Choosing Straight vs. Spiral Bits

I’ve heard of spiral upcut, spiral downcut, straight, mortising, and rabbeting bits! When should I choose a spiral bit over a straight bit, and when do I choose an upcut over a downcut? What’s the difference between a straight and mortising bit? Will a straight bit with an edge guide give me the same kind of rabbet as a rabbeting bit? I’ve got a lot of stop dadoes and some long rabbets to make on my next project, and I will likely be using the router for most of this. Just want to make sure I’m using the right bits.

Michael Dresdner: Straight bits cut clean and fast, but tend to grab and will “lead” the router, making it tough to control. Spiral bits cut clean, but not as fast. They don’t grab or lead as much, so they are far better for freehand pattern routing (if you do that sort of thing) or in situations where grabbing can be a problem. Upcut spirals may chip the top edge of a cut, especially if it is brittle (like wenge wood) or if it is veneer. However, they cut a clean slot and eject the chips upward and out of a groove. Downward spirals leave a very clean top surface, but push the sward workpiece cuttings, chips, dust, oil, grain particles] downward. They will clog in a closed cut, such as cutting a deep groove the same width as the cutter. You’ll have to stop and clean out the chips frequently. Straight bits cut on the side. Mortising bits also cut on the end (for plunging.) Yes, a straight bit with a guide will give you the same kind of cut as a rabbeting bit, though not necessarily as clean a cut. If you have a lot of rabbeting to do, you’d be wise to invest in a good rabbet bit.

Lee Grindinger: For rabbeting, use a straight bit. For deep plunges, such as mortises, use an upcut; it will clear the waste better but will not leave a clean entry wound. For very clean entry wounds, use a downcut; they leave very clean cuts but don’t clear the waste as well as an upcut. The spiral shapes are there to clear the waste. Bits are dulled much quicker when they are having to re-cut waste, so removing waste is good for the bit and speeds cutting rates.

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