You can also determine the quality of the product thread form, by taking full advantage of System 22 indicating gaging. This gaging method provides the tools for efficient manufacturing and reliable quality control. What follows is a comprehensive menu of advanced thread gaging methods that are available for today's demanding quality control requirements in the screw thread manufacturing industry.

System 22 specifications may not require that thread form variations be analyzed, but advanced thread gaging techniques are maximized by using System 22 gaging methods. Differential Gaging measurements are used for efficient manufacturing set-up and process control. These advantages will maximize output, the process and tool life, while reducing costs. Also, the time to inspect and gage wear is reduced. In addition, time consuming calibration is reduced.

The first object is to minimize the difference between Functional diameter and Pitch diameter sizes at the time of set-up; then the quality of the thread form is ideal. The next objective is to position the actual size of the screw thread within the Pitch diameter tolerance limits.  The correct positioning of size should allow for maximum use of the Functional size tolerance that remains.

Systems Of Gaging

Federal Standard H28/20 can be referred to as the what to do document. It references the American Society of Mechanical Engineers (ASME) B1.3M standard as the how to do document. ASME B1.3M defines how to measure threads for dimensional acceptability. It also defines the gage geometry for Maximum and Minimum material limits of size measurements. ASME B1.3M suggests the proper gage contact designs for discriminating out-of-round, taper, lead, and flank angle variations.

Federal Standard H28/20 suggests inspection methods for acceptability. These methods are Systems 21, 22 and 23. Federal Standard H28/20 states that: "System 21 provides for interchangeable assembly with Functional size control at the maximum material limits within the length of standard gaging elements; and also control of characteristics identified as NOT-GO. Functional diameters or as HI (Internal) and LO (External) Functional diameters. 

These Functional gages provide some control at the minimum material limit when there is little variation in thread form characteristics such as lead, flank angle, taper and roundness." 
Federal Standard H28/20 simply states that System 21 is performed by Functional type gaging, for example; GO and NO GO ring and plug gages.

These are called attribute gages and provide no size information within Pitch diameter tolerance limits. GO and NO GO gages only expose product that exceeds tolerance limits. These Functional gages alone provide only a check for assembly.  "System 22 provides for interchangeable assembly with Functional size control at the maximum material limits within the length of standard gaging elements; and also control of the minimum material size limits over the length of the full thread.

Other thread characteristics such as lead, flank angle, taper and roundness variations are confined within these limits with no specific control of their magnitudes. For UNJ and MJ external threads, control is also provided for the thread root radius and rounded root minor diameter." System 22 introduces minimum material size, in addition to Functional diameter size. Minimum material size is the dimension called Pitch diameter size.  Pitch diameter size is the actual or pure material size; meaning no variations in lead, flank angle, taper or out-of-round."

System 23 provides for interchangeable assembly with Functional size control at the maximum material limits within the length of standard gaging elements; and also control of the minimum material size limits over the length of the full thread. The magnitude of other thread characteristics such as lead, flank angle, taper and roundness are further controlled within these limits.

For UNJ and MJ external threads, control is also provided for the thread root radius and rounded root minor diameter." Magnitude control of individual and total variations is required within given percentages of Pitch diameter tolerances. The best description of these requirements can be referenced in Military Standard 8879C (Mil-S-8879C) under category "Safety Critical." With System 22 gaging, we are able to identify many of the System 23 characteristics. However, this is not required by System 22. 

Variations

Federal Standard H28/20, System 22, speaks of maximum material limit of size and minimum material limit of size.  Good metrology practices should be used for dimensioning maximum and minimum material conditions; information exists in ANSI Y14.5, Dimensioning and Tolerancing.  ANSI Y14.5 uses a term called "Virtual size." The Virtual size idea equates to thread gagings' Functional diameter size. Therefore, on outside diameter (O.D.) threads, the threaded shaft grows virtually larger with increased amount of variations.

The Functional diameter size can only be equal to or greater than the Pitch diameter size. On inside diameter (I.D.) threads, the threaded hole virtually shrinks with increased amounts of variations. The Functional diameter size can only be equal to or less than the Pitch diameter size.

When the Functional and Pitch diameter sizes equal each other, no variations in lead, flank angle, taper or out-of round exists. Federal Standard H28/20 states there is no specific control of the magnitudes for thread form variations. However, System 22 indicating gaging is used to determine the magnitude of thread form variations.

