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 requirements for various bit diameters
• 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 bits. They're most often 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 bevels for joinery.
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 roman ogee bits (pictured here) edge beading bits - used to cut a 1/2 half circle profile called a "bead" into an edge cove router bits - used to cut a concave 1/4 circle "cove" into a material A large number of edge forming bits are piloted. 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.
Stile and Rail Bits Stile and rail router bits are used for a special type of frame and panel construction joinery called "cope and stick", and are used primarily for constructing frame and panel 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" meets the profiled edge of the frame's "stile". 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. A vertical frame and panel bit is used to cut the panel profile with the stock tipped up on its edge. Vertical raised panel bits are not piloted, and rely on a router table fence to guide the workpiece. A horizontal raised panel bit cuts the panel profile with the workpiece laying flat on the table, and is most often piloted.
Other Joinery Bits Other 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 decorative box-making joinery projects.

Shank Diameter

Router bits are most often available in 1/4" and 1/2" shank diameters. When available, it is best to select 1/2" shank bits.  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. Most better quality router bits are available in either a 1/4" or 1/2" shank.

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 too fast and produces a rough cut or too slow and 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

Anti-Kickback Design

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 regrindings. Pictured on the left is an example of a bit with this safety design: Rockler's Drawer Lock Router Bit. 

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 regrindings. They are also made of high quality, fine grained carbide, which allows for a sharper cutting edge.

Rockler offers high quality router bits from respected manufacturers including Freud and Amana. Rockler also offers it's own line of router bits designed for the serious woodworker. Rockler router bits are made using high quality ISO K10 and K20 carbide, are finish sharpened with 600-800 diamond abrasives, and are precision balanced and are geometrically designed for superior chip ejection. Rockler, we're confident that the router bits we sell will provide long-lived, safe and smooth cutting - but to make you their ultimate judge, Rockler Router Bits are also unconditionally guaranteed.

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