Frequently Asked Questions

Definitions from ASME B1 and Fed-Standard H28.
Disclaimer: These are for guidance purposes only and always consult with the Industry Standards or customer contracts for verification of requirements.

Specifications

What is ASME B1.3?

ASME B1.3 is a standard that presents screw thread gaging systems suitable for determining the acceptability of UN, UNR, UNJ, M and MJ screw threads on externally and internally threaded products.

What is a UN Thread?

The UN Thread form is the basic of all thread forms.  The basic angle measured in an axial plane is 60 degrees, the crest of the basic thread is flat, the roots clearances are allowed to provide round root forms when specified, clearance at minor diameter of the internal thread is provided.  Refer to ASME B1.1 for specific details.

What is a UNC Thread?

A UNC series thread is a Coarse Thread Series. Generally used for production of general fastening hardware. General applications include threading into lower tensile strength materials to obtain resistance to stripping of the internal thread. Refer to ASME B1.1 for specific details.

What is a UNF Thread?

A UNF series thread is a Fine Thread Series. Generally used for production of fastening hardware where the UNC is not appropriate. External threads have a greater tensile stress area that comparable UNC series. Refer to ASME B1.1 for specific details.

What is a UNR Thread?

The UNR applies only to an external thread and is similar to the UN thread with the exception of a rounded root contour for the UNR external thread. Refer to ASME B1.1 for specific details.

What is a UNJ Thread?

The UNJ is a screw thread form with a controlled root radius of the external thread. The minor diameter of both the external thread and internal thread are designed at a basic height of .5625H to accommodate the external thread maximum root radius.  This basic thread form, dimensions and acceptability are outlined in SAE AS 8879 Aerospace Standard. Refer to SAE AS 8879 for specific details.

What is AS8879?

SAE AS8879 is the Aerospace Standard that provides guidance for the design and acceptability requirements for the UNJ Thread profile.

What is a Buttress thread?

A standard Buttress Thread is a thread that has a designed angle of 7 degree / 45 degree with 0.6p basic height engagement.  The Buttress thread is utilized for one directional high stress applications, for thrust purposes, often times in tubular style assemblies.  For specifics refer to ASME 1.9.

Thread Size and Form

What is Functional Diameter?

Also known as Screw Thread Maximum Material and GO Size. The functional diameter of a screw thread is the pitch diameter of an enveloping thread with perfect pitch, lead, and flank angles and having a specified length of engagement.  This diameter includes the cumulative effect of variations in lead (pitch), flank angle, straightness, taper and roundness.  Crest and Root are excluded.  Often referred to as Assembly size it is one requirement for dimensional acceptance.

What is Pitch Diameter Size?

The pitch diameter size of a screw thread on a straight thread is the diameter of the pitch cylinder.  The pitch cylinder is an imaginary cylinder that passes through the thread profile of a straight thread in such a manner as to make the widths of the thread ridge and the thread groove equal. Pitch Diameter is an essential measurement for machine set up, process control and dimensional acceptance.

What is the Thread Differential?

Thread differential is the quantitative measure of the entire effect of thread form and profile error on a screw thread.  Easily achieved with indicating gaging the result can isolate thread form error and be the basics of part analysis or a process control strategy.

Inspection Systems

What is Method A Thread Inspection?

Method A Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also the inspection/evaluation of characteristics identified as NOT GO functional diameter or as HI (external) or LO (internal) functional diameters.  Often utilized for parts with no specific mechanical strength requirements. Refer to Specification for specific details.

What is Method B Thread Inspection?

Method B Thread Inspection Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also inspection/evaluation of the minimum material size thread pitch diameter or thread groove diameter over the length of the full thread.  The cumulative effects of all other thread characteristics such as lead, flank angle, taper, and roundness variations are confined within product tolerance limits with no specific inspection/evaluation of their magnitudes.  Often the minimum acceptability requirement for industry System 22 is used for parts that need to maintain mechanical strength requirements. Refer to Specification for specific details.

What is Method C Thread Inspection?

Method C Thread Inspection Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also inspection/evaluation of the minimum material size thread pitch diameter or thread groove diameter over the length of the full thread.  The magnitude of other thread characteristics such as lead, flank angle, taper, and roundness are further inspected/evaluated for System 23 compliance.  System 23 is most often used for critical fastening application. Refer to Specification for specific details.

What is Other Thread Inspection?

Other Thread Inspection Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also inspection/evaluation of the minimum material size thread pitch diameter or thread groove diameter over the length of the full thread.  The cumulative effects of all other thread characteristics such as lead, flank angle, taper, and roundness variations are confined within product tolerance limits with no specific inspection/evaluation of their magnitudes.  Often the minimum acceptability requirement for industry System 22 is used for parts that need to maintain mechanical strength requirements. Refer to Specification for specific details.

What is Safety Critical Thread Inspection?

Safety Critical Thread Inspection Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also inspection/evaluation of the minimum material size thread pitch diameter or thread groove diameter over the length of the full thread.  The magnitude of other thread characteristics such as lead, flank angle, taper, and roundness are further inspected/evaluated for System 23 compliance.  System 23 is most often used for critical fastening application. Refer to Specification for specific details.

What is System 21?

System 21 Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also the inspection/evaluation of characteristics identified as NOT GO functional diameter or as HI (external) or LO (internal) functional diameters.  Often utilized for parts with no specific mechanical strength requirements. Refer to Specification for specific details.

