How to use and read a micrometer caliper

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The basic principle upon which the micrometer caliper is based is the fact that a screw will advance a fixed distance when it is rotated in a nut which has been threaded to fit it. Thus, in the micrometer caliper as shown, the screw A fits a hole threaded in the frame C. The screw has been turned to the right until its end is in contact with the stop, or anvil, D. If the screw is turned to the left, its end will move from the anvil D by an amount determined by its pitch, or number of threads per inch. If the screw and hole of the micrometer caliper were threaded ten threads to the inch, then one full revolution to the left would cause the distance between the end of the screw and the anvil to be one-tenth of an inch.

Principles of the micrometer caliper

Figure shows a skeleton view of a modern micrometer. This micrometer has many features which are lacking in the simple measuring device just described, but it employs the same basic principles in its operation.

The screw A in our simple device has become the spindle C. Both have a frame and both have an anvil. The spindle, like the screw, is threaded; but with this important difference: it is threaded with forty threads per inch. This means that when it is rotated one revolution the gap between its end and the anvil is increased or decreased by one-fortieth of an inch. One-fortieth of an inch is expressed in decimals as .025, which is 25 one-thousandths of an inch.

In the micrometer caliper, the sleeve D is fastened to the spindle C. When the spindle turns, the sleeve turns with it. The circumference of the sleeve has been divided into 25 equal parts, as shown at Y. When the sleeve of the micrometer caliper is turned the distance included in one of these divisions, the threaded spindle rotates one twenty-fifth of a revolution; the gap between the end of the spindle is increased or decreased by one one-thousandth of an inch.

The divisions are numbered, and as the sleeve is turned the numbers indicate the linear advance of the spindle in thousandths of an inch. The hub inch the micrometer has a scale Z on it with divisions .025 of an inch apart.

When the sleeve rotates one full revolution, its edge moves away from the anvil a distance equal to one of these divisions. When the end of the spindle rests on the anvil, all the divisions in the scale Z are covered by the sleeve D.

Starting from this setting, if the sleeve of the micrometer caliper is rotated two revolutions, two divisions on the scale Z will be uncovered and the space between the end of the spindle and the anvil will be 2 X .025 thousandths or .050 of an inch. If the sleeve is now rotated a twenty-fifth of a revolution — that is, through one of the divisions on the scale Y —the distance will become .051 of an inch.

Figure shows a random setting of a micrometer caliper. The sleeve has uncovered the 1 on the scale. Included between the 0 and the 1 are four divisions. Each division represents one revolution of the spindle, or .025 inch.

Four revolutions mean 4 X .025, or .100. Two more divisions are exposed beyond the 1, so .050" is added. The sum now is .150. The scale on the sleeve shows that in addition to the six revolutions indicated on the hubscale, the spindle has rotated 4 twenty fifths of a revolution. When this is added to the .150, the reading becomes .154".

Refinements incorporated by the manufacturer increase the usefulness and wearing qualities of the micrometer caliper. An important refinement is the ratchet stop attached to the top of the sleeve. In closing the micrometer against the object measured, the spindle may be rotated by applying the turning force to the thimble E on the sleeve or by turning the ratchet stop.

When the spindle is turned by the thimble, a slight variation in measurement may occur because the fingers may apply more force at one time than another. Toolmakers refer to this variation as light and heavy touch or feel. When the ratchet stop is used, the amount of torque applied is determined by the tension to which the stop has been set. Thus, if several users of the same micrometer used the ratchet stop, they would all get the same reading.

Another useful refinement is the lock nut. By its use the spindle can be locked at a given setting. The micrometer can then be used as a snap gage; that is, the spindle can be locked in position when the same dimension is to be checked a number of times. It is important when this lock nut is used to remember to loosen it before attempting to turn the spindle.

The internal micrometer caliper

Sometimes it is necessary to make an internal measurement. The internal micrometer caliper is used for this. Two forms are shown. In one form, accurately calibrated rods are used to increase the range. The reading of the micrometer head is added to the measurement marked on the rod. By dividing the circumference of the sleeve into twenty-five equal divisions, we are able to read to one twenty-fifth of an inch. If each of the twenty-five divisions were divided into ten equal parts, then each of these very small divisions would represent a measurement of one ten-thousandth of an inch. These divisions, however, would be so small that we could not read a measurement without the aid of a powerful magnifying glass. To overcome this difficulty, the vernier principle is employed.

The vernier scale

Figure shows a micrometer setting of .1546". We may read a micrometer equipped with a vernier scale by taking the following steps. First, take the reading to thousandths, which in this case would be .154. Then look at the scale on the hub and at the same time look at the scale on the thimble. At one point a line marked on the hub appears as a continuation of a line marking one of the divisions on the sleeve.

The line on the hub which coincides with the line on the sleeve in this case is the line marked 6. Attach this 6 to the .154 and the reading becomes .1546. With a little practice the eye automatically selects the coinciding line; the corresponding number and a dimension in ten-thousandths is read as quickly as one involving thousandths.

