Technical Information

Quick Navigation: Table of Contents

Section 1 - Understanding Thread Calibration Methods 

• Calibration Procedures for Thread Ring Gages | Quick Jump
• Adjustable Thread Ring Gage Calibration: How Accurate Is It? | Quick Jump
• Gages and Gaging for Unified Screw Threads Per ANSI/ASME B1.2-1983 (R2017) | Quick Jump
• Rules For Determining How Many Turns A Not Go ( Lo ) Ring Gage Can Enter A Part | Quick Jump
• Checking the Pitch Diameter of a Thread with a "Not Go" Gage | Quick Jump
• The Three Wire Method Of Measuring Pitch Diameter | Quick Jump
• White Paper on Resolving Measurement Disputes | Quick Jump
• Selecting Gage Blocks | Quick Jump
• Why Measurement Uncertainty is Better Than a Simple "Reading" | Quick Jump
• Dimensional Inspection of Parts Using Fixed Limit Gages | Quick Jump
• How to determine the proper gagemaker tolerance for a GO/Not Go cylindrical gaging application | Quick Jump
• Dimensional Inspection of Parts Using Go and Not Go Gages | Quick Jump
  

Section 2 - Understanding Threads

• Thread Systems | Quick Jump
• Terms Relating to Screw Threads and Gages | Quick Jump
• ANSI and International Thread Series Designations | Quick Jump
• What’s The "J" In UNJ Screw Threads? | Quick Jump
• What is the difference between UNR vs. UN Screw Threads? | Quick Jump
• Taper Pipe Thread Gages | Quick Jump
• Gages Required To Check Other Pipe Threads | Quick Jump
• NPT and NPTF Pipe Gage Information | Quick Jump
• British Thread Gages | Quick Jump
• Calculating Pre Plate Screw Threads & Gages | Quick Jump
• About Engineering Metrology Toolbox | Quick Jump

Section 3 - Thread Tolerances

• Basic Screw Thread Designations | Quick Jump
• Fixed Limit Gage Fact Sheet | Quick Jump
• Gage Tolerance | Quick Jump
• Standard Inch Pitch Diameters | Quick Jump
• Metric Pitch Diameters | Quick Jump
• Unified Miniature Screw Threads per ASME B1.10M 2004 | Quick Jump
• NPTF Class 2 Specifications | Quick Jump
• Thread Measuring Wire Formulas | Quick Jump

Section 4 - Thread Gage Best Practices

• Proper Care And Usage Of Gages | Quick Jump
  

ThreadGageCalibration.com calibration procedure sheet provides step by step detailed instruction for properly setting and calibrating thread ring gages by use of AGD standard truncated setting plug gages. The setting of thread ring gages is specific to the particular set plug that the ring is set to. It is common for ring gages to have a different feel on another setting plug of the identical size. 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 can be set slightly snugger than Not Go 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 gages past burrs will reduce gage life.

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.

A Not Go or Lo thread ring gage inspects the Not Go or Lo functional diameter limit of product external thread. The Not Go thread ring gage, when properly set to its respective calibrated thread setting plug, represents the Not Go (Lo) functional diameter limit of the product external thread. Not Go (Lo) thread ring gages must be set to the applicable truncated or Hi-Lo setting plugs.

A Not Go or (Lo) thread ring gage inspects the Not Go or (Lo) functional diameter limit of product external thread. The Not Go thread ring gage, when properly set to its respective calibrated thread setting plug, represents the Not Go (Lo) functional diameter limit of the product external thread. Not Go (Lo) thread ring gages must be set to the applicable truncated or Hi-Lo setting plugs.

The pitch diameter of a thread is the most important characteristic because it reflects the widest spectrum of possible defects in a threaded part or gage.

Reference Standards

A common issue of measurement problems is the use of standards and specifications that may be outdated. User of these standards should contact the reference sources to ensure they are working to the most current standard. The following list covers many popular gaging and measurement standards.

The accurate measurement of pitch diameter of a thread, which may be perfect as to form and lead, presents certain difficulties which result in some uncertainty as to its true value. The adoption of a standard uniform practice in making such measurements is, therefore, desirable to reduce such uncertainty of measurement to a minimum. The so-called "three - wire method" of measuring pitch diameter, as herein outlined, has been found to be the most generally satisfactory method when properly carried out, and is recommended for universal use in the direct measurement of thread plug and thread setting plug gages.

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.

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 required. Add wear block sets if necessary.

Measurement Uncertainty – This term refers to a measurement plus or minus a particular range that best represents the true value. This allows for the fact that a true, actual reading may not be entirely accurate, and so the range accounts for that possibility.

Choosing the proper inspection instrument is a critical quality control decision; the wrong device or method could produce inaccurate results and also affect the cost and performance of the parts that are inspected. The following will give you a better understanding of fixed limit gages.

The industry standard is referred to as the Ten Percent Rule. This common rule of practice requires that 10% of the product tolerance is divided between the GO and Not Go gauges. For plug gages a plus tolerance is applied to the GO member and a minus tolerance is applied to the Not Go member. Ring Gauges receive the reverse tolerance direction so that the GO member is minus and the Not Go member is plus.

Go and Not Go Gages are typically simple devices like Go and Not Go plug gages, Go and Not Go ring gages or Go and Not Go Snap Gages that contain no moving parts and are easy to use. As such, their use is very common. Go and Not Go gages are also easily portable, do not suffer breakdowns in service, and typically do not require power.

