CNC Router Cutter Bit Types (Upcut bit, Downcut bit and Compression bit) and How to Use Them
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    CNC Router Cutter Bit Types (Upcut bit, Downcut bit and Compression bit) and How to Use Them

    upcut downcut compression cnc router bits

    CNC Router Cutter Bit Types and How to Use Them

    Some cnc router users have a confusion on Upcut bit, Downcut bit and Compression bit.

    Let’s do a quick survey with some tips:

    Upcut vs Downcut

    Upcut router bit – aka conventional endmill

    Depending on which way the spiral goes on a cutter, you get a cutter that either moves the chips up or down.  Whichever way the chips are going, there is also force exerted on the workpiece in the same directions.  So, an upcut will move the chips up and out of the cut, and it will tend to pull up on the workpiece.  By the way, the conventional endmill world refers to the cutters as left handed or right handed.  This is not quite the same thing because it refers to the actual spindle rotation.  You can get left hand (counter-clockwise) rotation router bits, but 95% of downcut bits are right hand rotation.

    On the down cut, the end plug is often not “cut up”. It ends up spinning on the end of the bit. I have on more than one occasion had a plug spin inside a cut…. they get so hot that they catch a vacuum table’s low density particle board on fire.  Since the vacuum pump exhausts outside, you often don’t realize you got a fire for a while.


    Downcut router bit – aka lefthand endmill…

    Why would you care?

    An upcut is a conventional endmill for all intents and purposes.  For wood and materials that may chip, it has the disadvantage that it wants to pull chips along the top edge. Since a downcut pushes down, it leaves a cleaner cut.

    Workholding is another issue. The upcut wants to pull up on the workpiece. If you’ve got a flexible workpiece (think plywood sheet vs a 1/2″ aluminum plate–the plywood is flexible), pulling up on it may bow it and make the cut inaccurate.  A vacuum table helps a lot with this.

    Okay, why not always use the Downcut, get cleaner cuts, and keep the workpiece firmly pressed down onto the table?

    The answer is that the downcut forces the chips down to the bottom of the hole, which is far from ideal. It is less a problem with wood than metal, but it is still very much a problem.  Consider using a downcut for a final finish pass where there are fewer chips to clear and where you want to avoid chipping the top edge of the cut.

    One other tip:  a lefthand endmill smaller than the diameter of a stuck screw can make a great way to remove the screw.  The opposite rotation puts “unscrewing” pressure as material is cut away to loosen the threads.

    Compression Cutters

    Compression cutter

    Imagine combining an upcut and a downcut and you’ve got a Compression Cutter.  The flutes literally go one way for the bottom half of the flute length and the other way at the top.  Compression cutters are for plywood, composites, and laminates.  They’re used because the pull towards the middle of the cutter, which reduces chipping on both the top and bottom.  Use one to cut completely through a sheet of plywood and you’ll have cleaner edges on both sides.

    Don’t try to use a Compression Cutter on metal!


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