Have you ever tried threading without a pre-drilled core hole? It’s something most people quickly learn isn’t possible. Creating that initial hole is a fundamental part of threading, and while it may sound simple, it can be tricky for beginners. Sure, drilling itself might not seem overly complicated, but there are nuances that can make or break your threading process. Don’t worry though—this guide will walk you through why drilling a core hole is so important and what steps you need to take to get it right. Once you’ve mastered this, threading will feel like second nature.

Why Is Drilling a Core Hole Essential?

A tap doesn’t actually drill the hole—it uses its sharp cutting edges to remove material from the inside walls of the pre-existing borehole. Without this initial hole, threading simply won’t work. Every tool serves its own unique purpose, and the core hole is no exception. Skipping this step will leave you frustrated and your project incomplete.

What Tools Do You Need for Drilling a Core Hole?

So, what kind of tool do you need for this job? Good news: for most cylindrical holes, you can use a regular twist drill bit. Tapered threads require a reamer, but we’ll cover that another time. When selecting the right drill bit, make sure it’s compatible with the material you’re working with. If you’re using a handheld drill, it’s wise to mark the spot first with a center punch to prevent slipping. A stationary drill press makes the process much smoother and cleaner.

Common Mistakes When Drilling a Core Hole

Drilling a core hole can present several challenges. Poor cooling, incorrect cutting speeds, or improper feeds can lead to excessive wear on your drill bit or produce an undersized or oversized hole. Vibrations or uneven surfaces often stem from unstable workpiece clamping or excessive runout errors. However, the biggest issue is typically using the wrong drill size. An incorrect size can result in a thread that doesn’t meet specifications, and worst-case scenarios include the tap getting stuck or breaking. This can render your tool unusable and potentially ruin your workpiece. In extreme cases, the entire component may need to be scrapped. To avoid these problems, always double-check your measurements.

The Correct Dimensions for Drilling a Core Hole

At first glance, determining the right pre-drill diameter for threading might seem daunting. Online searches can sometimes yield conflicting advice, but it’s not as difficult as it seems—especially for threads with a 60° flank angle. Let’s start by looking at this common thread type.

Drilling a Core Hole for Threads with a 60° Flank Angle

Many standard threads, including metric standard threads (M) and metric fine threads (MF), fall under the 60° flank angle category. These are widely used across Germany and Europe, so chances are your project involves one of these. Other examples include American coarse threads (UNC) and American fine threads (UNF). Calculating the core hole diameter for these threads is straightforward using this formula:

Thread diameter (D) – Pitch (P) = Core hole diameter (Dk)

For a metric standard thread M 10 x 1.5, the calculation would be 10 mm – 1.5 mm = 8.5 mm. Thus, you’d drill a core hole with a diameter of 8.5 mm.

Drilling a Core Hole for Threads with Different Flank Angles

Not all threads have a 60° flank angle. British threads, for instance, feature a 55° flank angle, and their formulas are significantly more complex. We strongly recommend avoiding self-calculations in these cases, as even minor errors can lead to costly mistakes. So, how do you get accurate information for drilling a core hole?

Dimension Tables for Drilling a Core Hole

Luckily, others have already done the hard work for you. Dimension tables exist for drilling core holes based on the thread type you plan to use. These tables eliminate guesswork and save valuable time. Did you know that such tables are also available for metric ISO threads? Even if you’re not fond of math, you can still find precise values for drilling a core hole for threads with a 60° flank angle. Your secret is safe with us!

You can find dimension tables for drilling a core hole on countless websites. For example, in the BAER online shop! As a leading screw tap manufacturer, we provide detailed information and reliable values for drilling a core hole for each product. You can also purchase the appropriate twist drill bits directly from us. Check it out!