A compression bit is the combination of both up-cut and down-cut spirals. Typically, a compression spiral has an up-cut part, at the tip of the cutter, approximately the length of the cutting diameter, The remaining is down-cut.
By combining both geometries, you achieve a win/win situation. The bottom of the material is pulled upwards while the top of the material is pushed downwards thus eliminating the risk of damaging the laminated surfaces. When run at full depth, the bit leaves a sharp, clean edge on both sides of your workpiece.
Compression bits are used extensively for cutting double side laminates such as melamine or plywood. These tools can also be used on natural woods where edge finishing is a problem with standard spiral tools.
A dedicated up or downward spiral may only give a good finish on one side of the material if the material is laminated. Where both sides of the material require the best possible finish, a compression bit is your best solution.
At SpeTool, we hold a vast range of compression CNC router bits. These bits range from 1/8” to 1/2” cut diameters and can be found at
We recently designed and produced a 0.1” up-cut, 1/4” diameter compression bit(W02010), which you can find on this link:  
Firstly, When making a mortise, you need to change a down-cut router bit or a compression bit in which the up-cut part is short than the mortise depth. Normally the up-cut part length is approximately the cutting diameter when using the standard 1/4” compression bit, you could not get a 1/8” inch depth hole or you will get a bad surface, with this 1/10” up-cut compression, you will get a smooth top surface.
Secondly, when plugging the solid woods, the compression bit can work just as well. But to get their major benefit you need to keep in mind the depth of that first pass. For very hardwoods, .250" per pass might be too deep, so we need to change the 1/10" up-cut compression.

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