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What are the parts of an expansive bit?

Shop for Expansive bits

Diagram showing the parts of an expansive bit including the guide screw, shank, outrigger cutter and adjuster screw

Guide screw or gimlet point

A guide screw on an expansive bit Most expansive bits have a guide screw that helps to pull the bit through the wood as it is rotated thanks to the spiral thread that originates at its tip. This type of bit will need a little less downward pressure than one without a guide screw.
An illustration of tear out caused by a drill bit with a guide screw such as an expansive bit Guide screws can bite into the workpiece aggressively, making these bits difficult to control if used in a powered driver such as a drill press. To combat this problem, some expansive bits have a gimlet point, which looks like a guide screw with no thread, instead.

Outrigger cutter

Close up of the outrigger cutter on an expansive bit The outrigger cutter (often referred to simply as the cutter) is the part of the bit that makes it ‘adjustable’ or ‘expansive’. The position of the cutter can be altered relative to the centre of the bit, allowing the tool to bore both narrow and wide holes. All cutters have a rule marked onto them to help the user to set the diameter.
Image of an outrigger on a boat to explain how the outrigger cutter on an expansive bit earned its name This cutter gets its name from the bar that projects from boats to keep them from capsizing, as they extend at a similar angle.

Spurs

Labelled diagram showing the location of the primary spur on an expansive bit

Primary spur

The primary spur is close to the guide screw. It is one of the first parts of the tool to penetrate the workpiece with its sharp edge.

Labelled diagram showing the location of the outrigger spur on an expansive bit

Outrigger spur

The outrigger spur is at the end of the outrigger cutter. This spur is responsible for cutting around the outer rim of the hole.

Lips

Labelled image showing the location of the primary lip on an expansive bit

Primary lip

The primary lip is located near to the guide screw and shaves out wood between the guide screw’s place of insertion and the primary spur. It carves out a pilot hole that helps to keep the bit on course as you drill.

Image showing the location of an outrigger lip on an expansive bit

Outrigger lip

The outrigger lip can be found along the bottom edge of the outrigger cutter. This is a flat, chisel-like blade that scrapes wood out from the bore hole and is responsible for cutting it to the right diameter. This lip is responsible for most of the removal of excess wood during the boring process.

Locking screw or adjuster screw

Image showing the location of an adjuster screw on an expansive bit Depending on which type of expansive bit you are using, there will be a locking screw or an adjuster screw just above the outrigger cutter.

For more information on the differences between types of expansive bit, see: What are the different types of expansive bit?

Close up of the head of a clark patent expansive bit showing the clasp adjuster mechanism Clark patent expansive bits have detachable bits that must be held in place by a locking screw. Tightening the screw pulls two jaw-like sections of the bit’s body together, firmly clamping the adjustable cutter in place.
Close up of the head of a wright patent expansive bit showing the cog adjuster mechanism Wright patent expansive bits feature an extendable cutter with cog teeth cut along its upper surface. This is attached to a cog that will pull the blade forwards or backwards as you turn the adjuster screw.

Shank

Diagram showing how to check the cross section of a drill bit by inspecting it end on The cross-sectional shape of the shank of your expansive bit will tell you what type of driver it should be used with. To work out the cross-section, look down the bit lengthways, with the shank pointing towards you.
An expansive bit with a hexagonal shank If your bit’s shank is hexagonal, it is designed for use in powered hand drills and drill presses.
Image of an expansive bit with a square shank If your bit’s shank is square, it is designed for use with a hand brace.