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Forstner bits

With a Forstner bit, you can drill small and large holes as well as blind holes precisely and with minimal tear-out in both hardwood and softwood. Forstner bits are suitable for a wide range of applications. Find practical tips for working with Forstner bits in our Expert Knowledge section.


Did you know? The Forstner bit is named after its inventor, Benjamin Forstner, who developed it in the 19th century.

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Forstner bits

EXPERT KNOWLEDGE FORSTNER BITS

What are Forstner bits used for?

Typical applications can be found in both furniture making and carpentry. Since Forstner bits are often used for drilling hinge cup holes, they are also commonly used as hinge boring bits. Blind holes for joints, for example with screws or wooden dowels, are another common application.

Forstner bits are also frequently used for drilling out knots. Simply drill out the unwanted knot with the Forstner bit and fit a cross-grain plug from the same piece of wood, or from a piece with a similar grain pattern, into the drilled-out knot hole — and the knot will be barely visible.

What should I bear in mind when working with a Forstner bit?

There are two things you should consider before positioning the Forstner bit:

  • Pre-drill a pilot hole for the Forstner bit so that the centre point is guided.
  • Always pay attention to the correct speed.

Pre-drilling

You should always pre-drill when using a Forstner bit: the centre point is usually not a cutting point. Pre-drill a hole to match the depth and diameter of the centre point. This means you do not have to force the blunt centre point into the workpiece — instead, it enters the pre-drilled hole easily and accurately and remains guided as you drill.

Diameter

For hinge cup holes or blind holes, the required diameter is usually predetermined. If you want to remove a knot hole, choose a diameter as close as possible to the size of the area to be removed. However, it is better to remove one millimetre too much than one millimetre too little. Remember that you still need to pre-drill in the centre to guide the centre point.


Forstner bits are available in many different sizes. Diameters between 10 and 50 mm are the most common, with special sizes available up to 150 mm.

Speed

The correct speed is important when drilling with a Forstner bit. As a general rule, it should be used at a lower speed than a twist drill bit. The bit must not run hot. Excessive heat not only risks scorch marks, but also quickly reduces the sharpness of the main cutting edges. Always start at a low speed and increase it only gradually.


The speed can be slightly higher when working in softwood than in hardwood. For a Forstner bit with a diameter of 10 mm, use approx. 2000 rpm in hardwood and 2400 rpm in softwood.


The larger the diameter, the lower the speed should be. For a diameter of 20 mm, the speed should be around 1000 rpm in hardwood and 1800 rpm in softwood. For diameters of 40 mm and above, set the speed to a maximum of 500 rpm in hardwood and 1000 rpm in softwood. Less is more.

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