What are the best drill bits for metal? Is it worth buying high-quality metal drill bits? When can we cope with cheap black oxide bits? What bits are good for drilling in stainless steel? The above are some of the issues that I will cover in this article.
Let’s put ourselves in context. The qualities of metal bits are very different from wood, masonry, or porcelain bits.
Generally, the best metal drill bit is made entirely of the same material. Not always, there are exceptions. This means that we can use, sharpen and grind the metal bits until they become useless. That is, when they are too short to fit on the drill chuck.
The wood drills include a needle feature: a tip very sharp, spike-shaped, we can focus the bit.
Masonry bits, when of a certain quality, cut thanks to the carbide inserts welded to their tips. Except for the plate, the rest of a brick or concrete bit is made of carbon steel or, in the case of SDS hammer bits, of stainless steel. In both cases, a much cheaper material than that of the inserts.
On the other hand, with the exception of the bits for lathes and automatic milling machines with numerical control (CNC), the most common metal bits do not have welded carbide plates. Also, the design of its tip is different.
Types of drill bits for metal
To drill soft metals such as aluminum or brass, we will choose HSS-R high-speed steel bits (black ones) with a 118º tip angle. For large thicknesses or hard metals, for example, stainless steel, it is advisable to buy higher quality bits. The steel drills fast alloyed with cobalt (HSS-Co) or coated with titanium give better results than the above without being prohibitively expensive.
HSS-R metal drill bits
Black high-speed steel bits are the most common and also the cheapest. Its characteristic color is given by a black oxide coating, one of the cheapest. The fundamental characteristic of these bits is the angle of their tip. The cutting edges form a sharper angle (118 degrees) than other bits. This makes it easier to center the bit and improves feed.
These bits are suitable for drilling soft metals such as aluminum or brass. They also offer good results on ordinary steel, especially if the thickness is not too high.
Cobalt bits, or better said, cobalt-alloyed metal bits are a type of high-speed steel bit. The speed steel is an alloy of steel and other metal elements that allow drilling at high speed, and hence its name: HSS stands for High-Speed Steel.
A cobalt drill bit with a tilt angle of 135º is suitable for cutting hard metals such as stainless steel.
Titanium drill bits
The titanium bits can be found with their conventional shape (with a pair of cutting edges) or, as in the following image, with a double cutting geometry that facilitates centering and reduces the probability of breakage. This is equivalent to drilling with various diameters.
The coating of the drill with titanium alloy increases its hardness, facilitates progress in the metal, and improves the resistance to wear. It is important to note that this type of drill bit is coated with titanium, more specifically, titanium nitride. However, the core of the bit is made from high-speed steel or high cobalt steel. This means that sharpening will reduce the anti-wear properties. This does not happen with cobalt ones.
Angles drill bits drill metal
The most commonly used angles for metal drill bits are 118º (sharper) and 135º (flatter head). These inclinations are not random, they are the inheritance of the first bits that were manufactured to cut metal, conical, and with a flat surface.
Currently, the tip of metal drill bits is much more complex and many of them include several planes with different inclinations. The newest drill design achieves up to 50% less cutting effort and improves the removal of long chips on soft metals and short filings on hard ones.
Why is it important to buy the right metal bit?
Choosing the correct metal bit is essential for accurate and efficient drilling. This is true for all materials, including wood, but in hard materials such as stainless steel or reinforced concrete, buying the correct bit is decisive. We can make a punctual hole in metal even with the cheapest bit, however, to do serious work we need to have the right bit. Otherwise, we will be much less efficient or we may not even be able to finish the job.
The bit configuration (the shape of its body, the number of cutting edges, and the attack bevel), as well as the material of the bit (high-speed steel, titanium, and tungsten carbide), have a crucial influence on the speed of the perforations and the precision of the finish.
The tolerance of a metal hole in which we will subsequently carve a thread with the male does not have the same importance as a through-hole in a structural angle that is to be screwed to a wall.
How can you prevent the bit from slipping?
To prevent the bit from slipping on the chuck of our hammer drill we can do two things:
Buy bits with a triangular or hexagonal shank: the shank of the hexagonal drills make it impossible for them to slip in the drill. Even when the chuck doesn’t offer proper torque, this metal bit design will prevent it from slipping.
