Know-How Notes: How To Choose A Drill Bit
When you need to make a hole is something that isn’t paper, you need a drill bit. While the basic drill bit will make a hole, there are many different types of drill bits, and what you are working on makes a difference when you need to choose a drill bit that is right for the job. Drill bits come in different materials, shapes, and types of cut, making the drill bit aisle a confusing place for the novice driller of holes. This article will focus on the types of bit generally used in DIY use as it relates to automotive applications.
There are many types of drill bit styles, but the main ones you will deal with are twist, step, unibit, and hole saw. While all drill bits spin, twist bits refer to the spiral fluting. The others cut in a different manner.
Twist Drill Bits
This is what you think of when you hear the word “drill bit”. The twist bit uses two spiral flutes that begin at the tip of the bit and continue to the shank, where the bit transitions to a solid cylinder for the drill chuck. Some twist drill bits have a hex for use in quick-change drill-drivers. The spirals are essentially cutting screws that are designed to pull the swarf out of the hole. Swarf is the material being removed from the hole. The rate of spiral varies by manufacturer and use. A fast spiral (more twists per inch, called a compact flute) is used for low speed with a high feed rate, to remove large amounts of swarf. A low spiral rate (elongated flute) is used in high-speed cutting to remove the swarf as quickly as possible. Most consumer grade twist drill bits have a low spiral rate.
Point angle is the angle of the cutting head on the tip of the bit. There are 3 commonly used angles in consumer drill bits: 90-degrees, 118-degrees, and 135-degrees. 90-degree bits are only used for soft materials such as plastic and aluminum, they get dull very fast. If you are not sure what you have, you can always use a protractor to mark the angles on paper like we have done here.
118-degree bits are the most commonly used angle for consumer bits. They are suitable for just about every material you will deal with, including steel, aluminum, wood, stainless steel, brass, cast iron and plastic. These bits may walk on harder materials such as stainless steel and steel, requiring a center punch to keep them in place for the initial cut.
For hard materials such as steel and stainless, 135-degree bits are best. The shallow angle allows the bit to cut into the material without dulling quickly, however that same shallow angle makes for a bit that wants to walk around, requiring a center punch.
A split-point bit has a small cut on either side of the cutting flutes at the tip. This is a critical feature for 135-degree bits and is useful on 118-degree bits as well. This eliminates the tendency for the bit to walk on the material, as the split point starts cutting quickly.
What your twist drill bits are made of is another critical feature. There are four main types for consumer bits: low carbon steel, high carbon steel, high speed steel, and cobalt steel.
Low carbon steel is only used for wood, and they get dull very quickly. Even using these on hardwoods dulls the bit, these are not suitable for any type of metal.
High carbon steel are low carbon steel that has been tempered. They are economy bits, the kind that you usually find in multi-pack tool kits. They will get the job done when they are new, but get the bit too hot, and it loses the temper and will never stay sharp again.
High speed steel (noted with “HSS” on the shank or box) bits are more resistant to temper loss, these are quite common in drill bit sets, and are suitable for just about any automotive use. If you don’t need to drill very often, these are a good value as they are inexpensive and durable.
Cobalt (noted with “Co” on the shank or box) bits are very durable in terms of staying sharp, as they do not lose their edge even at higher temperatures. If you need to drill stainless steel, cobalt is the best option. Like most things, there are drawbacks for Cobalt bits. They are more brittle than high speed steel bits, so they can break easier, especially when stressed on the side or if the bit binds in the hole. They are also much more expensive, easily costing over $100 for a basic bit set. They do last a long time when properly taken care of.
Bits can be coated in titanium nitride to increase the lifespan of the bit as well. These are an in-between to HSS and cobalt bits. The HSS bit shown in the above image has a titanium nitride coating.
Step Drill Bits
Step drill bits are not commonly used in basic automotive applications, these are custom-made for each use. A true step drill bit has smaller cutting tip that gets the hole started (usually an inch or so) and then steps up to the final hole size. Think of these as a pilot drill bit and the final drill bit all in one piece.
Unibit Drill Bits
Unibits are often referred to as step drills, which is why the above definition was included here. A unibit is a single bit that is shaped like a cone with multiple steps machined into it. Each step is a different hole size. These commonly come in three sizes, up to about 2 inches. They feature a small cutting tip (a separate pilot bit cuts faster than the built-in tip), and are convenient for enlarging existing holes in sheet metal. These DO NOT work well in thicker materials or hard materials such as stainless. They may have a single cutting edge or two, depending on the manufacturer. The name “Unibit” is a trademark of Irwin tools, so the commonly used name is “step bit”. Once these are dulled, they cannot be sharpened.
A hole saw is a completely different animal from a twist drill bit, but they are very useful for automotive applications. You won’t use these for small holes for bolts, rather these are best used when you need make a big hole for wire, hoses, and other pass-thru items. A hole saw is essentially a toothed blade formed in a circle. The arbor holds the blade and typically uses a twist bit for a centering pilot to keep the blade from walking on the material. These come in many sizes and materials. If you are cutting metal, you want a bi-metal saw. Bi-metal blades are not alloys, rather they are two steels formed together, providing better cutting power, and flexibility. The teeth are high speed steel formed over a flexible steel alloy. These last much longer than plain steel saws when cutting harder materials.
The style of arbor and saw makes huge difference in the quality of the cut. Cheap hole saws fit loose to the arbor and can walk around, making cuts on the surface that you don’t want. Solid hole saws, known as Rotabroach or Annular cutters don’t walk very much, but they are more susceptible to breakage from cutting too fast and lateral pressure. They are also really expensive, but they make very clean holes. A locking arbor that has a threaded ring which inserts tow pins into the hole saw is the best kind.
Knowing how to choose a drill bit for your garage project can save you time and frustration. Stick with the best that you can afford with the features that you use the most, and you will be in good shape. Most DIYers will be fully served by a good quality set of high speed steel twist bits with a 118- or 135-degree cutting angle. When in doubt, check with your local NAPA Auto Parts Store for more assistance.
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