The terms countersink and counterbore are considered by many people to be about the same thing. However, there are important differences between the two which makes it important to understand when to use one or the other.
Difference Between Countersink and Counterbore
Counterboring and countersinking are done on top of a drilled hole to allow the screw or bolt head to stay below the surface of the workpiece.
The most noted difference between a countersink and counterbore is the shape of the recess and the type of fastener head it is designed to accept. A counterbore hole is deeper and has a flat-bottom where the shoulder of the screw head can seat. It allows a socket head cap screw, bolt head, pocket hole screws, or washer to seat on a flat surface.
A countersink will create a conical-shaped hole that is identical to the angled shape found on the underside of a screw which has a flat top. The hole can vary in depth so that the flat head can rest even with the surface or be driven in deeper and covered with a plug to hide its appearance.
Comparison Chart
| Countersink | Counterbore |
| Conical shaped head | Cylindrical shoulder-type head |
| Generally less depth (depends on the angle) | Deeper than countersink |
| Sufficient holding strength | Provides stronger holding |
| Works best for PCB, woodworking, plastic, thin sheet metal, etc. | Best for heavy-duty applications such as automotive, machinery, construction, etc. |
So, while both are used to set a fastener below the surface of the material, a countersink creates a conical seat, while a counterbore makes a cylindrical, flat-bottomed recess.
A counterbore will create a hole that is flat on the bottom. This will allow the bolt or screw that has a flat underside as opposed to the angled shape of a countersink to rest inside. A washer is often used in this situation. It should be noted that the hole created by a counterbore is normally larger than the head of the screw which allows for a washer to be placed.
Both countersinking and counterboring are done co-axial to the screw hole, the difference is in their geometry.
Symbols for Counterbore and Countersink
The counterbore is denoted by a ⌴ shaped symbol whereas the countersink is denoted by ⌵ symbol (V-shaped).
- Counterbore Symbol: ⌴
- Countersink Symbol: ⌵
The above screenshot of an engineering drawing shows the sectioned view of a counterbore and countersink in a steel plate.
Cutting Tool Bit
The cutting tools used to create a countersink and a counterbore have different geometries.
- Countersink Bit: The V-shaped cutting tool used for machining a countersink hole is called a countersunk or a countersink bit. The 90-degree countersink tool can also be used to chamfer hole edges.
- Counterbore Bit: You can use a specialized counterboring tool, end mill, or a flat drill bit to create a flat-bottom counterbore hole.
When to Use a Countersink vs. Counterbore?
Choosing between a counterbore and a countersink depends on the material’s thickness and the type of material you are drilling into. For metal and wood, both can be used to equal effect. It is why many people think of a counterbore and countersink as much the same thing, even though their differences are important.
- Metalworking: Use a counterbore when you want to create a stronger joint between heavy metal plates where a flat seating surface is required for a socket head screw or bolt head with washer. You use countersink screws when working with thin sheet metals or when self-centering screw location is required.
- Woodworking: Most wood screws are countersunk screws with either 82-degree angle (North America) or 90° angle (Metric standard). However, there are exceptions like pocket hole joinery where the special drill bit creates a flat-bottom hole.
- PCB: When it comes to printed circuit boards the differences become more pronounced. A countersink is well-suited for printed circuit boards that will be placed in a device where space is limited. This is because the conical-shaped hole that is created for the countersink requires less depth and the flat screw head can sit flush with the surface of the PCB.
Another advantage is that countersink screws can be placed with greater precision and assembled faster, which is useful when working on small circuit boards such as those found in smartphones and watches.
However, a counterbore has its place on printed circuit boards as well. While it does require more space and will not leave as smooth a surface compared to a countersink, it does create a stronger connection.
Countersink Vs Counterbore Strength
For a given screw size, the counterbore usually has more holding strength than the countersink. This is because the force applied by the ⌴-shaped bolt or socket cap screw head is parallel to the axis of the screw hole. On the other hand, the force delivered by the countersunk screw is dispersed at an angle.
That said, the actual joint strength depends on the fastener type, material, depth, and overall design.
The strengths of a countersink vs. a counterbore will depend largely on its specific use. There are determining factors when the difference may count in choosing which one is right for the task. For a countersink, you will need to know the following.
- Diameter, Drill Angle, and Depth
- Location of the Sink
- Hole Shaft Diameter
- Plating or Not Plated
The needs for a counterbore are similar, with the added caveat of the space that is available on the board and in the device. Plus, the strength of the connection that is created is another important consideration.
Spotface vs Counterbore
Spotface is very similar to a counterbore in geometry, but with less depth. Spot facing is done to clean the uneven or curved surface to provide a flat seating surface for the screw head. For example, forged and die-cast parts where the screw head seating area is uneven.
This means in the case of spot facing, the screw head is usually protruding out of the workpiece surface. Whereas in a counterbore, the screw head is completely inside the bore.
How deep is a spot face?
There is no standard depth for spot facing. As a general rule, the spot face depth can be 1/32” to 1/16-inches (approximately 1 to 1.5mm). When you use a screw with a washer, tthe spotface is usually made deep enough to provide a clean, flat seating surface for the washer.
Which One is Better?
Put simply, a countersink is better if you want to retain a smooth surface, have a limited space to work with, and need the job done at a faster speed. It also looks more attractive, especially on a printed circuit board compared to a counterbore.
A counterbore is not nearly as neat, but it is a better option when you need to create a stronger connection. You will need more space to create a counterbore, but if you have it then it will be worth the effort. You can create a socket connection that will last longer and be more durable compared to a countersink.
For wood and metal, the choice depends mainly on the fastener head style and the required strength or appearance.
The differences become greater when creating holes for connections in printed circuit boards and thin sheets. So, be sure to evaluate the situation properly before choosing between a countersink and counterbore for your needs.
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Dan Miller is a professional toolmaker with extensive experience in the metalworking and tooling industry.
An avid DIY enthusiast and passionate hobbyist woodworker, Dan shares his expert knowledge here.