Drill Bit Size: Perfect Match For 3/8 Machine Screws

Choosing the right drill bit size for a machine screw is crucial to ensure precision and avoid damage to the material. While self-tapping screws can penetrate softwood without a pilot hole, larger diameter holes and harder materials like hardwood and metal require one. The correct drill bit size depends on various factors, including screw type, material, and thread design. In the US, fractional inch and gauge drill bit sizes are commonly used, while metric sizes are prevalent in most other countries.

Drill bit sizing by screw type

Drill bits come in a wide range of sizes, with several naming conventions, including fractions, numbers and letters. In the US, fractional inch and gauge drill bit sizes are most common, while metric drill bit sizes are used in most other countries.

Self-Tapping Screws

Self-tapping screws are sharp-pointed screws that can split wood or distort metal if they don't have a properly sized pilot hole. Type A and Type AB self-tapping screws require a pilot hole with a recommended hole diameter that varies according to the screw threads per inch.

Tapping, Thread Cutting, Thread Forming or Thread Rolling Screws

Blunt-tipped screws like Type B Tapping, Type F Thread Cutting and Thread Forming Screws for Plastics or Metals require the correct sizing to allow for insertion into the material. These screw types have machine screw threads with tapered ends.

Wood Screws

Woodworkers often use traditional screws in antique or reproduction pieces that require matching hardware. Tapered drill bits are used to drill both the body hole and the pilot hole in one pass for wood screws. Tapered threads require careful depth control—too deep and the threads won't hold, too shallow and the screw becomes jammed.

Modern production screws have a reduced shank diameter that results in a deep thread profile offering great holding power. Drilling a pilot with a straight drill bit yields excellent results because the shank of the screw is straight throughout its length.

Self-drilling vs. other screw types

Self-drilling screws, also known as Tek screws, are a subtype of self-tapping screws. They are commonly used in construction, commercial, and industrial applications. Self-drilling screws have a distinct advantage over other screw types, as they can drill their own pilot hole, eliminating the need for pre-drilling or pre-punching. This makes them a convenient and versatile choice for a wide range of materials, including metal, wood, and plastic.

Self-drilling screws feature a sharp point and cutting threads that allow them to drill, tap, and fasten in a single action. The tip of the screw is designed like a twist drill bit, enabling it to create a hole in the material as it cuts its own threads. This makes self-drilling screws highly efficient and time-saving during installation. Additionally, they can be used with various tools, such as power drills and screwdrivers, and are available in different head and drive styles, including Phillips, hex, or square.

However, it is important to note that self-drilling screws have more specialized applications compared to self-tapping screws. They are particularly useful for fastening metal to metal, wood to metal, and working with light, low-density materials. They are commonly used in metal building, light gauge metal assemblies, HVAC applications, cladding, metal roofing, and general construction tasks.

On the other hand, self-tapping screws require an initial pilot hole before installation. They are often referred to as metal screws, sheet metal screws, tapping screws, or tapper screws. Self-tapping screws have different tip shapes, including pointed, blunt, or flat, and they can be thread-forming, thread-cutting, or thread rolling. Pointed self-tapping screws are thread-cutting, creating threads in a pre-drilled hole, while flat-ended screws are thread-rolling, extruding threads and creating zero clearance between the screw and the material.

The selection between self-drilling and self-tapping screws depends on the specific application and material being used. Self-drilling screws offer the convenience of drilling and fastening in one step, saving time and reducing errors during installation. In contrast, self-tapping screws are versatile and suitable for a wide range of materials, including thinner sheet metals, wood, and plastics. By considering the unique characteristics and applications of each screw type, users can make an informed decision for their projects.

Pilot holes for wood screws

Pilot holes are used to improve the holding power of a fastener while reducing the likelihood that a board will split. They are also called "pre-drilling" or "clearance holes". The goal of the pilot hole is to remove material from the wood so that the screw doesn't force the board apart and cause it to split.

When joining two boards together, you should only pre-drill a hole through the first board, not the mating piece. Many wood screws don't have threads at the head end of the screw, and a pilot hole clears enough space for the shaft to fit through.

