Common Mistakes When Drilling Hardened Steel

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

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

Why Hardened Steel is Difficult to Drill

Low carbon steel is quite hard when compared to other materials like wood or plastic, but hardened steel can be significantly harder still. Common hardened steels include 4140 and 4340 or tool steels such as D2, A2, or S7.

These steels have a hardness level exceeding 60 HRC (Rockwell), meaning the crystal structure of the metal is excellent at resisting local plastic deformation, aka abrasion, indentation, or cutting.

Using the Wrong Drill Bits for Hardened Steel

HSS drill bits are usually the most readily available type of bit in the average workshop or toolbox, perfect for cutting mild steel and even stainless steel. However, the typical hardness of an HSS machine tool, such as a drill, is around 66 HRC, only just higher than that of the materials mentioned above.

This means the drill bit will cut into the metal but will not do so easily. This will vastly increase the heat generated, which in turn increases the likelihood of the drill overheating and losing its hardness.

Cobalt drill bits are a good choice, as they have a higher hardness rating. Even better still are tungsten carbide drills, although these are incredibly expensive.

Incorrect Drill Speed, Pressure, and Heat Control

Every machine tool, be it a drill bit, miller cutter, or saw blade, has an optimum speed and feed rate for a given material. These parameters are readily available from the manufacturers.

Not adhering to these numbers can quickly result in blunted drills and unsuccessful drilling operations.

Even when using the correct combination of drill speed and feeds, it is still possible to overheat the drill bit by not using sufficient cooling or lubrication. Specialist cutting fluids are used in professional environments to remove heat and aid processes such as drilling.

Skipping Lubrication and Cooling: What It Costs You

There are many reasons why you may attempt to drill without using the correct cutting fluid or coolant, such as the coolant reservoir has run out, or the coolant pump is clogged, or you simply don’t have it at hand.

However, the costs of not using the correct fluid for cooling and lubricating can cost far more than the corner-cutting ever would. In certain materials, an expensive cobalt drill bit can be rendered useless in a matter of minutes.

What’s more, a work-hardened part-drilled hole will require even more care to finish, as the cutting face is now significantly harder than the raw material was to begin with.

How to Drill Hardened Steel Correctly Without Damaging Tools

There are several steps to successfully drilling holes in hardened steel:

• Use the correct machine. A hand-held drill is not likely to achieve the required drilling pressure or speed accuracy but can be used in a pinch. A drill press or mill is better.
• Select the correct bit: Use at least a cobalt drill bit, preferably a carbide insert drill if speed and efficiency are important
• Use appropriate cooling: it is important to check what the recommended coolant or lubricant is for your selected drill type and the material
• Prepare and commit: using the incorrect tools or bits can cause problems, so too can being too gentle; hard materials need a high feed rate to avoid rubbing

Get your Hardened Steel from Metal Supermarkets

From tool steel to wear plate and normal low carbon grades too, Metal Supermarkets is likely to have it. Choose from a wide range of sizes and sections, and have anything you need delivered and cut to size!

Check out the range here.