Thread Gages Information
Last revised: February 3, 2025
Thread gages are dimensional instruments for measuring and/or verifying thread size, pitch or other parameters. A variety of thread gaging instruments and tools exist such as measuring wire, tri-roll comparators, thread plug gages, thread ring gages and thread micrometers. The appropriate variable or fixed limit gage for an application should be selected based on internal and external thread type, specific thread designation (UNS, UNF, UNC, NPT, ACME, Buttress), part tolerances and gaging frequency (shop vs. high volume production).
Types of Thread Gages
Thread gages can be one of any number of types of gages. These include plug, ring, 3-wire, micrometer, tri-roll comparator, measuring wire, screw thread insert (STI), and thread gaging roll thread gages. Thread plug gages measure go/no-go assessment of hole and slot dimensions or locations compared to specified tolerances. Thread ring gages measure go / no-go assessment compared to specified tolerances of the dimensions or attributes of pins, shafts, or threaded studs. 3-wire thread gages are gages that use thread wires to gage thread size with one wire mounted in one holder and two wires mounted in a second holder. The holders are placed in the measuring gage and brought in contact with the threads. Thread micrometers are micrometers for measuring thread. A tri-roll comparator is a specialized thread gage employing three thread rolls and a digital or dial display. The thread gaging rolls can be interchanged to measure different thread sizes. A measuring wire is a specialized wire manufacturing to precise gage sizes for measuring external threads. The wire is wrapped or placed in the thread cavity and then a measurement is made with a micrometer or other OD gage. STI gages, also referred to as helical coil or helicoil, are used where a screw thread insert will be used. STI gages are widely applied in the automotive industry. Thread gaging rolls are threaded rolls for use on roll thread comparators.
Thread Types, Profiles, and Geometries
Different thread types, profiles, and geometries provide different functionalities. Thread designations include UNC, UNF, UNEF, UN, M/MJ (metric), NPT, NPTF, NPSF, ANPT, BSPT, BSPP, ACME, and buttress. Thread gages measure the size or diameter of the feature being measured. English pitch is the threads per inch that the gage can measure. Metric pitch is the metric thread spacing that the gage can measure.
Thread verifiers, thread detection probes or thread verification systems detect or verify the presence of threads, threaded holes and/or specific thread pitches. Thread verifiers often use a high speed, non-contact technology to rapidly inspect parts. Eddy current thread verification probes are a common type of thread or thread pitch verifier.
Common shapes or geometries measured include cylindrical and tapered or pipe. A go gage provides a precision tool for production comparative gaging based on a fixed limit. Go gages consist of a fixed limit gage with a gaging limit based on the plus or minus tolerances of the inspected part. No-go or not-go gages provide a precision tool for production comparative gaging based on a fixed limit. No-go gages consist of a fixed limit gage with a gaging limit based on the minimum or maximum tolerances of the inspected part. Go / no-go gages configured with a go gage pin on one end and a no-go gage pin on opposite end of the handle. Go / no-go gages provide a precision tool for production comparative gaging based on fixed limits. Go / no-go gages manufactured in the form of stepped pins with the go gage surface and the no-go gage surface on the same side of the handle. The gage can save type in gaging since the gage does not have to be reversed for no go gaging. Master gage blocks, master or setting discs, and setting rings are types of master gages used to calibrate or set micrometers, comparators, or other gaging systems. Fixed limit or step gages are specialized thread plug gages for gaging taper pipe threads. Notches or external steps indicate maximum and minimum allowable tolerances. Tolerance classes for thread gages include Class XX, Class X, Class Y, Class Z, Class ZZ and thread Class W.
Measurement Units
Measurement units for thread gages can be either English or metric. Some gages are configured to measure both. The display on the gage can be non-graduated meaning that the gage has no display, dial or analog, digital display, column or bar graph display, remote display, direct reading scale, or Vernier scale.
Thread Gages FAQs
What is the three-wire method for measuring thread gages?
The three-wire method is a widely used technique for measuring the pitch diameter of external threads. Here are some key points about this method based on the information available:
Purpose: The three-wire method is employed to accurately measure the pitch diameter of a threaded component. This is crucial for ensuring that the threads will fit properly with their mating parts.
Procedure: In this method, three wires of known diameter are placed in the thread grooves. The wires are positioned symmetrically around the thread, and a micrometer is used to measure the distance over the wires. This measurement, along with the known diameter of the wires, is used to calculate the pitch diameter of the thread.
