First, we need to distinguish between drill bits and milling bits:

• Drill bits: These drive into the material. They’re the best choice for tasks that require material to be removed straight down through the workpiece. They’re often used for pre-drilling holes for screws.
• Milling bits (or cutters, carving bits, and end mills): These cut laterally across your material. They’re designed to move across the surface of the workpiece, clearing away chips to achieve 3D designs.

Even though bits fall into these two main categories, other variables are always at play, like bit material (solid carbide, HSS, carbide-tipped), flute type (straight, upcut, downcut, compression), number of flutes (1-2 flutes, 2+ flutes), and bit end (fishtail, engraving, V-bit, ball nose).

There are also specialty bits, which have more sophisticated profiles for all manner of specialized applications. They can be used to drill holes, etch glass, or make dovetails, for example. That means it’s up to you to investigate which bit is best for your needs. Some notable specialty bits include boring bits, miter fold bits, round-over bits, and “veining” bits.

Some common bit materials include the following:

• High-speed steel (HSS): This is a tool steel bit with high heat resistance, wear resistance, and hardness. It’s much better than carbon steel when used for cutting tools and drill bits and is mainly recommended for cutting foams. HSS is processed from raw ore and is heated to its melting point before several elements and minerals are added to enhance the internal structure. HSS tool life is shorter compared to carbide.
• Solid carbide (SC): This is a sturdy and wear-resistant steel. SC bits come in different types, ranging from cheap ones to high-quality ones that cost top dollar. When shopping for SC bits, you have to be extra careful or you’ll end up with a knock-off. There are many forms of carbide (tungsten carbide, beryllium carbide, silicon carbide, among others). Note that these bits can be brittle and should be handled with care.
• Carbide-tipped: Since carbide is costly, large-diameter CNC router bits have chunks of carbide brazed to a steel body. You then get the longevity of carbide with the lower cost of a steel core.
• Solid tungsten carbide bits: Tungsten carbide bits are made using hard particles of tungsten carbide bound with metallic cobalt. These kinds of bits are super strong and durable, so they remain sharper for a much longer time than HSS bits.
• Polycrystalline diamond (PCD) bits: These bits are made up of diamond particles sintered with a binder. The diamond is blended onto a carbide substrate to make an extremely hard and abrasive-resistant bit.

Spiral bits are great all-purpose tools that can be used in several applications, but you need to determine the cut direction, which will be informed by the type of material or other variables.

Upcut

Upcutting bits pull chips (and the material) up. When using them, your material will need to be securely held down. These bits are preferable if you want to clear chips from your cuts to prevent overheating the bits. They’re great for making cuts through the material. An upcut is the most common type of cut.

Downcut

Downcutting bits press the chips and the material back into the cut. Their down shear action is excellent for preserving the top surface of your material. These bits are preferable for cutting thin, flexible material but aren’t the best for drilling holes because the bottom layer of the workpiece will be pushed away from the bit by chips. Downcut bits leave a clean cut at the top of the workpiece but may fray the bottom edge.

Compression Cut

Compression bits (up-down bits) offer the benefits of both upcut and downcut bits. The bit is a standard downcut, but its tip is an upcutter. Therefore, when cutting materials like plywood, you’ll end up with a clean edge on both sides because the top is pushed down while the bottom is pulled up. Combining both geometries eliminates chipping and reduces the risk of damaging the workpiece.

Note that compression bits should never be used to drill holes. Once the bit goes deep enough beyond the direction change, the chips get compressed because they have no way of escaping; this creates friction and heat. These bits are a little costly because of their sophisticated nature, but they’re recommended for laminates, composites, and plywood.

Bit manufacturers go to great lengths when engineering their cutters, but selecting the right bit still requires some due diligence. There are plenty of variables to take into account when picking the right bit, but the type of project and material are two factors that you must always take into consideration.

Project Suitability

What project are you undertaking? The project will drive what type of bit you should use.

For instance, detailed work like lettering requires a V-bit (sometimes called engraving bits, V-groove bits, or V-carving bits). As the name suggests, V-bits have a cutting profile in the shape of a ‘V’. They come with different “included angles”, and this will determine what they’re used for. A 60° V-bit, for example, will give you smaller details compared to a 90° bit, which is well suited for larger or shallower details or letters.

