How I Use a CNC: Cutting Parts Using a CNC

Precisely cutting a narrow triangle-shaped opening with power and/or hand tools is anything but simple. Cutting parts and precise details are tasks well suited for digital woodworking and CNCs.

Cutting a narrow triangle-shaped opening with power and/or hand tools is anything but simple. Cutting parts with precise details is a task well suited for digital woodworking and CNCs.

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.

Stock Preparation

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.

The red area in this CAD drawing is a narrow triangle that I wanted cut out of the board.

The red area in this CAD drawing is a narrow triangle that I want to be cut out of the board.

 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.

— Tim Celeski

6 thoughts on “How I Use a CNC: Cutting Parts Using a CNC

  1. Cellarat

    We had a CNC vendor give a talk to our group on this subject. We tend to machine very small and precise elements. We learned that inside cut outs of rectilinear nature will have rounded corners that are the radius of the cutting tool. These have to be hand worked. In addition for sharp cuts such as the triangle one could run past the end point of the wood (say the hypotenuse) and do the same on the converging cut (say the adjacent side) creating the sharp end of the triangle and eliminating any round overs. Does that make sense?

    1. Tim Celeski Post author

      Just to clarify, the objective of the cut was the triangle pocket in the board, not the triangle that was cut out. Therefore, the cut was clean. But I make use of the left-over triangles for wedges 🙂

      This is what is referred to as the “inside corner” problem. Because the bit is round, when you hit a corner with a router plus temple or with a CNC you end up with a rounded inside corner. If you want the corners to be squared up, you take chisels and files to clean them up. Take’s maybe a minute to clean up all three. In this case, the rounded corners of the triangle are what I happened to be after and the look is actually part of the intended design.

      There are some CNC techniques for getting past the inside corner problem that will certainly be the subject of upcoming posts or articles. Because they all involve running past the corner and sometimes show, these techniques are usually used for cutting plywood parts where aesthetics may be less critical rather than solid wood parts.

      The CNC shown is a custom machine I designed and had built for me. And, being that it’s a special size 3×6 and has various tweaks and additions like a second spindle it cost more than some hobbyist machines. However, there are now several machines available that are well designed and engineered in the 2×3 and 2×4 range that are well suited to woodworking hobbyists needs and priced accordingly.

  2. BlueEnamel

    I can see where the CNC router helps with cranking out parts by performing the bulk of the work. However, you still have the 3 corners to work by hand if you want them to be sharp points. did you try a router and templates for cutting out the part? Also, the CNC shown looks more advanced than most of the hobbyist ones on the market. How much did it cost, and what is its capacity?

  3. David Anglin

    I’m enjoying Tim’s blog postings about CNC stuff, incorporating CNC into a woodworking shop, whether pro or hobbyist. I am dabbling and learning using 2nd hand Rockler stuff. I am still a 90+ % hybrid woodworker using a mixture of hand and power tools, with a trend toward more hand tools, but I see many possibilities for CNC in a workshop, for carving, sign making, template production, etc. I know some craftsmen using CNC in fine guitar production, especially electrics, but also acoustics, and I don’t mean factories.
    I’m sure you will receive some negative feedback about using CNC, but I see no reason for not supporting the use of CNC for the woodworker who can become facile with the methods and software and employ this technology in efficient and unique ways.

    1. Tim Celeski Post author

      As long as the parts are wood, I don’t see any reason why a CNC router couldn’t make wedges and for that matter, machine most of the wood parts for hollow and round planes.

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