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Many customers will soon see a change in how the gaging and metrology industry presents the  N.I.S.T. number on calibration certificates. NIST is the acronym for National Institute of Standards and Technology.

NIST, A2LA, and many auditors are REQUIRING that we no longer list the NIST test report number on calibration certificates. The NIST number would typically change depending on the type of master used for calibration as well as each time the calibration masters were calibrated at NIST.

Thread Check Inc. is substituting the NIST number with the statement “ ….traceable to SI units through NIST”. This change has already taken place with many of our certificates of compliance and accuracy forms. SI is defined as System International

Many customers will be asking about this missing NIST number on their certificates. We are referring them to A2LA document P102-A2LA policy on measurement Traceability available at  and the NIST policy on traceability located at

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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.



Our company was evaluated by ABS Quality Evaluations and accredited by ANAB.
Our entire staff is extremely proud of this achievement and is committed to consistently achieving customer satisfaction for product quality, on-time delivery, and customer service.




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.



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 :

ASME B1.1 Unified Inch Screw Threads ( UN and UNR Thread Form )
ASME B1.2 Gages and Gaging for Unified Inch Screw Threads
ASME B1.3 Screw Thread Gaging System for Dimensional Acceptability – Inch and Metric Screw Threads ( UN, UNR, UNJ, M and MJ
ASME B1.7 Nomenclature, Definitions, and Letter Symbols for Screw Threads
ASME B46.1 Surface Texture ( Surface Roughness, Waviness and Lay )
ASME B47.1 Gage Blanks

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

Designations Thread Series
UN Basic Unified National Thread Series
UNR Basic Unified National Thread Series with external thread controlled root radius
UNS Special Unified National Thread Series
UNRS Special Unified National Thread Series with external thread controlled root radius
UNC Special National Coarse Thread Series
UNRC Unified National Coarse Thread Series with external thread controlled root radius
UNF Unified National Fine Thread Series
UNRF Unified National Fine Thread Series with external thread controlled root radius
UNEF Unified National Extra Fine Thread Series
UNREF Unified National Extra Fine Thread Series with external thread controlled root radius

For questions or comments regarding the difference between UNR vs. UN Screw Threads, contact Thread Check Inc.


Ordering Cutting Taps and Trouble Shooting


Ordering Cutting Taps

It is highly recommended that a drawing with the thread information be supplied whenever possible. This will assist engineering staff in providing the right cutting taps for the job and avoiding long delays and costly problems in manufacturing.

Critical information required to supply the correct cutting taps are as follows:

  1. Nominal Size, TPI or Thread Per Inch, Thread Form, and if the thread is a multiple start.
  2. Class of thread to be manufactured – 2B or 3B.
  3. Right hand or Left hand thread. Right hand is always assumed if not specified.
  4. Material to be tapped.
  5. Nature of hole to be tapped.
    1. Drilled, punched, or cast.
    2. Tap drill or hole size.
    3. Depth of hole.
    4. Through hole or blind hole.
    5. Required length of full threads.
  6. Style of cutting taps required – Taper, plug, bottoming, gun tap, and spiral fluted tap.
  7. Our manufacturing engineer will determine this based on the information supplied.

Any additional information concerning the type and condition of the machine or equipment being used can be helpful in manufacturing the best design of cutting taps for the job.

Trouble Shooting Checklist For Tapping

  1. Using an incorrect style or design for the job.
  2. Using an incorrect tap limit size for the class of thread.
  3. Using dull cutting taps, cutting dies, and cutting tools that require re-sharpening.
  4. Using poorly re-sharpened cutting taps, cutting dies, and cutting tools.
  5. The part material is too hard or too soft.
  6. The part material is of poor quality and not uniform in structure or analysis.
  7. The part material is galled on the tap threads.
  8. Over packing of chips in the flutes.
  9. Tapping machine is too heavy or too light for job.
  10. Tapping machine lacking a quality lead screw.
  11. Speed range of tapping machine is too limited.
  12. Table of tapping machine or work piece not perpendicular to the tap.
  13. CNC machine not properly programmed.
  14. Adverse pre-tap hole conditions ( size, depth, straightness, roundness, glazed or work hardened surface, chips in the bottom ).
  15. Tap and prepared hole are not aligned.
  16. Work piece and fixture lifting on reversal when tapping vertically.
  17. Lack of proper lubrication on application.

Measuring Cutting Taps

The major diameter and pitch diameter are two critical measurements on cutting taps. It is important to monitor the wear on these dimensions as they will eventually wear with prolonged use. Back taper, thread relief, and built up major diameter are three factors in the design of cutting taps, cutting dies, and cutting tools that should be taken into consideration when making measurements on the major diameter and pitch diameter of cutting taps, cutting dies, and cutting tools.

Back taper is a gradual decrease in the thread diameter towards the shank. This is usually about .001" in the diameter per one inch of length. Thread relief gradually decreases the thread diameter towards the heel of the land and as a rule covers about two thirds of the land width although in certain cases it may cover the entire width. The function of the back taper and thread relief is to provide easier cutting action and reduce friction. Measurements for size should always be taken across the full threads immediately behind the chamfer and as near the cutting face of the land as possible. Cutting taps, cutting dies, and cutting tools are always made larger than basic on the major diameter to allow for wear, and to provide major diameter clearance in the tapped hole.

A basic micrometer can be used to measure the major diameter of cutting taps. The micrometer anvil should contact the teeth at the cutting taps and cutting dies face. The pitch diameter can be measured by use of the Three Wire Method Thread Measuring System.


Selecting Gage Blocks

Select gage blocks in accordance with the combination range required. If a large length is required, add a long block set.

Select gage blocks in accordance with the minimum length step required. Add wear block sets if necessary.

If a set containg a large number of gage blocks is selected, the number of combination gage blocks required for a length is reduced and the number of combinations is increased. The accuracy will be retained and damage will be reduced.

The specific gage block set for micrometer inspection and caliper inspection is available.

If using only one length repeatedly, it is a good idea to purchase discrete gage blocks.

The 2mm-based gage blocks, which take the base of the minimum length set as 2mm, are easy to handle and will not warp, as compared to the 1mm-based gage blocks.

Filed under: Calibration No Comments

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.


  • 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.

Thread Check Inc/TCI Industrial Supply Ltd passes ISO 9001 2008 Audit

We are pleased to announce that Thread Check Inc passed its stage 1 and stage 2 ISO 9001 2008 audit with zero non conformances and expects to receive its certification within the next month. I would like to congratulate all of our employees in our organization on their tremendous efforts.

As in the past our focus and commitment will be in consistently achieving customer satisfaction for product quality, on time delivery, and customer service. Thread Check Inc/TCI Industrial Supply will achieve this through continuously improving our business operations.