How to Countersink a Screw

Countersunk screws have different heads. Screws that can be countersunk have the following screw heads:

countersunk head
lens countersunk head
cylinder-head
round head
hexagonal

Nuts can also be countersunk.

If you need to cut an internal thread in the drill hole for the screw, countersinking is the second step in the work process after you have drilled (and deburred) the drill hole. Once you have made the countersink, all you have to do is cut the internal thread. How to do these other steps is explained in our article "How to cut an internal thread in three steps".

The right countersink

You need a suitable countersink to countersink screws. Countersinks have a countersink angle of 90° (and for US standard 82°) for countersinking screws, e.g. the countersink angle for rivets is 120°. For the flat countersink, the countersink angle is 180°.

Screw	Countersink
Countersunk head screws	Countersink / Slotted taper
Lens countersunk head screws	Countersink / Slotted taper
Cylinder-head screws	Flat countersink
Round-head screws	Flat countersink
Hexagon screws	Flat countersink
Nuts	Flat countersink

Countersunk screw in through hole

Cylinder-head screw in through hole

Countersinking with the countersink

You only need the diameter of the screw head. The best way to measure is to use a calliper.

When you have measured the diameter of the head, use the corresponding countersink with a lager diameter. Use the tool to make your countersink.

Countersinking depth

When countersinking a screw you cut with the countersink the depth by feel.

Lowering with the flat countersink

With the flat countersink, in addition to the head diameter, you also need the height of the hexagon socket screw to determine the countersink depth.

With a hexagon head screw (also called a wrench screw), which you can only tighten with a socket wrench or pipe wrench, you need a larger diameter for the countersink. For this you need to measure the diameter of your wrench.

Note: In very rare cases a flat washer is used. Then the height of the washer is added to the countersinking depth. It is also likely that the washer will have a larger diameter than the head of the hexagon head screw. This diameter is then also needed for the countersink. Use the next larger countersink to make sure that the washer fits into your countersink.

When selecting the correct flat countersink, note that it is available in two versions: for through hole and thread core hole. With the through-hole there is a thread only in the second material. With the threaded 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. For a through hole for thread M12 (grade fine) the pin diameter is 13.0 mm. For the thread core hole, the pin diameter for the M12 thread is 10.2 mm.

Applications

There are countersinks made of HSS, HSSE-Co 5 and hard metal, which are finished with a TiN, TiAlN or RUnaTEC coating, for different fields of application. The following overview will help you to find the right countersink.

Make sure that the cutting speed is very low when lowering, especially when using hard materials such as hardwood or metal. Apply less force when processing, because countersinking only requires light pressure.

Conclusion and summary

Sinking a screw is basically very simple.

You need the diameter of the head of your screw and off you go. With the head's diameter, whether for countersinks or flat countersinks, you have the measurement for the selection of the right countersink.

In the application, you drill with the countersink up to the maximum diameter. For the flat countersink, you will also need to measure the height of the screw head to know how far into the workpiece you need to drill and what type of hole (through hole or thread core hole) you need to drill. Then, you can countersink your screw in the material flush with the surface.

You can find our countersink 90° and 82°, and flat countersink at Products > Countersink.

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

FAQs

How to Countersink a Screw | RUKO? ›

Start by drilling a pilot hole for your screw so you can thread it in easily without splitting the wood. After that, you can use a countersink cutter to drill a larger hole for the screw head. Once the screw is in place, you can either leave it exposed or conceal it using wood putty!

How do I know which countersink bit to use? ›

> A simple rule of thumb is to choose a countersink which is 50% larger than the diameter of the hole. > Countersink diameter = 1.5 x Bolt Size (hole) diameter. > Example: For a 1/4-20 bolt, multiply the diameter (. 250) x 1.5 = .

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What will happen if you don't use a countersink? ›

When drilling a screw into softwoods, like pine boards, you can indeed get the screw to go below the surface, but it typically chews up the wood at its entry point, leaving you with a rough finish. Countersinking your pilot hole prevents that chewed-up look from happening.

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What is the standard angle for countersinks? ›

Countersunk-head screws that follow the Unified Thread Standard very often have an 82° angle, and screws that follow the ISO standard very often have a 90° angle. Throughout the aerospace industry, countersunk fasteners typically have an angle of 100°.

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How do you countersink nail heads? ›

Place the tip of the nail set in the center of the nailhead and tap it lightly with the hammer until the top of its head is about a sixteenth of an inch below the surface of the finished wood. If you've never done this before, you should practice using scrap finish lumber.