What is an Expansive Bit – What it is Used For?

Put simply, an expansive bit is a tool used to bore holes of different widths into variable materials.

The bits themselves are created with cutters that are adjustable. This means that they can be expanded or retracted depending on the need.

This allows for one bit to do the job of several, saving time and expense.

The functional part of the bit is known as the cutter. You may hear expansive bits called ‘adjustable bits’.

You can find expansive bits used mostly to bore holes in wood.

However, they can also be used for other materials such as plasterboard or plastic.

Metal is generally too hard for expansive bits to work. Despite their obvious versatility, expansive bits are now considered specialty tools.

This is because other types of wood bits have become less expensive.

How Does it Work?

At first glance, the expansive bit works in a similar fashion to an auger. The outer edge of the bore hole is cut by a spur.

The waste material created is cleared by two sharp cutting lips that clean it all away.

As the bit turns, the thread on the guide screw bites into the material and draws it down which means less force is needed.

The primary spur, which is located closer to the guide screw, will contact the material first when the primary lip creates a small hole.

Then, the outrigger cutter spur, which is followed by the cutter lip, creates a wider hole.

The width of the wider hole is set by the user. In other words, two holes are being cut, a pilot hole and the drilling hole which eventually becomes a single hole in the material.

What are Expansive Bits Used For?

You will find expansive bits used in projects that require different sizes of holes in the same material.

Many electricians and plumbers use expansive bits.

They are used to drill through wooden boards to create holes for cables or pipes. They can even create wider holes for bundles of cables.

Another common use of expansive bits is to create a round hole in the doors to accommodate the locking mechanism.

Because of their versatility, the bit can be used to create the large hole that holes the lock.

Plus, they can be used on the side of the door to create the holes necessary for the lock to engage with the door frame.

Because you only need to purchase one bit, an expansive bit may save you a considerable amount of money over purchasing a set of bits.

Plus, the expansive bit may save time as well since you can adjust it as needed without having to remove it from the drill.

How to Use an Expansive Bit Correctly?


You’ll need to attach the bit to the appropriate brace.

Be sure to set the width and secure it with a chuck.


You’ll want to bore the bit into the material slowly to keep the hole clean.

Plus, because of the design, not as much force is needed to keep the bit on target when creating the hole.

Break the Surface:

Check the reverse side at regular intervals until the guide screw breaks through.

Then turn the material over and drill from the bottom side.

Otherwise, if you keep drilling the material will tear outwards creating a messy exit point.

Of course, if you are drilling through the material in which you cannot turn it over, such as plasterboard, just keep drilling until the bit is all the way through.

To protect your drill, set the revolutions at the lowest speed to help prevent overheating.

If you feel the motor start to overheat, stop drilling. Remember that you only need apply light pressure as the guide drill is boring into the material.

What are the Parts of An Expansive Bit?

There are several parts to an expansive bit.

Understanding the function and purpose of each part will help you better use the expansive bit.

Guide Screw:

Sometimes called the ‘gimlet point’, this helps pull the bit through the material.

As the bit rotates, the spiral head of the guide screw pulls the bit into the material right from the tip.

Because of the guide screw, less pressure is needed.

The downside is that guide screws may be too aggressive depending on the hardness of the material.

The result is a lack of control when driving the bit using a drill press.

This is why you will find some expansive bits that have gimlet points with no thread.


There are two types of lips on an expansive bit, the primary and outrigger.

The primary lip is located near the guide screw. Its job is to shave out the material between the place where the guide screw penetrates and the primary spur.

The purpose of the primary lip is to create a guide hole that keeps the bit pressing forward.

The outrigger lip can be seen along the bottom edge of the outrigger cutter.

This operates in the same manner as the primary lip.

The chisel-like blade is flat and scrapes out the material from the bore hole.

It is also responsible for cutting the material to the proper diameter.

Most of the excess material that is removed from the bore hole is accomplished by the outrigger lip.

Locking Screw:

Sometimes called the ‘adjuster screw’ depending on the type of expansive bit being used, this will be located just above the outrigger cutter.

There are two types of patents on this part of the locking screw.

The first is the Clark patent which has a detachable bit.

The bit is held in place by the locking screw. Tightening the screw pulls the two sections of the bit together which clamps the adjustable cutter into position.

The Wright patent features an extendable cutter that has cog teeth that cut along the upper surface of the material.

This is attached to the cog, so the blade pulls forward or backward as the adjuster screw is turned.

Outrigger Cutter:

Sometimes called the ‘cutter’, this is the expandable or contracting part of the expansive bit.

You can alter the position of the cutter in terms of its location to the bit’s center.

This allows the bit to create holes that are narrow or wide.

It’s called the cutter because its appearance is similar to a part of a ship’s bar, which is also called a cutter, that extends outward to keep the boat from capsizing.

The outrigger cutter has the same general appearance although its function is somewhat different.


There are two types of spurs, the primary and outrigger. The primary spur is set in close proximity to the guide screw.

It is one of the first parts to come into contact with the material.

The outrigger spur is located at the end of the outrigger cutter.

The spur is used to cut around the outer rim of the hole.


If you look at the cross-section of the shank, you’ll know what type of driver should be used.

You’ll need to look at the end of the shank to see its exact shape.

If the shape is hexagonal, then it is primarily designed for drill presses and powered hand drills.

However, if the shank is squire, then it is designed to be used with a hand brace.

What are the Different Types of Expansive Bits?

There are two types of expansive bits, each named after the person who filed the patent for their respective designs.

They are known as the Wright patent and Clark patent bits.

They are different in terms of how the cutter is lengthened or shortened.

Wright Patent Bit:

Invented by Alfred M. Wright and patented in 1939, this version has adjustable cutters with teeth cut into the top.

You can adjust the length of the cutter by turning the adjuster screw.

The screw is part of a cog mechanism that pushes the cutter either forward or backward using the teeth located on the upper edge.

Clark Patent Bit:

Patented by William A. Clark in 1858, this version is more commonly found compared to the Wright patent bit.

This bit uses adjustable cutters that have no teeth on their upper edge.

They attach to the main body of the bit using a wedge and clamp system.

Adjusting the bit requires loosening the lock screw and sliding the cutter to its desired length.

Then the screw is tightened which set the bit in place.

What are the Different Available Sizes of Expansive Bits?

The size of the bits will depend on the cutters being used.

Small cutters can bore holes from 5/8” up to 2” in diameter depending on the brand being used.

Large cutters can bore holes from 7/8” up to 6”.

Most expansive bits are roughly 8” long, which is about the length of the hole that can be bored.

However, expansive bits that are made for plumbing tend to be shorter, around 5” in length.

You may want to use some scrap material and create test holes first if you have never used an expansive bit before.

This will give you a good idea of how they work before applying them to your project.

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