What, Why & Hows of CNC Machines

What is a CNC? Why might I want one & How does it work?

Over the last decade, CNC machines have become a common topic in woodworking. Most know that they cut things out, but that’s the extent of their knowledge. If you had a stronger understanding of what CNCs are, how they can be used, and the basics of their operations, I guarantee that you too will agree why this tool has become a topic of interest.

What is a CNC Machine?

CNC stands for computer numerical control. Simply put, it’s a computerized process in which software is pre-programmed with code to tell the machine the exact movements to make. CNC machines can include routers, lathes, milling machines, 3D printers, vinyl cutters, and much more and break down into two categories: additive and subtractive. As the name suggests, additive CNCs add material to create the design. A good example of this type is a 3D printer. For our purposes, we will be talking about subtractive machines, which remove material with cutting tools.

Why Might I Want One?

Believe it or not, CNCs can be used to create not only more accurate and complex projects, but improve traditional woodworking as well. How? Well, CNCs provide higher accuracy as the machine moves exactly as instructed. This leads to more reliability and repeatability than we can do by hand. They can easily make intricate cuts or carve advanced shapes. And, most importantly, you can review or simulate how the machine will move before it cuts the digital file. This allows you to make corrections or edits before beginning the project.

As for improving traditional woodworking, think of a CNC like an almost unlimited jig. This jig can be used for different needs and once a task is complete, it can repeated over and over again. Let me give you a few examples:

• Accurately cut mortises in
complex parts

• Make jigs and fixtures for other tasks (for example, create a steady rest for the lathe)

• Quickly create new throat plates for the band and table saw

• Save money by batching out custom hold-down clamps, push sticks, or even guards

How Does a CNC Work?

CNCs work through a process called Digital Fabrication: a design and manufacturing workflow that results in program-generated instructions given to a computer to ingest and inform the CNC controller on how and what to do. Before we break down this process, we need to review the X, Y, and Z axes on the Cartesian Coordinate System and how it relates to the CNC machine. This will make understanding the process of digital fabrication much easier!

Cartesian Coordinate System:

The Cartesian Coordinate System defines points or coordinates on a 2D or 3D space. The coordinate is described numerically (such as 10, 10). To define coordinates on a 2D space, we need two axes: the X-axis which runs left to right and the Y-axis which runs from front to back (perpendicular to the X-axis). The third axis or the Z-axis allows us to create a 3D space as this axis runs up and down (vertically). It represents depth, which allows us to lower or raise the cutting bit when applied to a CNC machine.

How Does this relate to the CNC?

Let’s think first about how the machines creates a 2-dimensional space. As shown in the diagram on the previous page, the gantry spans from left to right. This entire gantry travels from front to back along the crossbars, allowing the bit to move in the Y-axis.

To move the bit along the X-axis, we need to look at the carriage. The carriage holds the motor (a router, spindle, or laser) and travels left to right along the gantry. This moves the cutting bit along the X-axis. The Z-axis, or depth, is also controlled via the carriage. On most CNC machines, a lead screw will raise or lower the cutter motor in relation to the rest of the carriage and gantry.

Now, let’s talk about how all of this relates to the Cartesian Coordinate System. In the CCS, the starting point is 0,0,0— where the X, Y, and Z axis all converge. Often, you can have a point in one of the negative planes, however, for CNCs, we only work in the positive space. Therefore, the home point (0,0) on most CNC machines will be at the lower left position. You can see this as the orange area in the illustration above.

The Z-axis is a little different however. The Z-axis home (or zero point) is set based on your project’s workpiece. Most of the time, you’ll lower the Z-axis until the bit touches your workpiece and set that as “0.” In the instance of cut depth, we do work in the negative space. For example, to make a 1/8“ cut, the Z-axis point will be set as –0.125“.

Digital fabrication

Let’s jump back into the workflow of the CNC. There are 4 main steps to take an idea and turn it into a  CNC program:

1. Create Design (using CAD)

2. Generate Toolpaths (using CAM)

3. Translate to G-Code

4. Send to Post-Processor to run on each specific CNC

I’ll go into a little more detail about each step, but first let’s sum it up. You begin by designing a project in 2D and then transition it into a 3-dimensional space. Those details are then converted to a language that the computer can share with the CNC. Lastly, a post-processor reviews that information and reformats it to the specific machine you are using and sends it to the machine.

1. Create Design

Most commonly, this is done in a program that runs using CAD, Computer-Aided Design. Programs include V-Carve, Easel Software,
Carbide Create, AutoDesk, and more. (More on these programs in a future article.) The goal is to create a design made up of a series of points (X, Y) that form a drawing. That drawing is then exported to a  CAM (computer aided manufacturing) file such as .dwg, .dxg, .svg, .eps, or .ai. These are  all vector-based files. At this point, the 2D drawing can guide the bit in X, Y directions to trace a shape.

2. Generate Toolpaths

The CAM file is now uploaded into a CAM-based program. For most programs, including those stated above, the CAD and CAM programs are the same program — once you complete the drawing you can jump right into generating toolpaths without switching software. Toolpaths are instructions given to each line (or series of lines) such as the type of cut, the tool needed, and depth of cut. Type of cuts can include simple cuts, pockets, holes, engraving, and carving. As the toolpaths are generated, a 2-dimensional drawing is transformed into a 3-dimensional shape.

3. Translate to G-Code

In order to make these precise cuts, the toolpaths created in step 2 has to be translated into a computer language called G-Code. The G-code is sent to the controller which runs the CNC machine. This language (G-code) contains hundreds of lines of code that specifies the necessary movement of the router or spindle from point to point. Each line of code begins with a G followed by a number to instruct a change in geometry.

Today, we have programs available to create this G-Code for us. However, not that long ago, programming the G-code was completely manual and time consuming. It’s still useful to know at least a little bit about how it works though.

There are two parts in each line of G-code. The first part tells the machine how to move and the second tells it where to move, how fast, etc. Check out two examples above.

4. Send to Post-Processor to Run on the CNC

This last step simply uses a post-processor, which is specific to each CNC manufacturer, to adjust the G-code. The post-processor simply alters the code just a little bit so that it can be read by the controller, which is the brain of the CNC.

Now that you know what a CNC is and the basics of how they work, I hope that you may see why many people have chosen to add one to their shops, and hopefully it’s a little less mystifying and scary. To be honest, they’re not as complex as people think they are, and I think that adding one to your shop is something that a lot of people should consider. Be on the lookout for our next article in this series where we will discuss a few of the CNCs available on the market, and what accessories that are handy to keep on hand in your shop for use with your CNC.

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