Calipers, Micrometers, and Vernier Scales

11-19 Diestock Tap wrenches It is very important to use a scribe only where it is defining a line to be cut as scribed lines may introduce stress points where failures can occur. 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 lead inserted. Dividers have both legs tapered to needle points. When using pencil compasses or dividers, the 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 graduation of the rule that gives the required measurement. 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. 4. 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 are later 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 inside, outside, and hermaphrodite calipers, such as gear tool calipers. [Figure 11-34] Outside calipers are used for measuring outside dimensions— for example, the diameter of a piece of 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 gauge 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: outside micrometer, inside micrometer, depth micrometer, and thread micrometer. Micrometers are available in a variety of sizes, either 0 to 1⁄2 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. In addition to the micrometer inscribed with the measurement markings, micrometers equipped with electronic digital liquid crystal display (LCD) readouts are also in common use. The AMT uses the outside micrometer more often than any other type. It may be used to measure the outside dimensions of shafts, thickness of sheet metal stock, the diameter of drills, and for many other applications. [Figure 11-35] The smallest measurement that 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 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 11-20 National Coarse Thread Series Medium Fit Class 3 (NC) Size and Threads Diameter of Body for Thread Body Drill Preferred Diameter of Hole Nearest Standard Drill Size Tap Drill Size and Threads Diameter of Body for Thread Body Drill Preferred Diameter of Hole Nearest Standard Drill Size Tap Drill National Fine Thread Series Medium Fit Class 3 (NF) 0-80 0.060 52 0.0472 ³⁄64" 1-64 0.073 47 0.0575 #53 1-72 0.073 47 0.0591 #53 2-56 0.086 42 0.0682 #51 2-64 0.086 42 0.0700 #50 3-48 0.099 37 0.078 ⁵⁄64 3-56 0.099 37 0.0810 #46 4-40 0.112 31 0.0866 #44 4-48 0.112 31 0.0911 #42 5-40 0.125 29 0.0995 #39 5-44 0.125 25 0.1024 #38 6-32 0.138 27 0.1063 #36 6-40 0.138 27 0.113 #33 8-32 0.164 18 0.1324 #29 8-36 0.164 18 0.136 #29 10-24 0.190 10 0.1472 #26 10-32 0.190 10 0.159 #21 12-24 0.216 2 0.1732 #17 12-28 0.216 2 0.180 #15 ¹⁄4-20 0.250 ¹⁄4 0.1990 #8 ¹⁄4-28 0.250 F 0.213 #3 ⁵⁄16-18 0.3125 ⁵⁄16 0.2559 #F ⁵⁄16-24 0.3125 ⁵⁄16 0.2703 I ³⁄8-16 0.375 ³⁄8 0.3110 ⁵⁄16" ³⁄8-24 0.375 ³⁄8 0.332 Q ⁷⁄16-14 0.4375 ⁷⁄16 0.3642 U ⁷⁄16-20 0.4375 ⁷⁄16 0.386 W ¹⁄2-13 0.500 ¹⁄2 0.4219 ²⁷⁄64" ¹⁄2-20 0.500 ¹⁄2 0.449 ⁷⁄16" ⁹⁄16-12 0.5625 ⁹⁄16 0.4776 ³¹⁄64" ⁹⁄16-18 0.5625 ⁹⁄16 0.506 ¹⁄2" ⁵⁄8-11 0.625 ⁵⁄8 0.5315 ¹⁷⁄64" ⁵⁄8-18 0.625 ⁵⁄8 0.568 ⁹⁄16" ³⁄4-10 0.750 ³⁄4 0.6480 ⁴¹⁄64" ³⁄4-16 0.750 ³⁄4 0.6688 ¹¹⁄16" ⁷⁄8-9 0.875 ⁷⁄8 0.7307 ⁴⁹⁄64" ⁷⁄8-14 0.875 ⁷⁄8 0.7822 ⁵¹⁄64" 1-8 1.000 1.0 0.8376 ⁷⁄8" 1-14 1.000 1.0 