In Gage Threadcheck's Official Blog

Understanding Tri-Roll & ITC Thread Comparator Systems

A variable thread gage system such as external and internal thread comparators can be an effective means of monitoring and controlling screw thread fabrication in high production environments.

The tri-roll comparator system provides a fast and efficient means of definitive thread inspection. Also, variable data output makes this gage a perfect statistical process control device. Routinely, the tri-roll thread comparator is used to analyze and distinguish the following characteristics: Functional diameter size, Pitch diameter size, Lead and/or flank angle deviation, Taper, and 120-degree out-of-round (lobing).

The functional size of a screw thread is defined as that diameter which relates the diameter effects of variations in the above characteristics as they apply to assembly with its mating part. In other words, it is the result of the cumulative effect of variations in these characteristics along the full length of engagement. Any variation in these characteristics will always cause the functional diameter of an external thread to increase, and in an internal thread to decrease. This diameter is commonly referred to as the Maximum Material Condition of the product thread.

Pitch diameter size is defined as an imaginary cylinder, concentric and parallel to the thread axis, at which the thread ridge and thread groove are of equal width. Pitch diameter is the predominant factor used in the control of size and fit of product threads. This measurement isolates one thread, disregarding the cumulative effect of variations of functional size. Therefore, it is proper to refer to minimum pitch diameter as the Minimum Material Condition of the thread.

Using the pitch diameter rolls (best wire radius or cone and vee) set the comparator with the same master used for functional size inspection. Place the product thread in the gage locating two full threads from the front. Rotate the part 120 degrees. Record the smallest reading. Repeat the process at the center and back of the thread. Record the smallest reading as minimum pitch diameter size. Record the difference of the three readings in the same plane as taper. Record the rotational T.I.R. as out-of-round.

Lead: The axial distance a screw thread moves when rotated about its axis one complete rotation. On a basic single start thread, the lead is equal to the pitch (1/TPI).

Flank Angle: The angle created by the thread flank and a line perpendicular to the thread axis. This is often referred to as the half angle of the thread.

The term drunken lead is a broad reference to variations in helical path. This must be added to the axial lead variation to obtain total lead variation.

Understanding Differential Analysis: Lead/Flank Angle

Deviation in lead/flank angle is inspected by comparing functional size and pitch diameter size. Theoretically, identical readings would indicate perfect lead and form. In actual practice, functional size is almost always larger than pitch diameter size. If the difference between the two is excessive the lead/flank angle variations inherent in the thread can cause problems with assembly or compromise integrity of fit. The value at which these variations can be termed excessive is usually dependent upon application. Military specifications for safety critical threads limit the acceptable differential to 40% of the total pitch diameter tolerance. However, as a general rule, 50% is sufficient for most commercial manufacturing.

Example: .375” – 24 UNF -3A
Min. P.D. .3479
Max P.D. .3450
Tolerance: .0029 x .50 = .0015 Max. allowable differential, functional diameter to pitch diameter with limit set at 50% of total pitch diameter tolerance.

Important notes:

1. No single gaging method can guarantee that a product thread will assemble and conform dimensionally. USE A GO THREAD RING TO VERIFY ASSEMBLY!!!

2. Avoid the production of thread parts in the upper or lower 15% - 20% of the pitch diameter tolerance. Most disagreements between gages and problems in assembly occur in these zones.

3. Do not turn product parts through the tri-roll as you would a thread ring. This will cause excessive and unnecessary wear on the gaging rolls, as well as prohibit consistent readings. Always lift the lever arm and index the part unless inspection requires rotation of the part.

4. Set roll screws so that rolls rotate without lateral movement.

CHROME THREAD PLUG GAGES REDUCE REPLACEMENT COSTS AND SAVE MONEY

Thread Check Inc. provides chrome plating on all standard sized thread plug gages up to 1 ½ in diameter typically for about the same price that other manufacturers charge for tool steel. The chrome process brings the Rockwell hardness up to 70-72 Rc from tool steel which has a hardness of 60-62 Rc. The Chroming of thread plug gages extends the wear life by more than 100% which provide dramatic savings on replacement cost. Wear issues that lead to undersize and out of tolerance gages are one of the major root causes for thread problems with suppliers and their customers. Thread plug and ring gages can wear within a day’s worth of inspection and cause misleading results creating scrap and rejections.

