The Rules for Calculating the Diameter of the Hole to Be Threaded

The pre-hole for threading is that element which is performed with the drill bit in the metal. The passage of a die will be useful to create the threading profile. In this way you can use, inside it, screws and pins. But how do you determine its exact diameter? What are the measurements to follow to calculate the pre-hole for threading? The answer should not be improvised.

Because there is a delicate balance to respect. In this guide you have all the details for successful threading: now we explain which are the steps useful for calculating the diameter of the pre-hole and making sure it matches the pin you want to screw in.

The Concept to Follow for the Hole Diameter

Before you start, it's right to consider an essential passage that will be your guide for preparing the pre-hole for threading. The core to be threaded and treated with tools equipped with blades for cutting metal must be slightly smaller than the nominal diameter of the threading. In this way it's possible to create the ideal spaces and cuts to grip the pin or screw and tighten the closure. Now there's only one concept to clarify: how to calculate the diameter of the hole to be drilled for threading? Here you find the answer.

To read: how to choose taps and dies

Preliminary Steps to Define the Diameter

An essential point to determine before starting the drilling work: identify the type of threading. Only in this way can you determine the diameter consciously, knowing that the creation of the profile for screwing and unscrewing takes away part of the material.

So, you must understand whether it's a metric threading (like M8, M10) or another standard that requires different steps. To have a clear reference, we introduce the concept of nominal diameter of a threading geometry.

The thread with a designation equal to M10, for example, has a nominal diameter of 10 mm. Beyond this data, we must become familiar with the term of pitch: the distance between two crests of a threading (image source Wikipedia).

Nominal diameter (which represents the widest part, in opposition to the inner diameter or core diameter) and the pitch that defines the distance between two crests are the two necessary parameters to calculate the diameter of the hole that must be drilled.

Although, generally speaking, all taps and dies always have accompanying tables to precisely indicate the diameter of the starting hole.

How to Calculate the Diameter of the Hole to be Drilled for Threading

The formula to calculate the hole diameter you find on the packages of the various threads, as mentioned. If you want to proceed independently there's a formula to apply with a simple operation that includes threading pitch (P) and nominal diameter (D).

Hole diameter = D − P

Let's make a concrete example: for an M10 threading with pitch 1.5 mm the hole diameter is calculated with this operation: 10 mm − 1.5 mm = 8.5 mm. There's a small detail to consider: if the material is very soft, it might be necessary to adjust the hole diameter which can be slightly smaller. In any case, remember to buy and use only the ideal tools for this work: the best taps for threading and a slide caliper to measure the hole diameter precisely.