How to Choose the Right Bit for Your CNC Projects

In this Instructable, we are looking at how to choose the right bit, or cutting tool, for your CNC projects. Makera is a manufacturer of high precision and easy to use desktop CNC machines, as well as a company built by makers, for makers, in the era of makers. This Instructable is intended to guide users of all levels through getting started with CNC machines, specifically by looking at bits.

While we are discussing bits for the Carvera Desktop CNC and Carvera Air, similar concepts are applicable to a wide range of CNC mills or routers, suitable to different resources that you might have available. While we sell bits in our store, the Carvera and Carvera Air can be equipped with any bit ranging in collet diameters from 3mm to 6.35mm, including 1/8 and 1/4 inches.

In addition to writing and sharing how to select the right bit in this Instructable, we've also created a YouTube video as part of our Getting Started with CNC Machines series for additional information and support.

For additional information, support, and resources, visit Makera.com or check out our YouTube Channel and Wiki Page.

For this, you need a CNC machine and bits of course! As mentioned earlier, these concepts are applicable to a wide range of CNC machines, including mills or routers, as well as with a wide range of material choices. Through this Instructable, we're going to look at when and why you might choose one bit over another depending on the material you're working with, or the types of projects you're looking to create. But if you're interested in learning more about the materials you can choose for your CNC projects, check out this instructable too!

But if you need a CNC or bits, we recommend you check out the resources in our store.

Before we dive into the different types of bits and when you might choose them, let's cover the basics and discuss what bits are. Bits, sometimes referred to as Cutting Tools or simply Tools, are what do the work in a subtractive manufacturing process. CNC Mills and Routers are subtractive manufacturing machines, meaning material is removed during the machining process through a cutting or drilling action in order to make your part.

This is very similar to how a drill, saw, or even a lathe works, but CNC (computer numerical control) machines do this process autonomously based on the file you prepare in CAM software. If you're interested in learning more about the different types of CNC's, check out this video.

Like a drill bit, a cutting tool for a CNC cuts through the material (also know as stock) to remove material in the form of chips or dust. As a cutting tool works across the stock in multiple axis, such as X, Y, and Z, it not only cuts in a vertical direction, but back to front and side to side too. Think of this as a combination between a drill press and saw. And if you're working with a CNC Mill with a 4th axis, then you can rotate your stock around for even more intricate machining by combining the operations of a drill, saw, and a lathe all in one!

As you will find through the next few steps, you need to choose the right bit based on the type of cut you want to make, and the type of stock you're working with. In a detailed project, you might find that you need several bits to perform all of the different operations needed to make the part you've designed. In this case, it's best to work with a CNC that has an automatic tool changer, such as the Carvera, to make tool changes autonomous. Alternatively, the quick tool changer on the Carvera Air does require you to change the bits, but in an easy and quick way with no wrenches needed.

With all the basics covered, let's get started!

Bits are typically measured in shaft diameter, cutting diameter, overall length, cutting length, number of flutes depending on the type of bit, and cutting angles depending on the type of bit.

The shaft diameter defines the outer most width of the cutting tool's shaft, and you must ensure you select bits which fit within the collet of your CNC. A collet is the part of the spindle that holds the bit in place. As mentioned earlier, the Carvera and Carvera Air can be equipped with bit collets for 3mm through 6.35mm diameter bits, including 1/8 to 1/4 inches.

The cutting diameter measures the diameter of the cutting tip, and this will define the minimum pocket or cutting width that the bit can achieve in a single pass.

When we see the length of the bit, that typically refers to the overall length, which includes the cutting length, and also the length that sits in the machine's collet as well. In general, you always want to work with the shortest possible bit, as the longer a bit extrudes from the spindle collet, there is a greater possibility for flex and bit breakage.

The cutting length is the usable length that can be used for machining our parts. This is important, because a bit that is 40 mm in length overall can't machine material that is 40 mm in thickness, you need to understand how much of the bit includes the flutes and cutting surfaces as that will define the workable area.

Some bits include flutes, which are the sharp corkscrew like slots that travel upwards. Flutes work to cut and evacuate material during the machining process, and bits with higher flute counts are typically stronger, provide a smoother finish, but also have a smaller cutting area. Additionally, multi-flute bits create smaller chips which can easily accumulate more heat, especially when machining metals.

Some bits also include cutting features that are on angles, which are also part of the measurements to be considered. Larger angles typically make for stronger bits, but smaller angled bits can create smaller cuts and therefore more detailed projects.

And you may notice that our bits have a bit collar. This plastic ring is used to hold bits in place when loaded in the Carvera's Automatic Tool Changer, or to ensure that a bit is installed at the correct height for the Carvera Air's quick tool changer. The bit collars can be installed and removed on any bit you want to use, as shown in this tutorial.

Spiral O' bits get their name based on their shape, and they are a very common type of cutting tool that works with a wide range of materials. This bit typically only has a single flute, and also comes in a wide range of lengths and diameters to accommodate a range of stock sizes.

