Quality 101: Measure Precisely with Hand Tools
August 29, 2008
Developments in precision measurement have made modern hand tools more accurate and easier to read. Reviewing basic tips on the use of precision instruments can help operators use the tools to their full potential and ultimately save time, reduce errors and increase productivity.
Presented here are tips for measuring with three popular hand tools: micrometers, slide calipers and micrometer depth gages.
MicrometersThe precision micrometer is the most accurate handheld tool available to skilled operators. Some examples of micrometers include digital, vernier, inside and bench.
Micrometers combine the double contact of a slide caliper with a precision screw adjustment that may be read with great accuracy. To use the instrument, the work piece is placed against the anvil with the left hand while the spindle is turned with the thumb and index finger of the right hand. The measurement should not be forced, as light contact pressure ensures a correct reading.
Measuring with MicrometersDigital micrometers make readings faster and easier for every machinist, regardless of experience. The frame-mounted counter saves handling time because it can be read without removing fingers from the thimble or the micrometer from the work.
Adjusting a Micrometer
Slide CalipersWhile slide calipers do not provide the same degree of precision as micrometers, they are versatile, 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 inch every 6 inches. The best vernier calipers are accurate to 0.0005 inch per foot.
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 of 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 The slide also has a stop, preventing it from being entirely withdrawn from the body.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. Generally, a minimum measuring pressure should be set in the half-pound 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 inch. Also, it is important to keep the sliding surfaces clean and lightly oiled.
Micrometer Depth GagesA micrometer depth gage can be used to measure the depth of holes, slots, recesses and keyways, for example, 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 to 1 inch, it is necessary to consider the additional rod length. For example, if the 1- to 2-inch rod is being used, 1 inch must be added to the reading on the sleeve and thimble.
Before using the micrometer depth gage, be sure that the base, end of rod and work piece are wiped clean, and that the rod is correctly seated in the micrometer head. Hold the base firmly against the work piece, and turn the thimble until the rod contacts the bottom of the slot or recess. Tighten the lock nut and remove the tool from the work piece to read the 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 a new position, then check against a known standard.
To achieve reliable measurements from precision hand tools, a common sense approach on basic usage can produce the best results.
Sight, Touch and EstimationThe 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 inch. 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 is correctly balanced in the hand when held lightly and delicately in such a way that uses the fingers to handle or move the tool. If the tool is clumsily 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 inch. Variations in size much smaller than a thousandth of an inch can readily be felt and judged by the eye with reasonable accuracy.