20 Dec The Difference Between Upcut, Down-cut and Compression Spiral Bits
Straight router bits (or ‘tools’) are used in both traditional and CNC router applications. Many have a cutting edge on the bottom as well as the sides, so that they can plunge vertically into or through material to start a cut, but most of the work is done horizontally by sharp edges on the carbide spirals.
Upcut bits are more common than down-cut. With upcut bits, the chip and dust waste is channeled upward along the spirals of the bit and evacuated at the top. That means the bit isn’t always cutting material multiple times – first as a virgin cut and then again as waste. Upcut bits are designed for solid material (hardwoods, MDF) and usually thicker material. If the wood part being milled is too thin, an upcut bit can emerge violently and leave a ragged, torn exit wound. So, most situations call for the best side of the board to be facing down when using an upcut bit.
Down-cut, or down spiral bits are the opposite. They work very well in thinner material, especially when it is supported by a sacrificial panel below and the best side of the workpiece is facing up. Thin sheet goods (veneers and other products in or about the 1/8” range) can also be milled very successfully with a down-cut bit when held in place by vacuum pods or a vacuum table. But obviously, the emerging tip of the bit below the surface must not be allowed to contact the CNC table. A down-cut bit is a good choice when a part needs to be milled with the good face up. These cuts are also made slower – both in revolution or RPM of the bit, and also in spindle movement. That’s because the waste often has nowhere to go, so it is continually being chopped by the sharp flutes. Down-cut bits will leave a cleaner top edge on a dado or groove.
A compression bit combines both upcut and down-cut features. Usually, the top half of the bit (near the shank) is down-cut and the part of the bit closest to the plunge tip is upcut. Manufacturers have come up with a lot of different geometries, some of which look very dramatic. Setting the depth of cut is critical here, so that both the top and bottom edges are cut cleanly. Compression bits come as plunge bits and also as bearing guided trim bits, so that parts can be trimmed to a pattern and both the top and bottom faces are cleanly cut.
To visually illustrate the differences, the image above shows three bits from Amana Tool. (WoodEzine has no connection whatsoever with Amana – we just admire and use their products.) At right in the image (vertical, blue/red) is 46002-K, which is a solid carbide Spektra™ brand coated up-spiral plunge bit with 3 flutes. At left (angled, blue/red) is T46055-K, which is a solid carbide Spektra™ down-cut bit with three flutes. And in the middle (angled, silver) is 46372, which is a solid carbide compression spiral bit designed for nesting operations on a CNC.