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THE MICROMETER In using the jack you exerted your effort through a distance of 2pr, or 150 inches, to raise the screw 1/4 inch. It takes a lot of circular motion to get a small amount of straight line motion from the head of the jack. You will use this point to your advantage in the micrometer; its a useful device for making accurate small measurementsmeasurements of a few thousandths of an inch.In figure 5-3, you see a cutaway view of a micrometer. The thimble turns freely on the sleeve,
Figure 5-3.-A micrometer.
Figure 5-4.Taking turns. rigidly attached to the micrometer frame. The spindle attaches to the thimble and is fitted with screw threads that move the spindle and thimble to the right or left in the sleeve when you rotate the thimble. These screw threads are cut 40 threads to the inch. Hence, one turn of the thimble moves the spindle and thimble 1/40 of inch. This represents one of the smallest divisions on the micrometer. Four of these small divisions make 4/40 of an inch, or 1/10 inch. Thus, the distance from 0 to 1 or 1 to 2 on the sleeve represents 1/10, or 0.1, inch. To allow even finer measurements, the thimble is divided into 25 equal parts. It is laid out by graduation marks around its rim, as shown in figure 5-4. If you turn the thimble through 25 of these equal parts, you have made one complete revolution of the screw. This represents a lengthwise movement of 1/40 of an inch. If you turn the thimble one of these units on its scale, you have moved the spindle a distance of 1/25 of 1/40 inch, or 1/1000 of an inch0.001 inch. The micrometer in figure 5-4 reads 0.503 inch, that is the true diameter of the half-inch drill-bit shank measured. This tells you that the diameter of this bit is 0.003 inch greater than its nominal diameter of 1/2 inch0.5000 inch.
Figure 5-5.A turnbuckle.
Figure 5-6.-A riggers vice. Because you can make accurate measurements with this instrument, it is vital in every machine shop. |
 
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