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Threaded Hole Location Gages

Threaded Hole Location Gages

GENERAL CONCEPT
All product features are designed with tolerances which are set to provide optimum functionality of the end product. A threaded hole may have to be concentric to another feature. A threaded hole may need to be perpendicular to another feature. A threaded hole may need to be positioned in relation to other holes or part features. This sounds like it would be a simple thing to do, just use a Coordinate Measuring Machine (CMM), but it is easier said than done.

Internal threaded hole centers are not the easiest product feature to quantify. There are actually two centers directly related to in an internal threaded hole. There is the center of the internal thread pitch diameter cylinder and the center of the internal thread minor diameter cylinder. The main feature that establishes the threaded hole center is the thread pitch diameter cylinder. The thread pitch diameter cylinder is a theoretical cylindrical plane that passes through the thread at the point where the width of the thread tooth is equal to the width of the thread valley. The center of the thread pitch diameter cylinder is the actual functional center of the thread. There may also be other centers present for counter-bore, relief, bearing surfaces or other required diameters.

The internal thread pitch diameter cylinder center may be independent of, and different from, all other hole centers, except the internal thread major diameter cylinder center. The internal thread major diameter cylinder will always be concentric with the thread pitch diameter cylinder because they are both cut with the same tool at the same time, but the internal thread major diameter is very difficult to measure.

The internal thread minor diameter cylinder center is much easier to locate than the internal thread pitch diameter cylinder center, but the center of the thread pitch diameter cylinder can be different from the center of the internal thread minor diameter cylinder in any aspect. This is because the internal thread minor diameter may have been manufactured with a separate cutting tool and even in a separate piece set-up. Typically, before the thread is cut, the internal thread minor diameter is machined along with other diameters located on the same center. Then the threading tool is inserted to cut the thread form. If the threading tool is a single point cutter or a threaded insert being machined in the same set-up as the minor diameter, the chance of the thread pitch diameter cylinder being concentric with the other diameters is high. If the thread was cut with a tap, a thread mill, or in a separate set-up the thread pitch diameter cylinder may be different from the other diameters.

Regardless of how the thread was manufactured, the best way to locate the thread center is to locate the internal thread pitch diameter cylinder center. This is done with the help of a Threaded Hole Location Gage. The Threaded Hole locator Gage is screwed into the threaded hole and then the hole center is located with a CMM, or some other measurement tool. The end of the Threaded Hole Location Gage that protrudes out of the threaded hole has a 'known cylinder' is the measurement reference and a knurl to ease insertion and removal.

A special note of caution here. All of these Threaded Hole Centerline Location Gages should be used with caution, regardless of the manufacturer's claims. None of these tools are defined in any ANSI specification so design, dimensions, tolerances and effectiveness may vary from manufacturer to manufacturer.

 

Types of Threaded Hole Locator Gages

FLEXIBLE THREADED HOLE CENTERLINE LOCATION GAGE

SOLID THREADED HOLE CENTERLINE LOCATION GAGE

TAPERED THREADED HOLE CENTERLINE LOCATION GAGE

LEAD LOCK THREADED HOLE CENTERLINE LOCATION GAGE

VARIABLE THREADED HOLE CENTERLINE LOCATION GAGE

Call or e-mail us for a quote on anyone of the above styles.

COMMENTS:
Initial posting: 8/5/2008
Last Revised: 12/10/2014
Refer comments/corrections to gageguy@gagecrib.com.

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6701 Old 28th St SE, Suite B
Grand Rapids, MI 49546-6937
Phone: 001-616-954-6581 • Fax: 001-616-954-6583
Email: gageguy@gagecrib.com

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