While CNC machines have opened all kinds of creative possibilities, the magic lies in the bit. It determines whether you will end up with a perfect or okay design. Plus, it also makes the difference between cutting down on waste or increasing material costs significantly.
The bit is used to remove material in a specific way. Initially, routers used to be handheld tools that were guided by templates or ball-bearing pilots, and they were moved manually around the workpiece. But today, routers are controlled using computers that precisely determine the toolpath, and these routers require bits to make intricate cuts.
The range of materials and designs cut by router bits has grown significantly in diversity, and so have the types of CNC router bits. Today, the market is flooded with so many kinds of CNC router bits that picking the right bit for a project isn’t always intuitive.
Before going any further, let’s familiarize ourselves with some CNC routing jargon that will be used throughout this article:
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, up cut, down cut, 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.
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.
Up cutting bits pull chips (and the material) up. When using them, your material will need to be securely held down. Upcut bits are preferable if you want to clear chips from your cuts to prevent overheating the bits. They are great for making cuts through the material. An up cut is the most common type of cut.
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 are not the best for drilling holes because the bottom layer of the workpiece will be pushed away from the bit by chips. Down cut bits leave a clean cut at the top of the workpiece but may fray the bottom edge.
Compression bits (up-down bits) offer the benefits of both up and down cut bits. The bit is a standard down cut, but its tip is an up cutter. Therefore, when cutting materials like plywood, you will 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, and 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.
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.
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 will find different grades of carbides used for different materials. Going for the wrong bit could be detrimental to the bit or even the machine. Plastic will require a specific bit (specifically one that will not melt it) and so will aluminum and plywood.
In case you’re new to CNC routing and you want a multi-purpose bit, go for two-flute spiral bits since 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.
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 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 are 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 will be dulling your bit. It’s all about getting the right balance.
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.
Lastly, always remember that proper tuning is critical if you want your CNC router bits to perform well and to last a long time.
License: The text of "Simple Guide to CNC Router Bits" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.
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