Standards and Calibration: The three-wire method is performed at the recommended force as per ANSI/ASME standards, specifically B1.2 for English threads and B1.16M for Metric threads. These standards provide guidelines on the calibration forces to be used during the measurement process.
How do plain ring or plug gages work for diameter checks?
Plain ring and plug gages are essential tools used for checking the diameter of components to ensure they meet specified tolerances. Here's how they work:
Plain Ring Gages
Purpose: Plain ring gages are used to check the external diameter of cylindrical parts. They ensure that the part's diameter falls within the specified tolerance range.
Operation: The part to be measured is inserted into the ring gage. If the part fits into the "go" ring gage, it means the diameter is within the acceptable lower limit. If it does not fit into the "no-go" ring gage, it means the diameter is within the acceptable upper limit. This provides a quick pass/fail assessment of the part's diameter.
Plain Plug Gages
Purpose: Plain plug gages are used to check the internal diameter of holes or slots. They ensure that the hole's diameter is within the specified tolerance range.
Operation: The plug gage is inserted into the hole. A "go" plug gage should fit into the hole, indicating that the diameter is not smaller than the minimum tolerance. A "no-go" plug gage should not fit, indicating that the diameter is not larger than the maximum tolerance. This method provides a straightforward way to verify the dimensional accuracy of holes.
Both types of gages are typically used in a go/no-go configuration, which allows for efficient and reliable inspection without the need for complex measurements or calculations. They are often used in manufacturing environments where quick and accurate checks are necessary to maintain quality control.
What are the common methods for measuring internal threads?
When it comes to measuring internal threads, there are a few common methods that are typically used:
Set Plugs: These are used to verify the dimensions of internal threads. Set plugs are inserted into the threaded hole to check if the internal thread meets the specified tolerances. They provide a go/no-go assessment, ensuring that the thread is neither too small nor too large.
Ball Method: This method involves using a ball of a specific size to measure the internal threads. The ball is placed in the thread, and the measurement is taken to determine the pitch diameter. The ANSI/ASME standards provide recommendations for the calibration forces to be used during this measurement process.
Thread Plug Gages: These are used for go/no-go assessments of internal threads. A "go" plug gage should fit into the hole, indicating that the diameter is not smaller than the minimum tolerance, while a "no-go" plug gage should not fit, indicating that the diameter is not larger than the maximum tolerance.
These methods are essential for ensuring that internal threads meet the required specifications and function correctly in their applications.
How do set plugs work for measuring internal threads?
Set plugs are used to measure internal threads by providing a go/no-go assessment to ensure that the internal thread dimensions meet specified tolerances. Here's how they work:
Purpose: Set plugs are designed to verify the dimensions of internal threads. They help ensure that the threaded hole is neither too small nor too large, which is crucial for proper fit and function.
Operation: A "go" set plug is inserted into the threaded hole. If it fits, this indicates that the internal thread is not smaller than the minimum tolerance.
A "no-go" set plug is also used. If it does not fit into the hole, this confirms that the internal thread is not larger than the maximum tolerance.
Application: This method provides a straightforward and efficient way to check the dimensional accuracy of internal threads without the need for complex measurements or calculations. It is commonly used in manufacturing environments for quality control purposes.
What are the differences between thread plug gages and set plugs?
Based on the information available, here are the differences between thread plug gages and set plugs:
Thread Plug Gages
Purpose: Thread plug gages are used for go/no-go assessments of internal threads. They help verify whether the internal thread dimensions fall within specified tolerances.
Operation: A "go" plug gage should fit into the hole, indicating that the diameter is not smaller than the minimum tolerance. Conversely, a "no-go" plug gage should not fit, indicating that the diameter is not larger than the maximum tolerance. This provides a straightforward pass/fail assessment of the thread's dimensional accuracy.
Set Plugs
Purpose: Set plugs are also used to verify the dimensions of internal threads, ensuring that the threaded hole is neither too small nor too large.
Operation: Similar to thread plug gages, set plugs provide a go/no-go assessment. A "go" set plug fits into the threaded hole, confirming that the internal thread is not smaller than the minimum tolerance, while a "no-go" set plug should not fit, confirming that the internal thread is not larger than the maximum tolerance.
While both tools serve a similar purpose in verifying internal thread dimensions, the specific design and application context might differ. Unfortunately, the documents do not provide more detailed distinctions between the two.
Thread Gages Media Gallery
References
GlobalSpec—Ring Gages Information
GlobalSpec—Air Gages Information