Use the ladder with the correct diameters: in holes 6-8 mm in diameter (or less if the thickness to be drilled is very high), it is recommended to drill with a smaller diameter first and gradually scale up to the final diameter. This reduces effort and helps us to center the bit.
Use a manual chuck with key lock. The best-corded drills include automatic chuck jaws with very effective jaws that prevent the bit from slipping. However, in mid-range models, the automatic chuck tends to slip over time. If the automatic chuck of our drill no longer offers a precise adjustment we can replace it with a new one or switch to one with a key. Replacing the bits is more cumbersome but the tightening is more powerful and effective.
Use reduced shank bits
Why do carbide bits have a flatter tip?
135-degree metal bits are used to drill hard metals such as stainless steel or manganese reinforced steel. This tip is flatter than that of ordinary 118-degree bits. As a consequence, the full length of the cutting edge comes into contact with the metal earlier. In other words, the bit will be cutting its entire diameter much sooner than with the sharpest tips.
In general, the softer the material, the sharper the bit can be. Sharp bits are easier to focus on soft materials and vice versa.
The cutting edge of a carbide drill bit is shorter. There are even completely flat drill bits that are used when you need to drill a blind hole with a flat bottom. As the cutting edge is shorter, these bits generate less heat since the friction metal surface is also smaller.
That is, the carbide drill bit cuts slower, progresses more slowly, power is used to tear away hard material. Instead, with sharp bits you advance faster, mechanical power is used to start faster.
Why are they helical?
The bits are helical in shape for two reasons. First, to remove the material torn off by the cutting head of the drill bit from the hole. And second, to allow the oil or the drill (when used) to easily reach the bottom of the hole.
The type of material we are going to drill will determine the shape of the bit’s propellers.
To drill hard metal, the bit will have fewer spirals and will be shallower. What we need in these cases is that the bit is solid and rigid. Chip removal capacity is sacrificed for high torsional strength. So we avoid that the bit breaks.
On the other hand, when drilling wood or soft plastics, there is no danger of breaking the tip and bit manufacturers can afford to add more coils and deeper. This facilitates the extraction of the torn material, which in this case has the shape of long chips.
What is a reduced shank bit?
Reduced shank metal bits are a special type of bit designed to prevent the bit from slipping on the drill chuck.
The bits generally maintain the diameter along the length: both the shank and the tip are equally wide. This system works well with small bits (3-8 mm) where the cutting efforts are not too high and the bit does not usually slip. However, from 8-10 mm in diameter, the bit is easy to slip, especially on low-quality automatic chuck.
To avoid this problem, reduced shank bits were invented. These metal bits have a smaller diameter shank (generally half an inch) regardless of the bit cutting diameter. In this way, the chuck with three claws of the chuck presses them with more force and we avoid the slip. A similar system is the triangular
Materials used to make metal bits
The harder the metal we have to drill the harder the bit tip has to be. Ordinary carbon steels (better known as mild iron) are very soft for drilling metal and only the cheapest, poor quality bits for cutting wood are made from this material.
Instead, HSS (high-speed steel) bits or tungsten carbide bits (also known as widia) are used to drill steel. The latter, in turn, are divided into two groups. Carbide tipped bits (use welded inserts) or solid carbide bits (all are carbide).
Tungsten carbide is an extremely hard but very brittle material: it breaks easily. For example, if you fall to the ground or get hit. It is common for carbide tipped metal bits to get damaged for this reason. Of course, these bits cannot be recovered by sharpening them, as only the plate is the one that actually cuts the metal, the rest of the bit is just ordinary steel that supports it.
High-Speed Steel (HSS)
HSS bits are used for their low cost and the ability to drill carbon steel, aluminum, and copper relatively easily.
Cobalt-alloyed high-speed steel bits allow us to work faster. The incorporation of a percentage of cobalt of between 4-9% allows the tip of the bit to retain the sharpness of the cut for a longer time.
Thanks to these bits we can cut harder steels, cast iron, and even harder alloys. These bits are similar to titanium bits.
Tungsten carbide (widia)
They are the toughest of all. No other metal drill bit achieves the good performance of these tools. They withstand wear well and also very high temperatures. They are much more expensive than any other. They are used to drill hard materials such as titanium alloy stainless steel.