ASME B1.25:2019 pdf free download – Measurement Uncertainty Factors in the Calibration of Screw Thread Gages
lt is important to understand that there is no genericuncertainty for a given process.Two laboratories havingidentical principal equipment will have different uncer-tainties due to variations in the factors noted insection 5.
often several laboratories report the same or similarvalues for uncertainty due to rounding of the actualvalues to the nearest convenient value. For example,12 uin. may be rounded to 15 uin. in a conservative budget.uncertainty budget: a listing of all of the factors affecting aparticular measurement and the mathematical methodused to process each one to arrive at a total expandeduncertainty. Each factor is processed to show itscommon dimensional effect even though it may not beof dimensional origins, such as temperature or measure-ment force. Also called uncertainty statement.
3.2 Dimensional calibration Definitions
The following terminology is commonly used in dimen-sional calibration; however,some terms may refer tothread gages and related devices only:
accuracy: an indication of the performance of an instru-ment, the dimensional state of a master, or the outcome ofa process when compared to specified values.Per JCGM200:2012,”accuracy” is not a quantity and is not given anumerical quantity value.
comparison: a dimensional measurement process inwhich the unknown size of a gage is compared tothe known size of a master, such as a gage-blockbuild-up. Some instruments, usually referred to ascomparators,are designed for this type of short-range measurement only. Other devices with a longmeasurement range may be set up in a similarmanner for improved precision. The performance ofall devices used in this manner is limited by the uncer-tainty in the calibration of the dimensional master(s)used.
cosine error: an error that results when the feature beingmeasured, such as a diameter, is not normal (square) tothe axis of measurement.This error may be encounteredwhen plug gages rest on a worktable that cannot beadjusted to offset it. lt may also appear in the directmeasurement of solid thread ring gages. In both cases,it occurs because either the face or table the gage is resting on is not square to the axis. It can also be found indevices that mount the gage on centers that are notalignedproperly.
functional size: a size, such as a diameter, that contains allelements it is comprised of such that it could be larger onan external thread or smaller on an internal thread thanindicated by discrete measurement for the dimension(e.g., diameter) alone (see ASME B1.7).
lead: the axial distance a thread gage will travel over onerevolution in a mating thread.When single start threadsare involved,the lead will be equal to the linear pitch of thethread.
NOTE: A thread with no lead or pitch variation could still containconsiderable helical path error.
linearity: a measuring instrument characteristic compar-able to accuracy.An instrument is said to be “linear” when,for every unit of input, it displays an equal unit ofmeasured value. A simple example of this is a dial ordigital indicator. For every 0.0001 in. (0.001 mm) thecontact point is moved, the display reading will show achange of the same amount.
National Institute of Standards and Technology (NIST): thenational legal authority for measurements in the UnitedStates through which measurements may be linked tointernational standards for length, such as Sl,etc.Formerly called the National Bureau of Standards (NBS).pitch,linear: the distance between corresponding pointson adjacent thread forms in the same axial plane on thesame side ofthe thread axis. It is often incorrectly referredto as “lead.”
reading: a measuring device’s display of a measured valueor its variation from a master.
repeatability: the range within which a device will repeat agiven measured value over a short period of time. While animportant characteristic of the measurement process,repeatability on its own should not be confused with accu-racy or uncertainty.
resolution: the smallest measured value an instrument candisplay. While a critical feature of any instrument, it is notnecessarily an indication of that instrument’sperformance.
simple pitch diameter: on a parallel thread, the imaginarycylinder or diameter that passes through the thread formin such a way that the widths of the thread ridge andgroove are equal.This diameter is equidistant from thetop and bottom of a perfect unrelieved thread form.Also known as thread groove diameter.
thread form: a thread’s profile in an axial plane for a lengthof one pitch of the thread.