0.9072 ⁴⁹⁄64" Nominal Size Inches Number of Threads Per Inch A (inches) B (inches) L2 (inches) L1 (inches) Pitch Diameter Size and Threads Pipe OD (inches) Depth of Thread (inches) Size Drill Minor Diameter Small End of Pipe Tap Drill for Pipe Threads ¹⁄8 27 0.36351 0.37476 0.2638 0.180 0.405 0.02963 0.33388 R ¹⁄4 18 0.47739 0.48989 0.4018 0.200 0.540 0.04444 0.43294 ⁷⁄16 ³⁄8 18 0.61201 0.62701 0.4078 0.240 0.675 0.04444 0.56757 ³⁷⁄64 ¹⁄2 14 0.75843 0.77843 0.5337 0.320 0.840 0.05714 0.70129 ²³⁄32 ³⁄4 14 0.96768 0.98887 0.5457 0.339 1.050 0.5714 0.91054 ⁵⁹⁄64 1 11 ¹⁄2 1.21363 1.23863 0.6828 0.400 1.315 0.06957 1.14407 1 ⁵⁄32 1 ¹⁄4 11 ¹⁄2 1.55713 1.58338 0.7068 0.420 1.660 0.06957 1.48757 1 ¹⁄2 1 ¹⁄2 11 ¹⁄2 1.79609 1.82234 0.7235 0.420 1.900 0.06957 1.72652 1 ⁴⁷⁄64 2 11 ¹⁄2 2.26902 2.29627 0.7565 0.436 2.375 0.06957 2.19946 1 ⁷⁄32 2 ¹⁄2 8 2.71953 2.76216 1.1375 0.682 2.875 0.10000 2.61953 2 ⁵⁄8 3 8 3.34062 3.8850 1.2000 0.766 3.500 0.10000 3.24063 3 ¹⁄4 3 ¹⁄2 8 3.83750 3.88881 1.2500 0.821 4.000 0.10000 3.73750 3 ³⁄4 4 8 4.33438 4.38712 1.3000 0.844 4.500 0.10000 4.23438 4 ¹⁄4 11-21 ¹⁄32 ⁶⁄32 ¹⁄64 ³⁄64 ⁵⁄64 ⁷⁄64 ¹⁄8 ¹⁄16 ³⁄16 ⁵⁄16 ⁷⁄16 ⁹⁄16 ¹¹⁄16 ¹³⁄16 ¹⁵⁄16 ¹⁄4 ³⁄4 ⁷⁄8 1 1 12 ³⁄8 ⁵⁄8¹⁄2 ²⁄32 ⁵⁄32 ³⁄32 ⁴⁄32 Soft Metals 300 FPM Plastic and Hard Rubber 200 FPM Annealed Cast Iron 140 FPM Mild Steel 100 FPM Malleable Iron 90 FPM Hard Cast Iron 80 FPM Tool or Hard Steel 60 FPM Alloy Steel Cast Steel 40 FPM Diameter of Drill ¹⁄16 (No. 53 – 80) 18,320 12,217 8,554 6,111 5,500 4,889 3,667 2,445 ³⁄32 (No. 42 – 52) 12,212 8,142 5,702 4,071 3,666 3,258 2,442 1,649 ¹⁄8 (No. 31– 41) 9,160 6,112 4,278 3,056 2,750 2,445 1,833 1,222 ⁵⁄32 (No. 23 – 30) 7,328 4,888 3,420 2,444 2,198 1,954 1,465 977 ³⁄16 (No. 13 – 22) 6,106 4,075 2,852 2,037 1,833 1,630 1,222 815 ⁷⁄32 (No. 1– 12) 5,234 3,490 444 1,745 1,575 1,396 1,047 698 ¹⁄4 (A – F) 4,575 3,055 2,139 1,527 1,375 1,222 917 611 ⁹⁄32 (G – K) 4,071 2,715 1,900 1,356 1,222 1,084 814 542 ⁹⁄16 (L – N) 3,660 2,445 1,711 1,222 1,100 978 7,333 489 ¹¹⁄32 (O – R) 3,330 2,220 1,554 1,110 1,000 888 666 444 ³⁄8 (S – U) 3,050 2,037 1,426 1,018 917 815 611 407 ¹³⁄32 (V – Z) 2,818 1,878 1,316 939 846 752 563 376 ⁷⁄16 2,614 1,746 1,222 873 786 698 524 349 ¹⁵⁄32 2,442 1,628 1,140 814 732 652 488 326 ¹⁄2 2,287 1,528 1,070 764 688 611 458 306 ⁹⁄16 2,035 1,357 950 678 611 543 407 271 ³⁄8 1,830 1,222 856 611 550 489 367 244 1 ¹⁄16 1,665 1,110 777 555 500 444 333 222 ³⁄4 1,525 1,018 713 509 458 407 306 204 11-22 Spring outside calipers Firm joint screw adjusting outside calipers Firm joint screw adjusting inside calipers Hermaphrodite calipers Spring inside calipers 112 0 180 90 3 4 5 8 9 10 Scriber Stock head Center head Protractor headLevel 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. [Figure 11-36] Reading a Micrometer The lines on the barrel marked 1, 2, 3, 4, and so forth, indicate measurements of tenths, or 0.100 inch, 0.200 inch, 0.300 inch, 0.400 inch, respectively. [Figure 11-37] Each of the sections between the tenths divisions (between 1, 2, 3, 4, and so forth) 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, and 20. When 25 of these