I've routed miles of grooves in my life, but the quality of cut I've been getting from spiral bits still amazes me. I love these bits! The secret is in the spiral design, which cuts the wood through a shearing action. This offers three big benefits. First, spiral bits leave a silky smooth surface, even on end grain. Second, they can eliminate chipping on the face of chip-prone materials, such as veneered plywood and melamine. Third, it takes less effort to rout with a spiral bit. This reduces the load on your router and means faster, easier routing for you. The spiral can be either an up-cut spiral, a down-cut spiral or a combination up- and down-cut spiral. The bits can even be found with bearings for trim and pattern cutting. Spiral bits are available with one, two or three flutes. Single-fluted bits cut the fastest; bits with three flutes produce the smoothest surface. Two-fluted spiral bits are the most commonly available and provide an excellent balance of speed and quality of cut. You can buy spiral bits with cutting diameters ranging from 1/8 in. to 1-1/8 in. They cost between $10 and $100 each, depending on size and whether they are carbide or high-speed steel. A 1/2-in-diameter solid carbide spiral bit, for example, costs about $45; the same size bit in high-speed steel will run about $15. Carbide will stay sharp longer than high-speed steel, but is brittle and easily broken or chipped if the bit is dropped or is pushed too hard when routing. This is especially true of bits that have a 1/4-in. cutting diameter or less. A high-speed steel spiral bit will save you money, but use it only on solid wood. Manufactured materials, such as plastic laminate and melamine, will quickly dull a high-speed steel bit. Solid carbide and high-speed steel spiral bits can be sharpened for about $10 to $15 each. Your local saw blade sharpening service should be able to do it for you or to send it to someone who can. All major router bit companies sell spiral router bits.
Up-Cut Spiral Bit
The up-cut spiral router bit is an excellent bit for cutting deep mortises and grooves. The end of the bit has square cutting edges that permit plunge routing and leave a flat bottom to the mortise. The up-cut spiral of the cutting edge pulls the chips up and out, preventing the chips from getting packed into the mortise, which is common with a standard straight router bit. Up-cut spiral bits also work great in a router table because they pull the workpiece down onto the table during cutting.
Down-Cut Spiral Bit
The down-cut spiral router bit excels at routing dados and through slots. The downward shearing action of the bit guarantees a chip-free surface on chip-prone materials, such as melamine (shown below), plastic laminates and delicate wood veneers. The bit's downward cutting action puts downward pressure on thin material and helps hold it down. Down-cut spiral bits should not be used in a router table, because they can push the workpiece up off the table and create a safety problem.
Compression Spiral Bit
This bit combines up and down cutting action. It's called a compression bit because the up and down cutting actions pull toward the middle of the bit. This bit is particularly useful when you want to guarantee a chip-free surface on both sides of your workpiece. The compression spiral bit performs equally well when routing melamine (shown below) or veneer plywood.
Trim-Cut Spiral Bit
It you want to experience pure routing bliss, try a down-cut trim-cut spiral bit the next time you flush-trim solid wood edge banding. Trim-cut spiral bits with bottom guide bearings are available in 1/8-in. to 1-in. diameters in either up- or down-cut styles. The shearing action nearly eliminates all risk of chipping and grain tear-out. I've even routed the ends of edge banding strips without a single end grain splinter. I can't guarantee these results with every type of wood, but if you do solid edge banding or similar work that requires flush trimming, you should have one of these bits in your router bit arsenal.
Pattern-Cutting Spiral Bit
Pattern routing commonly involves unusual shapes and end-grain cutting. Routing end grain with a regular straight-edged bit can easily result in grain tear-out. That's why a pattern-cutting spiral bit makes so much sense. Not only does it remove the stock with ease, but it also produces incredibly smooth results on end grain. This bit is carbide-tipped rather than solid carbide. It's big, too, with a 1-1/4-in. cutting length and 1-1/18-in. cutting diameter.
See the Bit Type Chart