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The Calibration Bench | Blog

Threads and Calibration: ISO 1502 and ASME/ANSI B1

Posted by Phil Wiseman on Nov 13, 2017 3:07:38 PM

 

 

 

 

The reality is it can become confusing when talking about Threads and Calibration. 

 

go no go gage calibration rings alliance calibration.jpgsolid thread ring.jpeg

Image Sources:https://blog.threadcheck.com/page/7/

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ISO1501:1996 (which was reaffirmed in 2016) and ANSI B1.* both address threads.

So why is there confusion?

Wouldn't the standards be identical?

These two standards are taking a different perspective on threads.

 

"The ASME philosophy is that the maximum limits of thread gages should be exactly equal to the maximum product limits. When applying the tolerances to the gages the tolerance always comes into the product tolerance. Using the ASME gaging philosophy an ASME gage may fail an acceptable threaded product which is within the gages tolerance band, but AN ASME GAGE WILL NEVER ACCEPT A NON-COMFORMING PRODUCT THREAD!

The ISO philosophy is that thread gages should start below threaded product limits and be allowed to wear slightly beyond those product limits. USING THE ISO GAGING PHILOSOPHY NEW ISO GAGES MAY REJECT CONFORMING THREADED PRODUCTS AND USED ISO GAGES, AT THEIR EXTREME ALLOWABLEWEAR LIMTIS, MAY ACCEPT NONCONFORMING THREADED PRODUCTS!

Article Exclusively prepared for FASTENER WORLD Magazine By Joe Greenslade on Feb. 19, 2006"

For detailed explanation read this article in Fastener  World.

 

Threads can be course or fine and diameter  characterized as major, minor and pitch.

"Major diameter

The major diameter of threads is the larger of two extreme diameters delimiting the height of the thread profile, as a cross-sectional view is taken in a plane containing the axis of the threads. For a screw, this is its outside diameter (OD). The major diameter of a nut may not be directly measured, but it may be tested with go/no-go gauges.

The major diameter of external threads is normally smaller than the major diameter of the internal threads, if the threads are designed to fit together. But this requirement alone does not guarantee that a bolt and a nut of the same pitch would fit together: the same requirement must separately be made for the minor and pitch diameters of the threads. Besides providing for a clearance between the crest of the bolt threads and the root of the nut threads, one must also ensure that the clearances are not so excessive as to cause the fasteners to fail.

Minor diameter

 
The basic profile of all UTS threads is the same as that of all ISO metric screw threads. Only the commonly used values for Dmaj and P differ between the two standards.

The minor diameter is the lower extreme diameter of the thread. Major diameter minus minor diameter, divided by two, equals the height of the thread. The minor diameter of a nut is its inside diameter. The minor diameter of a bolt can be measured with go/no-go gauges or, directly, with an optical comparator.

As shown in the figure at right, threads of equal pitch and angle that have matching minor diameters, with differing major and pitch diameters, may appear to fit snugly, but only do so radially; threads that have only major diameters matching (not shown) could also be visualized as not allowing radial movement. The reduced material condition, due to the unused spaces between the threads, must be minimized so as not to overly weaken the fasteners.

Pitch diameter

 
Variants of snug fit. Only threads with matched PDs are truly snug, axially as well as radially.

The pitch diameter (PD, or D2) of a particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points, when viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance. Equivalently, a line running parallel to the axis and a distance D2 away from it, the "PD line," slices the sharp-V form of the thread, having flanks coincident with the flanks of the thread under test, at exactly 50% of its height. We have assumed that the flanks have the proper shape, angle, and pitch for the specified thread standard. It is generally unrelated to the major (D) and minor (D1) diameters, especially if the crest and root truncations of the sharp-V form at these diameters are unknown. Everything else being ideal, D2D, & D1, together, would fully describe the thread form. Knowledge of PD determines the position of the sharp-V thread form, the sides of which coincide with the straight sides of the thread flanks: e.g., the crest of the external thread would truncate these sides a radial displacement D - D2 away from the position of the PD line.

Provided that there are moderate non-negative clearances between the root and crest of the opposing threads, and everything else is ideal, if the pitch diameters of a screw and nut are exactly matched, there should be no play at all between the two as assembled, even in the presence of positive root-crest clearances. This is the case when the flanks of the threads come into intimate contact with one another, before the roots and crests do, if at all.

However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing the galling of the threads. For this reason, some allowance, or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing interference and to expedite hand assembly up to the length of engagement. Such allowances, or fundamental deviations, as ISO standards call them, are provided for in various degrees in corresponding classes of fit for ranges of thread sizes. At one extreme, no allowance is provided by a class, but the maximum PD of the external thread is specified to be the same as the minimum PD of the internal thread, within specified tolerances, ensuring that the two can be assembled, with some looseness of fit still possible due to the margin of tolerance. A class called interference fit may even provide for negative allowances, where the PD of the screw is greater than the PD of the nut by at least the amount of the allowance.

The pitch diameter of external threads is measured by various methods:

  • A dedicated type of micrometer, called a thread mic or pitch mic, which has a V-anvil and a conical spindle tip, contacts the thread flanks for a direct reading.
  • A general-purpose micrometer (flat anvil and spindle) is used over a set of three wires that rest on the thread flanks, and a known constant is subtracted from the reading. (The wires are truly gauge pins, being ground to precise size, although "wires" is their common name.) This method is called the 3-wire method. Sometimes grease is used to hold the wires in place, helping the user to juggle the part, mic, and wires into position.
  • An optical comparator may also be used to determine PD graphically "

 Source:https://en.wikipedia.org/wiki/Screw_thread#Lead.2C_pitch.2C_and_starts

It is common practice to have this information stamped on thread gages and this facilitates calibration. However, you really do need to communicate with your calibration provider if a thread has a custom feature. In addition, if you have a specific measurements you need to clearly communicate this information. The standards mentioned above may not call out the specific measurement you need for your application.

Here is a useful article on Choosing and Using the Right Thread Gage.

 

Bottom Line: Have open communication with your calibration provider and put all requests in writing.

 

You may want to look at out International Standards in Calibration page.

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