Upcut spiral bits are the most popular type of spiral fluted tool. 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.
Downcut spiral bits are best used for thinner materials that will be pushed down into the machine bed rather than being lifted with an up-cut spiral. This can be particularly useful when using vacuum hold down where maintaining the seal between the material and sacrificial bed is crucial. These tools are predominantly used when a high-quality cut finish is required on the top surface of cut parts. Downward spiral tools often require reduced cutting speeds because the chips are pushed back into the material. The chip extraction is generally less effective with this type of tool than it is with an upcut spiral tool. Downcut spiral tools should be used when processing thin materials, 1/8” (3.175mm) or thinner.
At SpeTool, we hold a vast range of upcut and downcut CNC router bits. These bits range from 1/16” to 1/2” cut diameters and can be found at www.spetools.com/collections/spiral-bit.