Walk into any home center or fine woodworking tool supplier and you'll find a various assortment of router bits. If you look closely, you'll see that these router bits fall into two categories, those with 1/4" shanks (the cylinder that fits into the router collet) and those with 1/2" shanks. Most profiles are available in both shank sizes, and most router kits come with collets to fit both sizes of router bit shanks.
So, the question begs to be asked, why the two sizes of collets and shanks? Which size is better?
There are some differences and advantages to each. 1/2" shanks have nearly four times the mass of the 1/4" shank bits, which translates into greater stability when in use. The extra mass will help reduce what is known as "chatter," or vibrations caused by the high speed of the spinning bit. As one might expect, a more solid bit means a cleaner cut.
The advantages of the extra mass don't stop there. The additional mass also helps dissipate heat that is generated by the bit, which will help prevent burning the stock. The extra size provides the collet more gripping area on the shank, making it less prone to slippage in the collet. And finally, if you need a guide-bearing on the tip of the bit, you'd definitely be wise to choose 1/2" shank bits, as the 1/4" models don't have a large enough diameter for a guide-bearing. Instead, 1/4" shank bits have a rounded guide post on the end that can mar or burn the edge of your workpiece.
Now, with all of that in mind, let me also say that there are some advantages to 1/4" bits. First of all, they're typically much less expensive and more readily available. You'll often find 1/4" bits in many hardware stores and home centers that would never carry a 1/2" bit. Second, some inexpensive routers don't come with a 1/2" collet, meaning you'd be restricted to using 1/4" bits only. And finally, when you need to route a very thin profile, a 1/4" shank bit is simply more practical than a 1/2" shank bit.
When choosing between 1/4" shank bits and 1/2" shank bits, the choice is up to you. Personally, I have a number of each size. I prefer the larger shank for the added stability, but do keep a number of thinner profile bits in the 1/4" size. That being said, if I have the same profile in both shank sizes, I'll almost always reach for the 1/2" shank bit first.
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