Metrology
Metrology
The Ins and Outs of Fixed Body Mechanical Plug Gages
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The Ins and Outs of Fixed Body Mechanical Plug Gages
George Schuetz, Mahr Federal Inc.

 

Fixed body mechanical plug gages are one of the best ways to measure hole diameters. With this type of gage there is no rocking, as with an adjustable bore gage, and therefore it is fast and provides high performance for tight tolerance holes. Because these gages are so easy to apply, they are usually used to measure a high volume of parts. Even in cases where the volume of parts is not high, if the parts are expensive and the tolerance tight, a fixed body plug may still be an economical choice for the application. There simply may be no other way of measuring the critical part tolerance required.

With a fixed plug type bore gage, the body size of the gage is very close to the diameter it is made to measure. Therefore, the plug "locks" the gaging part of the plug in the hole. This requires very little operator skill and results in virtually no operator influence. Obviously, there has to be some body clearance between the plug body and the hole diameter just so the plug will fit into the hole being measured, and control of this clearance is critical for the gage builder. Limited clearance will provide shorter measuring range but better performance. More clearance increases measuring range but allows for more "centralizing error" (actually measuring the chord rather then the diameter).

In fact, the plug body acts as a way of holding and centralizing an internal mechanical transfer mechanism. Typically this consists of a set of contacts, mounted to some linear spring arrangement that is in contact with a 90 degree transfer rod (see Fig 1.). The transfer rod has a precisely ground vee in it that takes the diameter change seen at the contacts and transfers it to an indicator out the rear of the plug. The ground-in vee is made at such an angle that there will be a one-to-one 90 degree transfer, and no compensation is needed by the indicating device.

The rear of the vee rod will either have a spherical or flat end. It's important to note the style of the end, as the indicating device monitoring the rod needs to be the opposite type of contact. For example, a flat ended vee rod should be used with an indicator having a spherical contact, and vice-versa. This assures a point to point motion transfer.

The transfer mechanism is also a two-point floating differential system. This means that once in the hole, the plug may be moved (albeit slightly) side to side in the direction of the contacts. When this is done, one contact will move out a little while the other one moves in. Thus, they cancel each other out and the result is always the diameter of the hole, regardless of plug position. This configuration is very important for exploring the bores for various geometric conditions, including ovality, barrel, taper or hourglass shapes.

Another issue to consider when applying the plug gage is the location of the measurement (or the contacts). Exploring the part for barrel, taper or hourglass conditions usually requires the plug to be moved axially throughout the hole. This is generally done with a through-hole style plug where the contacts are set at a certain distance back from the front of the plug and allow the plug to pass freely through the hole being measured. Not all holes are through holes, however. Blind holes often need their diameters measured very close to the bottom. In this case a "blind hole" plug is used in which the contacts are placed closer to the end of the plug body. Various configurations of contact position to the end are available, and they can be made to be within 1mm (0.040") from the end if required.

In most cases tungsten is used for the contacts to provide best wear characteristics for longest gage life. However, if the part material is not best served by this type of contact, other materials such as tool steel, ruby or diamond may be required, and are readily available.


Finally, the most critical thing to remember about the fixed bore gage is that it is a dedicated instrument. Specifying the requirements for the application is really no different than specifying for an adjustable core gage, snap gage or any other measuring device: you need to know the size, tolerance, and where the measurement is to be made. However, unlike these gages, the fixed bore plug is custom made for the application. Get any one of the parameters wrong and the gage becomes a very nice looking desk ornament. Always double check the application when specifying to make sure you get the right custom gage.

Figure 1  
 

Figure 1. The plug body acts as a way of holding and centralizing an internal mechanical transfer mechanism which typically consists of a set of contacts, mounted to some linear spring arrangement that is in contact with a 90 degree transfer rod. The transfer rod has a precisely ground vee in it that takes the diameter change seen at the contacts and transfers it to an indicator out the rear of the plug.

 

With a fixed plug type bore gage, the body size of the gage is very close to the diameter it is made to measure. Therefore, the plug "locks" the gaging part of the plug in the hole.