All spiral bits make clean cuts. This veneered plywood shows the effects of the three types of spiral bits on the top and bottom edges.
Spiral Bits Leave a Clean Edge
The three basic cutter
configurations for spiral bits are up-cut, down-cut, and a combination of the
two, known as a compression bit. ... A down-cut bit sends
the chips downward; an up-cut bit sends them up to ward the shank. (On a router
table, all direc-tions are reversed.)
Besides directing the chips, the advantages of these configurations are best
illustrated by the quality of cut, especially on veneered plywood (see the photos
above).A down-cut bit will leave a clean edge on top but a ragged edge on the
bottom; an up-cut bit will accomplish the opposite. This is great until you want
to cut a dado with no tearout on the face. 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.This virtuosity comes at a hefty
price:A typical compression bit will cost about $90.
Straight Bits Come in
Many Sizes and Bearing
Router-bit manufacturers have difficulty making solid-carbide spiral bits with
cutting diameters larger than their shanks. So for small-shop hand routers you
won’t find many bits with a cutting diameter larger than 1/2 in., the size of
the largest bit shank. Spiral bits also come pretty much in a few standard fractional
sizes up to 1/2 in. Straight bits, on the other hand, go through dozens of fractional
sizes, all the way up to 2-in. dia. cutters. 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. It is really too bad that solid-carbide spiral bits can’t
accommodate bearings a little more readily. 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. There are some problems with this: The cost is high (about
$80); it precludes cutting only partway through the work, which means full-thickness
cuts only; and the template has to be under the work, an inconvenience. 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.
For this type of routing, you’ll
have to stick with straight bits.
This article is excerpted from The New Best of Fine Woodworking Working With Routers.