To confirm the accuracy of a tapped hole, always use a Go-No Go gage. Never use a fastener, like a screw or bolt.

Using a screw or bolt does not enable you to verify thread size. Instead it only indicates the bolt used will fit in that particular hole.

Go-No Go gages can indicate the thread produced is within the thread specification. A screw or bolt cannot provide this information.

Accurate gages help predict remaining tool life by indicating tightness or looseness of the gage in the part.

Technical Tip #5 - Gaging Threaded Holes

• Clean the part before gaging. Get rid of dirt and chips so the gage is checking product size that is not affected by debris.
• Keep the gage clean and lightly oiled, both in use and storage. Light oil will greatly increase wear life. It also helps to prevent "loading" or "smearing" of product material onto gage threads. There can, of course, be some circumstances or materials where oil is not recommended.
• Align and start threads carefully. Cross-threading can damage end threads of both product and gage.
• Don't force the gage. Use reasonable light pressure only. Use common sense to prevent damage or wear on the gage and to correctly evaluate acceptance or rejection of parts.
• Protect gages from damage, rust, nicks, or jams that could ruin the gage and allow inaccurate results. Furnish some kind of protective box at the workbench. Seal/peel plastic dip on gage threads is recommended when storing the gage.
• Use thread ring gages equally from both faces. Many times ring gages, submitted for reinspection or reconditioning, are found to be worn/tapered only on the stamped side. The ring is designed for use on either face. Alternating gage face use will prolong the wear life.
• Gages must be checked/recalibrated periodically to ensure they are still within proper tolerances and in suitable condition for continued usage.

Technical Tip #56 - Care of Gages

Go-No Go plain cylindrical gages are used to ensure plain holes are within tolerance. If the Go member is long enough to check the entire depth of the hole, it will be used to check hole straightness from top to bottom.

Special design "locating" gages can be used to check the position of holes. Plain gages are commonly used to check the minor diameter of tapped holes produced by a drill prior to tapping.

Plain cylindrical gages are available in four classes: "XX", "X", "Y", and "Z". "XX" gages are made to the tightest tolerance and used for master or setting gages, or for part tolerances that are very precise. "X" tolerance gages are sometimes used for master gages or gaging close work. "Y" and "Z" gages are generally used for inspecting workpieces only.

Selecting the right class of cylindrical gage is very important to ensure that "bad" parts are rejected, and that "good" parts are not rejected. If a lower class gage is used on a tight tolerance part, the gage tolerance will consume most of the part tolerance and reject the part as "bad". In this case, a "good" part might be rejected. Gage manufacturers call this "thievery" when gage tolerance "steals" part tolerance.

To ensure acceptance of the maximum number of good parts, select a gage class using the "5% rule". That is, the gage tolerance should be no more than 5% of the part tolerance.

For example, if the part tolerance is 0.001", the gage tolerance should be no more than 0.00005". Select a class that has a 0.00005" tolerance or less.

See the Kennametal Gages catalog in the Metalworking/Tapping section of the www.kennametal.com website for gage standards charts for classes and tolerances.

Technical Tip #68 - Plain Cylindrical Pin Gages

Gaging NPT pipe threads is a single-step process, compared to the multiple steps involved with gaging NPTF threads.

Internal NPT Gaging Process: Using an L-1 NPT plug gage, rotate the gage into the threaded hole until it stops. This is referred to as "hand tight." If the notch on the gage stops flush with the top of the threaded opening, the part is "basic size." The threaded opening is allowed +/- 1 full rotation to still be considered basic size. Note that minus 1 turn above the notch is the minimum size allowable and 1 full turn below the notch is the maximum size allowable.

External NPT Gaging Process: Using an L-1 NPT ring gage (which is the total thickness of "hand tight"), rotate the gage onto the threaded section until it stops. If the small end is flush to the end of the threaded section, it is considered to be "basic size." The gage may stop up to 1 turn short of the thread and be considered maximum basic size. The gage may stop 1 turn beyond the end and still be considered minimum size.

Remember that NPT-class threads require sealant or tape wrap to prevent leakage.

Technical Tip #113 - Gaging NPT Internal & External Threads

NPTF (National Pipe Taper Fuel) or dry seal threads are for more critical applications than NPT.

Therefore, a greater variety of gages are required to check size, taper, and actual thread form.

The crests of the external thread are "sharper" or more pointed than the roots of the internal threads, which are flat.

When hand tightened, these crests interfere, but the flanks remain separated. When wrench tightened, all these areas are crushed together and form a leak-proof seal.

Technical Tip #114 - Gaging NPTF Threads