In order to achieve reliable measurements from precision hand tools,
a common-sense approach on basic usage produces the best results.

Developments in precision measuring have made modern hand tools more accurate and easier to read. Reviewing basic tips for “how to” use precision instruments can help operators use the tools to their full potential and ultimately save time, reduce errors, and increase productivity. 

This article covers tips of the trade for measuring with three popular hand tools: micrometers, slide calipers, and micrometer depth gages.

Micrometers

The precision micrometer is the most accurate hand-held tool available to skilled operators. Types of micrometers include digital, vernier, inside, bench, and special models including Mul-T-Anvil, V-anvil, ball-anvil, etc.

Micrometers combine the double contact of a slide caliper with a precision screw adjustment that may be read with great accuracy. The work is placed against the anvil with the left hand while the spindle is turned down the work with the thumb and index finger on the right hand. Do not force measurement, as light contact pressure assures correct reading. Measuring tips include:

Digital micrometers make readings faster and easier for every machinist, regardless of experience. The frame-mounted counter saves handling time since it can be read without removing fingers from the thimble or the micrometer from the work.

Keep the work to be measured and the micrometer anvil and spindle faces clean.

For very fine measurements, the micrometers should be set to zero or to a standard by your “feel,” by the friction thimble, or by the ratchet, whichever is being used.

To minimize any frame flexure influence, large micrometers should especially be set to a standard in the same approximate position, either vertical or horizontal, in which they will be used. 

Avoid rushing measuring work, as this may result in inaccurate results.

Do not remove work from a micrometer before taking a reading. If a reading cannot be seen without removing the micrometer, locking the spindle at the final setting with the lock nut and sliding the micrometer off the work by the frame will be useful.

If a micrometer has been set to a flat standard, operators can get approximately 0.0001” (0.0025mm) difference when measuring over a round because the same pressure is being applied to a point or line contact.

Adjusting any micrometer can be done in two easy steps:

Step 1: To eliminate play in the spindle, back off the thimble, insert spanner wrench (likely furnished with the micrometer) into the adjusting nut and tighten just enough to eliminate play. 

Step 2: To adjust zero reading, clean all dirt or grit from measuring faces by gently closing the spindle to the anvil with a clean piece of paper between them. Pull the paper out with pressure applied, then close the faces using feel and insert spanner wrench in the small slot of the sleeve. Next turn sleeve until its zero line coincides with zero line on the thimble.

The Starrett 456 Gear Tooth Vernier Caliper is designed to measure in .001 inch or 0.02mm, the thickness of gear teeth at the pitch line (the chordal thickness of the teeth) using the distance from the top of a tooth to a chord.

Slide Calipers

Slide calipers are very versatile tools, and while they do not provide the same degree of precision as a micrometer, they are very accurate and offer much more range than a single micrometer. Slide calipers include electronic, mechanical dial, vernier, and plain versions.

The best digital and dial slide calipers, regardless of resolution, are accurate to within 0.001” or 0.03mm, every 6” or 150mm. The best vernier calipers are accurate to 0.0005” or 0.013mm per foot or 300mm.

Slide calipers have two knurled thumb pieces on the slide, which make it easy to open or close the jaws, and a knurled clamping screw with a left-hand thread for locking the slide at any desired setting. The thumb on the same hand that holds the tool can be used for both these adjustments. The slide also has a stop, preventing it from being entirely withdrawn from the body.

Because slide caliper measuring surfaces are not in-line with the beam of the caliper, some care should be taken not to use too much measuring pressure. This will lessen the possibility of springing the jaws. A general rule of thumb is to use good judgment for setting a minimum measuring pressure, perhaps in the 1/2 pound (0.2 kilogram) range.

To check or set the separate ID nibs on a caliper, a micrometer or ring gage can be used. Individual feel is important when measuring an ID because the measuring surfaces are so thin that small pressure changes that are normal from person to person can affect the reading by as much as 0.001” (0.03mm) or so. Also, it is important to keep the sliding surfaces clean and lightly oiled.

Micrometer Depth Gages

A micrometer depth gage measures the depth of holes, slots, recesses, keyways, etc. and is available in electronic, mechanical digital, and standard readouts. The tool consists of a hardened, ground, and lapped base combined with a micrometer head. Measuring rods are inserted through a hole in the micrometer screw and brought to a positive seat by a knurled nut. 

The reading is taken exactly the same as with an outside micrometer except that sleeve graduations run in the opposite direction. In obtaining a reading using a rod other than the 0-1”, it is necessary to consider the additional rod length. For example, if the 1-2” rod is being used, one inch must be added to the reading on the sleeve and thimble.

Before using the micrometer depth gage, be sure that base, end of rod, and work are wiped clean, and that rod is properly seated in micrometer head. Hold base firmly against work, and turn thimble until rod contacts bottom of slot or recess. Tighten lock nut, and remove tool from work to read measurement.

Adjustment to compensate for wear is provided by an adjusting nut at the end of each rod. Should it become necessary to make an adjustment of a rod, back off the adjusting nut one-half turn before turning to new position, then check against a known standard such as a Starrett Webber gage block.

Conclusion

To achieve reliable measurements from precision hand tools, a common sense approach on basic usage can produce the best results. 

Sidebar

Sight, Touch, Estimation

The sense of touch becomes important when using contact measuring tools. A skilled machinist with a highly developed sense of “feel” can readily detect a difference in contact made by changes in a dimension as small as 0.00025” (0.006mm). While the acuteness of the sense of touch varies with individuals, it can be developed with practice and proper handling of tools.

In the human hand, the sense of touch is prominent in the fingertips. Therefore, a contact measuring tool should be properly balanced in the hand and held lightly and delicately in such a way as to bring the fingers into play in handling or moving the tool. If the tool is clumsy or harshly grasped, the sense of touch or “feel” is greatly reduced.

Sight and touch are frequently combined by the skilled worker to estimate measurements finer than the graduated limits of a tool. For example, on the average micrometer graduated to read in thousandths of an inch, the space between the smallest graduations of the thimble is approximately 1/ 16”. Variations in size much smaller than the thousandth of an inch can readily be felt and judged by eye with reasonable accuracy.

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Scott Robinson is technical support manager at the L.S. Starrett Company. For more information, go to www.starrett.com.