Sorting through router bit terminology and making selections from a seemingly endless variety of router bits can be confusing, especially if you are new to working with a router. Reading up on the basic router bit facts will help you to choose the best router bits, and to make the best use of them. In this article we’ll cover a few of the most useful things to know about router bits, including:
- Common types of router bit
- Shank diameter
- Router speed
- Anti-kickback design
- Router bit quality
Common Types of Router Bit
There are a numerous types of router bit, including many designed for highly specialized applications. We won’t be able to cover every type of router bit in this article, but here are some of the most common:
Straight Router Bits
Straight router bits are among the most common frequently used of all bits. Straight bits used to make cuts straight down into a material to form a groove or dado or to hollow out an area for a mortise or inlay. Straight bits come in a variety of cut diameters, most commonly in the range from 3/16″ to 1-1/2″.
Rabbeting Router Bits
Rabbeting router bits produce a straight vertical and horizontal cut, and are designed specifically to cut a rabbet (notch) in the edge of a material.
Rabbeting router bits are an example of a “piloted” router bit, meaning that a bearing of the top of the bit is used to guide that bit along the edge of the material. Rabbeting bits usually come in a set that includes a range of pilot bearing diameters, allowing a single bit to produce a variety of rabbet dimensions.
Flush Trim Router Bits
Flush trim router bits are guided by a pilot bearing that is the same size as the bit’s cutting radius. They’re used to trim the edge of one material flush with the edge of another material. Trimming a veneered surface flush with a substrate, or using a pattern to create multiple identical shapes are examples. The pilot bearing may be on the top of the bit, at the base of the cutting edge, or both.
Chamfer Router Bits
Chamfer Router Bits produce a bevel cut at a given angle. Chamfer router bits are sometimes used to decorate the edge of a material, and can also be used in joinery to create beveled edges for multi-sided constructions.
Edge Forming Router Bits
Edge forming router bits are most often used to cut a decorative edge into a material. The variety of edge forming profiles are practically unlimited, but some of the most common include:
- Round over bits -Used to cut a rounded edge of a given radius.
- Ogee bits – The term “ogee” refers to an “S” shaped profile. Ogee bits are available in a number of configurations; the Roman ogee bit pictured here is one of the most common
- Edge beading – Used to cut a 1/4 or 1/2 half circle profile called a “bead” into an edge or corner
- Cove router bits – used to cut a concave 1/4 circle into a material
Many edge forming bits include a pilot bearing. In most cases, the bits are used for final decoration of a project where edges are already established and can serve as the guide for the bit.
Molding Router Bits
Designed to architectural molding profiles, molding bits are typically larger than basic edge forming router bits. Molding bits may incorporate multiple basic edge forming profiles into a single router bit. Because of their size, molding router bits most safely used in a router table.
The classic Multi-Form Bit pictured here is designed to make several decorative profiles possible by combining basic profiles included in a single bit.
Stile and Rail Bits
Stile and rail router bits are used in frame and panel construction, primarily for constructing frame members of cabinet and passage doors. These bits cut a decorative profile and a panel slot into the edge of door frame stock, and also a corresponding cut into the end of the material where the frame’s “rail” (horizontal member) meets the profiled edge of the frame’s “stile” (vertical member).
Stile and rail bits are available as either as a set of two “matched” bits, or as a single bit that can be arranged to cut both of the necessary components of the cope and stick joint.
Raised Panel Bits
Raised panel bits are often used in conjunction with stile and rail bits to produce a profiled edge on a door panel. The profiled edge fits into the corresponding slot in the frame’s stiles and rails. Raised panel bits are available in both a horizontal and vertical configuration. Horizontal raised panel bits cut the panel profile with the panel stock laying flat on the table. A vertical frame and panel bit is used to cut the panel profile with the stock tipped up on its edge and run along a fence.
Vertical raised panel bits are considered by many to be safer to operate because of their much smaller radius. A horizontal raised panel bit are necessary for panels with curved edges, such as those used in arched top or “cathedral” cabinet doors.