The largest difference between the indicated values for Functional and Pitch diameter sizes is the total magnitude. The Functional diameter size is expressed as the cumulative effect of Pitch diameter plus the variations in lead, flank angle, taper or out-of-round. These characteristics and elements create a measurable diametral growth.

By subtracting Pitch diameter, from Functional Diameter, we are left with variation only. Taken a step further, we can separate the elements and characteristics that make up the total magnitude of the variation. This is referred to as "Differential" gaging. Variations in thread elements and characteristics are the causes for the diametral additions to Pitch diameter sizes.

The variations that seem to cause the most trouble are the lead, flank angle, taper, and out-of-round. Lead is the axial advance per unit rotation for a given pitch distance. Lead equals pitch under ideal conditions. Machine capability, process, speeds, feeds, approach, programming, and mis-aligned dies can be culprits for lead variations. Lead variations are called short, long, or drunk (helical deviations). Flank angle of a Unified National thread design calls for a 60o included angle.

Worn tooling and tooling not square to the centerline of the work piece are the usual causes for the flank angle being other than a centered 60o included angle. Taper is caused by part deflection, worn tooling, thread blank configuration.

Out-of-Round is usually an even lobe 180 degrees or odd lobe 120 degrees, also called egg-shaped and multi-lobed, respectively. Roundness variations are caused by thread blank configuration, geometry of tooling, such as two flat or roll dies at 180 degrees positions, or improper chucking. Also, other forces may distort product parts in a secondary operation.

Major diameter of O.D. threads can be measured by using a caliper or micrometer. Major diameter of I.D.  threads is usually considered a clearance dimension. If undersized, a proper Functional gage profile will detect it. Minor diameter of O.D. threads can be gaged using an indicating gage with sharp contacts that penetrate to the root of the thread. This works when specific minor diameter measurements are required; for example, of UNJ type threads.

Minor diameter and root radius of O.D.  threads can also be measured by an optical comparator.  Minor diameter of I.D. threads can be measured using a caliper or inside micrometer. Indicating gages are also used to measure thread related features, such as concentricity and perpendicularity. Other features can be measured on fasteners or product parts, such as related diameters, overall length, head diameter, shank diameter, thread length, width across flats and corners, washer face, and recesses.

Measurement

The key to variation measurement is to have the proper geometric contacts, tools and gages that will detect the full magnitudes of these variations, as they relate to maximum and minimum material conditions. Taper and Roundness variations have two indicated values-one high, one low.  The difference between the high and low dimension gives the total magnitude. On O.D. threads, the Functional diameter gage captures the, high of the high, indicated reading and the Pitch diameter gage captures the, low of the low, indicated reading.

This method is advised because if the largest Functional diameter size measurement exceeds the maximum of the Pitch diameter size tolerance limit, parts may not assemble. If the smallest Pitch diameter size measurement exceeds the minimum of the Pitch diameter tolerance limit, your thread is undersized. On I.D. threads; the Functional diameter gage captures the, low of the low, indicated reading and the Pitch diameter gage captures the, high of the high, indicated reading.

This method is advised because if the smallest Functional diameter size measurement exceeds the minimum of the Pitch diameter size tolerance limit, parts may not assemble. If the largest Pitch diameter size measurement exceeds the maximum of the Pitch diameter tolerance limit, your thread is oversized. Anytime a screw threads Functional diameter size exceeds the maximum material condition, parts may not assemble.  Anytime a screw threads Pitch diameter size exceeds the minimum material condition, there isn't sufficient material, per the standards.

This is typically caused by a machine operator making size offsets to an O.D. thread, to compensate for increased thread form variations. What prompts the operator to make an offset is that the GO ring gage is tight or the part cannot enter the GO ring gage. The way to fix this problem is to make a size offset, run another part, and check that GO and NO GO gaging passed. After a while, no more offsets can be made without entry of the NO GO gage.

At this time, a tool change is made. By this time, the Pitch diameter size can be extremely undersized. This typical undercutting of the Pitch diameter size is done to compensate for the virtual effect of the Functional diameter size while using System 21 gaging.

Differential Gaging

Now that we understand variations and how they relate to maximum material (Functional diameter size) and minimum material (Pitch diameter size) conditions, differential gaging techniques can be practiced properly. Lets assume we are using System 22 indicating-type thread gaging.

Federal Standard H28/20, System 22, doesn't require dissecting the differential, however, System 22 gaging is capable of doing this. The differential is considered the size of the thread, with all of its variation in lead, flank angle, taper, and out-of-round, minus the Pitch diameter.  This being the case, all you're left with is pure variation.

