This is a refinement of Cut 3 Heckballs; Heckballs is an edge-lap construction-set design I developed based on some ideas Matt Heck showed me in 2005 or 2006.
I spent a couple of hours making these changes from cut 3 tonight:
The basic octagon size across the flats is reduced from 100 mm to 50 mm.
The tabs that prevent the thing from falling apart during cutting are increased from 750 μm to 1 mm in width. Also, they include the scraps.
The slop on the slot width is reduced from 150 μm to 30 μm. I still want to get rid of that by designing a snap joint.
I also laid it out in 9 rows of 4 octagons instead of 4 rows of 8, in order to nest better on the sheet.
I eliminated the top and right of the outer box in order to eliminate the possibility of double cuts.
The slot bottoms have 6-mm-diameter outside divots to prevent breakage. For now I’m using regular octagons rather than circles, since laser-cutting circles seems to be a lot slower. I want to use hexadecagons but I also need to sleep.
Here are the things I still want to do but haven’t done yet:
Beams!
Slot the scrap squares so they also work as connectable pieces.
The octagons and divots should be hexadecagons so their corners aren’t as sharp, both to reduce the stress concentration factor and to make them easier on your hands.
I’m still just using PostScript for this version, but I needed a way to describe the divots in a way that allows rotation.
PostScript’s imaging model allows you to rotate and translate the
coordinate system, so I could just use that. But I thought it would
be easier to use turtle graphics instead, so I wrote a simple turtle
graphics system in PostScript. This allows you to draw with the
PostScript currentpoint using Logo commands: seth to set the
turtle's angle in degrees (0 being straight up, as in Logo or the
compass, not to the right) lt and rt to rotate the turtle left and
right in degrees, fd to move it forward by some distance drawing a
line if the pen is down, pu to pick up the pen, and pd to put down
the pen. Additionally you can save the turtle’s state (pen state,
angle, and position) onto the PostScript stack with here and restore
it with return.
You need to position the turtle with moveto before you can start
drawing with fd.
Here’s the whole implementation of turtle graphics for PostScript:
/seth { /theta exch def } def
/rt { theta add seth } def
/lt { neg rt } def
/pd { /turtle-pen {rlineto} def } def
/pu { /turtle-pen {rmoveto} def } def
/here { [ /turtle-pen load theta currentpoint ] } def
/return { aload pop moveto seth /turtle-pen exch def } def
/fd { dup theta sin mul exch theta cos mul turtle-pen } def
pd 0 seth
I was hoping that this version would take something like 9 minutes to cut, because of the smaller size and cost between AR$200 and AR$300, but instead it took 13'52" and cost AR$320. Also, I was hoping that it would be less breakable in two ways: the outside divots on the slots would reduce stress concentrations, and the smaller size would reduce the leverage available to break it with.
Instead, it was more breakable: both the divots and the smaller scale reduced the distance between slots, with the consequence that it was far more fragile. The divots also probably accounted for the longer-than-expected cutting time.
Still, it has enough elasticity that I was able to throw an assembly against a wall repeatedly with no visible damage.
I was also hoping to have time to add slots to the square scraps in between the octagons, but my attempts to do that failed, in part because of the big divots, and in part because I was at a loud party that made it hard to concentrate. The whole thing looks like a jigsaw puzzle, which is cool, and the scraps do hold together a little bit better with their knobs, but not really well enough.
I gave some of the previous (100-mm) Heckballs to a baby, who enjoyed them and was not able to put them together or to break them. If you broke one of the tabs off, it would be 34 mm wide, probably too wide to choke on. This version might be less suitable for that, because it has lots of small pieces that could break off pretty easily.
The smaller amount of slop (30 μm) was intended to eliminate the problem of assemblies simply falling apart under the weight of the octagons. It did greatly reduce it, but it didn’t eliminate it. Also, it made them noticeably trickier to assemble, since I still haven’t managed to add corner chamfers.
The 1-mm tabs to hold the sheet together during cutting were annoyingly difficult to break apart.