Below we discuss different topics related to cnc-machining that help understand problems and difficulties related to the process along with potential solutions. Check also our Terminology page for cnc-related slang

Types of milling bits

Flat bits

Flat bits mean they create a flat bottom surface when using them to cut and come in a variety of forms. The most commonly used ones at fellesverkstedet are the Upcut, Downcut and Compression bit.

Image showing the different flat milling bits.

• Upcut: Multi-purpose tool which removes chips very well from the slots being milled, so higher speeds/pass depths can be used. The thread of the bit goes upward while rotating. Great for plastics, foams, woodwork en metals. Does cause tear-out on the top side of plywood, as it pulls up the wood fibers up.
• Downcut: Thread of the bit goes downward while rotating so it pushes the chips downwards. This can cause tear out on the bottom side of the plywood, if there are gaps below the material or slots in the sacrificial layer. Mainly used for woodwork. Advantage of this bit is that when cutting mdf (>9) or plywood (>12) it pushes the dust downward causing it to pack around the pieces. This side-effect can be used to avoid using tabs, if the pieces are not too small.
• Compression bit: Combination of Up-cut and down-cut, which gives a clean edge on both sides. Great for cutting of plywood. Not good for drilling though as you get burn marks and it heats the milling bit, thus making it less strong. Also compresses dust around your pieces like with the downcut, which potentially means avoiding
• tabs.

Check out this video where they compare these different types of bits (including a straight edge cutter, which we’ve never gotten a good results with) cutting wood in slow-motion.

Ballnose

Mostly used for 3D-milling. The tip of this milling bit is round off which makes it excellent for machining smooth surfaces as it can match the angle of the surface. Because of the geometry of the milling bit you might get a difference in finish, due to changing diameter influencing the speed at which the bit cuts in a particular spot. This is especially visible when milling Aluminium or (Transparent) plastics.

Vbit

Great for engraving or putting a chamfer on edges. Since it's conically shaped it will make a wider slot the deeper you cut. Have a look at this video to better understand how that process works:

CNC Vbits for Beginners