Types and Sizes of Drill Bits

In our early edition, we had discussed about the definition of drill bits and some quick facts about these fastening devices. Here, we are going to have a complete cytology of these items. Do have a read!


Drilling Machine with a Drill Bit

Let’s rewind!

What is a drill bit?

Drill bit is a special kind of cutting tool, which is specially designed to fit into a rotating drill. It comes along-with a bit and is used to cut cylindrical holes of various sizes in various kinds of surfaces. The terminologies, such as drill bit and drill are quite often confused, as these terms are used to define the cutter as well as the cutter holder.

These fastening objects are available in different types and sizes. Because of their extensive use in US, as per American standards, a defined set of sizes has been set for these metal pieces.

Various Types and Shapes of drill Bits

The British standardization was enacted in 1959 for the first time for different gauge and metric sizes. Today the US standard is followed everywhere for determining the metric sizes.

The metric drill bit sizes are usually measured in terms of standard metric lengths. The definite size pattern has been set determining the traditionally manufactured and stocked pieces. For instance, a BS 328 item (British Standard) will have size from 0.2 mm to 25.0 mm.

List of Drill bit sizes in MM

The price and availability of a particular size does not change when the size range ascends. Bits are more commonly available at size increments of 1 mm and can be easily found at any web store. If you’r looking for a unique size, say 2.55 mm then you’ll have to specially place an order from a  a specialist drill bit supplier.

Most Commonly available sizes of drill bits

Fractional Size

The fractional inch drill bit sizes are quite common in U.S.

The number and letter gauge is mostly used for twist drill bits. It is used as – size 80 (smallest) to size 1 (largest) and size A (smallest) to size Z (largest). Such standardization is set to offer accurate clearance holes for screws as well as bolts.

Decimal fractional Size equivalents of drill bits

Different types of drill bits

Twist drill bits

The twist drill bit, which was invented by Steven A. Morse of Massachusetts in 1861, is the most frequently used type in the industry today.

It features a cutting point at the tip of a cylindrical shaft with helical grooves. These grooves act as an  Archimedean screw and lift swarf out of the hole.

Length and Diameter specifications of twist drill Bits

The twist bits range in diameter from  0.002 to 3.5 inches and can be as long as 25.5″. The most common twist drill bit features a point angle of 118 degrees and is ideal for use in wood, metal, plastic etc.

The 90 degree type, which is a quite aggressive one, is used for soft plastics. This kind of bit is self-starting and cuts very quickly.

Step drill bits

It is a kind of bit, whose tip is grounded down to a different diameter. The instability between the original diameter and this ground diameter is either angled, forming a countersink or straight, forming a counterbore.

Step Drill Bits Description

The best part of this kind of bit is that both the diameter can have same groove characteristics, which prevents the bit from clogging. Most of these are custom-made, so they are comparatively expensive.

Metal drill bits

These are mostly used in metalworking, so as to enable a hole for a large-sized drill bit. These are also used to create a conical dent in the end of the surface, so as to easily mount a lathe center.

Drill Bits for structural Steel

These are mostly used to create a conical hole between the centers, in places, say lathe or cylindrical-grinder work.

Core drill bit

This kind of rivet is used for dual purposes. It is used to enlarge an existing hole, created from casting or a stamped hole. It is also used for extracting core. In this case, the drill creates a hole with an annular cross-section, thereby making the inner core intact.


Core Drill Bit

In this case, the purpose is only to retrieve the core rather than creating it.

Ejector drill bit 

Description of a Ejector Drill Bit

This kind of bit is exclusively used for drilling deep holes of medium to large diameters (from 3/4″ to 4″). The bit uses a specially designed carbide cutter and the entire device looks like a tube within a tube.

Gun drill bit

Gun Drill Bit

These bits are usually straight grooved drills, enabling the cutting fluid to get injected into the drill’s hollow body.

Indexable drill bit

It is mostly used in CNC and in other high precision equipment. This kind of bit.