The frequency of calibration for any gage or instrument should be based on usage. If a thread plug gage is being used to check 200 pcs per hour I am fairly certain the gage will be undersize after just one day or even sooner. I would suggest having multiple gages on hand and a master gage that is used to settle internal disputes between production and final inspection. The most common wear issue involves the go member which takes the most amount of friction.

Create “USAGE and WEAR” charts for gages that are being used in high volume inspection.

Keep Thread Gages clean and lubricated to reduce friction and increase wear life.

The most common wear issue for thread gages involves the go member which takes the most amount of friction.

Thread plug gages can be easily checked for wear using a 3-Wire Thread measuring System.

Thread Ring Gages can be checked using a matching thread setplug gage.

WHAT’S THE “J” IN UNJ SCREW THREADS?

UNJ, UNJC, UNJF and UNJEF threads are almost identical to UN, UNC, UNF, and UNEF threads except that the external J thread has a much larger root radius than the standard UN threads and inspection must be performed on this element. The Special UNJ threads are designated by the nomenclature UNJS. J screw threads feature a root radius which improves the tensile stress area of the fastener and helps to reduce the stress concentration factor in the thread thus making the thread stronger. Additionally, the requirement for high strength is achieved with 3A and 3B classes of fit. All pitch diameters and tolerances are based on the Unified Inch Standard. 

SAE AS8879 was introduced to provide an alternative to the inactive government specification Mil-S-8879 for screw threads – UNJ profile, inches.

Per SAE AS8879, UNJ Thread profiles are not interchangeable with Metric system MJ profiles or UN profile inch screws.  The internal UNJ thread will assemble with the standard UN external thread however assembling external UNJ threads with UN series internal threads should be avoided due to potential interference at the minor diameter.

External threads are of Unified form per ASME B1.1 and modified at the root so that the flanks of the adjacent threads are joined by one continuous, smooth and blended curve tangent to the flanks. The radius is specified in the applicable standard.

Internal threads are also of Unified form per ASME B.1. and modified  at the minor diameter to the values specified in the applicable standard.

Standard working Go/No Go thread plug gages are used to inspect internal J threads. The gage is typically marked without any modifications to the Plugs.

For external J threads, the Go member is modified to clear the minor diameter. A standard No Go thread ring gage may be used to inspect J threads. Both gages are typically marked accordingly.

Applicable Standards :

What is the difference between UNR vs. UN Screw Threads?

The UNR thread form and UN thread form are identical except that the roots of the external threads are rounded with a radius of curvature between 0.108 and 0.144 times the pitch. The rounded root improves fatigue strength compared to the UN thread form.

The UNR thread designation standard was developed during the 1950s due to the request for fatigue-resistant safety- critical fasteners. Almost all externally threaded fastener manufacturers produce UNR rolled threads rather than plain unified form threads.

ANSI/ASME B1.1 – 1989 para 2.3.1: “The rounded root (UNR thread) reduces the rate of threading tool crest wear and improves fatigue strength over that of a flat root (UN) thread.”

Internal Screw Thread

There is no UNR for internal threads. Use only UN Thread.
ANSI/ASME B1.1 para 1.3: “UNR applies only to external threads”
ANSI/ASME B1.1 para 2.3.2: (there is no internal UNR screw thread)”

External Screw Thread

Both UNR and UN are used for external threads.

External UN threads per ANSI/ASME B1.1

Para 1.3: UN threads have “a flat or optional rounded root contour” at the minor diameter.
Para 2.3.1: UN threads have “a flat root contour” at the minor diameter, but “a rounded root contour cleared beyond the 0.25P flat width of the basic profile is optional”…”to provide for some threading tool crest wear.”