The principle involved may be grasped by studying the figure. In this figure the scale A is movable and can be slid along the edge of a fixed part B. Readings in thousandths of an inch are taken from the arrow C, which is engraved on the fixed surface. In the setting, the reading is .008 because the graduation marked 8 is over the arrow.

Attached to the fixed surface upon which the arrow C is engraved is a vernier scale, D. At the setting shown, the vernier extends from the division 10 on the movable scale to the division marked 19 on this scale. Its length is equal to 9 of the divisions on the fixed scale. It is divided into 10 equal parts. Each part, then, must be equal in length to .9 of one of the parts on the fixed scale.

With the scale set as it is in the figure, the distance between the zero and the 9 on the vernier scale is 9/10 as long as the distance between the 19 and the 18 on the movable scale. The distance between the line marked 9 on the vernier and the line marked 18 on the movable scale must then be equal to one-tenth of a division on the movable scale.

If the movable scale is moved in the direction indicated by the arrow E until the line on it marked 18 coincides with the line 9 on the vernier scale, it will have been moved one-tenth of a division. Since one division on the scale represents 1 one-thousandth, then one-tenth must represent 1 ten thousandth of an inch and the new reading is .0079.

Following the same reasoning, if the scale is moved farther until the line marked 17 on the scale coincides with the line marked 8 on the vernier, the distance moved will represent 2 ten-thousandths and the new reading will be .0078. So we can proceed through each number until the zeros on the vernier coincide with lines on the movable scale; and the reading then will be .007 of an inch.

Note that we disregard the numbers marked on the graduations on the movable scale. The only number we read is the number of the line on the vernier which coincides with a line on the scale.

A ten-thousandth of an inch is a very small dimension. If a piece of steel rod were measured for length to an accuracy of 1 ten-thousandth of an inch, it would be found to vary as temperature changed.

The micrometer caliper itself is made of metal and its readings are influenced by temperature. It is customary to maintain an even temperature of about 70° F. in shops where work requiring great accuracy of measurement is done. When using a micrometer to measure to ten-thousandths, it is held lightly and measurements are taken quickly to minimize the effects of heat from the hand. The micrometer used for these fine measurements should be confined to this use.

Coarser measurements should be made with other instruments. In spite of every precaution, the reading of a micrometer to ten-thousandths cannot be taken as absolute. When a reading is taken under the best conditions, it is assumed to be correct within 1 ten-thousandth, plus or minus. For example, a reading of .0075" is taken to mean that the part measured is larger than .0074" and smaller than .0076".

One of the precautions taken to insure accuracy in the use of precision measuring instruments is to check their measurements frequently against standards of known dimension. These standards may be hardened discs, blocks, or rods, made with extreme accuracy to a given dimension.

In use, the standard and the micrometer are brought to standard temperature, and the standard measured. The extent to which the reading on the micrometer agrees with the dimension marked on the standard determines the accuracy of the micrometer.

The decimal system

Sometimes a machinist must use dimensions closer than a sixty-fourth of an inch (1/64”), but he would encounter difficulties if this fraction were divided further. Half of 1/64”is 1/128”; a quarter is 1/256”.

Combining fractions like these would not only consume time but would lead to many errors. All this can be avoided simply by dividing the inch into multiple of ten, as is done according to the Ford Decimal System.

As you know from your arithmetic, you can divide a number by ten or multiply it by ten simply by moving the decimal point to the left or the right. So, if you want to know the length of one-tenth of 1/16” you can rewrite it as 0.0625 and then, by moving the decimal point one place to the left, find the answer to be 0.00625.

Moving the decimal one point to the right would have multiplied the dimension by ten, giving 0.625 as the product.

Adding and subtracting decimals is almost as easy. When two or more decimals are added, the only precaution to observe is to add together only integers which are the same number of places to the right or left of the decimal point. From the drawing a machinist sometimes learns that a part may be made a certain amount larger or smaller than a given dimension.

This leeway is called tolerance and is indicated on the drawing in thousandths or ten thousandths. If the diameter of a shaft is indicated as 1.5005/1.4995 the shaft must be not more than .0005 larger or smaller than an inch and a half.

A hole diameter might be given as 2.1600 +0.0009/-0.0005 The machinist would add 0.0009 to 2.1600 and get 2.1609 as the maximum size of the hole. He would subtract 0.0005 from 2.1600 and get 2.1595 as the minimum size.

To convert a common fraction into a decimal requires only a simple process in arithmetic but it takes time. To save the time and avoid possible errors, a machinist can use a decimal equivalent table. Such tables list common fractions with their corresponding values expressed in decimals.

In studying the circular gage, we found that by depending on touch we could achieve accuracy far beyond that which depended on our ability to read graduations on a scale. In using the circular gage we fitted a part of unknown dimension to an opening.

If the part fitted this opening, its diameter or thickness was the measurement corresponding to the opening. If we could adjust this opening and have some way of accurately indicating what the opening was, we could apply the principle of touch measuring to an unlimited variation of measurements.