CNC routers need bits. They determine the type of carving, the resolution of the finished designs, and the speed at which you can move through the material. They have cutting edges that pull up or push down (sometimes both), they have square or shaped ends, they are designed for speed or accuracy, and they come in diameters from a needle point to over two inches for standard CNC routing.
Choosing the right bits for your CNC router can be confusing. There are many variables to consider when searching for the best bit for you and your project. Here are some important features to consider.
The world of Computer Numerical Control (CNC) machining has revolutionized manufacturing, prototyping, and various craft industries. At the heart of this technology lies an oft-underestimated component: the CNC bit. These bits, sometimes called end mills or cutters, are the tools that remove material and shape the final product. But with so many variations in terms of material, geometry, and size, how does one navigate the intricate maze of CNC bits? This comprehensive guide aims to clarify everything you need to know about these essential tools.
Understanding the Basics: What is a CNC Bit?
At its core, a CNC bit is a cutting tool used in a CNC machine to remove material from a workpiece. The CNC machine moves the bit in three-dimensional paths, allowing for precise cutting, carving, and shaping of materials like wood, metal, plastic, and foam. The design and geometry of the bit determine how it cuts, the quality of the finish, and the speed at which it operates.
Types of CNC Bits: An Overview
There are numerous types of CNC bits, each tailored for specific applications and materials. The most common types include:
- Flat End Mills: These are standard bits that make flat-bottomed cuts. They’re ideal for general purposes and can produce smooth surfaces.
- Ball Nose Mills: With a round tip, these bits are used for milling 3D contours and can leave a smoother surface finish.
- V-bits: These have a pointed tip and are used for engraving, carving, and producing fine details.
- Roughing End Mills: Designed with serrated edges, these are used to remove large amounts of material quickly. However, they don’t leave a smooth finish, so a finishing pass with a different bit is often needed.
- Drill Bits: While traditional drill bits are for drilling holes, CNC-specific drill bits are optimized for the sideways loads experienced in CNC applications.
Material Matters: Choosing the Right Bit for the Job
CNC bits are made from various materials, and the choice often depends on the hardness and type of material you plan to cut. Common materials for CNC bits include:
- High-Speed Steel (HSS): This is a basic, affordable option ideal for beginners. HSS bits are suitable for soft materials like wood and some plastics but wear out faster than other bit types.
- Carbide: Offering more durability than HSS, carbide bits are harder and can retain their sharpness longer, making them suitable for cutting harder materials like metals and hard plastics.
- Diamond-tipped: For extremely hard materials, such as stone or tough metals, diamond-tipped bits are the go-to option. They offer longevity but come at a premium price.
Bit Geometry: Understanding the Nuances
The geometry of a CNC bit plays a pivotal role in its performance and the finish it delivers. Key parameters include:
- Flutes: Referring to the spiral cutting edges on the bit, the number of flutes can range from one to many. Fewer flutes allow for faster material removal, while more flutes give a finer finish.
- Helix Angle: The angle of the flutes affects the quality of the cut and the heat produced. Low helix angles (around 30°) are ideal for softer materials, while high helix angles (around 45° or more) are suited for harder materials.
- Cutting Diameter: This refers to the width of the bit. Smaller diameters allow for detailed work, while larger diameters are used for bulk material removal.
Maintenance and Longevity
Even the best CNC bit will dull over time. Regular inspection for signs of wear, chipping, or dullness is crucial. Keeping bits clean and free from resin or material buildup can extend their life. When a bit starts to dull, it may produce burn marks, rough finishes, or require more force to cut. Such signs indicate that it’s time for sharpening or replacement.
Feed Rates and Speeds: Optimizing CNC Bit Performance
One of the essential parameters to consider when using CNC bits is the combination of feed rates and spindle speeds. These parameters determine how fast the bit moves through the material (feed rate) and how quickly it rotates (spindle speed).
- Feed Rate: Measured in inches per minute (IPM) or millimeters per minute, it determines how quickly the bit advances through the material. A too-fast feed rate might result in a rough finish or even break the bit, while a too-slow rate can cause burning and reduce bit lifespan.
- Spindle Speed: Given in revolutions per minute (RPM), this denotes how fast the CNC bit rotates. A high RPM is ideal for softer materials to ensure clean cuts, while harder materials might require a reduced RPM to prevent excessive heat and wear on the bit.
Balancing these two parameters is crucial. Many manufacturers provide recommended speed and feed charts for their bits, considering different materials. It’s also wise to do test cuts to determine the optimal settings for a specific project.
Specialty Bits for Advanced Operations
As CNC machining grows in complexity and application, specialty bits have emerged to tackle specific jobs:
- Tapered End Mills: These bits are used for intricate carving in molds and dies. They offer both strength (due to their thicker base) and precision (thanks to their narrow tip).
- Dovetail Cutters: As the name suggests, these are used to create dovetail joints, a popular and robust woodworking joint.
- Face Mills: These have multiple cutting edges on the face and are used for achieving flat, smooth surfaces, especially in metal machining.
- Thread Mills: Designed to cut threads into holes, these bits are a boon for applications where threaded parts are necessary.
Safe Practices: Protecting Both Machine and Operator
Safety should always be paramount when working with CNC machines and bits. Always ensure that:
- The bit is securely mounted to prevent wobbling or dislodging.
- You’re using the correct bit type and size for the material and operation.
- Proper protective equipment, like safety glasses and ear protection, is worn during machine operation.
- The machine’s enclosure or shield is in place to protect against flying debris.
Regularly inspect your CNC machine for any signs of wear or damage. Regular maintenance ensures both safety and consistent performance.
The Evolving Landscape of CNC Bits
With advancements in materials science and manufacturing techniques, the world of CNC bits is continuously evolving. We’re seeing the rise of bits made with nano-composite materials, coatings that reduce heat buildup, and geometries optimized using machine learning and AI.
Choose the right router bit shape for your project
Are you making straight cuts in plywood? Then get a high-quality spiral end mill. End mills come in many different diameters. You can choose an upward or downward cut.
Are you carving a large three-dimensional outline or sculpture? You’ll need a ball-nose bit of the right size for the level of detail in your model. Ball-nose (aka contouring) bits are great for 3D carving (think of a topographical map or the grape relief on a wine store sign). (Think of a topographical map or a wine store sign.) You can use just the tip to carve details and smooth contours, or you can move a lot of material like an end mill.
For complex 3D engraving, consider a tapered ball nose bit like this one. A slight angle on the cutting edge can make tool marks parallel to the material surface less noticeable.
If you are flattening a large board, or if you are responsible for maintaining the spoilboard on your CNC router, a spoilboard cutter or flycutter bit is useful. These bits are made to scoop up the surface and leave a smooth, flat finish. I recently upgraded to a 2 1/2″ bit and I love it. Its got four cutting edges and is what is known as an “insert” bit, which means the blade can be removed for resharpening or replacement. It’s an investment bit, but it’s worth every penny. I love to use it to flatten table tops.
Choose the right bit for your material
Hardwood? Plywood? Laminated particle board? Plastic? Aluminum?
Many manufacturers produce bits that are specific to the material. Bits for hardwood are designed to leave a clean edge. Bits for plywood and laminates are designed not to crush the outer veneer layers. Bits for plastics are designed not to melt excessively. Aluminum cutting bits are designed to efficiently remove chips to avoid rewelding (heated chips fusing to the hot cutting tool). Many bits can be used for multiple purposes, so if you are in the prototyping stage of a project, you may not need to purchase 30 bits right away. A good all-purpose bit is a two-flute upcutting spiral bit.