A Peek into Peeling Cuts

I've received several questions about peeling cuts and removal of the extra material, so here's a brief peek at peeling cuts with my recent screw & nut projects.

The SD300 builds by ironing layers of PVC film on top of the build platform one-at-a-time, and selectively bonding and cutting each layer of PVC film. Regions that belong to the model are bonded to the layers above and below, and unused material is left in-place (as to act as support material) until the build is finished.

Before building these four bolt models (yellow) I added one long peeling cut (blue) to divide the unused material into two halves. The layers of PVC will be stacked parallel to the table, so the peeling cut is perpendicular to the actual layers of the model.


There's no single "right" way to arrange peeling cuts. You could add as many peeling cuts as you want so long as there are enough cuts to free the model from the surrounding material. Here I arbitrarily added an extra peeling cut to isolate some of the support material into two sub-regions.


Thanks to the extra cut, the material peeled away from two of the screws at a time instead of all four. It took longer than it might have, but there's no harm in making extra peeling cuts. Peeling cuts only affect the unused material, they don't cut into the model itself.


The nuts were a little more complicated. I don't put small cuts inside hollow models like I used to because I've learned a better way to remove the material inside holes. (I'll explain that below.) I added peeling cuts between adjacent nuts, but not inside.


Solido provided strong forceps to help peel away material. Those forceps are great for most models, but they're awkward for cleaning material from holes like these so I bought a set of hobby probes.


The probe's sharp point stabs neatly through several layers of unused material in the hole. Tipping the probe lifts the material so it can be pulled away.


Once the ends are free, the material can be pried out by hand.


The layers of extra material have been glued together in a chain. But these layers didn't just pull out freely; I carefully tugged at the edge of each sheet to tear out the next one, one-at-a-time.


Sometimes one layer of support material tears free of the next, so it's necessary to insert the probe and pry the next layer loose before continuing the chain.


After clearing the holes, I removed the rest of the support material from the outside.


Most observers notice there's a high proportion of unused material in each build, which is true. It's roughly like a CNC mill that custom-builds a block for each model, building up the layers additively and then peeling them away material afterward. But it's generally economical because the process is energy-efficient and uses raw material that costs 10 to 15 times less per unit-volume than other 3D printers in its price class. Nevertheless, the cost per model can vary widely from one sample to the next depending on how efficiently the material is utilized.


The SD300 can't directly re-use the leftover material, but it's recyclable as PVC scrap (recycle code 4) by bagging and tagging it and delivering it to a commercial recycler. It can't go with household recycling, so I collect the SD300's scraps in a separate bin. It could also be discarded as ordinary trash since it doesn't contain anything harmful.

Wrong-Way Nut video on YouTube

I built an enhanced version of my Wrong-Way Nut model and posted a new video on YouTube here.



Watch for better pictures and...perhaps...an explanation. Better still, can you explain how it works?

George Bell's Dice Box

George Bell designed this box puzzle as a conventional 3D object. It's an example of a coordinate-motion puzzle in three dimensions: its four pieces interlock and slide together to form a box if-and-only-if all the pieces start in exactly the right position and all four must be moved simultaneously.



Here's a full set of parts built on the SD300 with a contrasting red color on some sides. (It rather resembles a stop sign, doesn't it?) George's puzzle is perfectly functional, but I was still compelled to tinker so that I could exploit the SD300's peculiar capabilities.


My first modification was to transform the puzzle into a flat form with living hinges and a tuck-in tab.


When my modified piece is folded and the tab tucked in, it forms a solid shape identical to George Bell's original 3D model.


It can be assembled exactly the same as George's original model, but now all the faces have a lustrous sheen because they're all built parallel to the shiny surfaces of the PVC material.


Then I added small cylinders, raised about 0.18mm above the surfaces to resemble dice pips. And I changed material for exactly one layer while building to give the dice pips a contrasting color.


But there were sharp corners when the flat panels were folded, so George took over again and modified the design.


George's next revision rounded off the corners, so the puzzle really does look like a chance die now!


Because the puzzle starts as a set of flat panels, I decided to package it in a plastic bag with the four panels intertwined in a flat configuration. To solve the puzzle, the user removes the pieces from the bag, separates them, folds them into solid objects, and must figure out how to slide them together.


The puzzle was now named Dice Box. We packed instructions inside each bag.


I built over a dozen sets of Dice Box puzzles over the next few days, so George could take them to the International Puzzle Party.


About half the puzzles are transparent and the other half white.

Orange, Green, and Blue Marble

George Bell graciously took several sets of my Bag of Marbles puzzles to the International Puzzle Party in Berlin this year. Currently I have three styles of marbles, distinguished by color.



Orange Marble was my first design, a puzzle with two interlocking pieces that slide together unintuitively. It was created by accident as part of a design that I abandoned while studying Bram Cohen's Trapped Marble prototype. Although the two pieces are different from each other, they can be flipped relative to each other and they slide together symmetrically.


Green Marble is another two-piece design. This time the two pieces are identical, but they only fit together when the pieces are oriented in one specific orientation.


Blue Marble is based upon the same curves as Green Marble but divides a sphere into three pieces instead of merely two. Most people fit two of the pieces together easily, but instinctively try to insert the third piece upside-down...which doesn't work.


Here's how a batch of Blue Marble pieces looked shortly after I'd removed them from the SD300 and removed some of the support material.


Unprocessed pieces have a translucent sidewall finish. It's attractive, but I wanted a shinier smoother finish.


To smooth the side walls I dipped each piece in Weld-On 2007 solvent for 30 seconds.


