7. Computer-Controlled Machining
Group Assignment
During the group assignment, our instructor Rudolf introduced us to the basics of safe operation of the CNC milling machine.
How to properly fix the material, change the tool, and set zero points along the axes. I also learned how to choose the right type of milling bit depending on the task.
In our lab, we have a ShopBot CNC milling machine. We learned how to fix the material, change the tool, and set zero points. I also learned how to choose the right milling bit for different tasks.
We studied various types of milling cutters and their applications in milling.
-
Upcut End Mill – Provides effective chip removal but may cause tear-out on the top surface. Best for deep cuts in soft materials.
-
Downcut End Mill – Creates a clean top edge but directs chips downward, requiring good dust extraction.
-
Compression End Mill – Minimizes tear-out on both the top and bottom surfaces when working with laminated materials like plywood and MDF.
-
Straight End Mill – Provides stable cuts with no lifting or pushing, suitable for wood, MDF, plywood, and acrylic.
We conducted a series of tests.
-
Compared cutting accuracy using 6 mm and 3 mm diameter end mills.
-
Tested the difference between Climb and Conventional cutting methods, finding that Climb provides more accurate results.
-
Tested the fit of "dovetail joints," optimizing the gap for a tight assembly.
-
Measured depth accuracy when milling at different feed rates.
Individual Assignment
This week, I decided to create a chair inspired by the A-CHAIR by Austrian industrial designer Thomas Feichtner. His style is characterized by sharp geometric shapes, a balance between functionality and aesthetics, and an innovative approach to structural solutions.
Ergonomic Sketch
I started the drafting process in Rhino, following the main ergonomic dimensions but slightly adjusting them to my needs. I used the Polyline tool for drawing lines and the Fillet tool for rounding the corners.
I decided to use 18 mm plywood. To increase the reliability of the structure, I used two layers of plywood in the leg area of the chair and three layers for the backrest. Before starting the work, I measured the thickness of the plywood, which varied from 17.3 mm to 17.9 mm. To simplify the calculations, I took the average value of 17.5 mm and proceeded with creating the sketch.
Joints Test
I decided to make a table without using glue, so the joints had to fit together with the smallest allowable gap. To achieve this, I started cutting out the pieces with joints, adjusting their sizes as needed.
The first version did not fit into the hole, the second one fit with difficulty but eventually broke due to insufficient clearance and shoulder thickness. The third and fourth versions fit without issues, but the locking wedge had a 2 mm gap. I decided to shorten the length of the wedges because, in my opinion, they spoiled the appearance. In the end, I settled on the fifth version, as the sixth one also worked but required slightly more effort to assemble.
Drawing Process
Considering all the previous tests, I decided to make the holes for connecting the backrest and seat 0.1 mm wider so that the parts would fit together easily.
To join the two layers of plywood, I added recesses across the entire surface with a depth of 8.75 mm, into which I later installed rectangular plywood inserts. Thanks to this, the two pieces were connected, forming a smooth side surface. In the image, the red fragments indicate the locations of the inserts, ensuring the strength and precision of the joint. The green part is intended for securing the backrest of the chair to the sides using a wedge.
This is roughly what the entire structure looks like when assembled, with all the elements properly connected and fastened, forming a stable and functional shape.
After assembling the chair in the program, I created the layout and placed all the parts on a sheet measuring 1220 x 1520 mm. For convenience, I separated all the parts for the grooves into a separate layer and saved the file in DXF format.
First, I needed to make pockets for connecting the parts. For this, I gathered all the vectors related to the pockets into a separate layer. This makes it easier to select them all at once.
Cutting process
In VCarve, I opened the Layers tab, selected the needed layer, and clicked Select Layer Vectors to highlight all the vectors on this layer. After that, in the Toolpaths Operations section, I chose the Pocket Toolpath operation.
In the Pocket Toolpaths window, I configured the main parameters for cutting the pockets
-
Start Depth is set to 0 mm, meaning the cut starts from the top surface of the material.
-
Cut Depth is set to the required depth for the pockets. In my case, this is specified based on the thickness of the material and the joint design.
In the Pocket Toolpath settings, in the Clear Pocket to improve the cutting process I selected the Offset strategy, where the tool moves in concentric paths from the center outwards. This gives cleaner edges, which is important for precise joints. I also chose the cutting direction — either climb milling or conventional milling — depending on the material and desired outcome.
Climb milling (cutting in the direction of the tool's rotation) generally provides a smoother surface and is better for materials that are less prone to tearing.
Conventional milling (cutting against the rotation of the tool) is used for more aggressive cuts in harder materials, helping to increase tool life and reduce tool deflection.
After generating all the pockets and outer cut paths, I clicked Save Toolpaths to File to generate the G-code and prepare it for CNC cutting.
For cutting, I used a Compression End Mill, which keeps both the top and bottom surfaces clean when cutting plywood or MDF. This prevents tear-out and leaves smooth edges. After cutting, I only needed to clean up the areas where I added tabs to hold the parts in place during machining.
Assembly process
This is what I got after cutting — this part will be used to assemble one part of the chair.
After a little processing, I insert the guides into their places on the chair leg, then place the outer part of the chair legs on top of them and press everything together with clamps to secure it in place.
In the same way, I carefully assemble the chair's backrest, ensuring that all components are properly aligned and securely positioned before proceeding with the next steps.
And then, I thoroughly sand all surfaces, paying close attention to any rough areas, to ensure a smooth and even finish across the entire piece.
As it turned out, after conducting several tests, the chosen connection method was not suitable, and I was unable to assemble the chair because the parts fit together too tightly, making it difficult to join the components properly and causing issues during the assembly process.
Additionally, during the assembly process, I applied force to ensure the parts fit together properly. However, as a result, I ended up breaking several components, as the connections were too tight, and excessive force caused the material to crack in some areas.
This is how the chair appears in its assembled form without the fixing wedges in place. While I had hoped for a different outcome, this is not exactly what I expected. Nevertheless, I am quite pleased with the overall appearance and design, even though the functionality didn’t turn out as planned.
So, sitting on this chair, I can’t help but reflect on what went wrong during the entire process. I start to think about the mistakes I made during the design phase and where I could have made better decisions to avoid the issues that came up during assembly. And then, I realize how some of the challenges could have been prevented with more careful planning. The chosen connections didn’t work as expected, so in the future, if I create it again, I will use glue and avoid using wedge connections, as this will be much more effective.
Conclusion
During the execution of this task, I acquired knowledge about safe operation of the CNC milling machine, learned how to properly secure the material, change tools, and set the zero points. I also realized the importance of choosing the right end mill and cutting methods to achieve precise results. This project has been a valuable lesson, emphasizing the significance of thorough planning and testing during the design phase. Although the final result didn’t fully meet my expectations, it provided insight into areas that need improvement and confirmed the necessity of precision both in design and in the assembly process. In the future, I will definitely take these lessons into account to achieve better results in upcoming projects and avoid the mistakes made here. Specifically, it’s important to consider the material thickness and allow extra space in the connection dimensions to make assembly easier.