If you've ever shopped for router bits, you've probably noticed that they come in a huge variety of types and sizes. This is a good thing because it's the variety of bits that makes routers such versatile tools. If you look closely, you'll also find that router bits fall into two categories, those with 1/4-inch shanks and those with 1/2-inch shanks.
Most bit types are available in both shank sizes, and most router kits come with collets to fit both sizes of a shank. If you can use either size, you probably wonder which is better. The short answer is: all things being equal, 1/2-inch is better. There are some cases where 1/4-inch is the only option and many cases where shank size simply doesn't matter.
Router Bit Shank
The shank is the solid, perfectly cylindrical part of a router bit. It's the part of the bit that goes into the collet of the router and is secured with the collet nut. At the bottom end of the shank is the bit body, the part that contains the cutter or cutters that shape the wood. Bits with large bodies often are available only with 1/2-inch shanks, while bits with very small or narrow bodies may have only 1/4-inch shanks. But the majority of bits out there are available with both 1/4-inch and 1/2-inch shanks.
Router bits with 1/2-inch shanks have nearly four times the mass of 1/4-inch-shank bits, which translates into greater stability. The extra mass helps reduce what is known as "chatter," or vibrations caused by the high speed of the spinning bit. As you might expect, a more stable bit makes a cleaner cut. The greater mass of a 1/2-inch bit also helps dissipate heat that is generated by the bit cutting into the wood, and it slows the transfer of heat from the router's motor to the bit. Heat is a common concern with routing operations, and minimizing heat buildup is necessary to prevent burning the stock.
The larger diameter of 1/2-inch shank router bits means there's more surface area for the router's collet to grip onto, making the bit less prone to slipping in the collet. Slipping isn't a common problem if you tighten the collet carefully, but with large bits or rough-duty work, a better grip can be a real advantage.
In addition to the centrifugal forces at work on a spinning bit, there are also can be significant sideways forces imposed by the operator pushing the bit into the wood. Sideways forces can cause the bit to deflect, or flex, affecting the precision of the cut. A 1/2-inch shank resists deflection better than a 1/4-inch shank. This can be especially important when using a long bit, which is subject to greater deflection, and when using a piloted bit, which has a bearing that is pushed up against the wood throughout the cut.
When 1/4-Inch Bits Make Sense
The advantages of 1/2-inch-shank bits don't always apply (or they are negligible), and sometimes 1/2-inch shanks aren't even available. As a result, most woodworkers who use routers end up with a collection of bits containing both 1/2-inch and 1/4-inch shanks.
One potential advantage of 1/4-inch bits is that they can be less expensive. With router bits, you certainly get what you pay for, but if you need a bit for a specific project and might not use the bit much otherwise, an inexpensive 1/4-inch bit might be the best option. Availability is another potential advantage of 1/4-inch bits, as some stores carry a wider range of 1/4-inch bits than 1/2-inch.
Finally, if you have a small router or a laminate router, the tool may accept only 1/4-inch bits, in which case the question of shank size is moot. But even if your router can accept both 1/2-inch and 1/4-inch bits, and the bit you need isn't large or particularly long, you'll probably get similar performance with either shank size.