How I Use a CNC: Cutting Parts Using a CNC
How I use a CNC: Part Two
In the previous post, I gave an overall explanation of the process that I use to make furniture parts with a combination of various power tools and hand tools. Essentially, it’s a hybrid process of power and hand tools that many other professional and hobbyist woodworkers use. The goal is consistency and accuracy. I mill and square up materials with jointers, planers and saws. Then, I rough cut and final shape parts to my patterns.
So, how does the process change when you use a CNC to cut out your parts? For me at least, some things are the same. I still prepare my stock with the same tools as before with saws, jointers and planers. Getting your boards flat and square is even more critical when using a CNC. You want the boards you’re going to CNC very flat. From this point on, everything changes.
Below is a simple example of the CNC doing what it does well: accurate part cutting. In this example, we’re not cutting out a complete part, just a shape in the middle of a board. A long, thin triangle. As simple as it looks, I’ve had to make this exact cut many times. It’s been difficult with any combination of power tools and hand tools. The red triangle in the drawing below shows you the area I need to remove.
A Challenging Cut for Woodworkers
Here’s the problem: The triangle is very narrow. It’s about 2-1/2” wide at the top and tapers down over 9” to a sharp point. It might seem simple but in practice it’s been a difficult detail to cut out accurately. I’ve tried all the woodworking processes that you’d expect. The most successful technique has been to drill holes in the corners, then jig saw or scroll saw out the triangle. So far, I’ve been unhappy with the results. Even holding a jigsaw to a guide, the walls of the triangle are not perfectly straight and smooth. So, after rough cutting, I set up a two-step shaping process with the pattern to get the walls straight. But, we’re just getting started. The sharp end of the triangle keeps it challenging. Time for rasps, files and lots of sanding in a difficult-to-access area before the job is done. After a lot of time-consuming work, I’ve never been completely happy with the quality of the results. This job is all about precision. That makes it a perfect task for a CNC.
First Draw the Part, Then Cut It
After drawing the board and the triangle in CAD and then programming the CNC with CAM software to make the cut, I place the board on the bed of the CNC, align it, clamp it tightly and begin. I cut my way down in steps to the full 1” depth of the board just as you’d do with a router. Small connecting pieces of wood at the bottom of the cut called “tabs” or “bridges” are left to keep the triangle just slightly attached to the main board. That keeps it from rattling around during the last pass and potentially damaging the part.
You can see the tabs being machined in the video below when the CNC pauses, rises up and then drops down. Total time to cut the triangle opening: 116 seconds. The results are perfect. When you add in the time for changing boards and clean up, eight boards were completed in 20 minutes.
Sometimes a small detail is a big challenge. Cutting this narrow triangle out of a board is particularly difficult using other woodworking techniques. With a CNC it’s done quickly and accurately.
Though this triangle is only a simple example, it should give you an idea of how accurately and quickly a CNC router can machine parts. If we were cutting a complete part with any combination of curves or shape, there’s a bit more involved but in general, the process is pretty much the same.
As I said previously, a CNC can do many things. Certainly, it does some tasks better than others. And, as always, some tasks are better left to hand or power tools. Accurate part cutting just happens to be one woodworking task a CNC does very well.