Common Mistakes When Drilling Hardened Steel

Drilling holes is a vital part of fabrication and any project involving metalworking. With the correct drill and tooling, drilling almost any material is achievable. However, using the wrong tool or technique can result in incomplete holes, burnt-out tooling, and frustration, especially when drilling hardened steel.

In this article, we’ll look at some common mistakes people make when drilling hardened steel.

Why Hardened Steel is Difficult to Drill

Low carbon steel is relatively hard compared to materials like wood or plastic, but hardened steels can be significantly harder still. Common hardened steels include EN19, EN24, or tool steels such as D2, A2, or similar shock‑resisting grades.

These steels have a hardness level exceeding 60 HRC (Rockwell), meaning the crystal structure of the metal is highly resistant to local plastic deformation, including abrasion, indentation and cutting.

Using the Wrong Drill Bits for Hardened Steel

High-speed steel (HSS) drill bits are usually the most readily available type in the average workshop or toolkit, ideal for cutting mild and even stainless steels. However, the typical hardness of an HSS cutting tool, such as a drill, is around 66 HRC, which is only marginally higher than that of the hardened materials mentioned above.

This means the drill will cut into the metal, but not efficiently. This significantly increases heat generation, which in turn raises the likelihood of the drill overheating and losing hardness.

Cobalt drill bits are a good option, as they provide a higher hardness rating. Better still are tungsten carbide drills, although these are considerably more expensive.

Incorrect Drill Speed, Pressure and Heat Control

Every cutting tool, whether a drill, milling cutter, or saw blade, has an optimum speed and feed rate for a given material. These parameters are typically provided by manufacturers.

Failure to follow these values can quickly result in blunt drills and unsuccessful machining operations.

For example, applying insufficient pressure (feed rate) when drilling hardened steel will cause the drill to rub rather than cut, rapidly increasing temperature. This can damage the heat treatment at the drill tip and may also work-harden the material.

Even when using the correct speeds and feeds, it is still possible to overheat the drill by failing to use adequate cooling or lubrication. Specialist cutting fluids are used in professional environments to dissipate heat and improve machining performance.

Skipping Lubrication and Cooling: What It Costs You

There are many reasons why drilling may be attempted without proper coolant or cutting fluid, for instance, the coolant reservoir may be empty, the pump blocked, or the fluid simply unavailable.

However, failing to use suitable cooling and lubrication will often cost far more than the time saved. In certain materials, an expensive cobalt drill can become unusable within minutes.

In addition, a work-hardened, partially drilled hole will require even greater care to complete, as the cutting surface becomes significantly harder than the original material.

How to Drill Hardened Steel Correctly Without Damaging Tools

There are several steps to successfully drilling hardened steel:

• Use the correct machine: A hand-held drill is unlikely to provide sufficient pressure or speed accuracy, although it may be used if necessary. A pillar drill or milling machine is preferable.
• Select the correct drill: Use at minimum a cobalt drill or, preferably, a carbide drill for improved performance and efficiency.
• Use appropriate cooling: Always verify the recommended coolant or lubricant for both the tool and material.
• Prepare and commit: Incorrect tooling can cause problems, but so can excessive caution—hard materials require a firm feed rate to prevent rubbing.

Get Your Hardened Steel from Metal Supermarkets

From tool steels to wear plate, as well as standard low-carbon grades, Metal Supermarkets is likely to have what you need. Choose from a wide range of sizes and sections, with delivery and cutting services available.

Check out the range here.