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Measuring Devices

There are a large amount of measuring devices available for different applications. For the sake of brevity , only a few will be discussed here . The direct visual inspection method is frequently augmented by the use of several common tools used to measure dimensions, discontinuities, or range of inspection. Among these are linear measuring devices, outside diameter micrometers , ID/OD calipers , depth indictors , optical comparators, gauges , templates , and miscellaneous measuring devices.
The most common linear measuring device is the straightedge scale. Typical scale lengths are 6 inches(15cm) and 112 inches (30cm) ling. Additionally , 25-foot to 50-foot tape measures are frequently utilized. The 6-inch pocket scale is frequently misused by placing the thumb on one edge of the specimen and abutting the end of the scale against the soft and pliable thumb flesh. This is inherently inaccurate when a movable surface is the beginning point for a measurement. Preferably, the first whole unit of measurement (metric or imperial) should be aligned with one edge of the specimen and the other edge read for total distance. Caution: one must then remember to subtract the first unit of measurement from the total measurement noted.
Micrometers are use to measure outside or inside diameters. They are very accurate measuring devices commonly utilized to measure to the nearest one thousandth of an inch. Micrometers are available that can be used to measure to an accuracy of one ten including the anvil, stirrup, spindle , sleeve, thimble, internal screw, ratchet, and knurled knob. It is worth noting that the essential component is the internal screw. It has been turned to have 40 threads per inch. One divided by 40results in 0.025 thousandth of an inch spindle movement per 360°turn of the thimble . the micrometer may be outfitted with attachments for different applications. For example , a 0.2″ball may be attached to the spindle to accommodate the inside radius of a pipe when measuring pipe wall thickness. Additionally , a pointed attachment may be utilized when measuring pits or very localized variations in wall thickness.
The vernier capiper is a variation of the basic caliper. A caliper is usually used to measure the outside diameter of round objects. A common application is the measurement of remaining stock on a part in a machine lathe. If transfer calipers are used, the calipers are placed with the two contact points touching opposite sides of a round object. The calipers are removed and compared to the distance between the contact points as measured on a linear scale. Vernier calipers have inside diameter(ID) and OD capabilities, depending on which set of points or jaws are utilized. The handle that forms part of the stationary jaw has one or two inscribed scales. There is also a vernier slide assembly that is integral with the movable jaw. The correct OD or ID set of calipers or jaws makes contact with the specimen. The user reads the vernier scale zero mark upon the stationary scale for the whole unit measurement . the vernier scale marks are matched with the stationary scale marks and the best match-up of lines are utilize to read the linear distance to the nearest one thousandth of an inch or millimeter as the case may be. These two basic forms of calipers have since been replaced with direct read-out dial or digital calipers. Some digital gaguges can be interfaced with a laptop or hand-held computer so that a large number of readings may be stored.
Depth indicators (dial indicators) are frequently used to measure surface discontinuity depths . examples of discontinuities are pits, corrosion, or wastage . verification of dimensions can also be conducted. In either case, zero depth mist first be verified. The dial indicator or the digital read-out indicator is placed on a flat surface and zeroed. It is then moved over the depression. The spindle movement indicates the depth of the depression. When using the older dial indicator the sweep hand rotations must be counted if the movement exceeds one 360°sweep. If a digital indicator is used, one must note the minimum and maximum point to which the indicator or its extensions are to range, i.e. 0?to 1″or 4″to 5? etc. it is a common mistake not to count the number of rotations or not to note the beginning and end range or the indicator, resulting in only the last increment of measurement being accurate.
Optical comparators are frequently used in machine shops where close tolerance measurements are desired. A comparator produces a two-dimensional enlarged image of the object on a large smoked (frosted) glass screen. The reflected light or background lighting is utilized to cast a magnified image onto the glass screen. This image is compared with a template of the object to check for dimensional accuracy.
The welding, fabrication, and construction industries use templates extensively to measure fillets, offset, mismatch, weld reinforcement , undercut , and other dimensional attributes. A common application is a template with minimum and /or maximum dimensions made from sheetmetal stock. The actual weld or base metal is compared to the template to determine “go?or “no-go?status. Accurate and actual measurements would still require linear measuring devices.
Miscellaneous measuring devices come in many sizes and configurations. Snap gauges, feeler gauge, radius gauges , temperature gauges, pitch gauges and diameter gauges are just a few. Some applications could have a gauge custom for a specific need or requirement.