Thursday, December 22, 2011

Rapid Evolution of "Candy Ball"

Over the weekend I built a simple test model that inspired me to work on a new (but familiar-looking) puzzle design that rapidly evolved in the next two days...

As an experiment I recently built a two-part shell that fits over my existing Green Marble puzzle, following exactly the same contours when the two pieces are precisely aligned.

I added a dash of green color to the model while it was being built, just to help identify the parts.  Serendipity!  The green calls attention to a swirly shape like a yīnyáng on the surface of the outer jacket.

The marble sticks out of an opening in the jacket, opposite the green yin-yang, and it can turn freely inside the jacket.  By coincidence the swirly shape can be replicated by moving the marble around but doing so causes the marble to block the whole puzzle from being disassembled.  Could I design a new puzzle that deliberately includes such a hint at a false solution?

Next I built a whole set of parts based on the same curved geometry as Green Marble with a 2-part inner ball and a 2-part outer shell.  The shell includes a round dome centered on the swirly shape, opposite an opening where the ball sticks out.  I switched between red and white material, hoping the coloring scheme might emphasize the illusion.  The colors remind me of peppermint candy.

The ball pieces fit inside the shell pieces, then the whole thing screws together.  It looked cool, but the screw action was a bit too pronounced for my taste.

 The color coding worked perfectly.  The white dome formed a ball-like shape with the same swirly shape as the green puzzle.

On the opposite side, the marble could be turned until it also exhibited a similar swirly shape.  The contrasting white color on the ball further emphasized the swirly shape, but the puzzle won't open when the swirly shape is exposed.  The camouflage works very well, but there were plenty of other details I disliked: the correct solution required too much unscrewing, the jacket pieces were too flexible, and the ball was too hard to maneuver inside the jacket.

Applying what I'd learned, I designed a whole new model from scratch without using any of the model data from Green Marble.  It looks very similar to the last model but there's less twist and the geometry is better optimized for the ball to move inside the container.

As it turns out, the newest design has an easy-to-find method of taking it apart without aligning the pieces as intended.  If the ball is turned so the white portion sticks up at the top as shown here the outer shell can be gradually withdrawn by pushing it to the left while the ball rotates and unscrews itself within the jacket.

Looking back, the previous version didn't have that problem because the red section was wider and the twists had overlapped so there wasn't as much freedom to move the jacket independently of the ball inside.

This design evolved rapidly until it reached a dead-end, so now I have an opportunity to go back a few steps and see if I can develop it into a fresh puzzle.

Tuesday, December 13, 2011

Hands free memo holder

I had intended this memo holder to use a marble to grab papers behind a wedge-shaped panel.  But the marble didn't work very well so I 3D printed a plastic cylinder that worked better.  It works by sliding papers up into the slit behind the cylinder, whereupon the force of gravity traps the paper between the cylinder and the back plate.

There are two versions, one plain and one with slots added for hanging lightweight articles.  After a little more experimentation I modified the cylinder into a barrel with a fat middle section.  That held papers better than the cylinder.  I rebuilt the memo holder with side walls oriented for better transparency to illustrate how it grips papers by the wedging action of the red barrel.

Here's video that illustrates how it works...

This particular note holder was attached to the wall using a small Command brand adhesive strip.

I also made a refrigerator-magnet version by trimming a square from a worn out magnetic pad from my SD300 and gluing it to the back of a memo holder.

STL and STP files for this model are shared here.

Saturday, December 10, 2011

Helical Burr Followup

After my Helical Burr Experiment had required sanding to make the pieces fit together, I refined the data and built a second model just to fix the problems with the first.  The new one (on the right) fits together smoothly, but it still has impractically-thin walls so it's just another experimental prototype.

I'd intended to design it so the parts could only be put together in one specific sequence, but testing the new model revealed a slight flaw: one of the pieces could be put on (or taken off) out-of-sequence because I'd only blocked movement in one of the two directions it might move.  I probably wouldn't have discovered the oversight without building these test models.

Here's a movie demonstrating how the puzzle works and revealing the design oversight.

One of the pieces had some exceptionally thin edges, which came out kind of wispy the first time so I tried building it in a horizontal orientation for comparison.  The new orientation is probably a bit stronger, but the difference isn't compelling.  Both parts work about the same.

I will probably start over from scratch because of the thin walls and un-blocked movements, but I consider it a very successful experiment nevertheless.

Tuesday, December 6, 2011

Adapting the Magic Screw for Makerbots

Last month I shared my Wrong Way Bolt model, but home users found themselves unable to build functional copies of it using hobby-type 3D printers like the Makerbot Thing-o-Matic.  The model's geometry seemed to fall within the rule-of-thumb limits, but I looked closely at individual 'slices' of the model and thought about what would actually occur when it was built using a hobby extruder.

Here's what might have happened.