External UNR threads per ANSI/ASME B1.1

Para 1.3: UNR have “a rounded root contour” at the minor diameter.
Para 2.3.1 (a): “The root contour…shall have a smooth, continuous, non-reversing contour with a radius of curvature not less than 0.108P at any point and shall blend tangentially into the flanks and any straight segment. At the maximum-material condition, the point of tangency shall be at a distance not less than 0.625H below the basic major diameter.”

External UNR threads per IFI, Basic Elements of Screw Thread Design

Page A-3; UNR threads have a “single difference from UN threads…a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. The minimum radius of 0.108P is the largest radius that can be fitted into the UN profile without violating the minimum material condition of the external thread. The maximum radius of 0.144P is the largest radius that can be accommodated without causing theoretical interference with an internal thread at its maximum material condition.”

Screw Thread Ring Gages

It is not necessary that the gages be marked with UNR. The UNR or UN ring gages can be used to check either the UNR or UN screw thread form. Since the amount of radius at the root of the UNR thread form is too small there is no requirement for any special adaptation of the standard UN thread for ring gages.

Screw Thread Work Plug Gages

Use the standard UN Thread Work Plug as there is no internal (female) UNR thread.

How to check the root radius

1. For low volume applications, use an optical comparator to check the UNR thread form root radius at the minor diameter of the external screw thread.
2. For high volume applications, it is faster and more economical to use Gaging System 22 ( Tri-Roll thread Comparator ) to monitor the minor diameter.

Thread Nomenclature

For questions or comments regarding this information, contact Thread Check Inc.

Calibration Procedures for Thread Ring Gages

1. Select the corresponding master set plug. Confirm all pitch diameters and class size match the ring gages. The set plug must be clean and calibrated to ensure it is not nicked, tapered, or out of tolerance. Lubricate the setting plug gage with a thin film of light viscosity oil.
2. Clean and inspect ring gages for nicks and embedded metal filings and burrs. Carefully remove the sealing wax with a knife.
3. Turn the ring gage locking screw counter-clockwise until it is loosened.
4. Turn the adjusting screw clock-wise which opens the ring to a larger pitch diameter than the setting plug.
5. Turn the ring gage onto the setting plug all the way to the back (Full form section) so that approximately one thread extends beyond the last thread of the setting plug. This will promote more uniform wear over the entire thread length of the plug.
6. Turn the adjusting screw counter-clockwise and rotate the ring on the setting plug until there is a slight drag between the two gages. The ring should have a noticeable amount of drag when rotated on the set plug. This procedure may have to be repeated more than once to obtain the proper amount of drag. Be patient! The degree of drag is subjective. Smaller ring gages and those set to set plugs near the low limit would require less drag than larger rings or rings set to setting plugs on the high limit.
7. To ensure that the ring has been properly seated, tap the ring with a small hammer and then recheck the amount of drag to ensure it has not changed. If the drag has changed, the ring gage has not been properly seated. Repeat step 6.
8. Turn the ring gage from the full form section to the truncated section at the front of the set plug. The drag should be essentially the same. The ring should not feel "shaky" or loose. A loose or "shaky" gage indicates lose of root relief or flank angles are worn out of tolerance and the gage should be removed for possible rework or replacement.
9. Remove the set plug from the ring. Now turn the ring onto the set plug 1 to 2 threads at the front. There should be some drag or resistance even at this short engagement. Remembering the feel at the 1 to 2 thread engagement, turn the ring further onto the truncated section. The drag should remain approximately the same although it may be slightly greater at full engagement due to more flank contact. Repeat step 9 on the other side of the ring gage. The drag should be essentially the same on both sides.
10. The minor diameter of the ring gage should be measured with either a bore gage, internal measuring machine, or fixed limit GO/NOGO plug gages. For plug gages, the GO member plug gage should GO and the NOGO member should not.
11. The locking and adjusting screws should be sealed with wax to prevent tampering.
12. The gage is now ready for service.

