First things first, what is the root of a tap?

The root of a tap is the surface at the bottom of the thread-form that connects adjacent thread flanks and is expressed as width or as a diameter. The term root diameter is also called minor diameter, it’s one of those things, you say rain and I say precipitation, meaning the same thing.

The root of a tap, unless designed as a controlled root tap, normally does nothing, it is a nonfunctional surface. Its diameter is smaller than the tap drill size providing a clearance or space between them. It is a diameter located between the sharp vee thread and the minimum minor diameter on the part, typically located at the diameter where a root width is equal to P/8 (1/8 the width of the pitch).

The root diameter of a controlled root tap is held to a specific diameter with a tolerance of +/- .001 or .001 if necessary, based on the application.

The most common use of a controlled root tap is to manage what is called Spin Down.

When the material to be tapped is soft and ductile such as aluminum or brass, the tap may not cut or shear as efficiently as desired. This results in some displacement of material on the corners located at the intersection of the crest or minor diameter and thread flanks of the tapped hole.

This Spin Down can interfere with the assembly of the internal and external threaded parts. This Spin Down also can break free creating a fine hair-like burr that is very difficult to remove during the cleaning operation. In this situation, a controlled root tap with a minor diameter held close to the minimum tap drill size will minimize or eliminate Spin Down and the hair-like burrs. It also may be necessary to control the hole size prior to tapping to a closer tolerance than normal.

Another use for a controlled root tap, although not very common, is when the application requires that the part minor diameter be concentric with the part pitch and major diameters. During a typical tapping operation, the tap drill hole that creates the part minor diameter is produced separately and prior to the tapping operation that creates the part pitch and major diameters. Unless the tapping operation is fixtured exactly like the tap drill operation there will be some minor diameter to pitch diameter eccentricity. A controlled root tap designed to cut the minor diameter will do so at the same time the pitch and major diameters are produced resulting in the minor being concentric to the pitch and major diameters.

There is a current standard tap type/style that has a controlled root as part of its design, it’s a taper pipe tap, NPT and NPTF. The tap is not only responsible for producing the pitch and major diameters like a standard tap, but it must produce the root flat with as well. This is accomplished with a controlled root on the tap.

Our engineers have proven tap designs depending on the details of your application, ex. material to be tapped, size of thread and the number of threads per inch. The controlled tap design may include thread relief and or require the chamfer point diameter to be adjusted based on the controlled root diameter.

If you think your application can benefit from a controlled root tap, give us a call 800-872-8277.