The two major screw thread systems used today are ISO metric and the Unified thread standard. The advantages of using coarse threads and fine threads are covered in this document. A metric tolerance class 6g ring and 6H plug is suitable for use where a Unified class 2A Ring and 2B thread plug was previously used on a thread for comparison basis.

Thread Plug Gage is a complete internal thread gage of either single or double-end type, comprising handle and threaded gaging member or members, with suitable locking means.

ThreadGageCalibration.com. calibrates and distributes the following ANSI Thread Series Designations and international standards. All Unified threads including – UN, UNC, UNF, UNEF, UNS, UNJ, UNJC, UNJF, UNJEF, UNR, UNRC, UNRF, and UNREF. NC5, NC5HF, NC5CSF, NC5ONF, NC5IF, M, MJ, MJS, UNM, NPT, NPTF, F-PTF, PTF-SAE Short, PTF-SPL Short,, PTF-SPL Extra Short, SPL-PTF, ANPT, NPSL, NPSM, NPSC, NPSF, NPSI, NH, NPSH, NFPA, NHR, NGT, NGO, NGS, SGT, R, Rc, Rp, G Series, BSW, BA, ACME –C, ACME –G, Stub ACME, BUTT, PUSH BUTT, AMO, AWWA, DIN, JIS, and ISO standards.

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.

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.

ThreadGageCalibration.com distributes a complete line of tapered pipe gages including special and custom designs. NPT general purpose pipe thread plugs and ring gages are manufactured per ANSI/ASME B1.20.1. Threads are within the permissible limits of the tolerance. Feel free to contact us with any questions you have about taper pipe thread gages.

ThreadGageCalibration.com offers a complete range of standard and custom pipe thread gages. Review the chart for gages required to check other pipe threads.

NPT gages and NPTF gage document provides a complete overview of general purpose and dry seal pipe threads. This document describes the differences between class 1 and class 2 NPTF threads. L1, L2, L3, and 6 step gages are described in detail. Learn more about npt gages, npt pipe gages and npt pipe thread.

British thread gages provided by ThreadGageCalibration.com. as well as plug & ring gages manufactured to the latest BS and ISO specifications. ThreadGageCalibration.com. supplies full form ISO 228 - 2 ( BS2779 ) Go/Not Go thread British plug and ring gages.

It is important to understand how various plating affects manufactured parts featuring internal and external threads. All too often component parts are manufactured without considering the correct plating requirements. This results in rejection in final and incoming inspection or even in the field resulting in a recall. Either way proves to be very costly and time consuming.

The NIST goal is to provide world-class engineering metrology resources to the U.S. manufacturers. This web site is one of an array of mechanisms they make available to our customers to help them achieve high-accuracy dimensional measurements traceable to national and international standards. Their goal is to assist you with improving your measurement capabilities.

External and internal thread diagrams illustrate basic thread geometry. This technical sheet illustrates proper thread nomenclature and terminology for screw threads.

The fixed limit gage document provides a gage maker tolerance chart for selecting gage tolerances as well as tolerance direction for various gages. The fixed limit gage fact sheet also explains how to properly determine gage tolerance for measurement applications. This fact sheet has much to teach about the fixed limit gage. Learn more about fixed limit gages.

ThreadGageCalibration.com gage tolerances and gage maker chart are listed in one convenient document for easy reference. This document provides tolerances for class X and class W tolerances of pitch diameter, major diameter, minor diameter, and lead for thread gages.

ThreadGageCalibration.com. offers basic, 2B, 3B, 2A, & 3A pitch diameter dimensions in chart form. Pitch diameters are the most important characteristic because it reflects the widest spectrum of possible defects in a thread. Pitch diameters can be accurately measured thru several approved methods.

ThreadGageCalibration.com. offers 6H and 6g metric pitch diameters for thread gages. Pitch diameters are the most important characteristic because it reflects the widest spectrum of possible defects in a thread. Pitch diameters can be accurately measured by use of the 3-wire method.

Dimensional product data and tables for Unified Miniature Screw Threads per ASME B1.10M 2004.

NPTF gages are manufactured per ANSI/ASME B1.20.5. Standard practice for gaging NPTF threads calls for using 2 or 3 gages in coordination with each other. The standard calls out class 1 and class 2 NPTF threads. Class 1 signifies that specific inspection.

The following approximate formulas for computing the pitch diameter, from the measurement over wires, should be used only for screws having lead angles from 0° to 5°. These formulas neglect the effect of lead angle and give results which show the screw to be larger than the true condition. The formulas are for any wire size which will fit in the thread.

Part dimensions to be gaged should be cleaned and burr free to prevent gaging interference. Grit and part chips which become lodged in thread gages will create scratches and wear on the flanks of threads and on the outside and inside diameters of cylindrical plain gages. Various materials such as aluminum and castings are extremely abrasive and will tend to wear out gages more quickly than other types of materials. Finer pitch and smaller diameter thread gages tend to wear quicker than larger and coarser pitch gages and have less gage tolerance as well. Regarding thread gages, it only takes a small amount of wear to have a significant effect on the pitch diameter. The wear on each flank angle is multiplied by almost 4 times to determine the total impact of wear on the pitch diameter. 50 microinches or 1 micron of wear per thread flank will impact the measured size by .0002” which can be the total tolerance of many thread gages.