What is a Counterbore?

For screw mounting different types of screw holes need to be created. Countersunk screws need a countersink hole and other screws which needs to cover their heads need a counterbore. This is very popular in the product design industry due to its aesthetic view. Other than countersunk screws most of the other screws can be fixed to the counterbore holes.

What is a Counterbore?

This is a flat bottom cavity. It allows a bolt or a fastener to sit flush with the workpieces. This allows the head of the screw or bolt to cover the head inside the bore/hole. This makes a good aesthetic view for the final product.

Aesthetic View of the Socket Head Cap Screw with Counterbore.

Side/Section view of the Screw

How to Make this Hole?

There are different types of drill bits and drilling tools. For counterbore drilling you can use the counterbore tool. Normally counterbore allows screw head clearance. Whatever the thread type metric, unified, or some other type, there are standard tools for holes. You can use a drilling machine or milling machine. Today, we use CNC machines for a lot of machining tasks. So, you can use CNC or NC machines for this hole machining.

These drilling or milling tools are available in the local market as well as online markets like McMaster.

Counterbore Symbol in Engineering Drawings

In engineering drawings, we use different symbols, views, and lines. When come to symbols surface finish symbols and welding symbols the common. Other than those symbols each hole type also has symbols like countersink and counterbore holes.

This is the standard symbol.

Hole Dimension

The hole dimension is cover the thread size if included. Then screw clearance is included and hole depth and counterbore depth. Below image gives a better understanding of that.

Counterbore vs Countersink

There are some major differences between counterbore and countersink holes. However, there are two different types of screws are fitted to each hole type. The below table gives a better understanding.

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