laser
Laser cutter: 30W GCC
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/box_multiple_sizes.8acb05c6c5b5d964.jpg)
Final result
settings
Using the suggested settings as a baseline, I arrived at the following refined numbers for 1/8" cardboard:
speed | 2.3% |
---|---|
power | 100 |
ppi | 220 |
The largest item causing variance in cut quality was focus -- my cardboard sheet was bowed up, causing me to lose focus in the middle of the sheet, leading to a black burn on the upper surface blooming from the cut location and loss of effective cut power. I noticed this by comparison as I was watching Cayden and Lancelot cut their projects.
kerf characterization (part 1)
I initially tried to measure kerf with arrays of interlocking fingers of increasing kerf compensations.
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/fit_test_2.ec93474c9e0c6b23.jpg)
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/fit_test.cd43a9651ac0f4b2.jpg)
I measured the thickness of the cardboard using a set of calipers and found that it was approximately 4.15mm thick. The finger widths thus started at 4.15mm and increased in 0.2mm increments. I anticipated that this would make the first two indices too small, as Neil told us that the beam waist was in the ballpark of 0.010". 0.1mm is approximately 0.004", so I would have expected an interference fit between indices 2 and 3. However, index 1 fit perfectly (no kerf compensation).
I realized the error of my ways after the fact as I was building my project: the precision of my measurement was reliant on my ability to measure cardboard thickness precisely with calipers, and on cardboard having a consistent thickness in the first place. I remedied this later.
project summary (or: kerf characterization part 2)
I made parametric cardboard holders. Their function is simply to elevate an object. I started out making small boxes using box-jointed edges and realized that (obviously) they would not hold with just a box joint (I'm used to using adhesive).
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/kerf_good_fit.41909d79103f3c0e.jpg)
This, however, let me dial in my kerf, as the precision in the box joint is entirely in the vertical axis and not dependent on the thickness of the cardboard at all. I binary searched, printing repeated joints, and arrived at a kerf compensation of 0.15mm (= approx 0.006"), giving an interference fit inasmuch as one can be said to exist in cardboard.
wedged joints
Given that the joint was not going to hold as-is, I added a joint (topmost on the edge) that could accept a wedge.
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/small_box_no_wedge.1bfbfe7fcec91b66.jpg)
Wedges installed:
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/box_wedge_final.0cdc2519e454142e.jpg)
The complete support is relatively sturdy for what it is. Holding a power supply on my desk:
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/box_holding_power_supply.a024eca600341d6d.jpg)
I don't believe the wedges are actually in any significant shear loading while the structure is bearing weight -- I think the increased friction of the joint as a result of this construction is actually what's providing most of the stability.
cutting wedges
The wedges were difficult to cut, small as they were:
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/wedge_wrong_axis.7a8a4e83a0eb9bd2.jpg)
The above wedge failed because it was cut along the axis of the cardboard corrugations and could easily twist off of the single line of glue. I soon realized this and changed them to be cut across the corrugations.
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/wedge_good_axis.67b7242b88bb911f.jpg)
I still had some failures this way due to delamination, but under 20%.
Originally, I was cutting the wedges one-by-one in left-over areas on the cardboard between the walls, but I decided to develop a way to pack them more densely.
First, I simply went for a dense packing in an array:
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/wedges_array.559750655aad0962.jpg)
Then I built a "sprue tree" out of cardboard to retain several wedges as one complete unit that could be extracted from the laser cutter more easily.
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/wedges_sprue.ea888cfd82a5e69b.jpg)
Surprisingly for how thin the connecting member is, these came out without tearing any of the "sprues" very consistenly.
parametricity
Adjustable parameters:
- Height
- Width
- Leg height
- Number of box fingers
- Cardboard thickness
The cross-member thickness is a proportion of the width or height of the relevant direction.
A small box and a big box patterned from the same design file:
![](https://fab.cba.mit.edu/classes/863.23/CBA/people/Nathan/processed_images/box_multiple_sizes.8acb05c6c5b5d964.jpg)
corel draw tricks
The best way I could figure out to use Corel Draw was as follows:
- Set the "paper" as the full size of the bed
- Pick a color that is your no-cut color (I picked blue) and configure the printer not to cut it (uncheck vector, raster in the color configuration)
- Draw a rectangle within the bed outline that is the size of your cardboard (colored no-cut) as a reference
- Keep working off of this one file: each time you print a part, mark it no-cut but leave it in-place. Now you know what parts of the sheet you've used and can squeeze in other parts quite precisely.
Additional note: there is a palette of recent colors in the bottom left. Right-click a color to set the currrently-selected curve to that color (left-click is fill).
design files
Here.