Introduction to CNC Machines - Canadian Woodworking Magazine

Tool Technology: You may have heard the term “CNC” a lot in the last few years and found yourself wondering what CNC machines are all about. Read on to learn about how these machines are changing the way people work wood.

Introduction to CNC Machines

Introduction to CNC Machines



Photos by Rich Keller; Lead Photo by CWI Machinery
 
CNC (Computer Numeric Control) machines have been around for many years. A simple explanation of CNC is that a computer is used to control the movement of a tool – most often a router, with lathes being second most common. This movement can be controlled very precisely, in thousandths of an inch or decimal millimetres. You can use CNC machines to carve, cut out and shape just about anything, and one of the big pluses is that you don’t have to use any jigs. With a CNC, if you can imagine it, you can make it.
 
CNC machines have been used extensively in industry for decades, but they are now starting to find their way into home shops. In the last decade or so, several companies have started to make machines geared exclusively for small shop use. Machines are becoming more readily available and more affordable all the time. Many of the major retailers of woodworking equipment here in Canada have started to stock machines and display them in their showrooms. In this article we’re going to look at CNC routers specifically; what they do, how they work, and how they can be integrated into a shop.
 

Software

To accomplish all this fancy computer control stuff you need two things, computer software and equipment. All CNCs are programmed in their own special “language” called G-code, which gives the machine step-by-step instructions on how to move. Each line of code tells the machine how far to move and in what direction. The machine reads the file line by line, executing one line at a time to create anything, from very simple to highly complex designs. Unless you are a super geek, you will want computer software to make this code file for you.
 
Some software, like ArtCAM or VCarve, allows the user to design a project and create the G-code files your machine will need in one package. Other software, like Google’s Sketchup, will allow you to create designs, but you’ll need additional software to convert your Sketchup file into something the machine can understand. For example, VCarve can import Sketchup files and allow you to create the G-code your machine will understand.
 

The machines

The most important part of the equation is the CNC machine itself. CNCs come in many shapes and sizes, and it pays to do a little research and think through what you want to do before you buy. CNC routers are machines with special guide rails in (at least) three directions, X (left-right), Y (front-back) and Z (up-down) as they are called on the machine. The router is attached to these rails, and precision motors called Stepper or Servo motors (depending on the construction of the motor) are used to move the router. Machines are purchased with specific X-Y working capacities. Smaller machines are 20" x 15" or 24" x 36". Machines have a limited Z (up-down) capacity. Smaller machines have a Z capacity of 3" to 6". It’s important to keep this limit in mind because this is the maximum travel of the router. The thickness of the workpiece PLUS the height of the router bit you are using can never be greater than the Z capacity of the machine, and on some machines the workpiece can’t be bigger than the X-Y capacity of the machine. It’s also very important to realize that the capacity of a CNC typically can’t be increased later.
 
cnc-machines-screen
Screen Time – Some machines have a fully functional computer attached to run them. This one just has a simple keypad to operate it. You just plug in your USB drive, select the file, and run it.

What's in a Price?

There are some technical differences that can cause the price of CNCs to vary dramatically. This is an area where you will want to do some research before you buy. CNCs use two different types of motors. Router or spindle motors power the cutting tool, while Stepper or Servo motors power the movement of the router. To power the cutting tool, some CNCs use a standard router. Better machines use a spindle motor. One big difference is that most spindle motors are brushless, meaning less noise and maintenance, along with increased power output and durability. Spindle motors are usually more compact. Most of the small machines available for purchase today will have a small spindle motor, around 1/2 or 1 HP. Larger CNCs will use spindle motors of 2-3 HP or more.
 