What is System 22?

System 22 Thread Inspection Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also inspection/evaluation of the minimum material size thread pitch diameter or thread groove diameter over the length of the full thread.  The cumulative effects of all other thread characteristics such as lead, flank angle, taper, and roundness variations are confined within product tolerance limits with no specific inspection/evaluation of their magnitudes.  Often the minimum acceptability requirement for industry System 22 is used for parts that need to maintain mechanical strength requirements. Refer to Specification for specific details.

What is System 23?

System 23 Thread Inspection Thread Inspection provides for interchangeable assembly with functional size/limit inspection/evaluation at the maximum material limits within the standard length of gaging elements, and also inspection/evaluation of the minimum material size thread pitch diameter or thread groove diameter over the length of the full thread.  The magnitude of other thread characteristics such as lead, flank angle, taper, and roundness are further inspected/evaluated for System 23 compliance.  System 23 is most often used for critical fastening application. Refer to Specification for specific details.

What is Category 1 Thread Inspection?

Category 1 Thread is a design and conformance category for UNJ Thread as outlined in SAE AS8879.  Category 1 threads shall be within the limits of size specified in AS8879.  Variations of Category 1 threads; Lead, flank angle, circularity, taper and runout are reference only dimensions. Category threads are used for parts that need to maintain mechanical strength requirements. Refer to Specification for specific details.

What is Category 2 Thread Inspection?

Category 2 Thread is a design and conformance category for UNJ Thread as outlined in SAE AS8879.  Category 2 threads shall be within the limits of size specified in AS8879. Also the combined variations of Category 2 threads; Lead, flank angle, circularity, taper and runout are satisfactory with the difference between functional diameter and pitch diameter is less than or equal to 40% of the pitch diameter tolerance.  If the difference is greater than 40% additional requirements apply. Category 2 threads are most often used for critical fastening application. Refer to Specification for specific details.

Thread Gages and Measuring Equipment

What is a Johnson Gage?

Johnson Gage designs and manufactures screw thread inspection systems.  Typically, a Johnson Gage refers to an indicating thread gage that inspects features related to a screw thread.  Inspection systems can be provided for External or Internal thread, most all thread forms and most thread characteristics.

What is an Indicating thread gage?

An indicating thread gage is a product screw thread inspection system that measures a thread characteristic and provides data about the size and or quality of the thread form.

What is a Thread Comparator?

A Thread Comparator refers to the inspection system that uses a Certified Master.  That certified Master is used to set the thread gage then the gage is used to inspect the part.  The operator will compare the reading from the set value to the measurement of the part.

How do you Measure an External thread?

External Thread inspection systems are available in either a fixed limit type gage or an indicating/data results type system.  All specifies related to thread inspection systems and thread characteristics are outlined in the ASME B1.3 Standard.

How do you Measure an Internal thread?

Internal Thread inspection systems are available in either a fixed limit type gage or an indicating/data results type system.  All specifies related to thread inspection systems and thread characteristics are outlined in the ASME B1.3 Standard.

How do you measure Functional Diameter on an External thread?

Functional Diameter, at times referred to as Assembly Size, Maximum Material or GO Functional limit/size can be inspected using a GO Ring Gage or Measured using an “Indicating Ring Gage” called a GO Functional Segment or with Functional GO Rolls.  ASME B1.3 outlines the gage systems and ASME B1.2 outlines the design profiles for the gage profile.

How do you measure Functional Diameter on an Internal thread?

Functional Diameter, at times referred to as Assembly Size, Maximum Material or GO Functional limit/size can be inspected using a GO Plug Gage or Measured using an “Indicating Plug Gage” called a GO Functional Segment or with Functional GO Rolls.  ASME B1.3 outlines the gage systems and ASME B1.2 outlines the design profiles for the gage profile.

How do you measure Pitch Diameter on an External thread?

Pitch diameter inspection on external product screw threads is typically measured using a Minimum Material pitch diameter type cone and vee gage contact.  NOT GO Ring Gages do not inspect Minimum Material / Pitch diameter.  For a complete list of approved gages used to inspect Pitch Diameter refer to ASME B1.3.

How do you measure Pitch Diameter on an Internal thread?

Pitch diameter inspection on internal product screw threads is typically measured using a Minimum Material pitch diameter type cone and vee gage contact.  NOT GO Plug Gages do not inspect Minimum Material / Pitch diameter.  For a complete list of approved gages used to inspect Pitch Diameter refer to ASME B1.3.

What is a Tri Roll Thread Gage?

A tri-roll gage in the Screw Thread Industry is a thread inspection system consisting of a 3 point / 120 degree gage element contact.  Most often used to inspect thread Functional Diameter, Pitch Diameter, Minor Diameter or Major Diameter.

What are Multi Rolls?

A Multi Roll technically called GO Rolls refers to a gage element contact and profile for the inspection of product screw thread “GO” Maximum Material Condition as outlined in ASME B1.2. 

What is a Full Form Functional Segment?

A Functional Segment technically called GO Segments refers to a gage element contact and profile for the inspection of product screw thread “GO” Maximum Material Condition as outlined in ASME B1.2. 

What is a Single Element Roll?

A Single Element Roll technically called either a cone and vee or thread groove diameter type refers to a gage element contact and profile for the inspection of product screw thread Minimum Material Condition as outlined in ASME B1.2.