After their solvent bath I laid the pieces on baker's parchment to dry. Parchment is non-absorbent and the wet pieces don't stick to it, so it's an ideal disposable drying surface.


Eventually I plan to add other Marble styles (and colors) to the series as I think of new challenging features to add.

Wrong Way Nut

A moderator on my favorite puzzle-building forum called attention to a YouTube video entitled Magic Nuts and Screw Threads in which two nuts are shown unscrewing in opposite directions on the same threaded rod.

I don't have any inside information about the trick nut in the video, but I conjectured I could build a nut that would behave like that. The threas on my first test model turn a bit tightly, but it works!

I will record a better demo video later. It could use a little refinement, and the screw threads need to be a lot longer for a better demonstration. But it's sure gratifying when a new idea passes the proof-of-concept test on the first try!

My first functional Yoshimoto Cube

Ultimately I would like to 3D print a fully-functional Yoshimoto Cube in one piece, but I really need a solid reference model to help me visualize the design. So I used the SD300 to build the individual hinged panels and glued them together one-by-one.

This black one is almost assembled; the hinged panels at right will be joined with the already-assembled group at left.

I built and assembled a pair of Yoshimoto cubes, one in black and the other using transparent amber.

The geometry of a Yoshimoto Cube is peculiar and confusing, so it'll sure help to have a physical model for reference! So far I've observed a few details that could enable me to build the model in one piece.

Yoshimoto experimentation

In July 2010 I saw plans on Thingiverse for building a kit that could be assembled into a Yoshomoto Cube, which is a type of folding puzzle. At that time I didn't have much experience with hinged models, so tried to adapt the concept.

I built a pair of experimental models that had hinges in each axis, some horizontal some vertical. This picture shows the two models before I'd finished removing all the support matieral; they look like they're attached, but I subsequently separated them.


One separated, the models could fold along the hinged connections. They were somewhat delicate (and tiny!) but each could be folded into many shapes just like a Yoshimoto cube.


Careful folding and unfolding would permit the model to change shape until it was inside-out from its starting position. The model on the left has been inverted, while the one on the right has been returned to its starting position.


Also like a Yoshimoto cube, the inverted and non-inverted models could be nested together in such a way that the combined pair could still be folded in all the same ways as before.


After a few hours' of use both models broke along one of their vertically-oriented hinges. I didn't continue with the project at that time...and now a year has passed by.

Recently I've gained a lot of experience building models with built-in living hinges, so I want to try experimenting with the Yoshimoto cube again soon.

Flexible Burr...Cuburr?

Here's a picture of the current revision of my flexible burr puzzle. I've adopted a scheme with a distinct color for each type of part.


Assembling the parts depends heavily on the flexibility of the material, as the parts must be distorted to fit through each other.


It looks like a mess, but the puzzle is almost solved.


The last step is to fold the faces flat and hook the corners together so it holds a cube shape.


I carved the puzzle's name "Cuburr" into one of the faces in the 3D data, which caused the SD300 to trace it like line art.


That last trick exploits a limitation of the SD300's build process: in the STL data the letters were rendered as cutouts in flat wall, narrower than 1 mm. The SD300's XY plotter can cut with 0.1mm precision, but regions narrower than 1 mm are left bonded to the layer beneath; you have to pry them mechanically if you want them removed. For situations like this, it's a convenient way to emboss line art onto a flat surface.

A flexible burr puzzle

(7 Feb 2013) I notice some debate about this post elsewhere online -- if you have questions feel free to leave a comment or contact me via the email address in my profile (right).

 
In 2002 George Miller devised an unusual puzzle titled Three Card Burr, made by laser cutting slots into three ordinary playing cards so they could be interlaced together. Interestingly, there's no practical way to disassemble the cards after the puzzle has been solved.


I was inspired by George Miller's Three Card Burr puzzle to create a more complex puzzle to exploit the SD300's ability to build sheet-like structures by creating a 3D model whose geometry can be mapped to the build material. I designed this flat model whose middle section is nominally 0.51mm thick, so it will be built using three thicknesses of 172-micron PVC sheet. The adjacent areas have two 0.34mm thick panels stacked one above the other and attached to the middle section by a 0.17mm thick strip to act as a hinge. The whole model is about 120mm x 70mm x 0.9 mm.


Here are my first 3 sample models, built in transparent material.


Like Three Card Burr assembling the puzzle begins by carefully interlacing the sheets together. It wasn't as difficult as I'd expected because the resiliency of the PVC material enabled me to flex the puzzle without tearing it.


Once the pieces are interlaced together, the hinged panels fold out and interlock at the corners to form a cube. This part was far more difficult than I'd expected because each corner depends upon its neighbors for stability, which meant each corner would come apart when I tried to assemble the next corner. I'd forgotten to allow for the thickness of the material in the corner dimensions, so the panels warped from excess tension.


Not bad for a first try, though. It proves the concept is basically sound; now I just have to adjust the dimensions and add something to make the corners more stable. Maybe a small tab or hook?

Eurofighter

Solido provided a sample model that demonstrates an interesting way to exploit the flexible PVC material. The model prints out as a number of flat, semi-rigid parts with living hinges and flexible tabs.


Fold the parts at the hinges and pass the tabs through openings in another piece.


These tabs tuck securely into the underside of the wing piece, thereby snapping the pieces together.


When all the pieces are assembled in this way, you get a modernized plastic version of the good old 'paper airplanes' we used to make from paper.


This gives me some fresh ideas for foldable models!