How to choose bits or bits according to the job to be done – Part 2: metal bits

In another article we presented a detailed report on the kinds of bits for use in power drilling tools such as drills, hammer drills, screwdrivers / drivers or rotary hammers.

We also learned what we must take into account when choosing drill bits when drilling wood and other soft materials, as well as concrete and masonry.

However, when it comes to drilling holes in metal, the complexity surrounding the choice of drill Correctness increases due to a wide range of factors that not only concern the composition of the metal of interest, but also the composition of the metal itself. drill.

Therefore, this time we are going to stop at the main parameters that we must consider before deciding on a drill in particular for metals. The essential thing is to ask ourselves a series of key questions.

What kind of metal are we going to drill?

Metals have different densities, which translates into hardness and puncture resistance. Drilling lead is not the same as aluminum, for example. This characteristic of metals determines the design and composition of the drill bits, which must always be harder than the metal to be drilled, and for this purpose we can distinguish two large groups of materials used in drill bits for metals:

High speed steel or high speed steel (HSS): It is a steel alloy that consists of a metallic element combined with a chemical element that gives steel, known as alloy steel, a certain characteristic. HSS is the most common of all alloy steels in the design of drill bits for metal, giving rise to bits of great strength and good resistance, which can be sharpened several times. In addition to the chemical element that is part of the alloy, the drill bits High speed steel can have coatings with various metals that give them additional properties.

HSS drill

Carbide or carbide: is a sintered compound, tungsten carbide (i.e. carbon and tungsten) and binder metals which are usually titanium or cobalt, which provides the drill greater resistance and durability. Are drill bits can be made entirely of tungsten carbide or simply have tungsten carbide tips and are preferred in industries from construction, mining, agriculture, automotive and machine shops due to its long service life.

For the hobbyist and professional contractor, however, there is no question that HSS or HSS drill bits They are the must-have choice for any metal drilling job, and for that reason we will focus on this particular type of drill bit from now on.

Cobalt Drill Bits -Titanium

What chemical elements make up the alloy in an HSS drill bit?

Technology has always advanced in the search for chemical components that will present the best combination of properties required in a drill for metal, for example:

• High hardness
• High temper
• Good wear resistance
• Good toughness
• Fatigue resistance
• Good behavior against machining at high speeds and temperatures

HSS drill

That is why the first elements that formed the alloy of steel were carbon, chromium and tungsten in various percentages. However, over time these materials and / or their proportions were modified, allowing the introduction of vanadium, cobalt and molybdenum. All these additives, added to the fundamental role played by the carbon content, are what dictate the final properties of the high speed steel of the drill.

High speed steel wicks

In this table we present the current main components that are part of the HSS alloy and the properties that each one contributes to the final product, that is, to the drill.

What are the coatings on HSS bits?

Wearing a drill for metal it is always carried out with cutting oil, at a reduced speed and a constant pressure. However, many times additional properties are sought that the alloy alone is not capable of providing, particularly in the case of high productivity machining that involves drilling at high speeds, dry, or in difficult-to-machine materials.

Therefore, the trade also offers a range of drill bits made of high-speed steel that, in addition to presenting some of the alloy components detailed in the previous table, also have coatings with other metals that give them additional characteristics, some of which are the following:

• Increased hardness on the surface, for greater resistance to wear (abrasive and sticky wear, flank wear)
• Reduction of friction coefficients for:
  • better chip evacuation
  • reduce shear forces
  • prevent regrown edges
  • reduce heat generation, etc.
• Reduced heating of the tool
• Resistance to corrosion and oxidation thanks to the chemical barrier
• Resistance to crater formation
• Improved surface quality of finished parts

The following table summarizes some of these coatings and their benefits.

How many types of HSS drill bits are there?

As we have seen, drill bits HSS for metal have a helical shape and are composed of various materials that give them different degrees of hardness and durability depending on their intrinsic properties.

However, the propeller and / or tip may also be different and some manufacturers offer two distinguishable types of drill bits HSS according to its type of machining (grinding or rolling) as shown in the following figure.

What advice to follow when choosing a drill bit for metals?

Choosing the drill Which is best suited to our metal drilling job is subject to a number of factors. The following tips can help us resolve various issues in this regard.

• The tip angle varies according to drill, between 90 °, 120 °, 135 °, 140 ° or 145 ° and must adapt to the metal that we are going to drill. If the metal is hard, the angle of the tip should be large.
• The HSS-R or rolled drills constitute the basic acquisition. It is very convenient to equip ourselves with a minimum of HSS-G or ground drills for regular use and / or for a wider range of materials.
• The HSS drills alloyed with 5% cobalt They offer good value for money and, when used with cutting oil and at low speed, allow many drilling operations in metal.
• The drills made entirely of carbide or carbide They are high-end and pierce deep into metal.
• Most metals can be drilled with any medium quality drill bit using Cutting oil and one suitable rotation speed.

High speed steel wicks

For greater clarity, the following table presents an indicative summary that will also be useful when deciding on a drill suitable for metals, based on the drilling of a 10 mm diameter hole.