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 test, just use a Coordinate
Measuring Machine (CMM), but it is easier said than done.
Internal threaded hole centerlines are not the easiest product feature
to quantify. There are actually two centerlines directly related to
in an internal threaded hole. There is the centerline 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 centerline 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 ridge
is equal to the width of the thread valley. The centerline of the thread
pitch diameter cylinder is the actual functional centerline of the thread.
There may also be other centerlines present for counter-bore, relief,
bearing surfaces or other required diameters.
The internal thread pitch diameter cylinder centerline may be independent
of, and different from, all other hole centerlines, except the internal
thread major diameter cylinder centerline. 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; however, the internal thread major diameter is very difficult
The internal thread minor diameter cylinder centerline is much easier
to locate than the internal thread pitch diameter cylinder centerline,
but the centerline of the thread pitch diameter cylinder can be different
from the centerline 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 centerline is to locate the internal thread pitch
diameter cylinder centerline. This is done with the help of a Threaded
Hole Centerline Location Gage. The Threaded Hole Centerline Location 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 Centerline Location Gage that protrudes out of the threaded hole has a 'known cylinder'
which is the measurement reference.
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, ASME, or ISO specification;
so design, dimensions, tolerances and effectiveness may vary from
manufacturer to manufacturer.