INSPECTION & METROLOGY: The Benefits of Using Tapered-Tooth Master Plugs (TMPs) with Spline Gauges

There is both a cost-effective and timesaving approach to evaluating the quality condition of a spline ring gage — the simple-to-use Tapered-Tooth Master Plug (TMP). This concept has been around for many years, but not everyone is aware of its benefits or how it works.


Eliminating unnecessary expenses is a very important factor in running a successful business. At first glance, it might seem unnecessary to purchase a gage that does not actually measure a component, but if you understand the true value in using tapered-tooth master plugs (TMPs) and how they assist the production process, then you can find that they are well worth the expense. TMPs allow for in-house control over the wear condition of the plant’s ring gages (as seen in Figure 1).

Figure 1: Tapered-tooth master plug (TMP) with go spline ring gage

First, the difference between actual and effective space width on an internal gage must be understood. The actual size is the circular space width on the pitch circle of any single space considering an infinitely thin increment of axial spline length while the effective size takes into consideration any variation in the index, lead, and profile. This variation must be taken into account to analyze the fit between a go ring gage to the manufactured components and a go ring gage to its TMP.

It is possible to measure every opposing pair of spaces on a ring gage with a measurement between pins to check for variation in actual circular space width. However, this measurement does not take into account the elemental variations of index error, lead error, or profile error of the ring gage.  Since all of these elemental features have a tolerance on a gage, they will reduce the size between pins on a ring gage. This difference is the space width calculated from the between pins measured size and the size of the component that the gage will accept.

The more effective variation you have in a go spline ring gage, the more the component manufacturer’s tolerance is being used up to account for those gage errors.

When having a full-form go spline ring gage made, the gage manufacturer will work to the effective sizes given in the standard for the component. The manufacturers then determine the tolerance for the dimension between pins for the gage. This comes from the component part tolerance band, the wear allowance of the gage, or possibly both. While manufacturing the gage, there will be some manufacturing process errors induced. Although minimal, these errors still consume some of the manufacturer’s part tolerance. It is difficult to measure these small errors on the gage and to know how they all sum together for an effective space width size. These errors, or “effective errors,” will usually mean the effective size of the space width of the ring gage will be smaller than the measurement between pins would suggest.

By using a TMP gage to control the ring gage, the manufacturer is provided the maximum tolerance. An external plug gage can be manufactured and measured more accurately than measuring the effective size of an internal gage. By adding a small taper to an external gage’s teeth, a TMP can be ground accurately to functionally evaluate the effective size of the ring gage.

This TMP is taken into the gage maker’s laboratory, and features are carefully checked to determine the tooth size required along the taper. Precise lines are then marked around the major diameter of the TMP to easily visualize the acceptable “new” and “wear” ranges for the ring.

Three lines are marked around the major diameter on the TMP — the first ring indicates the minimum ring gage size when new, the second indicates the maximum ring gage size when new, and the third indicates the size when the ring gage is worn out and must be replaced. (As seen in Figure 2)

Figure 2: Tapered-tooth master plug (TMP) with tolerance lines marked around major diameter

The TMP is a simple, yet effective, tool for checking the initial quality of ring gages and where the ring is at in its wear life. It provides fast and precise measurement of the ring gage and gives a clear indication of when the worn ring needs to be discarded before an oversized component is made. Another benefit of using a TMP is the ability to quickly check for damage to the ring gage on every space width, along every edge all at one time. Damage can occur to a gage from mishandling or from rough treatment on the shop floor.

The TMP has excellent longevity and can be re-inspected and certified to check for wear or distortion providing an in-house consistent quality standard for many years.

With your next spline ring gage order, you may want to purchase a TMP to test for yourself or purchase one for your existing ring gage. It may be surprising to learn that some ring gages believed to be worn will fall into an acceptable range and those still in use may not fit on the TMP.  This would indicate that they are too small to give the full manufacturing tolerance even if the ring gages were correctly made to specifications.

While it is tempting to purchase low-cost TMPs in an attempt to keep the initial cost down, it is important that the greatest care be taken to ensure a high quality TMP. Errors can prove to be costly as TMPs made without using high-stability quality material before the marking of measuring lines can lead to oversized components and interference at assembly. Investing in well-made TMPs from a trusted gage manufacturer is of utmost importance in order to ensure high quality ring gages.

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is the owner of Slone Gear International, Inc., in Dayton, Ohio — a developer and distributor of micro-measurement solutions for gears and splines. He graduated with a Bachelor of Science in Mechanical Engineering Technology from Purdue University, and he has 28 years of experience in gear production and gaging/measurement solutions; supplying spline gages and master gears; and developing analytical gear measurement systems. He also currently serves on the AGMA Spline Committee. For more information about the measurement of splines and gears, visit