graduations have passed the horizontal line on the barrel, the spindle (having 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 multiplying 11-23 Anvil Spindle Barrel Thimble Ratchet stop Thimble cap Thread play adjusting nut Fixed nut Clamp ring Micrometer screw Frame Measuring faces the number of graduations by 0.025 inch.) Add to this the number of divisions on the bevel edge of the thimble that coincides with the line of the graduation. The total of the three figures equals the measurement. [Figure 11-38] Vernier Scale Some micrometers are equipped with a vernier scale that makes it possible to directly read the fraction of a division that is indicated on the thimble scale. Typical examples of the vernier scale as it applies to the micrometer are shown in Figure 11-39.

All three scales on a micrometer are not fully visible without turning the micrometer, but the examples shown in Figure 11-38 are drawn as 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. 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 graduations, the barrel scale reading is noted.

The thimble scale reading is added to it, then the 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 11-39 , the barrel reads 0.275 inch and the thimble reads more than 0.019 inch. The number 1 graduation on the thimble is aligned exactly with the number 4 graduation on the vernier scale. Thus, the final reading is 0.2944 inch. In the second example in Figure 11-39, the barrel reads 0.275 11-24 = 0.150 0 5 20 0 0.100 0.025 0.025 0.100 0.025 0.025 = 0.151 0.150 in.

0.151 in. 0.160 in. 0.175 in. A C D B 1 0 5 0.001 10 15 5 10 0 15 20 10 0.001 0.100 0.200 0.300 0 1 2 Barrel Thimble Horizontal line¹⁄10 of an inch inch, and the thimble reads more than 0.020 inch and less than 0.021 inch. On the Vernier scale, the number 0 graduation coincides closest with the line on the thimble. This means that the thimble reading would be 0.020 inch. Adding this to the barrel reading of 0.275 inch gives a total measurement of 0.2950 inch. The third and fourth examples in Figure 11-39 are additional readings that would require use of the Vernier scale for accurate readings to ten-thousandths 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 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. In any event, follow the manufacturer’s instructions for calibration procedures and types of gauges to be used, such as gauge blocks, gauge pins, or ring gauges. 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. A variation of the micrometer is the dial indicator, which measures variations in a surface by using an accurately machined probe mechanically-linked to a circular hand whose movement indicates thousandths of an inch or is displayed on a LCD screen. [Figure 11-40] A typical example would be using a dial indicator to measure the amount of runout, or bend, in a shaft. If a bend is suspected, the part can be rotated while resting between a pair of machined V-blocks. A dial indicator is then clamped to a machine table stand, and the probe of the indicator is positioned so it lightly contacts the surface. The outer portion