This reader uses a router to make mortises. He wants to know if a spiral bit is better for this than a straight bit.
Ellis Walentine: Spiral bits ‘right-hand’ or ‘up’ spirals, that is havethe advantage of lifting the chips from the cut. This allows more air around the bit, which keeps the bit cooler and prevents burning. They also have a cutting geometry that is better for plunging. And, they leave marginally smoother walls in the mortise, which isn’t really a huge advantage over regular straight-flute bits.
Michael Dresdner: I find that spiral bits cut slower, but are far more controllable, and I favor them over straight bits for this type of work. Be aware that you can buy upward spiral bits, which eject shavings just like a drill bit, or down spiral, which leave a clean top edge. You can decide which is more important for your application.
Simon Watts: A spiral bit would clear chips better than a straight bit, so it would generate less heat and give a cleaner cut. My guess is it would keep its edge longer too.
Ian Kirby: A spiral bit gives a smoother cut, which is fundamentally a better cut.