IMPORTANT NOTES

• The setting of a thread ring gage is specific to the particular set plug the ring is set to. The ring gage will have a different feel on another set plug without readjustment.
• It is recommended that a set plug be readily available in house to inspect gages being heavily used or for gages that have been dropped or impacted.
• For high volume inspection it is good practice to have a new backup gage for comparison and reference inspection against the heavily used gage.
• Go thread ring gages may want to be set slightly snugger than NOGO thread ring gages particularly when shipping components to customers for incoming inspection and assembly.
• Keep gages lubricated and handle with care for longer gage life. Spinning ring gages onto parts or forcing rings past burrs will reduce gage life.

White Paper on Resolving Measurement Disputes

Disputes over measurements can be costly for both parties to resolve and may hinder ongoing relations between suppliers and users of gages and instruments. Often it is simpler for both parties to agree to accept an average value of their readings as the final ‘size’ or the point at which their readings plus measurement uncertainties overlap.

The obvious way to avoid such problems is to agree beforehand on a method that will be used to resolve them if they arise. Often, the degree of separation between the readings dictates the best approach to take. Where the uncertainty of each party is significantly different, the party with the lowest uncertainty in the calibration would be considered more reliable.

The AMTMA offers the following methods as options you can choose from. If the Referee Method fails to bring a resolution, then the Universal Standard Method should be used due to the fact it is technically based and internationally accepted by metrologists in all disciplines.

The Referee Method

The two parties agree on a third party to provide a referee measurement that it is agreed will be considered as the actual value. An alternative on this is where the reading by either party that is closest to that provided by the referee is considered the accepted dispute.

Unless otherwise agreed to, the costs of using laboratories in this method are paid by the losing party.

The Unviersal Standard Method

National and international standards agencies have produced methods of resolving measurement disputes that focus on the uncertainty budgets of those that have produced measurements. The advantage of this method is that its technical base tends to remove personalities from the equation and may indicate that neither party to a dispute has the capability required to resolve it.

Using this method, the onus of proving a measurement falls on the party who has questioned the results of calibration. If requested, this party must provide a copy of their uncertainty budget for the measurement to the other party review. Budgets from both parties should be compared. Such a review should focus on seeking agreement between both parties respecting each element included in the budget since it will rarely, if ever, be all right or all wrong. The mathematics should take care of the rest. There may be cases where one or more elements have not been included in the budget included in the budget and when they are, the outcome changes significantly.

In the event one or more assumptions in the budget cannot be resolved, a third party can be asked to provide an opinion on them.

No Go Gaging Per ANSI/ASME B1.2-1983 An American National Standard- How many turns?

One of the most frequently asked questions that we receive so here is the answer pertaining to unified threads. Please check the applicable standard for other types of threads as the acceptance criteria does change.

NoGo ( Lo ) Thread Ring Gage

A NoGo or Lo thread ring gage inspects the NoGo or Lo functional diameter limit of product external thread. The NoGo thread ring gage, when properly set to its respective calibrated thread setting-plug, represents the NoGo (Lo) functional diameter limit of the product external thread. NoGo (Lo) thread ring gages must be set to the applicable truncated or Hi-Lo setting plugs.

NoGo (Lo) thread ring gages when applied to the product external thread may engage only the end threads (which may not be representative of the complete product thread).

Starting threads on NoGo (Lo) thread ring gages are subject to greater wear than the remaining threads. Such wear in combination with the incomplete threads at the end of the product thread may permit further entry in the gage.

As stated in ANSI/ASME B1.2-1983, the “NoGo (Lo) functional diameter is acceptable when the NoGo (Lo) thread ring gage applied to the product external thread does not pass over the thread more than three complete turns.” The gage should not be forced. Special consideration such as exceptionally thin or ductile material, small number of threads, etc., may require modification of accepting up to 3 turns of the gage. It is highly recommended that a manufacturer not allow a maximum of 3 turns during thread fabrication as every thread ring gage is set slightly different to due the gage maker tolerances of setplugs and the subjective nature of setting adjustable style thread ring gages.

