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ASME B1.25-2019 pdf download

ASME B1.25-2019 pdf download.Measurement Uncertainty Factors in the Calibration of Screw Thread Gages.
Often several laboratories report the same or similar values for uncertainty due to rounding of the actual values to the nearest convenient value. For example, 12 μin.mayberoundedto 15 μin.inaconservativebudget. uncertaintybudget: a listing ofall ofthe factors affecting a particular measurement and the mathematical method used to process each one to arrive at a total expanded uncertainty. Each factor is processed to show its common dimensional effect even though it may not be ofdimensional origins, such as temperature or measure- ment force. Also called uncertainty statement. 3.2 Dimensional Calibration Definitions The following terminologyis commonlyused in dimen- sional calibration; however, some terms may refer to thread gages and related devices only: accuracy: an indication of the performance of an instru- ment, the dimensional state ofa master, orthe outcome of a process when compared to specified values. Per JCGM 200:2012, “accuracy” is not a quantity and is not given a numerical quantity value. comparison: a dimensional measurement process in which the unknown size of a gage is compared to the known size of a master, such as a gage-block build-up. Some instruments, usually referred to as comparators, are designed for this type of short- range measurement only. Other devices with a long measurement range may be set up in a similar manner for improved precision. The performance of all devices used in this manner is limited by the uncer- tainty in the calibration of the dimensional master(s) used.
This would be shown by an uncertainty budget for such aprocess comparedto one usingsuitable equipment. In this section, the minimum requirements for equip- mentareshownto assistusers inevaluatingthesuitability of their equipment for calibration work. In all cases, it is assumed that the instruments, thread-measuring wires, andmasters beingusedareinaknownstateofcalibration. 4.1 Bench Micrometers and Similar Devices Bench micrometers and similar precision-measuring devices function much in the same way as outside micro- meters; the axis ofthe gage may be vertically oriented, or the gage may be horizontally mounted on the centers for measurement of pitch diameter, etc. Devices similar to bench micrometers include micrometer-like devices with adjustable tailstocks, floating carriage-measuring machines designed specifically for calibration work, and universal length-measuring machines (ULMs). Of these devices, ULMs offer the highest order of precision. The basic requirements for bench micrometers and similar devices are as follows: (a) changeable measuring forces that comply with the requirements of the thread standard (b) 0.00001-in. (0.0002-mm) resolution or finer (c) measuring faces that are flat and parallel within 20 μin. (0.5 μm) (d) retractable, adjustable measuring face(s) to facili- tate insertion of thread wires 4.2 High-Resolution Comparators High-resolution comparators incorporate a high-reso- lution indicatorsystem thatis setto a nominal size usinga Optical comparators project a magnified image of the threadformonarotatingglassscreen,enablingtheuserto measurethethreadformusingeitheron-screencalibrated angular values or, if provided, an integrated digital measuring system. Magnifications used vary with the pitch of the thread being measured. For inch threads coarser than 14 threads per inch (TPI), use 20X magni- fication; for 14 TPI to 40 TPI, use 50X magnification; and for finer than 40 TPI, use 100X magnification. For metric threads coarser than 1.75-mm pitch, use 20X magnifica- tion; for 1.7-mm to 0.6-mm pitch, use 50X magnification; and for finer than 0.6-mm pitch, use 100X magnification. The basic requirements for optical requirements are as follows: (a) interchangeable lenses to provide 20X, 50X, and 100X magnification per the standard (b) magnification accuracy precision ofapproximately 0.05% (c) adjustment to align the thread helix to the optical path for form measurement (d) linear measurementresolution of0.0001 in. (0.002 mm) or better (e) angle-measuring system with 0-deg, 1-min resolution While overlay charts with tolerance lines on them are used with optical comparators to inspect thread form on products, they are not accurate enough for thread gages. 4.6 Toolmakers’ Microscopes Toolmakers’ microscopes function in much the same way as an optical comparator except that the image of the thread formis viewed through an eyepiece. Some tool- makers’ microscopes have an accessory that provides a small screen for viewing the image in lieu ofthe eyepiece. The basic requirements are the same as for optical comparators.

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