You can use Spiral O' bits for softer materials, like woods, plastics, composites, and also for softer metals like brass or aluminum. Because Spiral O' bits remove chips efficiently, they tend not to accumulate very much heat whcih makes them well suited for machining metals without needing external cooling. This means you can typically machine softer metals without needing a CNC equipped with liquid (flood) coolant.

As for the types of cuts you can create with a Spiral O', they work great for less detailed cutting operations, like facing stock, creating pockets, contour cuts, and even for hole operations as well. All in all, a Spiral O' bit is one of our favorite bits to use thanks to its versatility and performance!

Ball Nose bits get their name from its rounded tip. Ball nose bits typically have two flutes and a smaller cutting length, making it more suitable for shorter operations rather than deeper cuts. Additionally, the rounded tip prevents a Ball Nose bit from making flat cuts efficiently, making it more suitable for curved surfaces.

Ball Nose bits are very strong, making them suitable for machining harder materials such as metals like aluminum. But they can work for softer materials that are dense, such as hardwoods or plastics too.

The Ball Nose is best used for creating rounded pockets and engravings. This allows you to machine curved surfaces, pockets, or relief milling operations in exceptionally high detail! But the Ball Nose is also best paired with a multi-process operation, where you might first rough cut the design using a Spiral O', then perform a finishing pass with the Ball Nose. This not only reduces bit wear by allowing the bits to perform at what they do best, but also cut down on overall machining time too.

The next bit we are looking at is a V Bit, sometimes referred to as an Engraving Bit. V Bits can be considered to be a single flute bit, but they also have a cutting angle to consider which determines the angle of the V-shaped tip.

V-bits are very versatile bits that can achieve great detail through engravings, reliefs, and small detail pocket milling. The cutting angle and tip diameter will determine the minimum area you can machine, smaller being more precise and larger being better suited for harder materials.

Depending on the type of V Bit you use, you can machine a wide range of materials from softer woods and plastics, to harder metals if the bit has been made for them. But V-Bits work best when machining a small amount of material at a time, as machining too quickly will often result in breaking the bit's cutting tip.

Another type of bit to consider is a Corn Bit, which is a versatile tool that can be used for many cutting operations. Corn bits have a distinctive geometry of its cutting edge, which has numerous small teeth that resembles the rows of kernels on a cob of corn.

Corn bits are very strong and and exhibit high wear resistance making them suitable for abrasive materials such as composites like fiberglass, carbon fiber, or printed circuit boards (PCBs). And because of their strength and design, a smaller-diameter Corn Bit can typically perform well without breaking when compared to smaller size bits of other styles. This is why you often see Corn Bits as small as 0.6mm in cutting diameter!

And Corn Bits are very versatile in the types of operations you can perform, including pockets, drilling, or contour cuts. This means you may be able to use a Corn Bit for your entire project when making things like PCBs!

Now you may be familiar with Drill Bits for your drill press or handheld drill, but drilling is a very common CNC operation too. As mentioned earlier, you may be able to use a Spiral O' or Corn Bit to make a hole depending on your project, but this is typically done through a pocket tool path rather than a drilling path.

Unlike the other cutting tools we've discussed, you would need a Drill Bit with the cutting diameter of the hole you are looking to create, and also use drill bits specifically with drilling operations rather than hole milling operations. But depending on the type of Drill bit you choose, these tools can drill through any material you want to work with from softer materials to metals.

A good strategy for working with Drill Bits is to perform a retract, sometimes referred to as a peck-drilling operation. This will retract the bit as it drills to allow for chips to evacuate, reducing bit wear and creating a cleaner cut. We discuss this more in our Speeds & Feeds Instructable.

Like Drill Bits, Threadmilling bits are intended for one specific operation which is cutting threads into a hole using your CNC! Before using a Threadmilling bit, you must first drill the hole into your stock. A Threadmiling bit will then work like a threaded tap to machine threads in your material around the inside of a cut shaft.

To choose a Threadmilling bit, you need to identify the type of material you are working with as well as the size of the thread you are looking to create as this is dictated by the size of the cutting tool tip. But Threadmilling bits can be used with a range of materials, including woods, plastics, and metals.

There are more specialized bits like the Threadmilling bits which only perform a single type of operation. Some of our favorites include Chamfer bits which can create a chamfered edge on a cut surface, or a Solder Mask Removal Tool used for machining the surface of a PCB that has been coated with a cured solder mask.

And despite all of these options, there are even more bits out there to consider too! From flat end mills to bits with up to 8 flutes, there is always an optimal tool for the job!

While we didn't look at every possible bit available in this guide, what we covered should be enough to get you started, and hopefully finding success for your CNC projects as you work to choose the best tool for the job!

More things consider which are equally as important as the bits you choose are the speed and feed settings when preparing your projects, as well as selecting the right stock for your projects. You can check out more guides and videos we've created to support learning these concepts, along with more how to's and guides on the Makera Instructables Page, YouTube Channel, and Wiki Page.

Thanks for reading, and happy making!

Written by @MrErdreich and @Makera_Inc