Drill Bit Selection Guide - How to Select a Right Drill

How to choose the right drill bit? For getting the best result, you should choose a drill bit providing proper chip clearance, good surface finish and precise hole positions. Understand the basics of drill bit selection involves material, coating, geometries, types, sizes, and specifications to make the best choice.

How to Select a Drill Bit - What Should You Consider When Choosing a Drill Bit?

There are a number of types of drills designed for different purposes. To separate one drill bit from another, you need to analyze the difference between them. Drills can be made from varying materials and apply multiple coating, or come in different geometries. Choose the drill bit based on the material of CNC drilling parts to be machined and characteristics of the tool.

1. Materials of Drill Bits - What's Your Drill Bit Made of

A drill bit is generally made from high-speed steel, high-speed steel with cobalt or carbide.

– High-speed steel: the most basic drill material, it is inexpensive and can be used in the drill press and hand drilling operations, resharpen the drill can extend its lifespan.

– High-speed with cobalt: holds up better than high-speed steel and more heat and wear-resistant, easy to resharpen as well.

– Carbide: the most expensive material, but it can last the longest time, also gives the best heat and chip resistance, allows for coolant through holes. Despite the price of carbide is much higher than other materials, the cost for drilling per hole usually is the lowest when using carbide drill, because it can produce more holes than cobalt and is capable of running 3 to 5 times faster. The drill bit is mainly used for producing deeper holes or working with tough-to-drill materials.

2. Coatings of Drill Bits

– Bright finish: the cheapest coating, often applied to drill low-carbon steel and aluminum workpiece.

– Black oxide: more lubricity than bright finish, resistant to oxidation, applying heat treatment process will increase its service life.

– Titanium nitride (TiN): an extremely hard ceramic material and bright gold coating applied to metallic surfaces, the most common option, ideal for beginners and applications where lots of heat won’t be transferred to the tool from cutting harder or tougher materials.

– Titanium carbonitride (TiCN): has the highest lubricity of all the TiN coatings, with improved hardness and wear resistance, as well as better performance. It’s typically blueish or purple in color.

– Titanium aluminum nitride (TiAIN): stands for a group of metastable hard coatings, a great choice for steel and stainless steel, but not suitable for drilling aluminum. It has higher performing than TiN and TiCN, excellent for high-temperature materials.

Overall, TiN coated drills are common options for mild steel, TiCN coated drills are ideal for cast iron, and TiAIN coated drills are recommended for high-heat applications. High-quality cobalt drill with a TiN or TiCN coating is a relatively inexpensive way to get higher productivity unless cutting difficult materials.

3. Geometry/Specification of Drill Bits

– Length: there are two common screw machine lengths, referred to as “stub” length and “jobber” length. Stub length drills are widely used on CNC machines due to its high rigidity, which can make more precise hole position, the shorter the bit, the more rigid the drill is. Make sure you have enough flute lengths to get the chips out of the hole, two times the drill diameter in flute length above the hole is the best.

– Drill Point Angle: you can choose from many different drill point angles, two common types are 118°and 135°. The 118-degree drill is more suitable for mild steels, aluminum and other soft metals, it’s usually on jobber length. The 135-degree drill generally with stub length and used to drill hard steels, and other tough materials.

– Helix Angle: helix in 30-degree is used for drilling most materials. There are also drills with helix angle 10 degree, which is suggested for harder steels, aluminum alloys; and larger option 40-degree helix angle is designed for difficult materials.

– Self-centering points: many cobalt and carbide drills have it, so it does not require to spotdrill with cobalt and carbide.

4. Types of Drill Bits

Based on the applications or objects they work with, the drill bit can be classified into the following types.

– Wood bits: used for drilling into the wood workpiece. Wood drill bits can be identified by a small, pointed tip at the very end. Titanium-coated wood drill bits can last a long time.

– Steel bits: used to drill soft metal parts, or softwood materials.

– Masonry bits: used for drilling into parts made from stone, concrete, bricks, plaster, etc. Masonry drill bits usually have a gently sloping tip. Carbide coating on the tip of masonry bit can prolong its sharpness.

– Metal bits: versatile for a variety of applications, with a wide-angled point at the end.

– SDS bits: usually used on rotary hammer drills.

Common industrial drill bits can be divided into two categories: insert and twist. Insert drill bits are relatively budget and easy to use, there is no need to sharpen the bit, but insert bits may leave a burr at the outlet of the hole. Twist drill bits are commonly used in drilling services for general purposes, they are characterized by the helix and spiral fluted profiles. In metalworking based on CNC machining technologies, some special types of drill bits are applied.

– Indexable bits: a type of insert drill bit features ceramic inserts as cutting edges for inner and outer spans of the cut. Indexable bits are generally employed in CNC manufacturing and able to produce large holes quickly.

– Spade drill bits: consists of a metal body and a removable bit that is clamped into a precise slot, the bit usually comes in a twist pattern, with special notches along the cutting edge, for eliminating long chips, used in manual processing.

– Ejector bits: can be equipped with single or multiple cutting heads, suitable for deep hole drilling.

For woodworking CNC drilling, there are flat bits, dowel bits, and augur bits, these tools are commonly applied on CNC lathes and milling machines. Knowing about more varieties of drill bits and their specifications can help you find the right choice faster.

5. Sizes of Drill Bits

A drill bit size chart and tap size chart are often required to determine the proper drill bit for the accurate size of the hole. The drill size chart contains the standard size in different measurement systems, including fractional, metric, wire gauge number, and letter.

Drill Bit Selection Guide – How to Select a Right Drill | CNCLATHING