Specialized joinery bits include dovetail router bits, drawer lock router bits, finger joint bits, and lock miter bits. Each of these bits is used to produce a specialized type of precision joint. A dovetail bit is often used in combination with a dovetail jig to quickly and accurately produce dovetail joints for drawer boxes and other box-making projects.
Router bits are most often available in 1/4″ and 1/2″ shank diameters. When available, 1/2″ shank bits are generally thought to be preferable. A 1/2″ shank diameter bit has four times the cross-sectional surface area of a 1/4″ shank, and is therefore much more rigid than a 1/4″ shank bit. The extra rigidity and support for the cutting edge translates into a smoother cut with less chatter. Except for very small and very large profiles, router bits are typically available in both a 1/4″ or 1/2″ shank diameter. Many large profile bits are available in only 1/2″ shanks size. A few smaller-sized trimming, edge forming and specialized bits are available only a 1/4″ shank only.
There are a few exceptions to the 1/4″ and 1/2″ shank rule. You will run across bits measured metrically.
Router Bit Speed
Better routers are equipped with a speed adjustment that allows for speed adjustment typically ranging from around 8,000 to 24,000 rpm (revolutions per minute). The reason is that not all router bits can be safely or optimally run at the same speed. Larger router bits have more mass and therefore a potential to create forceful vibration at higher speeds.
It’s important understand what router speed actually measures. “Revolutions per minute” is simply a measure of the number of times that the bit makes a complete revolution in a given period of time, and not a measure of the speed at which the body and cutting surfaces of the bit are actually traveling. The outer perimeter of a 3-1/2″ diameter bit is actually traveling significantly faster at 24,000 rpm (and much faster than it should be) than a 1/2″ straight bit’s cutting surface would be at the same shaft speed.
Manufacturers often supply maximum free-running speeds for the router bits they sell. The maximum speed for a given bit is the maximum speed at which it can be safely operated, and may not be the best speed for the task. There are other factors to consider: router horsepower (a less powerful router will slow down when it’s driving a large bit through a cut), feed rate, quality and condition of the bit, and the cutting properties of the material. It is impractical to give a list of specific speeds that will work well in every situation.
In general, a faster speed is more desirable than a slower one, for the simple reason that a faster speed provides more cuts per inch, and more cuts per inch normally yield a smoother cut. Using sharp, high quality bits and using the correct feed rate go hand in hand with bit speed in making a clean cut. Make practice cuts with a new bit until you are able to produce a smooth, even feed rate that is neither so fast that it produces a rough cut, or so slow that it results in burnishing and burn marks.
The chart below offers speed guidelines for the maximum safe free running speed of bits of various diameters. This is supplied for general reference – manufacturer’s guidelines should be consulted:
Router Bit Diameter @ Maximum Speed
Up to 1″ @ 22,000 – 24,000 rpm
1″ to 2″ @ 18,000 – 22,000 rpm
2″ to 2-1/2″ @ 12,000 – 16,000 rpm
2-1/2″ to 3-1/2″ @ 8,000 – 12,000 rpm
Many better-quality bits offer the additional safety feature of an anti-kickback design. The enlarged body of anti-kickback an anti-kickback bit prevents the bit from biting in too deeply and catching on the material. Bits of this design have more body mass than other carbide bits, which helps to dissipate heat and keep the bit sharp for longer service between re-grindings.
Router Bit Quality
A number of qualities distinguish better quality router bits from “bargain” bits. Good quality router bits are sharp, and stay sharp for a long time (with proper use). Good bits are well balanced and therefore create minimal vibration. They are designed for good cutting geometry and have excellent chip ejection properties.
Currently, most woodworkers prefer carbide tipped router bits. Carbide cutters can hold their edge between ten and twenty-five times as long as hss (high speed steel) bits. Better carbide bits are ground to a finer edge and have thick cutters to allow for numerous re-grindings. They are also made of high quality, fine grained carbide, which allows for a sharper cutting edge.