Beginner's Guide to CNC Bits | CNC Router Bit Types
Early in your CNC journey, choosing the right bit for your projects can be a little intimidating. As CNC machines, like the X-Carve, have become more and more diverse and nuanced, so have the bits they use to make the cuts. There are so many options and uses out there it can become overwhelming pretty quickly.
This article will provide you with a basic overview of the most common types of bits used in CNC machining, the pros and cons of each type, and for what material you’ll want to use them on.
Before we dive into the actual types of bits, it’s important to get a hang of their general geometry and terminology.
Feed rate: The feed rate is the speed at which the machine runs the bit laterally through the material.
Speed rate: The speed rate is how fast the spindle rotates the bit; measured in revolutions per minute
Flute: a flute is the cutting edge of a bit that is threaded along the bit’s axis. Bits will generally have between 1-4 flutes and different flute counts provide different benefits and drawbacks which we’ll discuss later.
Shank: A shank is the straightened part of the bit where the flutes end. This is where the bit will attach to the CNC router.
CNC machines make their cuts by using a hard material (the steel bit) to create friction against a softer material (wood, plastic, aluminum, etc.). However, friction creates heat. And too much heat is not good for a bit. So deciding which bit to use will largely revolve around reducing the heat generated. Keep that in mind as we move forward.
End Mill Bits
The first category of bits we’ll look at is spiral bits. These are one of the most commonly used bit types and can be designed to cut any material.
End mill bits can be further defined by three features: cut direction, flute count, and nose shape.
Cut direction refers to the direction in which the chips of the material are ejected. An up-cut spiral bit works similar to a drill bit, sending material upwards away from the cut. On the other hand, a down-cut spiral bit’s flutes rotate in the opposite direction, sending the material down towards the cut.
Evacuating chips upwards, as up-cut spiral bits do, provides a significant advantage in heat reduction because it removes the bulk of extra material out of the path of the bit. Less material means less friction which increases the lifespan of sharpened bits.
The downside to up-cut bits, however, is the frayed top surface they produce on the material you’re cutting. This tear-out can be a big drawback if your project requires a smooth finish.
Down-cut bits don’t produce this top surface tear-out, which is their big advantage. If you want a perfect, smooth surface on your material, a down-cut bit is the way to go.
However, because they press chips back into their cut path, they produce a much larger amount of friction and heat which dulls the edge much quicker.
End mill bits are also defined by the number of flutes that carve out material. Most bits have 1-4 flutes.
Higher flute counts create more heat because, while they are able to remove more material, more chips become trapped in the cut path because the layers of flutes prevent them from escaping. As a consequence, three and four flute bits need to operate at lower speed rates to prevent overheating.
Meanwhile, single flute bits generate comparably little heat and are great for cutting soft or heat-sensitive materials like plastic and acrylic.
Two flute bits are generally considered to be great all-purpose bits especially for cutting wood.
End Mill Nose Shape
The final defining feature of end mills is the shape of the nose. The standard shape of an end mill nose is a flat surface. However, ball-nose end mills contain a rounded end which is great for sculpting smooth 3D surfaces.
The final type of bit commonly used in CNC is the V-bit. As the name suggests, the tip of this bit is neither round nor flat as spiral tips are, but rather the flutes come straight from the shank to the tip forming a V shape. These bits are used for engraving and very fine woodcutting.
Because the flutes on V-bits aren’t spiraled around the axis, there is no down or up cutting with these. Instead, V-bits are categorized based on their angle and diameter. Most common are the 45-degree, 60-degree, and 90-degree bits which often come in ½” and ¼” diameters.
Smaller diameters and 45-degree bits allow for greater depth in cuts while 90-degree bits are better for removing wider surface areas.
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