When you need to make a straight cut, such as a dado or rabbet, should you choose a spiral-cutting router bit or a straight-fluted bit (with no twist)? Which one is better? Well, I guess that depends on your definition of "better," because there are distinct advantages to both.
Spiral-cutting bits come in two varieties, up-cutting bits (meaning that the bit's motion lifts the cut debris out of the groove) or down-cutting bits (debris is forced downward). Both leave a clean cut, but where down-cutting bit
s tend to leave a very clean top edge but rough bottom, an up-cutting bit leaves a clean bottom but somewhat rougher top edge. When working with a down-cutting bit, you'll need to proceed a bit slower, because you'll need to wait for the debris to clear the bit. An upward cutting bit doesn't have this problem, but has a tendency to lift the workpiece if it isn't securely clamp
ed to the work table.
In either case, the spiral-cutting bits tend to cut a bit more efficiently with less vibration than a straight-cutting bit. They also excel in plunge routing applications, due to the drill
bit-like motion of the cutter. However, they are considerably more expensive than straight-cutting bits, and they aren't available in nearly as many sizes.
On the other hand, straight-cutting bits have advantages over their spiral-cutting cousins. First of all, they're less expensive and come in many more sizes. They are much easier to sharpen as well. Additionally, if your application requires the use of a guide-bearing on either the shank
or the tip, you're pretty-much restricted to using straight-cutting bits.
The disadvantage of a straight-cutting bit comes into play when you want to plunge the bit into the work. Because the cutting carbides do not extend all the way to the center of the bit, plunging directly downward into the work isn't possible. Instead, you'll need to plunge downward about 1/8" and move along your path, then make another pass another 1/8" deeper until the desired depth is achieved.
So, back to our original question: which type is better? As we've seen, it really depends on how you need to use the bit.