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Progress in basic gaging.

Progress in basic gaging

A little over a hundred years ago, basic gaging was very basic - merely a matter of matching a machined part to a model part. Before the invention of the vernier caliper (in 1851), most machinists used simple calipers, machined rules, or fabricated their own special gages.

Today, gaging has come a long way. The mechanics and materials of gaging have been finely honed. Just this past decade, the addition of the microchip to transform and transmit manual measurements to a remote data-processing system has given each measurement the ability to immediately affect the manufacturing process. Here, based on material supplied by Mitutoyo Corp, is a review of the latest refinements in manual measurement tools for first-cut dimensional gaging.


The micrometer continues to serve as the most basic and easily manageable measurement tool in shops large and small. It effectively measures OD and ID dimensions, grooves, depths, heights, and pitch diameters, even in hard to reach places. Standard micrometers measure to thousandths of an inch, and with the addition of a vernier scale, ten thousandths. Newer compact LCD models with digital displays have internal circuitry to measure to 50 millionths of an inch, and with electronic output, directly communicate with minicomputers and machine controls.

Micrometers today are produced in a wide range of configurations, types, and sizes. Readings are in English or metric units, and electronic mikes often can provide either at the touch of a button. Typical sizes range from 1" to a maximum practical span of 18" (with 1" of spindle travel). Metric mikes are carried in near equivalent sizes: 0 to 25 mm measuring ranges and spans to 600 mm.

Typical mechanical features are friction sleeves or ratchet stops to allow slippage and prevent damage from over tightening the instrument against the part, and spindle locks to hold a reading for comparison. Slim-thimble designs reduce barrel diameter to make one-handed operation much easier.

Digital readouts have enhanced measurement ease and speed, and helped less skilled workers make reliable, repeatable measurements. They offer features like preset, zeroing, hold, error adding, and counting. Their electronic output supports SPC systems, sending part readings to miniprocessors for analysis, recording, and direct links with machine controls to make adjustments of machining accuracy. Battery driven, they go about six months between replacement or recharging, under normal use.

Micrometer sales, Mitutoyo has found, are a good barometer of a nation's economy: directly proportional to economic health. In the US, micrometers are sold in four ways: the company buys them and loans them to the machinist or inspector (sometimes demanding their return at the end of each shift), the company sells them at a discount, the company reimburses the user's outside purchase, or the user selects and buys on his own, and retains ownership.


Still a major industrial measuring tool, the vernier caliper is widely used to measure workpiece OD, hole and recess ID, and depths, inside and out. It's particularly useful where frequent, repetitive measurements are required. Its flexibility makes it equally useful to confirm center-to-center measurements on even and offset planes, and to measure depth of grooves and narrow tracks. Care must be taken, however, to avoid damage to the instrument from excessive force in use or inadequate gapping during storage, and to assure that it is positioned properly perpendicular for accurate readings.

Caliper materials are principally hardened steel (usually stainless), although carbon fiber is now gaining favor due to its lighter weight and excellent durability. The most popular sizes have 6", 8", and 12" capacities, and beyond that, special heavy-beam extensions are necessary. Some popular features are satin-chrome reading surfaces to reduce glare, spring-loaded thumb clamps to speed extension/retraction, carbide-faced jaws to prolong life, and lapped and ground measuring-beam surfaces to boost accuracy.

Vernier graduations can be 0.001", 0.02 mm, 1/128", or dual-scale combinations of any two of these units.

Dial calipers add a dial readout affixed to the movable jaw assembly, and are the most common caliper model in use. Beam graduations provide inches and tenths, and the dial adds thousandths. With each revolution of the dial hand measuring 0.100, readings in thousandths can be made from widely spaced beam graduations. By measuring a [+ or -] 0.025" range with 0.0005" graduations, a special version, the dial-snap caliper, can serve as a go/no-go gage.

As with the micrometer, the addition of microchip electronics transforms this elementary gage into an advanced input tool for SPC and statistical process updating. This boost in usefulness has reversed a recent decline in elementary caliper sales, Mitutoyo reports. The company predicts wireless communication for electronic calipers sometime soon, and a move toward ceramic jaw edging and/or inserts that make the gage more economical and durable.

Dial indicators

The mechanical dial indicator is commonly used for setup and measuring inside, outside, and over/under dimensions of turned, milled, and ground workpieces. Often associated with tool-setting gages, they are used to check cam throw, slide travel, and table movement of machine tools.

The simplicity of dial-face readings is a major contributing factor toward error-free shop measurements. Dials can be mounted on a single fixture or gage or on several gages to make multiple measurements simultaneously. They also can make pre-set or comparison measurements following a basic go/no-go in-process inspection.

Dial-face numbers are typically decimal inches or millimeters with graduations of thousandths of an inch or hundredths of a millimeter, but can go as low as 0.000 050" and 0.001 mm. Face diameters are available as small as 1.22" and as large as 3 1/8".

Based on American Gage Design standards introduced in 1945, dial indicator repeatability should be [+ or -] 0.2 of a graduation, accuracy should be within one graduation at any point in the measured range, and total spindle travel should be at least 2.5 turns of the dial, except special dials for monitoring variation from a high-noon zero position or single-revolution dials used to avoid measurement errors from misreading the smaller revolution counter.

Easily mounted on fixtures, dial indicators are particularly effective in hostile environments where special versions protect the gear train against dust, water, and oil mist. Sealed indicators may be immersed in coolant or oil bath for grinding or machining environments with heavy liquid flow.

Among the many variations of dial indicators: long-range indicators for measuring table movement or slide travel, dial calipers for quick ID comparisons, pocket thickness gages for quick checks of materials, dial snap gages for ODs up to 12", dial bore gages for three-point contact go/no-go measurements, and back plungers where the mounting stem is on the back for mounting on holding fixtures.

Electronic dial indicators, besides providing output to data-acquisition devices, magnify sensitivity to 50-millionth graduations. They may retain the conventional dial or provide a digital indication and/or signal lights indicating out-of-tolerance conditions.

A recent survey by Mitutoyo revealed that 31 percent of machining plants still use no electronic indicators, 61 percent use both dial and electronic types, and only 1.5 percent use electronic models exclusively. Electronic models are used most in defense and aerospace. The larger the plant, the more likely both types will be used. At smaller plants, 56 percent use only dial indicators.

Mitutoyo predicts that over the next five years, the cost differential between electronic and conventional dial indicators will remain 2:1, that ceramic and carbon-fiber materials will be used to reduce weight and extend life, indicators will be made smaller to fit into tighter places, and wireless systems will link the indicator to SPC.

PHOTO : The electronic micrometer makes basic measurements, outputs them to SPC data systems, and/or displays deviation from a preset master-part dimension.

PHOTO : The dial caliper remains an excellent basic instrument for quick manual ID/OD readings, if SPC output is not required.

PHOTO : The fixtured dial gage can be used for repeated go/no-go checks or monitoring critical positioning of tables and slides.
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Title Annotation:includes related articles on products
Publication:Tooling & Production
Date:Jan 1, 1990
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