CNC Router Essentials. Randy Johnson

      CNC Router Essentials

      3 : Toolpath Essentials

      rather than liing it, which can occur with an upcut bit. Because a downcut bit pushes the chips down when cuing, the chips usually get packed into the kerf. The packed-in chips will fall away when you separate the finished parts. If you need to remove them before that, try running the cut a second time to loosen up the chips and then remove them with compressed air or a vacuum.

      Compression bits combine upcut and downcut shearing action in one bit and thus leave a clean cut on both the top and boom side of your material.

      Router bits

      Many of the bits you use with your handheld router or router table can be used in your CNC. There are also specialized bits for CNC that you will find useful. Carbide-tipped or solid carbide bits are the most commonly used bits on CNC routers because they stand up to abrasive materials such as MDF and plywood. However, due to their hardness, they are also more susceptible to breakage than high-speed steel (HSS). HSS also has the advantage of being less expensive than carbide, although HSS bits generally dull quicker.

      You probably already own a few straight bits. These are good general purpose bits, but won’t give you the cut quality of a spiral bit. Spiral bits are available as upcut, downcut, or a combination, Image 3.1. Upcut bits act like a drill bit, pulling chips up out of the cut, which keeps the cuing path clear. The spiral creates a shearing action that reduces chipping and gives you a beer edge quality on your material than a straight fluted bit. An upcut bit may fray or chip the top surface of your material, but when through cuing, it provides a chip-free cut on the boom side.

      Downcut spiral bits push down toward the material. This downward shear action provides chip-free cuing on the top surface but may cause chipping on the boom of some materials, such as melamine and plywood. Downcut spiral bits are also useful when cuing thin materials because the downcut pressure helps to hold the material against the bed

      A router bit starter set

      Image 3.1: Straight bits come in a variety of configurations. The four shown here are (left to right): straight flute, upcut spiral end mill, downcut spiral end mill, and compression end mill.

      • 60 and 90° V-bits (V-carving) • "-, "-, and "-diameter upcut spiral straight bits (general cutting)

      • " ballnose bit (3-D finish carving)

      5

      CNC Router Essentials

       Toolpath Essentials : 3

      Additional useful bits

      V-bits are most commonly used for V-carving text and other fine details, Image 3.2. The smaller the angle of the V-bit, the finer the details it will be able to cut.

      Ballnose and core box bits are available in a variety of diameters and lengths, Image 3.3. Tapered ballnose bits are used for finishing intricate 3-D carvings. Like V-bits, smaller bits will create more intricate details.

      A spoilboard or fly cuer bit is helpful, but not imperative when starting out, Image 3.4. This type of bit is used to level the spoilboard on your machine. With its large diameter, it can cover a big area quickly. You can also surface your spoilboard with something more common, such as a " straight bit. It just takes longer.

      Which bits you’ll need for your work depends largely on the types of projects you plan to make, but here is a short list of bits that will get you started, and you can add to it as your needs expand.

      Image 3.4: A large diameter spoilboard bit is used to surface the MDF table when it becomes rough.

      Image 3.2: V-bits are available in different diameters and angles. They are mostly used for V-carving.

      Image 3.3: Ballnose and core box bits are available in a variety of diameters and lengths. Smaller diameters are tapered to increase strength and reduce breakage. Spiral ballnose bits are commonly used to machine intricate 3-D carving designs. For shallow low-detail carvings, a core box bit works well. Both bits can be used to create fluted designs.

      As you do more with your machine you may want to add: • 30° V-bit (fine V-carving and details)

      • "- and "-diameter ballnose bits (3-D carving)

      • "-diameter downcut spiral straight bit (general cutting)

      • "-diameter straight bit (fine profile and pocket cuts)

      6CNC Router Essentials

      3 : Toolpath Essentials

      Frequently used toolpaths

      A CNC machine can make many types of cuts, but the four most commonly used are the Profile, Pocket, V-Carve, and 3-D Roughing and Finishing Toolpaths, Image 3.5. With an understanding and mastery of these cuts, you’ll be able to complete a variety of projects.

      Pocket cuts are used to create a recessed area in your workpiece, Image 3.6. The recess might simply be a design detail in your project, or it can be sized to accept another piece of material, such as buerfly inlay. A pocket cut can be square, rectangular, oval, or nearly any shape you can imagine.

      V-carving is used to create text that simulates incised handcarved leering, Image 3.7. But it works equally well on any “closed” line shape, such as the outline of an animal. By contrast, a single line with ends that don’t connect is considered an “open” line shape and cannot be V-carved. V-Carve is a very popular toolpath with CNC users and is a very simple way to add decorative carving to your project.

      3-D carving toolpaths include both roughing and finishing passes. They are used to carve 3-D models. The roughing

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