In addition to reducing the likelihood that the wood will split, the pilot hole also serves several other purposes:

• Easier to get the screw started
• Increases the strength of the joint

The thickness of your screw will determine what size drill bit you'll need. You can find the size of your screw listed on the package. The higher the number, the thicker the screw shaft.

Most drill bit sets come with the standard sizes required to make pilot holes. Look for the size inscribed on the shaft of the drill bit, or in the storage case. Those tiny numbers can be hard to read, but you can check the diameter with a drill bit gauge or digital calipers if you're not sure.

You'll also need to consider the type of wood you are using for your project. If you're drilling into hardwood, opt for a slightly larger pilot hole. If you're drilling into softwood, you can get away with a smaller pilot hole.

• Wear safety glasses to protect your eyes in case of drill bit shattering.
• Clamp the board to your work surface to keep your hands free and away from the drill bit.
• Choose the right pilot hole bit size. The hole should be slightly smaller than the thickness around the screw's threads to ensure the threads have enough material to bite into.
• Mark the pilot hole locations with a pencil or a small center punch to create a divot that helps get the drill bit started.
• Use a guide or a stop collar to ensure you don't drill too deep and bust through to the other side.
• Drill the pilot hole slowly and straight, holding the drill perpendicular to the board.
• Countersink the hole if desired to create an angular hole for the conical screw head to sit in, so the top is flush or below the surface.

Screw machine (stub) drill bits

Screw machine drill bits are available in a variety of sizes, finishes, and materials such as high-speed steel, cobalt, and solid carbide. The high-speed steel models are ideal for drilling steel, cast steel, cast iron, and other ferrous materials, while the cobalt models offer improved performance when cutting harder metals like stainless steel or nickel alloys. The solid carbide designs can result in up to five times the penetration of HSS drills.

The flute length and overall length of screw machine drill bits are dependent on the diameter of the drill bit. They are designed for maximum rigidity and longer tool life. The round shanks of these drill bits allow for use with a wide range of toolholding systems.

When selecting a drill bit, it is important to consider the material properties. The harder the material or the softer the screw, the larger the pilot hole should be. Proper sizing of the pilot hole is crucial to ensure the screw can be inserted without splitting wood or distorting metal.

Drill bit naming conventions

Drill bits come in a wide range of sizes and use several different naming conventions, including fractions, numbers, and letters. These naming conventions are important to understand to ensure you are using the correct drill bit for your project.

Fractional Naming

The most common naming convention for drill bits is based on fractions. All fractions of 64ths, from 1/64th to 63/64ths, are named by their lowest fractional equivalent (e.g. 1/32", 1/16", 5/64", etc.). There are also size options that fall between many of these fractions.

Numerical Naming

Another naming convention uses numbers, with common # sizes going from #80 through #1, in ascending sizes, up to .2280. This system is often used for screw sizes, with the number indicating the major diameter of the screw. For example, a #8-32 screw has a major diameter of 0.1640”. It is important to note that the number does not always correspond to the exact diameter, as seen in the example.

Letter Naming

Sizes over .2280 that are not named for their fraction of 64ths are named by letters, ascending from A (.2340) through Z (.4130). This system provides a clear progression of sizes beyond the numerical system.

IADC Classification System

The International Association of Drilling Contractors (IADC) has developed a classification system specifically for PDC drill bits and tricones. This system uses a single letter and three numbers, each bearing critical significance. The letter designates the body type: M for matrix, S for steel, and D for diamond. The subsequent numbers delineate the geological formation type to be drilled, the cutting structure, and the bit profile. For example, M241 indicates a matrix body type, a geological formation type of 2, a cutting structure of 4, and a bit profile of 1.

Pilot Hole Sizing

When working with screws, it is important to select the appropriate pilot hole size. The pilot hole diameter and recommended drill bit size vary depending on the type of screw and the material being drilled. For example, when using Type A Self-Tapping Screws, the recommended hole diameter for hardwood is different from that of softwood. It is crucial to verify the correctness of the drill bit size before drilling and to pay attention to the depth of the pilot hole.

Frequently asked questions