What is a Forstner Bit?
A Forstner bit is a cylindrical drill bit used to bore flat, deep holes into wood. It is particularly popular in furniture making and other large-scale woodworking projects in part because of its ability to drill deeply and precisely, even against the grain or through various inlays and joined pieces. Forstner bits typically come in a range of different diameters, but in most cases all are intended for mounted drill presses rather than hand-held tools. They typically require a lot of force, which can be dangerous and sometimes even unwieldy on a smaller appliance.
How the Bit Works
Most drills are guided through material by a central point, but Forstner bits are primarily guided by the outside rim. This means that they can be used to drill pocket holes, which are holes drilled at an angle; partially overlapping holes; and holes that are on the edge of the material. They have central spurs, but these are normally used to locate and mark a center-point rather than to serve as the actual drill guide.
When the bit enters wood, it bores down by essentially spinning fragments up and out with a sawing, rotary motion. The sides of the bit often look jagged and when they first enter the wood they often shear it and leave it somewhat splintery. As the bit’s radial sides continue through the hole, though, the edges tend to smooth out. Most of the time the result is a flat-bottomed hole.
Where and When It’s Used
The bits are named for Benjamin Forstner, a 19th century American gunsmith who is credited with inventing them. Gunsmiths were the first to popularize the bit; they prized it for its ability to bore a smooth-sided hole in woodwork. Today, the bits are more commonly used by furniture makers and home builders. Big projects like these often require a drill that can quickly move through multiple pieces, join different types of wood, and cut through multiple surfaces. The Forstner bit fits that bill for many carpenters.
They are available in a wide array of diameters, typically ranging from 0.25 to over 3 inches (6 to 75 mm), and they are often sold in sets that come with an array of sizes. Even bits with very small diameters are often able to bore through deep, thick wood at most angles. Their unique shape makes them very efficient at removing a lot of material, known as “hogging,” in a short amount of time. They’re also really useful for making precise holes for hinges and other moving parts.
Carpenters often praise this bit’s ability to drill precisely and cleanly through imperfections, like knots and baubles, in wood. A lot of this owes to the rim. The Forstner bit’s rim guides itself, which means that it won’t be thrown off when it hits problem areas or end-grain the way a center point-guided bit could be.
Speed and precision are also important factors. Experienced woodworkers often say that the Forstner is able to do work faster and better than almost any other bit, and can do a number of different tasks, too. It can drill both complete holes and partial bores; it can cut on an angle and against the grain.
Drawbacks and Precautions
This bit typically requires a lot of force, which means that it isn’t usually very well suited for a hand drill. People who want to use it typically have to invest in a drill press, and even then they often have to have at least a bit of training in order to control and properly use it. The bits also tend to be somewhat difficult to sharpen, and they don’t have any way of blowing dust back out the way some bits do; what this means in practical terms is that carpenters may have to pause more often to clear the bores. These bits also tend to be a bit more expensive than their counterparts.
The article states that a lot of force is necessary, making it unsuitable for hand tools; this is quite incorrect. As stated in Post no. 2, they pull themselves down slightly if properly sharp. You will only need a lot of force if you're turning it too fast, or if it's too dull, two things which often go together.
It's easy to tell if it's sharp enough and you're turning it slow enough: if you get shavings, you're doing it right; if you get tiny pieces and dust, it's either too dull or too fast, or both (this assumes you're not drilling into the end grain).
Some people confuse a forstner bit with a multi-spur bit. The forstner bit does not have the extra teeth a multi-spur bit does on the largest dia cutting edge. The problem of the drill wandering is well known with multi-spur bits but almost unknown with true forstner bits. The ribbon edge around the periphery is what makes the forstner bit stay on center- -even when drilling a partial hole in wood. It is also what makes the drill so slow. The multi-spur design has several saw-teeth on the periphery, and cuts much faster. When smooth side of the hole or straightness of the hole are not critical, the multi-spur bit is the best choice.
I've just bought a set which contains a 13mm. They do exist! It has 6, 10, 13, 16, 19, 22 and 25mm in a nice wooden box all for $25, by Top Tools p/n 18570.
Actually, 12.8 does not equal 1/2". 1" = 25.4 mm exactly - divide that by 2 & 1/2" is therefore 12.7mm. For wood and plastics, however, 13mm is usually close enough as a 0.3mm difference does not generally matter - especially the way those materials close down after drilling anyway.
You can get Forstner bits in metric and imperial sizes in Australia. Guess that means we are a bit more advanced than some countries. The USA using imperial measurements probably means they only make/import imperial sizes.
13mm is CFE to 1/2" for woodworking. (that's "close enough" in case you can't figure it out).
12.8mm = 1/2 in.
13Mm does not=1/2
13mm = 1/2 in. Just use a 1/2 in bit.
why is it that no one sells a 13mm forstner bit? is this due to supertition, like buildings that have no 13th story? I need a 13mm bit, and can't find one anywhere.
Some forstner bits can be combined with a countersink bit to make a "Super-Bit" so you can drill mortise holes and countersink them in one operation.
While drilling, forstner bits have a strong tendency to pull themselves down into the workpiece. When used in a drill press, they will yank the workpiece up toward the chuck with great force, especially when the bit reaches the bottom side of the workpiece. The workpiece must be positively restrained for forstners. Spiral drill bits have a lesser tendency, but will grab the work if fed too rapidly into the workpiece. Straight-fluted bits, like reamers and rod bits have no tendency to self-feed into the work.
Forstner bits can also be used in wood with lots of voids and/or other imperfections. Since a forstner bit creates its own guide, imperfections in the wood will not lead the bit astray.
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