Benjamin Forester, the creator of best Forstner bits, designed this cutting tool to drill flat bottom holes in wood. Its special geometry makes it possible to drill blind holes.
High precision Forstner bits have other characteristics. For example, they are capable of cutting overlapping holes and can drill holes in the edge of a block of wood, rather than operations that cannot be done with twist drills.
The basic quality of this cutting tool is its high precision and the quality of finishing. For this reason, it is an accessory widely used in fine carpentry and joinery work. To achieve this precise cut, these special drills have side cutting blades, unlike other bits such as twist drills, which only have cutting edges at the tip.
There are different types of Forstner bits. Some have serrated teeth on the perimeter, others only a pair of blades. However, they all have three cutting surfaces:
Two straight edge blades at the base: these cut the bottom of the slot
A cutting edge, serrated or not, on the circumference of the base that shears the first perimeter fibers
Two cutting edges, on the cylinder walls, which cut the walls of the hole
Drilling with this type of bur requires a lot of effort. For this reason, some manufacturers offer a variant that facilitates the drilling operation. They are the best Forstner bits with self-feeding.
The operation of self-feeding is based on a screw, located in the center of the bit, which automatically threads into the wood. This system is also used in wood spade bits.
Self-feeding screw drills are much easier to use and also more effective. However, they do have a couple of drawbacks.
The first drawback is that they require a reversing twist drill driver to remove them (although this feature is available on almost any screwdriver). And the second, that they mark the wood more (in the center of the circumference).
Most common sizes and measurements
The most common diameters of these bits range from 10 mm to 65 mm.
Differences from twist drills
Forstner bits have a quality that makes them different from twist bits: they are guided by their edge. This sets them apart from most center-guided wood bits.
Perimeter guidance is the feature that allows Forstner bits to drill overlapping holes. Instead, when trying to do this task with a twist drill, the drill is pulled into the adjacent hole.
The other difference they have with respect to conventional wood bits is the absence of a spiral. As we have already said, this makes it difficult to remove the shavings and forces the cutter to be removed several times in order to continue advancing.
Quality, materials and price of Forstner bits
The material with which they are made and their degree of sharpening are two of the factors that most influence the quality of the bit. Like any other tool, the better they are the more expensive they will be.
Good quality ones are not cheap. There are some kits that include units of various diameters for little money: they can be used to drill softwoods such as pine or fir, but in hardwoods, they will not be effective.
These are some of the materials that manufacturers use to make these bits:
Tungsten carbide is a material that effectively dissipates heat. The cutting edge of Forstner Carbide Bits remains sharper longer and better withstands heat compared to other materials such as fast cutting steel. It is also known as widia, tungsten carbide, or cemented carbide.
Carbide bits are not built entirely with this material: their shank is made of steel while the teeth and cutting edges are made of carbide plates welded to it. Carbide inserts can be welded with oxyacetylene welding and softer metal filler rods, such as brass.
Although it is an extremely hard material, it is very fragile: we must avoid the bit falling to the ground, otherwise, the small plates could splinter.
Cobalt is a material of great hardness and resistance. Due to its ease of penetrating hard materials, it is also used in other types of bits, for example, in bits for drilling hard metals such as stainless steel. Cobalt bits are less prone to heat damage and are therefore useful for drilling in hardwoods. Like carbide, they are also fragile and delicate.
M2 high-speed steel
Fast cutting molybdenum steels, also known as high-speed steels (HSS), are a group of alloys that are classified within group M of the AISI standard. Furthermore, with the exception of the M6 type, tungsten is present in groups M1-M10. In neither case is cobalt used in these alloys.
M2 steel Forstner bits have a cutting performance similar to that of tungsten carbide. However, the cost of these bits is somewhat lower. To increase the performance and life of the tool they can be coated with titanium nitride or titanium carbide.
The process used to make this tool is forging followed by heat treatment. These bits are designed to work with minimal friction and thus avoid metal heating. They have the advantage that they can be resharpened (up to 25 times depending on models).
Although they are marketed as such, technically they are not bits with Forstner geometry but diamond core bits. Diamond dust is the hardest cutting material in the industry.
Diamond bits are not sharp but abrasive. They are not used in wood but in stone materials such as porcelain, granite, marble, reinforced concrete, and other stones and masonry materials (solid brick, sandstone, stoneware, etc.).
Due to the high cutting speed, they require to operate, they are not usually used with drills, but with angle grinders.
Where to buy Best Forstner bits?
If you want to buy Forstner bits, here are some links. There is a wide variety of sizes and qualities. At Amazon, you can find many other brands and models.
Forstner bits can also be physically purchased from industrial hardware stores and specialty woodworking establishments.
What brand to buy?
Two good brand of cylindrical shank bits with Forstner heads is Freud. The manufacturer specializes in wood cutting tools. Their Forstner strawberries are professional and of good quality but they are not cheap.
Less specialized brands in wood, such as Bosch, offer lots of Forstner bits with a good quality/price ratio. On the other hand, Wolfcraft, CMT, Hitachi, DeWALT, Makita (and many other brands) offer the possibility of acquiring Forstner drill bits individually.
A useful accessory: the depth gauge
To control the drilling depth you can use the regulation of the column drill or the milling machine. You can also buy a special accessory that allows you to adjust the depth with the hand drills.
The depth stops can be purchased in different diameters and include a ball bearing guide that avoids leaving marks on the piece of wood.
Its use is very simple: just set the desired working distance by sliding the stop along the shank of the Forstner bit. It is secured by means of a captive screw. When the set depth has been reached the stop shield collides with the wood and prevents the bit from advancing. Ball bearings prevent the stop from turning and damaging the surface of the wood.
How to drill with Forstner bit? Tools and instructions for use
Forstner bits are not always used with screwdrivers. The large contact surface with the piece of wood and the absence of a chip removal system require a great force to advance the bit through the wood. This makes it difficult to use with hand-held boring drills, especially if they are also underpowered.
This cutting tool includes a centering point that guides the cutter throughout the cut. The blades arranged along the circumference of the cylinder cut the wood fibers, thus forming the walls of the hole. Since there is no chip removal mechanism, the bit must be removed at regular intervals to remove sawdust from the hole.
It is not too difficult. In reality, the only precaution to take is to keep the drill as vertical as possible, to have the workpiece firmly fixed to the workbench, and to control the depth.
Safety measures and risk prevention
Since it is a cutting tool, the Forstner bit produces wood dust. Exposure to wood dust causes it to deposit in the nose, throat, and other airways. This exposure is associated with various types of respiratory diseases like cancer.
On the other hand, the electrical machinery used to drill with this type of drill is associated with other occupational risks such as hearing loss (exposure to noise), eye injuries (projection of wood chips or metal particles), and other mechanical risks (amputations, entrapment).
In order to prevent the occupational risks associated with the use of Forstner bits, it is necessary both to adopt certain safety measures and to use PPE (personal protective equipment).
The necessary protective equipment is the respiratory protection mask (obligatory to avoid the inhalation of sawdust), the hearing protection helmets (when using very noisy tools such as the milling machine) and the safety glasses whenever there is a risk of chipping and projection of wood or metal particles (for example, due to breakage of the cutter).
When these cutters are used in lathes, milling machines, or vertical column drills, the part must be firmly clamped in the machine jaw. If a portable tool is used, the part must be secured in a carpenter’s screw or on the workbench with the help of clamps, presses, or sergeants.
If we use gloves, extreme care must be taken to avoid the risk of entrapment. Avoid wearing loose clothing, long hair, or other accessories (chains around the neck, ribbons) for the same reason.