With their drill-like point, spiral bits are also better for plunge-cutting. This is a Veteran Owned site. I wrote a longish blog article long ago about the bits I use the most. Woodcraft Magazine. You'll likely want an blt as it will slice towards the router; the pattern is between the stock and the router so the pattern backs up the cut making it clean. You just have to sweep the router while you are plunging.

A down-cut bit will leave a clean top edge, but it sends the chips downward, into the dado where they have no place to go. You can make this cut, but you have to take it slower than usual to give the chips a chance to clear. For woodworkers who work with A-grade veneers on both sides of the stock and must have a clean edge top, bottom, and middle, the compression bit is a good choice. It has an up-cut configuration on the tip of the bit and a down-cut spiral ground on the shank. By lining up the bit just right, you can get a superior edge across the entire thickness of the wood.

Router-bit manufacturers have difficulty making solid-carbide spiral bits with cutting diameters larger than their shanks. Straight bits, on the other hand, go through dozens of fractional sizes, all the way up to 2-in. Depending on the job you have in mind for your router bit, straight bits also come in a variety of cutter lengths.

So you can buy close to exactly the length of cutter you need. Straight bits also have a huge advantage over spiral bits when it comes to template routing, because you can buy them with guide bearings. And those bearings can be mounted on the tip of the cutter or on the shank of the cutter, depending on your needs and your template. The bearings are made for a variety of cutter diameters and lengths. With their superior edge cut, spirals make great template cutters when used with collar guides.

But when it comes to bearing-guided bits, spirals seem to be available only with bearings mounted on the end of the bit. Shank-shod, bearing-guided, solid-carbide bits spiral bits with the bearings on the shaft end of the bit , which would permit template routing with the template on top of the work and trim cutting through only part of the work face, are not available.

Some woodworkers like to sharpen their straight bits, although I find it difficult to get it right and always send out my bits for sharpening. Carbide-tipped straight bits usually have enough carbide thickness to be reground four or five times, and the tech- nology to do so is common.

A few services claim to be able to sharpen spiral bits. To me, this translates into a substantial loss. Spiral bits might stay sharp longer than straight bits, but even so, the cost of using spiral bits will always be higher. Spiral bits work incredibly well in the production environment and especially in CNC computer numerically controlled router industrial applications. But in a hand router, their use sometimes imposes unusual risks not associated with the equivalent or bigger straight bits.

The down-cut spiral bit's screw-driven forces are sufficient enough to pick the router up and twist it out of your hands—with no warning. I know, because it has happened to me. On end grain the spiral bit is getting even more traction, so the risk is even greater—a pity, too, because a sweet end-grain finish is attractive.

The up-cut spiral bit can have the opposite effect. It wants to pick up the work. So you must secure the work in some kind of fixture or hold it by a clamp. I never rout anything that is not secured or clamped, but some people do. I do keep a few solid-carbide spiral bits because, when I want a beautiful face cut or I am cutting narrow mortises, and I have the money, there is just nothing better.

But my cabinet is mostly full of a wide variety of straight bits. I am in the process of beefing up my router bit collection and would like to understand the practical uses of each. The down cut reduces tearout because it presses the cut fibers against the work piece.

The up cut is useful for pulling the shavings out of a mortise or dado. That is helpful. You can use a down cut bit for any cut where you are more concerned about tear-out than you are concerned about clearing the chips.

I use the down-cut for dadoes and rabbets in plywood. While it leave the chips in the dado, it doesn't create any tearout on the plywood. I learned the hard way not to use an upcut bit So saying 'downspiral reduces tearout' is accurate if a backer is below the bit. Oddly enough, a lot of forum posts long ago would say the exact opposite.

Spirals are excellent for clean cuts, but there aren't general rules; you have to consider your setup then decide the bit. If you only have one bit, use how it works to setup your work. A downspiral slices away from the router; an upspiral slices towards the router.

Maybe they should have used Latin and called them trans-spiral and cis-spiral respectively. I digress In the case of dados and blind mortises: a downspiral will slice down, but down where Since it slices down, the top surface of the stock is supported in the cut, it is a clean cut.

All the crud in the dado can impair moving smoothly; loitering can smoke the chips. An upspiral slices toward the router lifting the chips from the dado so you have no issues with chips packing in the groove impairing motion allowing a faster cut, too.

Downside is that the upward slicing action at the surface can lift fibers. The decision is whether speed with a possibly ratty edge is okay.

If you are dadoing for a shelf and the shelf edge has shoulders, it will hide the ratty edge. If you are dadoing to insert the shelf into the dado, you need it pristine. In the case of mortises, typically your tenon stock will have shoulders that hide lifted fibers and typically you need a mortise depth that is difficult with a downspiral without a lot of passes. In the case of routing against a pattern, you again have 2 choices.

If you put the pattern on top of the stock, you'll use a bit with a shank bearing. You'll likely want an upspiral as it will slice towards the router; the pattern is between the stock and the router so the pattern backs up the cut making it clean. Use these for grooving or slotting, for upward chip evacuation and best finish on the bottom side of the part. These bits allow for rapid cuts since the tool clears the chips away from the material.

While ideal for cutting thicker materials, this type of tool is not recommended for thinner or softer materials. When working with these types of material, the upward force can pull the material causing a ragged finish on the top surface, or even worse, eject the part from the table.

This type of geometry is used whenever the best finish is needed on the bottom side of a part.

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