No Go ( HI ) Thread Plug Gage

The NoGo (HI) thread plug gage inspects the NoGo (HI) functional diameter limit of product internal threads. The NoGo (HI) thread plug gage represents the NoGo (HI) functional diameter limit of the product internal thread.

Thread plug gages when applied to the product internal thread may engage only the end threads (which may not be representative of the complete thread). Entering threads on product are incomplete and permit gages to start. Starting threads on NoGo (HI) plugs are subject to greater wear than the remaining threads. As Stated in ANSI/ASME B1.2-1983, the “NoGo (Hi) functional diameter is acceptable when the NoGo (Hi) thread plug gage applied to the product internal thread does not pass over the thread more than three complete turns.” The gage should not be forced. Special consideration such as exceptionally thin or ductile material, small number of threads, etc., may require modification of accepting up to 3 turns of the gage. It is highly recommended that a manufacturer not allow a maximum of 3 turns during thread fabrication as every thread plug gage may gauge differently due the actual size and gage maker tolerances allowed.

Adjustable Thread Ring Gage Calibration: How Accurate Is It?

While a new AGD - American Gage Design adjustable thread ring gage is initially calibrated to a master setting plug gage, the setting can be slightly altered during the transport from manufacturer to end user. This can result from a variety of factors, ranging from a dropped adjustable ring gage to exposure to radical changes in temperature during the shipping process.The end result of these factors is that the locking assembly can shift, and if the adjustable ring gage is not shipped with a setting plug, the end user might never even know that the setting is no longer valid.

A gage arriving with an invalid calibration setting is not uncommon, and can be easily remedied if the setting plug is included or the end user has an existing setting plug. However, all too often, this is simply not the case. The absence of a set plug means that not only is there any way to check if the setting has shifted; it also means the end user will be unable to monitor the ring gage during its use or check after an accidental dropping or for intentional tampering. The end result may be a production run of bad parts.

Many manufacturers avoid the potential for liability by specifying that calibration settings only apply at the time and place of initial calibration. European manufacturers eliminate the potential problems associated with adjustable thread ring gages by using non-adjustable or solid style thread ring gages.

Additionally, calibration of AGD adjustable style thread ring gages by direct measurement is not authorized by ANSI B1.2 1983; Gages and Gaging for Unified Inch Screw Threads; Reaffirmed in 2007. Some calibration laboratories utilize the direct measurement method with ULMs or CMMs when a matching threaded master set plug gage is not available despite the above fact. This method fails to account for the helix offset alignment effect of adjustable style thread ring gages.

The only prescribed method of calibrating AGD – adjustable style thread ring gages is by means of a master thread setting plug gage. Per ANSI B1.2-1983; Reaffirmed 2007; page 40; paragraph 5.1.1: "Adjustable Go thread ring gages must be set to the applicable W tolerance setting plugs". Also reference table 5 page 17 for further evidence. Even more evidence can be found in Table 12 page 151. Also refer to Note 6 in the table as well.

When a thread master setting plug has not been purchased or is not available, the end user may opt to have thread ring gages calibrated by means of direct measurement so that they have a record for their files. This common practice by both accredited and non accredited labs may create more issues by improperly adjusting the thread ring gage out of tolerance and causing the end user to have a false sense of confidence in the gage based on a calibration cert.

Whenever an AGD – adjustable style thread ring gage is purchased it is always advisable to purchase or have access to the matching master thread setting plug gage. This practice will help to eliminate the possibility of using an out of tolerance thread ring gage. It will also reduce any costly downtime of having to send out a thread ring gage or order one in the event that the gage is dropped or mishandled.

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Measuring Thread Measuring Wires

When you are calibrating thread plugs, thread setting plugs and AGD style thread ring gages it is important to know the correct size and proper constant of the thread measuring wires, as they provide the foundation for accurate thread gage calibration.Working thread plugs and thread master setting plug gages are calibrated by means of the three-wire method. The degree of accuracy in the results of your readings depends on the accuracy of the thread measuring wires. An error of one unit in the mean diameter of thread measuring wires will have a multiplying effect of three units in the pitch diameter of 60° thread plug gages.