 1.000 1.0 .8376 7/8" 1-14 1.000 I.0 .9072 49/64" FIGURE 12-30. American (National) screw thread sizes.
Tap Drills for Pipe Threads Diameter Length Size Drill 27 R 18 18 14 14 8 3-1/2 8 4 4.38712 1.3000 844 4.500 4.23438 4-1/4 FIGURE 12-31. American (National) pipe thread dimensions and tap drill sizes.
Diameter Soft Mild Malleable Hard Cast Tool or Alloy Steel of Drill Metals Steel Iron Iron Hard Steel Cast Steel 300 F.P.M. 100 F.P.M. 90 F.P.M. 80 F.P.M. 60 F.P.M. 40 F.P.M.
1/16 (No. 53 to 80) 18320 3667 2445 3/32 (No. 42 to 52) 122:2 2442 1649 1/8 (No. 31 to 41) 9180 1833 1222 5/32 (No. 23 to 30) 7328 1465 977 3/16 (No. 13 to 22) 6106 1222 815 7/32 (No. I to 12) 5234 1047 698 1/4 (A to E) 4575 917 611 9/32 (G to K) 4071 814 542 9/16 (L, M, N) 3660 733 489 11/32 (O to R) 3330 666 444 3/8 (S, T, U) 3050 611 407 13/32 (V to Z) 2818 563 376 7/16 2614 1222 524 349 15/32 2442 1140 652 488 326 1/2 2287 1528 1070 764 688 611 458 306 9/16 2035 1357 950 678 611 543 407 .: 271 3/8 1830 1222 856 611 550 489 367 244 11/16 1665 1110 777 555 500 444 333 222 3/4 1525 1018 713 509 458 407 306 204 Figures are for High-Speed Drills. The spced of Carbon Drills should be reduced one-half.
Use drill speed nearest to figure given.
FIGURE 12-32. Drill speeds.