This variation can be dissected to determine its contents. The contents consist of lead, flank angle, taper and out-of-round. A proper differential is measured by observing the largest Functional diameter size on O.D.  threads, with respect to taper and roundness. To do this, you must rotate the part and measure along the thread length, as explained below.

Then observe the smallest Pitch diameter size on O.D. threads, using the same technique. The opposite is true for I.D. threads-the smallest Functional diameter size and the largest Pitch diameter size measurements are observed.

ANSI/ASME B1.2-1983, paragraph 5.12 Differential Gaging states: "The concept of differential gaging for product external threads makes use of fundamental geometric theorems that relate directly to size, position, and form. For differential gaging, two methods are used for measuring screw thread size: (a) GO functional size and (b) pitch diameter (or thread groove diameter). Only when a screw thread has perfect position and form [i.e., zero variation in lead (including helical path), flank angle, taper, and roundness] are these two measurements equal.

Differential gaging is a variables method of in-process inspection, final conformance inspection, or both, that provides the actual numerical values for both GO functional and pitch diameter sizes. These are the two extreme sizes of any product screw thread. One of the sizes, pitch diameter, is the size of the thread with essentially zero variation in all other thread elements, while the other size, GO functional size, is the size of the thread with the effects of all variations in all other thread elements added to the pitch diameter.

The numerical difference between these two sizes is called a cumulative thread element variation differential and represents the diametral effect of the total amount of thread element variation differential. The inspection process that further refines the total amount of thread element variation so that the amount of variation for each individual element becomes known is called a single thread element variation differential." 

I urge anyone measuring threads to read the entire 5.12 section in ANSI/ASME B1.2.

The first thing you must do is isolate any taper and roundness so that a distinction for lead and flank angle can be made. See Illustration [Differential Gaging - Taper]. To measure Taper: Use the Pitch diameter gage (cone and vee) and measure at positions along the length of thread without rotating the part. Any deviation of the indicator readout will tell the direction and magnitude of the tapered condition. Also, reference ANSI/ASME B1.2-1983, page 143, paragraph 5.12.3.3.

See Illustration [Differential Gaging - Out-of-Round]. To measure Out-of-Round: Use the 180o Functional segment to capture the full magnitude of an egg-shaped (even lobe) out-of-round. Use the 120o cone and vee or multi-rib roll contacts for tri-lobe (odd lobe) out-of-round. When using the Functional diameter gage (multi-rib or segment), rotate the part to observe the largest indicated value for O.D. threads.

When using the Pitch diameter gage (cone and vee), rotate the part to observe the smallest indicated value for O.D. threads. For I.D. threads, rotate the part to observe the smallest indicated value for a Functional diameter measurement and the largest indicated value for Pitch diameter measurement. Also, reference ANSI/ASME B1.2-1983, page 143, paragraph 5.12.3.3. 

See Illustration [Differential Gaging - System 22]. In example number one, there are two dial indicators. One dial indicator is Functional diameter size and the other dial is Pitch diameter size. Both indicator needles are pointing in the same direction. This means the quality of the thread form is ideal. The thread size is positioned at the mean of the Pitch diameter tolerance. This example passes Systems 21, 22, and 23. Example number two, the differential, consumes almost the entire tolerance envelope.

The quality of the thread form is poor, however it is still within the acceptable Pitch diameter tolerance limits. The product passes System 21, 22, but may not pass 23. Example number three, the quality of the thread form, is poor. The Functional diameter size, maximum material limit is exceeded and product parts may not assemble.

This product will fail System 21, 22 and 23 gaging. Example number four, the quality of thread form, is also poor, but this time the minimum material limit of size is exceeded. This product fails System 22 and 23, but may pass 21.

A lot of information has been discussed about the different thread gaging Systems 21, 22, and 23. By utilizing advanced thread gaging techniques, the System 22 gaging will benefit most. 

Illustrations:

Pitch Diameter Size Pitch Diameter Size: Gage Contact Profile for Pitch Diameter Size Measurements Functional Diameter Size Functional Diameter Size: Gage Contact Profile for Functional Diameter Size Measurements Thread Class Variations: Lead Variations: Flank Angle Variations: Both have same Functional diameter size

Indicating Thread Gaging: Gage Contact Designs Differential Gaging: Taper Differential Gaging: Out of Round Differential Gaging: Lead Differential Gaging: Flank Angle Differential Gaging: System 22 Differential Gaging: System 23

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