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Screw Thread Plug Gage

Screw Thread Plug Gage

There are several names for Screw Thread Plug Gage: Work Plug Gage; Working Plug Gage; Set Plug Gage; Setting Plug Gage; Master Plug Gage. I am sure that I have missed some names given to Plug Gages.

The generic definition of Plug Gage: A tool used to check an internal feature.

The additional descriptors tell us more about the specific function of the Plug Gage. Screw Thread tells us that the Plug Gage is checking an internal Screw Thread. Work or Working tells us that the Plug Gage is checking a Product Thread. Master or Set or Setting tells us that he Plug Gage is being used to calibrate a measurement tool.

The Screw Thread Work/Working Plug Gage can be of different styles depending on the product screw thread.

For a Parallel Screw Thread a GO/NOGO Screw Thread Work Plug Gage set is used. Simply, this is two Screw Thread Work Plug Gages, one a GO which checks the maximum material condition of the product thread, and one a NOGO which checks only one feature of the product screw thread (usually the pitch diameter). Fully checking the internal product screw thread would include a plain cylindrical GO/NOGO plug gage for testing the minor diameter of the screw thread. The GO for checking the maximum material condition of the minor diameter and the NOGO for checking the minimum material condition of the minor diameter. Sometimes an additional NOGO Major Diameter Check gage is also required, but this is rare.

For a Tapered Screw Thread the Screw Thread Work Plug Gage is also tapered. Typically there is a step on the large end of the Tapered Screw Thread Work Plug Gage which is used as the reference point to determine the thread size. Care must be taken when evaluating a tapered product screw thread with a Tapered Screw Thread Work Plug Gage to assure proper understanding of the relation of the notch or the large end of the plug gage with the thread requirements. Different gages made to different standards, and sometimes within the same standard they are interpreted in different ways.

The Screw Thread Master/Set/Setting Plug Gage will be different depending on the intended use of the gages.

For Setting of Working Ring Gages for the American Gage Design (AGD) Adjustable Style; the terms Set and Setting can be used interchangeably when describing the Screw Thread Plug Gage. The word Master is used sometimes in place of Set or Setting, but its usage is only for including the full category of master gages, which includes Set or Setting Plug Gages. An additional definition division defines three variations on the theme of Screw Plug Set Plug Gage: Truncated; Hi-Lo and Full Form. Please be advised that it is inappropriate to calibrate the American Gage Design (AGD) Adjustable Style Ring Gage by utilizing direct measurement.

Truncated Set Plug Gages are the most common style of Set Plug Gage. There is one Truncated Set Plug Gage member for the GO Ring Gage and a separate Truncated Set Plug Gage member for the NOGO Ring Gage. The length of the Truncated Set Plug Gage member is near twice the thickness of the ring gage. The first half of the Truncated Set Plug Gage has the gage major diameter truncated (shortened) to a point just over the gage pitch diameter. The second half of the Truncated Set Plug Gage is left as a Full Form section. The calibration, or setting, of a Screw Thread Ring Gage with a Truncated Set Plug Gage results in a virtual metal to metal fit between the Truncated Set Plug Gage and the Ring Gage.

Hi-Lo Set Plug Gages are not as common as Truncated Set Plug Gages. There is one Hi-Lo Set Plug Gage member for the GO Ring Gage and a separate Hi-Lo Set Plug Gage member for the NOGO Ring Gage. The length of the Truncated Set Plug Gage member is near twice the thickness of the ring gage.Hi-Lo Set Plug Gages can be used for setting the American Gage Design (AGD) Adjustable Style Ring Gage, but IMHO they are better for calibrating ring gages which are not yet to the point of requiring re-setting. This is because the Hi-Lo Set Plug gage member design has two portions. One portion, the first part of the Hi-Lo Set Plug Gage member made at the low end of the gage tolerance. The second portion of the Hi-Lo Set Plug Gage member is made at the high end of the gage tolerance. Thus the name: Hi-Lo. In theory, if the Ring Gage is set within tolerance limits, the low part of the Hi-Lo Set Plug Gage member will screw into the Ring Gage, but the high end will not enter the Ring Gage more than 1 to 3 turns.

Full-Form Set Plug Gages are rarely used for setting of Working Ring Gages because the other two styles are preferred. However; in some cases Truncated Set Plug Gages and/or Hi-Lo Set Plug Gages are inappropriate, and a Full Form Set Plug Gage is the only logical way to set the Ring Gage.

For Checking of Solid Ring Gages different terminology is employed.

GO Check Plug Gage is used to assure that the maximum material condition of a Solid Style Ring Gage is not violated. This is a tool which may be used by a Solid Style Ring Gage maker or by the end user. In some cases, while in use a Solid Style Ring Gage may have the measurement surfaces become smaller in diameter. This occurs by galling and may only be noticed if a GO Check Plug Gage is used at the time of Solid Style Ring Gage calibration.

NOGO Check Plug Gage is used to assure that the minimum material condition of a Solid Style Ring Gage has not been violated. This is a tool which should be used by a screw thread ring gage maker.

NOGO Wear Check Plug Gage is used to assure that the minimum material condition plus the wear allowance of a Solid Style Ring Gage has not been violated. This is a tool which should be used by a screw thread ring gage end user or appropriate calibration laboratory.

For Setting of Adjustable Thread Comparators the standards recommend the use of a Class-W Full-Form Master Plug Gage, but with some intelligence any of the above Master Plug Gages are acceptable.

COMMENTS:
Original Posted: 11/14/2011
Last Revision:
If you wish to ask a question or expand on an existing question; email Wayne at: gageguy@gagecrib.com.

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