Laser cut wooden block tower

Maaike Kok
The goal of this project is to create a block tower for children, which consist of 6 differently sized cubes. Each cube is built out of five plywood sides, so the cubes function as boxes. The five wooden sides are connected to each other by means of a finger joint and are engraved with text or images. Before laser cutting the entire design, this study aims to identify the desired measurements for the finger joint. I want to find out if it is possible to connect the five sides to each other without the need for glue. In addition, I want to know which cutting method is best for engraving the text and images.
Both tests are performed on the smallest cube out of the six boxes.
Design software: Rhino 5 (Windows) Material: 3mm plywood Laser cutting settings in RDWorks V8:
scheme of RDWorks settings

RDWorks settings: Engraving type1 = raster engraving; Engraving type2 = vector engraving

1. Original dimensions

The smallest cube of the block tower is designed with dimensions of 50x50x50mm. The length of each finger joint is 10mm. The exact dimensions are shown in figure 1.
scheme of measurements

Fig.1 Sides’ dimensions in mm. From left to right: 1. Front- & back panel. 2. The two side panels. 3. Top lid.

In the earlier laser cutting workshop it became clear that the above mentioned dimensions for the finger joints were not optimal. The box did not held together on its own, glue was needed to construct the cube as the laser beam cuts away more than I anticipated. The dimensions I had drawn, where not the exact dimensions of the laser cut pieces.

2. Study on finger joint dimensions

In order to find out which dimensions are needed to design a cube that could be jointed together without the need for glue, I studied different offsets to compensate for the loss of material due to the laser beam. The offsets were drawn from the contours of the original dimensions of the cube as shown in figure 1. They were drawn outside the border of the original drawing, see figure 2.
offset scheme

Fig 2 Offset scheme

These were the offsets (in millimeters) I cut with: x = [0.01 – 0.025 – 0.05 – 0.1 – 0.125 – 0.15 – 0.16 – 0.17 – 0.18 – 0.19 – 0.2 – 0.21 – 0.22 – 0.23 – 0.24 – 0.25] mm

3. Study on engraving types

As mentioned earlier, I want to print text and images on the box side panels. Therefore I studied the difference in result between a text infill created by scanning and by cutting mode (engraving type 1 vs. engraving type 2).
VECTOR ENGRAVING – The reason for engraving text with the cutting mode was to create a text with a brown/black background colour. To do so, I drew, within the text boundaries, lines with an offset of 0.16mm (this offset distance was established from the minimal distance at which RDWorks V8 would not distort the lines, when exporting in .dxf from Rhino 5 and import the file into RD Works V8). The infill colour of the text (with engraving – type 2) was thus created by a set of lines and not by a hatch (infill). RASTER ENGRAVING – The infill colour of the text was created by an inner and outer boundary, RDWorks V8 recognizes this as an infill area. Additionally, I varied with the engraving of the text boundaries themselves (with engraving type 2), see figure 3.
screenshoot of RDWorks

Fig.3 Screenshot from RDWorks: Grey = Raster Engraving. Red = Vector engraving (offset lines 0.16mm) Black = Cutting.

laser cut pieces

Fig.3a Laser cut outcomes of the raster and vector engraving test.

I was not sure if a line offset distance of 0.16mm (for vector engraving) would create an aesthetic text infill. Therefore I varied with some offset distances too, see figure 4.
different offset test

Fig.4 Red = Vector Engraving. Black = Cutting – Top row from right to left, line offset = 0.5mm, 0.4mm, 0.3mm, 0.25mm. – Bottom row from right to left, line offset = 0.2mm, 0.175mm, 0.15mm, 0.13mm.

the pieces with different offset

Fig.4a Variations tested for text infill by vector engraving.

For the images which I want to print on the sides of the cube blocks, I made variations too (see figure 5). The alternations were all possible combinations of vector and raster engraving. Where vector engraving is used as infill, a line offset of 0.16mm is taken in all cases.
design of the front of the cubes

Fig.5 RDWorks screenshot: Grey = Raster Engraving. Red = Vector Engraving (offset lines 0.16mm). Black = Cutting.

The fronts of the boxes

Fig.5a Outcomes of the raster and vector engraving test for images.

4. Conclusions

When constructing the boxes, it became immediately clear that an offset of 0.1mm from the original dimensions was too big. The panels got damaged when jointed together. On the other hand an offset of 0.09mm is too small (joining a piece with an offset of 0.18mm together with an original piece without any offset). The pieces could be connected as such that they would be able to stand up-right without falling apart. While they could be separated easily. An offset of 0.095 seems the right one (joining a piece with an offset of 0.19mm together with an original piece without any offset). As result no pieces were damaged, and it became more difficult to separate the pieces.
Vector Engraving does not result in an aesthetic text infill. It might be optimized by adjusting the laser cutter settings. In my case the vector engraved text was almost cut through, and the infill was not of consistent colour. For the images, I noticed that vector engraving the circles results in an inconsistent line thickness and colour (see figure 5).
For the final design, I will use raster engraving for both text and images. That way an equally infill is created. In conclusion, in a next experiment I want to find out if there is even a more ideal offset for the connection (somewhere between 0.09mm and 0.1mm).
testing how the pieces fit together

Fig.6 Jointed pieces with an offset of 0.09mm. Cube dimensions: 50x50x50mm

final box assembled

Fig.6a Jointed pieces with an offset of 0.095mm. Cube dimensions: 50x50x50mm