Material Compatibility

What are you cutting? Is it hard or soft wood? Is it stainless steel, fiberglass, MDF, acrylic, or aluminum? The material matters.

It matters so much that you’ll find different grades of carbides used for different materials. Going for the wrong bit could be detrimental to the bit or even to the machine. Plastic will require a specific bit (specifically one that will not melt it) and so will aluminum and plywood.

Go-To Options

In case you’re new to CNC routing and you want a multipurpose bit, go for 2-flute spiral bits. They cut a wide range of materials, including plastics, aluminum, foam, and wood. The 2-flute cutting spiral bit is an excellent all-purpose bit if you’re still in the prototyping stage and don’t want to purchase 20 bits right away. The 60° V-bit is also a wise choice for engraving, making signs, and cutting letters.

Here are some of the other bits that you may consider having right away:

• 1/4″ upcut 2-fluted end mill
• 30° engraving bit
• 1/8″ end mill and ball nose
• 1/2″ end mill and ball nose

Most manufacturers make bits for different materials. For example, hardwood bits are designed to leave a clean edge, while plastic bits are designed to prevent excessive melting. Aluminum bits, on the other hand, are designed to effectively clear chips to avoid re-welding. (Heat is usually generated during the cutting process, and this heat can sometimes cause the heated chips to fuse to the hot cutting tool.)

Ask yourself, do you want to cut fast or cut smooth? You need to determine whether speed or accuracy is the most important at any given stage of processing.

Speed

Speed and thus feed rate will depend on the horsepower of the spindle. While a high RPM will generally result in a higher quality finish, it can also result in higher friction that will, in turn, increase the wear on your bit. Therefore, you’re better off referring to the spindle manufacturer’s speed parameters.

If you have to cut a large amount of material in a short time, go for a bit that can go through your material quickly. On the other hand, if you hate sanding hardwood, go for a bit that will leave a smooth finish.

Higher tool RPM produces smaller chips, while higher feed rates produce larger chips. Overall, if the chips are too large, your bit will be likely to break, but if your chips are too small (like fine powder), you’ll be dulling your bit. It’s all about getting the right balance.

Finish

Bits offer multiple cutting edges for extra flexibility. You have the option of going for 2-, 3-, or 4-flute bits. Having more flutes will give a finer quality of cut, but will go through your material much slower.

A CNC router can fabricate complicated designs, but only if you get things right. Here’s a great starting point when selecting CNC router bits:

• Single-edge bits: Use when speed is more important than edge quality.
• Double-edge bits: Use when edge quality is more important than speed.
• Spiral bits: Use when finish is a concern or when horsepower is limited.
• Upcut spiral bits: Use for slotting, grooving, or when chips need to be removed fast. Also suitable for when the material is coated or laminated on one side and is being machined while facing down. (This is the most popular type of spiral-fluted bit.)
• Downcut spiral bits: Best for thinner materials. They’re also great for leaving a nice finish on the top surface. However, chip extraction is less effective.
• Spiral, straight, or shear bits: Use for composites, plywood, natural woods, and particleboard.

Other Considerations

Before cutting, engraving, or pocketing, think about the following:

• Material: Pay attention to its type and thickness.
• Cutting length: Always go for the shortest cutting length because, whenever the cutting length is longer than necessary, you risk deflection, vibration, and an inferior finish. Generally, the recommended cutting length shouldn’t exceed four times the cutting diameter. Otherwise, the bits have a higher risk of breaking.
• Cut diameter: For a quality finish, increased rigidity, and longer tool life, use the largest tool diameter possible.
• Going for the strongest bit: Long bits are more susceptible to bending and vibration, while short bits tend to produce cleaner cuts.
• The sharpness of the bits: Sharp bits will always give you good cuts. They’re “kind” and quiet compared to dull bits, which are often loud and messy. Most manufacturers have a sharpening service for bits, but sometimes it’s much cheaper to buy a new bit rather than having an old one sharpened. Never attempt to sharpen bits by hand.
• Storage: Bits need to be stored well to prevent the edges from banging into each other. Carbide, for example, is brittle when it gets knocked on other pieces of carbide. Consider buying a bit holder (or design one that meets your needs). If you move your bits a lot, build a unique bag for the bits.

Lastly, always remember that proper tuning is critical if you want your CNC router bits to perform well and to last a long time.