Different models of machine will also use different styles of linear bearings. Linear bearings are the rails that the machine moves around on. Less expensive machines use a round rod with a sleeve that slides over it. The sleeve usually has a low friction plastic insert of Delrin, Teflon or some other hard-wearing low-friction material. More expensive machines will use a square or rectangular steel track with grooves in the side, precision ground to size, on which a steel bearing filled with roller balls will slide. The second method is better but more expensive. For smaller machines you will typically be limited by a number of other factors (such as spindle HP) on how much you can machine in one pass, so guide rails with plastic bushings are not necessarily a problem. Smaller machines do not typically move back and forth at high rates of speed. The last construction difference is the method of moving the machine table to and fro. There are stepper or servo motors involved to provide the motion. Either a gear and rack is used, or a precision ball screw. The gear and rack relies on the gear teeth meshing to provide the motion, and this type of system, if properly adjusted, usually has no more play than a ball screw. It typically allows the machine to move around faster, but this system tends to be a bit noisy, and dust in the gear can create bumping in the motion. Precision ball screws use a large diameter screw and a special nut filled with roller balls to provide a very smooth motion. Typically the nut has dust seals to keep it clean and smooth running.
 
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Spindle Motor – This CNC uses a spindle motor, which is made specifically for use in a CNC. You can also see the guide rails that the motor moves back and forth on.
 
cnc-machines-standard-router
Standard Router – A standard off-the-shelf router provides power to the router bit on this CNC.
 
cnc-machines-linear-bearings
Linear Bearings – This CNC uses heavy-duty linear bearings with roller balls instead of plastic sleeves.
 
cnc-machines-ball-screws
Ball Screws – The router on this CNC is moved up and down by precision ball screw. Some machines use this system on all three axes.
 
cnc-machines-rack-gear
Rack and Gear – The X and Y axis on this machine are moved with a rack and gear system. This system allows for more rapid movement of the router than ball screws do, but it is considerably noisier in operation.

Design work

Once you have decided to purchase a machine and set it up in your shop, the first order of business will be to design your part or project. This is one area where it pays to spend some time thinking things through carefully. If planning is important for a conventional woodworking project, it is many times more important with the CNC. Work-holding on the CNC can also be a big issue (depending on the project), so it pays to think about how you’re going to hold the piece in the machine. During the design stage you will need to either find or create CAD drawings (2D or 3D) for the project or part you want to make. There are several websites online where you can get CNC projects for free or at minimal cost, and of course the best fun is always to create your own project from scratch. Once you have the drawing, you will need to create what are called tool paths. This is done through specialized computer software such as ArtCAM or VCarve. In simple terms, you tell the software what size router bit you have, how you want the bit to cut in relation to your drawing, and the software figures out how to make the machine move around and do that.
 

Holding the workpiece

Once you have your tool path made and ready to run, the biggest challenge is figuring out how to hold your work on the CNC. CNC machines have a flat table on which your workpiece sits, and some very basic clamps to hold a square board in place. This works very well for carving a sign in which the routing is contained within the board, and assuming the board is square. However, if you wanted to hold an odd shaped board, or be able to cut right thru the workpiece, that is when things get tricky. Odd shapes are usually tricky to hold in conventional woodworking, but the CNC adds to the challenge because now your hands are no longer an option. CNCs may move suddenly and in unexpected directions while in operation, so you never want your hands on the work while the machine is on. It is also because of this possible movement in a sudden an unexpected way that clamps become tricky. Whatever you use to clamp your work, it can’t protrude above the workpiece because the router bit may hit it.  Remember that the router bit could still move above areas on the workpiece that are not being routed. This means you have to be creative. Sometimes double-sided tape or homemade clamps are required to keep everything in place. This also is one more element in the planning of the project.
 
Additionally, you may want to cut right through a workpiece, and this presents another challenge. Not only do you have to hold the piece you are cutting from, but you have to devise a way to hold the piece that’s being cut out to keep it from being damaged or grabbed by the router bit once it’s free from the original piece. Most machines you buy for the home shop come with aluminum tables, which you don’t really want to cut into, so you’ll want to put a sacrificial piece of wood on top to clamp your work to. Then, there are two simple ways to accomplish through-cuts. Software like ArtCAM that was designed specifically for CNCs can create “bridges” around your cut out part, essentially little tabs of material that are not cut all the way through. These tabs are then cut by hand after the machine work is done to free the part from the bigger workpiece. You can also glue your workpiece to a sacrificial board to be separated along the glue line later. Using a piece of newsprint in the glue joint makes this easy to do, and a chisel will separate the two parts very readily.
 
