Spade bits and Forstner bits are two drill bit types that you can use to cut large-diameter holes into wood and wood products. Each type has its advantages and limitations, and choosing between them is not always easy for the beginning woodworker. In this article, I compare these two bit types based on my experience, and I hope that I can help you in picking the right bit for each hole.
The main differences between spade and Forstner bits for the user can be summarized as follows: Spade bits are versatile, compact and affordable general-purpose bits, and allow you to easily drill short or deep holes up to 1½” in diameter in a variety of settings. Forstner bits are speciality bits, which excel at cutting very clean flat-bottomed holes up to 4” in diameter, but are more expensive, trickier to drive and drill only to a shallow depth.
In this article, I go through the differences between spade and Forstner bits one by one, so if you are interested of a detailed breakdown, read on. For those in a hurry, I have also summarized the list in the table below.
|sizes||1/4” … 1½”||3/4’’… 4’’|
|max. hole depth*||5”||2”|
|hole bottom||not flat||flat|
|drive (typ.)||1/4” hex||3/8” plain|
|physical size||flat, compact||bulky|
|cost||$1…$3 per bit||$10 … $100 per bit|
Spade bits are available in diameters from 1/4” to about 1½” (6…38 mm), a size range overlapping with that of standard spiral bits at the small end and auger bits at the large; the most common sets run from 3/8” to 1”.
Forstner bits are larger on average, with sizes starting at 3/4” and going up all the way to a whopping 4” (19…100 mm). Forstner bit sets usually include sizes between 1/2” and 1½”, although these bits are more commonly purchased individually.
2 Hole depth
Spade and Forstner bits differ in drilling depth capacity. Spade bits come with a long shank and can typically cut holes up 5” in depth. Forstner bits, on the other hand, have a short shank, and are suited for drilling relatively shallow holes only. Although shank extenders are available for both bit types, spade bits remain more suited for long holes.
3 Hole bottom
Spade and Forstner bits have different cutting edge geometries, which leads, among others, to different hole bottom profiles. Spade bits have a long tapered tip at the cutting edge center line, which may be threaded or plain. This tip assists the drill operator by guiding or even pulling the bit, but it has the negative side effect of creating a deep central recess into the hole bottom. While this recess is a non-issue in through-holes and often inconsequental in deep holes into thick material, it may become a problem when shallow blind holes are to be cut into thin board or sheet; a classic example of the latter case are hinge and handle recesses in cabinet doors.
Forstner bits, by contrast, have straight cutting edges perpendicular to the shank, and only a very small center spike. While this makes them less stable and more difficult to start than spade bits, it also allows them to create holes with a flat bottom. Thanks to this property, Forstner bits are optimally suited for cabinetry and other applications where the clean flat blind holes are required.
The shapes of the hole bottoms are illustrated in the photograph below.
4 Cut quality
While both spade and Forstner bits generally produce relatively smooth holes, the two types do differ slightly in cut quality. With straight long cutting edges and separate edge reamers, Forstner bits have the advantage when very clean cuts are required, and leave a surface finish to both the hole walls and the bottom equal to that of a plane. Although this is not really necessary in the vast majority of holes, it may be required in decorative items where the hole is left exposed, such as candle holders.
Spade bits, by contrast, have curved cutting edges with a long tip in the center and smaller spurs at the edges, the exact geometry of which varies with the manufacturer. The cutting edge has been milled into the flat bit head, and consequently has a relatively dull cutting angle close to 90 degrees. These both factors lead to somewhat rougher cut than with the Forstner bit.
The different cut quality can be indirectly observed in the shavings produced by the two bit types, as illustrated in the image below.
Spade and Forstner bits differ in ease of bit guiding. By virtue of the center tip, spade bits are strongly self-centering, and start a hole easily even on rough surfaces or at an oblique angle. The steady self-guiding also makes them well-suited for use with a hand-held driver.
Forstner bits, on the other hand, are trickier to start: due to the small center spike, they require a completely flat workpiece surface perpendicular to drilling direction to get a hold, and should preferably be used with a drill press for secure guiding. Although they can be used hand-held, you should note that even a small pre-drill on the center line will prevent the spike of the Forstner bit from taking a hold, and make the hole almost impossible to start. Once started, though, the bit guides itself securely.
Spade bits and Forstner bits also differ in the drive type. Spade bits often have a 1/4” hex end to their shank, which allows you to use them with cordless drivers and impact drivers, or even with a quick release adapter, and always get a good grip for secure torque transmission.
Forstner bits, on the other hand, almost always come with a somewhat thicker plain shank, usually 3/8” or 10 mm in diameter. While the diameter helps in getting a good grip, the largest Forstner bits at 2” and above require very large torques, and are prone to slipping in the chuck; personally, I have often struggled to get keyless chucks tight enough for them.
7 Physical size
Spade and Forstner bits are of different physical size and shape. While often not a primary concern, ease of storage is a valid consideration, as the space in your workshop or tool case is always at a premium.
Spade bits are flat, and store very compactly; actually, they are the clear winner in ease of storage across all drill bit types. The sets often ship with a flat case or a pouch which makes them easy to store in your workshop or your toolbox.
Forstner bits, by contrast, are not very efficient to store: they are neither slim like spiral bits nor flat like the spades; instead, they extend in all three dimensions. Whether you store them standing or on the side, they will take up around 4” x 2” x 2” per bit; you can reduce this to 4” x 2” x 1” per bit with clever stacking.
Spade and Forstner bits differ in construction complexity and therefore in price. Spade bits are simpler – essentially forged, milled and ground from a single steel piece – and very affordable: you can get the common 6- or 9-piece set of quality make at $10 to $15, and an extensive 12–13-piece set, including the larger sizes above 1”, at less than $30.
Forstner bits, by contrast, are either welded or soldered from multiple pieces (shank, head, spike, main cutting edges and edge reamers may all be separate components) or a complex casting with much machining to finish. This leads to a relatively high price per bit, from around $10 per bit for entry-level brands up to $100 per bit for premium brands such as Fisch. Due to the high price, it may be practical to purchase Forstner bits one by one, each size when need arises.