The What and Why of CNC

Computer numerical control (CNC) is a software-based method of moving tools and machinery. This method has traditionally been under the purview of factories and manufacturing facilities and well beyond the reach of a garage tinkerer. CNC machines have arms and carriages that can hold mills, grinders, lasers, and other cutting tools that move in multiple axes to shape an object via preprogrammed movement commands.

Our objective in this chapter is to provide a basic understanding of CNC from a hobbyist’s/tinkerer’s perspective as we prepare to take a deep dive into obtaining, operating, and customizing our own machine.

In this chapter, we will cover the following topics:

• Branches of CNC machining
• Differing approaches to motion in CNC machines
• How CNC works and when to use it
• A brief overview of G-code, the commands used to operate a CNC machine
• Safety considerations for CNC and lasers

Branches of CNC machining

CNC manufacturing can be traced back to the 1940s, when the first numerical control (NC) machines started to appear (https://en.wikipedia.org/wiki/History_of_numerical_control), and methods to automate handcrafted fabrication can be traced to three centuries ago. I’m sure you’ve seen videos of these machines perhaps fabricating the engine block for a car or cutting and shaping sheet metal. For all intents and purposes, a CNC machine is a type of robot. It takes stock material (a sheet of wood, a block of aluminum) and turns it into a product ready to be assembled or finished very quickly, very accurately, and, most importantly, repeatedly. CNC machining made fabrication at scale possible.

With the advent of desktop computing, more commonly accessible materials, and smaller, more powerful components, it also became possible to bring CNC machining to the home workshop. Now, anyone with a good guide can put together a robust desk or bench-top...

Differing approaches to motion in CNC machines

Depending on the nature of the machine itself, the motion system can vary based on the rigidity requirements of the toolhead. Very frequently, a compromise is struck between speed and rigidity (or stiffness) so that cheaper or more readily available components may be used. The most common of these tradeoffs is allowing X- and Y-axis motion to be driven by belts while the Z-axis motion operates using a leadscrew. Other designs use leadscrews throughout.

The following figure shows one of my four 3018 machines:

Figure 1.2 – A 3018 machine (see Figure 2.1 for an annotated close-up picture)

In the preceding figure, note the controller on the right. This picture was taken just prior to calibration. This is a machine that can be purchased online as a kit. It is made from some off-the-shelf parts (8020 aluminum extrusions, 8-mm steel rods, stepper motors (which are used for motion), leadscrews, and an engraving...

How CNC works and when to use it

The CNC machine is operated by an onboard controller that runs three or more stepper motors and the toolhead. Stepper motors have a lot more torque and can be controlled with greater precision than ordinary electric motors. Motor control is measured in fractions of revolutions, which allows for excellent precision in movement. Using precision leadscrews (or pulley/belt systems), the motors move a gantry along the Y axis while the toolhead moves left and right on the gantry (the X axis). The Z axis is nothing more than a small gantry that moves the toolhead up and down, typically with a leadscrew.

The commands being passed to the controller on the CNC machine are called G-code. The commands passed to the controller (either via the onboard software or a computer passing G-code to the controller) move the toolhead to various locations in all three axes and runs the spindle motor so that the milling end can then cut into the material as desired. The...

What is G-code?

G-code is nothing more than motion commands that the CNC machine’s controller interprets to move the toolhead. Of course, those commands are passed on to the motor as the number of turns the motors‘ shafts have to make in one direction or the other. In addition to motion commands, there are commands to start and stop the spindle motor. All these commands are generated when software running on a computer interprets a design as a series of movements. There are many applications like this available, some free and others that can be purchased. Some design applications are also capable of generating G-code (for example, Fusion 360), which can then be passed on to the sender application, which in turn passes it on to the CNC controller. Some sender applications also generate G-code, such as LightBurn (for lasers), Mach 3, and Easel. For the purposes of our projects, we will focus on freely available firmware and sender software.

A note on laser machines

...

Safety considerations for CNC and lasers

CNC machining involves the removal of material using a hardened metal bit that can fling fragments of your workpiece all over your shop. It should be needless to say that you should always have hand and eye protection on whenever you are working with and on your machine. This is especially true with lasers. The blue, green, or red laser on your machine can destroy your eyesight in an instant, so whenever the laser is on, you should have appropriate glasses on to filter out any harmful reflected laser light. Never operate a laser without those special glasses. Every laser unit I have ever purchased came with a suitable set of adjustable glasses (a green lens for a blue laser, for example). If your CNC bit shatters, you do not want metal fragments in your eyes or on your hands, and if your laser hits a reflective surface, you do not want to blind anyone looking over your shoulder or elsewhere in the shop with you. Do not allow pets anywhere near...

Summary

We now have learned some of the basics of CNC revolving around what it is, how it works, and what drives the design of various machines (for example, when to use belts instead of leadscrews). We also looked at how we might machine not just flat stock material, but also curved surfaces (with the rotary axis). All these are important concepts to grasp because they lay the groundwork we need to select, assemble, configure, and customize our own machines.

In the next chapter, we will look at setting up our own desktop CNC machine and get underway with fabricating some basic parts and shapes.