The structure and application of countersinks

Countersinks differ in shape and application. For example, countersinks can be used for deburring drilled holes or for countersinking screws. How countersinks are constructed and in which applications they are used are described in this article.

Countersink, cross-hole cutter, and flat countersink by RUKO

Countersinking is a process for the post-processing of existing drilled holes. This means that the countersink does not cut into the full material. There are different countersinks which fulfil different functions. They are divided into:

  • countersink
  • cross-hole cutter
  • flat countersink
  • shell sinker


The head of the countersink has a conical shape. The conical shape causes deburring or countersinking into the existing drill hole.

Features of a countersink

The countersink angle is decisive for the countersink. There are different angles which are intended for different applications. These are for the countersink:

  • 60° for deburring
  • 75° for rivet heads
  • 82° for countersunk screws US standard
  • 90° for countersunk screws
  • 120° for countersinking of sheet metal rivets

The number of cutting edges is normally odd for a smooth run when countersinking. Our countersinks have three cutting edges.

The chip is removed via the flute profile. The shank is available in four different versions: cylindrical shank, Morse taper, bit shank and 3-face shank. A bit shank or 3-surface shank, for example, prevent the tool from slipping in the drill chuck and make them ideal for use in hand drills.

Burr at the end of the drill hole

What does deburring mean?

Usually small shreds of the material are formed at the end of the drill hole edge , which were pressed out by the twist drill for a through hole. These shreds are called burrs. To remove these burrs the countersink is used which deburrs the edge of the hole, i.e. removes the burr. These countersinks have a countersink angle of 60°. In principle, however, deburring can be carried out with any countersink angle.

Especially when drilling metal a burr is formed. This can be very sharp-edged and represent a risk of injury. Therefore you should remove the burr after drilling. Not only for safety reasons but also to enhance the appearance of the drill hole.

The countersinking of the hole widens the entry area and facilitates the insertion of a thread drill, for example. How to cut an internal thread is explained in our article "How to cut an internal thread in 3 steps".

Countersink a screw

To countersink a screw flush with the surface of the workpiece, a countersink is used. The final result is as follows.

Countersunk screw in through hole

Cross-hole cutter

The special feature of this countersink is a drill hole across the countersink.

Features of a cross-hole cutter

Cross-hole cutters have a about 45° oblique hole to the countersink axis. This hole through the head thus forms two cutting edges. The chip is transported away via the hole, which prevents them from getting jammed between the drill and the workpiece. Cross-hole cutters are also extremely seldom caught in the material.

With a cross-hole core, clean results can be achieved and are easy to handle. The countersink is free of burrs and chatter.

The countersink angle for the cross-hole core is 90°. With the cross-hole core you can also make countersinks for the countersunk screw.

Cross-hole cutter

Flat countersink

Flat countersinks have a cylindrical shape compared to countersinks. Countersinks are used to produce countersinks for cylinder head screws, hexagon head screws and nuts.

Features of a flat countersink

At its tip there is a fixed guide which is inserted into the pre-drilled hole. This ensures a stable guidance of the drilling machine and precise working.

The cutting edges cut the material and make space for the head of the screw. Cylinder head screws and hexagon head screws are flush with the surface of the workpiece.

Cylinder head screw in through hole

Flat countersinks are available in two versions, for through hole and thread core hole. The difference lies in the two workpieces which are connected with a screw.

With the through hole, there is a thread only in the second material. With the thread core hole, both materials have a thread.

The diameter of the fixed guide for the through hole is larger than for the thread core hole. Thus, for a through hole for thread M12 (fine grade), the diameter is 13.0 mm. For the thread core hole, the diameter of the fixed guide for the M12 thread is 10.2 mm.

The diameter of the fixed guide for the through hole is larger than for the thread core hole

Shell sinker

In shell sinker a countersink is mounted on a twist drill. In this way, the drilling with the twist drill and the subsequent countersinking can be carried out in only one working step.

Since counterbores are only suitable for drilling in wood, wood-based materials or plastic, we do not include them in our product range.


Since our countersinks are made of the tool steels HSS, HSSCo 5 and tungsten carbide, you can use them to cut a wide variety of metals. The countersinks are available with the coatings TiN, TiAlN and our RUnaTEC.

In the application table (PDF behind the picture) you will find an overview of all our countersinks and their fields of application.

Extremely important for countersinks is the countersink angle. The different countersink angles are suitable for different applications.

Countersink angles Application Product
60°for deburring Countersink
75°for rivet heads Countersink
82°for countersunk screws US standard Countersink
90°for countersunk screws Countersink and cross-hole cutter
120°for countersinking of sheet metal rivets Countersink
180°for cylinder head screws, hexagon head screws and nuts Flat countersink

Cutting speed

When countersinking it is important to apply less force because countersinking requires only little grinding pressure.

Besides the grinding pressure you should also pay attention to the cutting speed. Especially with hard materials, like hardwood or metal, you should choose a very low speed. The slower the better. The countersink does not clatter and it does not get too hot. Thus the service life is not negatively affected.

How deep can I cut?

Depending on the application, you can cut to different depths. When removing the burr, for example, you only go a few millimetres into the material so that you only remove the burr.

If you want to countersink a screw, you can use the countersink to cut into the workpiece just before or up to the maximum diameter.

Maximum diameter

Conclusion and summary

Countersinks are mainly used for countersinking drill holes, countersinking screws and deburring.

Countersinking widens the drill hole and facilitates subsequent tapping. When countersinking screws, space is created for the screw head so that it closes with the surface of the workpiece. During deburring, the burr on the drill hole is removed for safety and/or optical reasons.

It is important that you select the correct countersink angle depending on the application and work with low cutting speed.

You can find our complete range of countersinks including product finder at Products > Countersink.

If you have trouble finding the right sheet drill, please do not hesitate to contact us.