What is Drilling ？
Drilling is a process that is used to create a cylindrical hole or bore in a solid piece of material, often as a precursor to tapping or to prepare the material for insertion of a fastener such as a bolt or a screw. The cutting tool used for drilling is called a drill bit, twist drill, or sometimes just a drill, but the latter may also refer to the entire tool as opposed to just the cutting bit itself.
Drill bits are some of the most common and versatile parts of any machine shop or home tool shed. They can drill into metals, solid wood furniture, porcelain, and even glass, and with a variety of different drill bits available, one can find an appropriate bit for almost any situation. One major aspect of drill bit selection is the material makeup of the drill bit itself. Not all twist bits will work on all applications. It is important to match the material of the bit to the material of the workpiece to produce the best results.
This article will present information on drill bits and the common materials from which they are manufactured to assist in the selection process for a particular application.
Best Material for Drill Bits
The best materials for drill bits are carbon tool steel, high-speed steel, and carbon high-speed steel. Standard twist drills are fabricated using steel whose mechanical properties allow the drill bit to maintain its toughness, exhibit good abrasion resistance, and resist changes in harness as a result of the heat that is generated during the drilling process. Generally, the selection of the drill bit for a particular application should be made by giving consideration to the amount of heat that will be generated.
Carbon Tool Steels
Carbon tool steels are used for applications where little heat is going to be generated. Both low and high carbon steels are both used for drill bits, but for different purposes. Soft low carbon steel cannot cut hard metals due to their poor tempers, but it can cut softwoods and plastics. They require sharpening to extend their lifespan. The primary bonus of low carbon steel is its relative inexpensiveness, especially when compared to some more exotic drill bit materials.
High carbon steels have better tempers than low carbon steels, so they require less maintenance, such as sharpening, and hold their form and effectiveness longer. They can cut both woods and metals, and if available, are preferred to low carbon steels when cutting extremely hard woods.
High-Speed Steel
High-Speed Steel (HSS) is a preferred material for use in drill bits since it has a higher red hardness and improved wear resistance. These properties allow for drilling at higher operating speeds and into harder materials. The friction created by high-speed turning can raise temperatures dramatically, but HSS is designed to operate at these higher speeds. HSS can function at normal temperatures, as well, but only at a level equal to standard carbon steel. HSS can also take coatings, such as titanium nitride, which give the drill bit better lubricity, decreasing friction and helping to extend the bit’s life.
Cobalt High-Speed Steel
Cobalt High-Speed Steel (HSS) drill bits have added Cobalt which gives the material a higher red hardness than standard HSS. This additional hardness permits these drill bits to be used for drilling materials that have a hardness of Rockwell 38C or greater such as treated stainless steel, cast iron, or titanium. They are also capable of being utilized at higher cutting speeds than conventional HSS and exhibit superior abrasion resistance.
Tungsten Carbide
Tungsten carbide is an extremely hard and wear-resistant material. Drill bits with tungsten carbide tips are effective for drilling into hard materials such as concrete, masonry, and some metals. Solid carbide drill bits are suitable for high-speed drilling applications.
Carbide Tipped
Carbide-tipped drill bits combine the toughness of steel with the hardness of tungsten carbide inserts. These bits are effective for drilling into hard materials and are commonly used in masonry and woodworking.
Diamond-Coated
Diamond-coated drill bits are designed for drilling into extremely hard materials like glass, ceramics, and tiles. The diamond coating provides excellent hardness and wear resistance.
Black Oxide-Coated
Black oxide-coated drill bits have a coating that enhances their durability and provides some corrosion resistance. They are suitable for general-purpose drilling in various materials.
Titanium-Coated
Titanium-coated drill bits have a layer of titanium nitride that increases their hardness and reduces friction. These bits are suitable for drilling into metal, wood, and plastic.
What is the Strongest Drill Bit Material?
Tungsten carbide is one of the strongest materials used for drill bits. Tungsten carbide is a composite material made from tungsten and carbon atoms, resulting in an exceptionally hard and wear-resistant substance.
Drill Bit Terminology
American National Standard B94.11M-1993 covers the relevant information relating to drill bits, including sizes, tolerances, nomenclature, and definitions. Below is a summary of some of the key terminology relating to drill bits.
Axis – refers to a centerline running down the middle of the drill bit longitudinally from the point of the drill bit to the end of the body.
Back taper – is a reduction in the drill bit’s diameter that occurs from the point of the drill bit to the end of the drill bit body. Most drill bits are machined with a small back taper to prevent the bit from binding in the workpiece when the drill bit becomes worn.
Body – refers to the portion of the drill bit that extends from the neck to the outer edges of the cutting lips and which contains the flutes.
Drill Diameter – the overall diameter of the drill bit measured at the point of the drill up to the margins.
Flutes – are the grooves that are formed in the drill bit which serve to allow the removal of chips from the hole being cut and to allow cutting fluid to reach the cutting lips of the bit. Flutes are usually helically shaped or straight cut.
Lips – the cutting surfaces of the drill bit.
Neck – a small section of the drill bit which has a reduced diameter that sites between the shank of the bit and the body.
Overall length – is the distance measured from the extreme end of the shank to the outer corners of the cutting lips. Note that this measurement excludes the conical shank end and the conical cutting point at the tip of the drill bit.
Point angle – the angle that is formed between the lips when a projection is made onto a planar surface that runs parallel to the axis of the drill bit and to the cutting lips.
Shank – the portion of the drill bit by which it is held in place and driven, i.e. the portion that does not contain flutes.
Types of Drill Bits
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