General Information about Measuring Instruments

The bar type indicator caliper is one of the more common, flexible and precision measuring devices. These usually permit outside, inside, step and depth measurements. Commonly measuring to about 6", very long sizes are available. Older types had a linear scale with a vernier to provide more accurate readings. Later geared dial types appeared, greatly easing accurate readings. Digital readouts have furthered the ease. On metal versions, tips are frequently sharpened for scribing arcs during layout.

CAUTION: Plastic versions are subject to wear, when used on sharp metallic edges. They can also be sprung by higher forces. While on metal types, sharp edges can scrape paint and finishes or even flick away fine detail, like rivets, on plastics .

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Available several years ago from Pacific Fast Mail (PFM), the top vernier reads to 1/4 HO scale inch while the lower reads to 1/1000". The top left blades are for inside measurements, while the lower are for outside. At the right end, the small extension is for depth. The top screw locks position.
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For quick and dirty measurements, while shopping or in the field, a small, shirt pocket , light 3" verier comes in handy. Included in a set with a slightly better 6", one was found in a cheapy bin in a hardware store at about $ 5. Surprisingly the accuracy is very good, within .001 when compare against a digital caliper.

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SMALL PLASTIC VERNIER

The upper scale reads to 1/128" = .0078" = .68" HO = .375" O = 1.25" N. While the lower states it reads to .05 mm, actually it reads to .1 mm = .003937" = .343" HO = .189" O = .63" N. This makes it a little awkward, requiring a calculator or conversion chart..
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GENERAL PLASTIC HO DIAL CALIPER.

Less expensive and accurate, main scale reads in feet, dial shows 3' and reads 1/5" per division. Upper scale reads directly to 1/4" in O scale.

NOTE: In the General caliper shown, both inside and outside tips had noticeable gaps when fully closed at zero, producing an error of about 1/2". The dial should be calibrated at some other point, by using accurately measured stock or feeler gauges and setting dial at this value.
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DIAL CALIPER.

Reads to .001".
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DIGITAL READOUT CALIPER.

The LCD is driven from minute magnets along the bar, using the Hall effect. The two buttons at bottom, switch between readouts in .0005" or .01 mm. and reset zero to find differences between points. There is a port at the top right to feed data to a computer, if the cable, driver and software can be found. For those with poor eyesight, a larger readout, 6" version is available at about $30 US and a 12" at about $50 US.

Less flexible, but considered more accurate is the micrometer. Resembling a question mark the most common type is a 1" capacity, reading to .001". Base on a precision 40 pitch screw, each full turn of the spindle equals .025". The frame is graduated in .025", while the spindle has 25 graduations of .001". Work is placed between the anvil and the spindle tip, which is tightened against the work. The measurement is the sum of the frame and spindle readings. Often with novices, repeated readings may vary by a couple of division, due to a lack of feel in tightening. More expensive versions have a pressure adjust to secure repeatability. Many have vernier scales on the spindle to obtain readings down to .0001". Precision ground blocks are often used to check accuracy.

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BASIC MICROMETER.

Usually in one inch increments, larger sizes are available, up to many feet. Most have only one inch range of adjustment. Some read to finer graduations. Today very expensive digital readout versions are available.