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Router Bits – Keller has a drawer under his CNC with all sorts of router bits. These are standard off-the-shelf router bits that will work well in a CNC router.
 
cnc-machines-hold-downs
Simple Hold-Downs – In this case, small clamps secure the workpiece to the table during operation. You sometimes have to get creative when securing a workpiece.
 
cnc-machines-sacrificial-table-board
A Clean Slate – The sacrificial table board on this CNC is well used. When it gets worn, a program can be run to resurface it with a large, flat router bit.
 

Misunderstandings about CNC machines

In my experience, there are three common misunderstandings about CNCs. It has been said that CNCs are not a “Santa Claus machine”, and I would agree. Experienced operators may make it sound like you just take a drawing and spend five minutes clicking around on the computer, and it’s done. Most projects take at least 30 minutes, if not several hours to complete the program, not counting the time to actually run the machine. While CNCs are getting more user friendly, especially the software part of the machine, it is not quite as simple as opening up a drawing and clicking on “print”. Some computer knowledge is a must. If you’re reasonably competent with Windows, then you should be fine, but if buying a CNC means you also have to buy a computer, you might want to think about taking a computer class first. All CNC software presupposes you can use a computer. While some CNC manufactures offer technical support for the machine, they generally are not going to explain to you how to turn your computer on.
 
The second major misunderstanding is that CNCs require no woodworking skill. I’ve often heard the CNC referred to as either “lazy way” or “cheater” woodworking, and after seven years and one hundred-plus projects on my own machine, I really find this amusing. CNC projects typically take as much time and effort as traditional methods. It takes time to come up with the concept, create the program, and run the program. It’s different than conventional woodworking, but I wouldn’t say it’s quicker or easier. The CNC allows me to create things that would be either too time consuming or difficult by hand, but it doesn’t come without effort. CNC projects typically take more thinking and less sweating than conventional projects. The greatest effort in making something on the CNC is designing the project. I can make small changes and modifications mid-stream in a conventional project, but with the CNC you can’t just cut a part a little smaller or bigger mid-stream – you have to go back to the beginning and start over. The CNC is also not a magic machine that turns a tree trunk into a finished project. You still must prepare raw stock to put into the machine. If you have not yet mastered the basic skill of making a board flat and straight, you should learn that first. A CNC will not work well with uneven stock, and especially with stock that doesn’t sit flat on the machine.
 
The third misunderstanding about CNCs is the learning curve. I, probably like most people, bought a CNC thinking that my first project would be perfect. Well, it was perfect, after about ten tries. The CNC has a learning curve just like any other machine, except maybe steeper. The machine will always do exactly what you tell it to, so you have to be sure that your instructions are correct. A CNC also can’t adjust speed on the go (at least for home machines) so there is no “feel” for feed rate, unlike a handheld router where you can change your feed rate on the fly. These little nuances make using a CNC a new experience and challenge for even a seasoned woodworker.
 
Despite these misconceptions, CNCs do have some major advantages. You can create intricate designs that would be either far too difficult or time consuming by hand. The CNC can also make duplicates of a design very easily. CNCs require no pattern or templates. Once you get a program working the way you want it to, it’s very easy to run the program hundreds of times, and the CNC will make each run the exact same as the last. On a project where I want several or even dozens of parts the same (especially intricate ones), the CNC is a much faster way to make those.
 
I hope that this article will encourage you to consider how a CNC might fit into your shop. Like any other tool, CNC machines do require some learning, but there is incredible potential to add new detail and precision to your projects.
 

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