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What are counterbores?

Counterbores have some similarities with their countersinking counterparts, and they are used in much the same way and for a similar purpose. Unlike countersinks however, counterbores feature a flat bottom instead of a conical point. They create an enlargement at the opening of a hole with a flat bottom, allowing tapered fasteners to sit flush with the surface material.

Why use counterbores?

Counterboring the workpiece helps fasteners inserted into the workpiece sit flush with the surface creating a greater aesthetic and a smoother surface for moving parts. Milling machines are usually used in counterboring with the tool slotting into the machine and then rotating into the opening of the pilot hole.

Types of counterbore

There are a number of different types of counterbores:

Piloted - These allow all in one drilling and counterboring of a pilot hole. One the drill reaches its depth the counterbore widens the opening of the hole.

Aircraft - These have a standard shank size and are designed for use with metals like aluminium, typically found in aircraft.

Cap screw - These counterbores help create the recess for the head of a standard socket head cap screw.

Blade - These counterbores use straight blade-like protrusions from the side of the body to cut

Back counterbores - These counterbores are designed so that the bore extends through to the opposite side of the counterbore chuck or holder.

FAQ

How does counterboring work?

Using a standard milling machine you can load the counterbore tool of your desired dimensions. Align the pilot with the hole and turn the spindle to let the machine run until you have bored to the required dimensions.

What is the difference between countersinking and counterboring?

Counterboring is used for screws that have tapered heads. On the other hand, countersinking is used for flat-headed screws. Whilst used in a similar way there are a couple of distinctions, the main one being that a counterbore produces a flat-bottomed hole, whilst a countersink creates an inverse conical recess.

When you need to decide between counterboring and countersinking, keep what type of screws you are working with in mind.