CALIPERING AND FITTING THE VERNIER AND HOW TO READ IT THIS method of measuring or of dividing known distances into very small parts is credited to the invention of Pierre Vernier in 1631. The principle is shown in Figs. 1 to 3 and its application in Figs. 4 and 5. In Figs. 1 and 2 both distances 0-1 are the same but they are divided into different divisions. Calling o - I = I inch then in Fig. 1 it is clear that moving the lower seal one division will divide the upper one in half. In Fig. 2 the upper scale is divided in half and the lower one in thirds. If the lower scale is moved either way untilor comes under the end line, it has moved of an inch but if either of these are moved to the center line then it is only moved of this amount or Figure 3 shows the usual application of the principle except that it is divided in four parts instead of ten. Here both the scales have four parts but on the lower scale the four parts just equal three parts of the upper scale. It is evident that if we move the lower scale so that o goes to 1 and 4 goes to 4 that it will be moved the length of the distance o 4 on the upper scale. If this distance was 1 inch, each division on the upper scale equals inch and moving the lower scale so that the line I just matches the line next to o on the upper scale gives of one of these divisions or of an inch. - Figures 4 and 5 show the usual application in which the lower or vernier scale is divided into 10 parts which equals 9 parts of the upper scale. The same division holds good, however, and when the lower scale is moved so that the first division of the vernier just matches the first line of the scale, it has been moved just one tenth of a division. In Fig. 4 the third lines match so that it has moved and in Fig. 5, of a division. So if A B is one inch then each division is of an inch and each line of the vernier is of that or of an inch. To find the reading of any vernier, divide one division of the upper or large scale by the number of divisions in the small scale. So if we had a vernier with 16 divisions in each, the large scale being I inch long, then the movement of one division is of or 26 of an inch. READING THE MICROMETER THE Commercial micrometer consists of a frame, the anvil or fixed measuring point, the spindle which has a thread cut 40 to the inch on the portion inside the sleeve or barrel and the thimble which goes outside the sleeve and turns the spindle. One turn of the screw moves the spindle or .025 of an inch and the marks on the sleeve show the number of turns the screw is moved. Every fourth graduation is marked 1, 2, 3, etc., representing tenths of an inch or as each mark is .025 the first four means .025 X 4 = .100, the third means .025 X 4 X 3 = .300. = The thimble has a beveled edge divided into 25 parts and numbered 0, 5, 10, 15, 20 and to o again. Each of these mean of a turn or of Too of an inch. To read, multiply the marks on the barrel by 25 and add the graduations on the edge of the thimble. In the cut there are 7 marks on the sleeve and 3 on the thimble so we say X 25 175, plus 3 178 or .178. In shop practice it is common to read them without any multiplying by using mental addition. Beginning at the largest number = = shown on the sleeve and calling it hundreds and add 25 for each mark, we say in the case show 100 and 25, 50, 75 and then add the numbers shown on the thimble 3, making .178 in all. If it showed 4 and one mark, with the thimble showing 8 marks, the reading would be 400 + 25+ 8 € 433 thousandths or .433. = THE TEN-THOUSANDTH MICROMETER THIS adds a vernier to the micrometer sleeve or barrel as shown in Fig. 7, which is read the same as any vernier as has been explained. First note the thousandths as in the ordinary micrometer and then look at the line on the sleeve which just matches a line on the thimble. If the two zero lines match two lines on the thimble, the measurement is in even thousandths as at B which reads .250. At C the seventh line matches a line on the thimble so the reading is .2507 inch. MEASURING THREE-FLUTED TOOLS WITH THE MICROMETER THE sketch, Fig. 8 on page 228, shows a V-block or gage for measuring three-fluted drills, counterbores, etc. The angle being 60 degrees, the distances A, B, and C are equal. Consequently to determine the correct diameter of the piece to be measured, apply the gage as indicated in the sketch and deduct one third of the total measurement. The use of this gage has a decided advantage over the old way of soldering on a piece of metal opposite a tooth or boring out a ring to fit to. Using a standard 60-degree triangle for setting and a few different sizes of standard cylindrical plug gages for testing, the V-block may be easily and very accurately made. -60 Deduct FIG. 8. Measuring Three-Fluted Tools PRESS AND RUNNING FITS Parallel Press, Drive and Close Fits TABLE 1, page 229, gives the practice of the C. W. Hunt Company, New York, for press, drive and close or hand fits for parallel shafts ranging between one and ten inches in diameter. In accordance with general practice, the holes for all parallel fits are made standard, except for unavoidable variation due to the wear of the reamer, the variation from standard diameter for the various kinds of fits being made in the shaft. This variation is, however, not positive, but is made between limits of accuracy or tolerance. Taking the case of a press fit on a two-inch shaft, for example, it will be seen that the hole that is, the reamer is kept between the correct size and 0.002 inch below size, while the shaft must be between 0.002 and 0.003 inch over size. For a drive or hand fit the limits for the hole are the same as for a press fit, while the shaft in the former case must be between 0.001 and 0.002 large and in the latter between 0.001 and 0.002 small. Parallel Running Fits Table 2, page 230, gives in the same way the allowances made by the same concern for parallel running fits of three grades of closeness. The variations allowed in the holes are not materially different from those of the preceding table, but the shafts are, of course, below instead of above the nominal size. In all cases the tables apply to steel shafts and cast-iron wheels or other members. In the right-hand columns of the tables the formulas from which the allowances are calculated are given, and from which the range of tables may be extended. |