At left is a slice of the trick nut, color-coded with shades of red to indicate the region to be built for the present layer.  Bright red areas are supported by the layer below while dark-red areas are unsupported overhangs.  When an hobby-type 3D printer tries to build the overhang marked with a circle its extruder would follow the path in white but the thread of extruded plastic won't have any support.  As a result, the thread will be pulled into a straight line between the two points where it's supported by the layer underneath.  The Makerbot would probably build some of the overhang, like the diagram at right.  It would probably need multiple layers before it could build that whole overhang, but each layer moves the overhang to a different place (because it's a screw) so the hobby-printer wouldn't have a chance to catch up.

The model could probably be adapted for hobby-type printers by designing the threads with a goal of eliminating the concave shapes.  Overhangs could probably be built with very high accuracy so long as they didn't have curves or corners in the unsupported regions.  So I designed a trick thread with a cross-section whose overhangs form straight lines, like so...

I don't own a Makerbot (nor any other hobby printer) so I don't really know if this will be any more buildable than the model I'd originally designed for my SD300.  But it's a starting point, in any event, as the trick threads function correctly and I'm sharing the source data so other users can adapt it if needed.

This new model and its source data are available from Thingiverse here so anyone can download it and edit it.

Saturday, November 19, 2011

Roxanne's Videos, addendum

In response to my last post, Roxanne graciously posted a new video honoring the Rox Box puzzle box I made for her last year, and which I discussed in an earlier post.

Just to clear up a little confusion, I engraved the gemstones but I didn't actually build them.  These were lab-grown gemstones, technically real rubies and sapphires but not natural rubies and sapphires.  Gem Select has a nice, balanced explanation of synthetic gem stones here if you're curious.  It's informative and respectful, but they don't trade in synthetic gems so they aren't pushing a product.

The clear plastic 'windows' in the Rox Box were literally 3D printed already-assembled by building the majority of the puzzle using white material, then switching to transparent material for the last few millimeters.  This is how the model looked when it came out of the printer, embedded in a solid block of white material with a few layers of clear material (at right).

After I cleared the material from the inside of the models I could see through the windows, but the parts were still embedded in unused support material from the build process.

Peeling away the material around the exteriors revealed the finsihed Rox Box parts, complete with their transparent windows for the top and bottom.  This is how the puzzle box looked the first time I got to see it!

Thursday, November 17, 2011

Roxanne's puzzle videos

Roxanne Wong is an extraordinary puzzle collector whose YouTube channel is crowded with reviews of puzzles by various ambitious designers throughout the world. A few are puzzles I've previously discussed or worked on.

George Bell's Dice Box
I built these hinged parts to George Bell's specifications, as described in an earlier blog post.

George Bell's Exploding Ball
George allowed me build my own Exploding Ball last year. In this video Roxanne exhibits a colorful version of it.

Air, by George Hart
This puzzle inspired my Rhombic Dodecahedron puzzle.  George Hart's web page shows examples of the pieces assembled and unassembled.

Cuburr is uniquely optimized for the SD300 build process, as described in an earlier post.

Tuesday, November 8, 2011

Helical Burr Experiment

After seeing my Rhombic Dodecahedron Puzzle Bram Cohen casually suggested a puzzle in which one particular piece has to be put in last.  That is to say, a puzzle in which the pieces must be assembled in a particular sequence.

The picture above shows how the idea might be adapted to one of my swirly 'marble' puzzles: the four pieces must be assembled in the sequence illustrated above because each new piece blocks the movements of the previously installed pieces.

Anxious to try it out, I built a set of pieces without checking their dimensions.  But I encountered some delicate, thin walls while I was peeling the models out of the support material.

 VisCAM revealed some astonishingly thin, wispy structures on the thinnest piece.  Some of the walls taper down to less than 0.1mm near some of the edges.  It held together only because the super-thin portions are anchored to the thicker 'backbone' up the center!
It took a lot of slow, cautious work to free this thin part from the leftover plastic but it came out intact!  Dipping the piece in plastic-welding solvent made it strong enough for functional testing, despite a few frayed-looking edges where the walls taper down to nothing.

Reviewing the measurements it became clear that the super-thin walls had occurred as a result of how two spiral-shaped channels gradually converged inside the puzzle.  Luckily the thin piece would be amply protected by the thicker piece when the puzzle is assembled.
As I'd hoped, the pieces can only be assembled in a specific sequence.  But unfortunately they fit together too tightly, so the last piece wouldn't go in.
I tried to improve the fit by sanding the pieces, but the PVC is just too soft for hand sanding so I resorted to a high-speeed abrasive wheel.  This improved the fit and gave the pieces a nice smooth feel.

Perhaps it's a bad habit to keep building models with such thin walls, as they're extremely vulnerable to breakage while cleaning the model.  But even though they could break during manual cleanup, they don't pose any sort of danger to the build process so the main penalty is the tedious labor.  The SD300 can safely attempt any sort of ridiculous geometry, even if the STL file is riddled with defects, and the worst that might happen is the model might be disintegrate or consume excessive labor during post-build cleaning.

Tuesday, November 1, 2011

Making signs with deeply-engraved letters

This amusing Do Not Disturb sign was adapted from a humorous sign in the animated TV series Moral Orel.  A Thingiverse user shared the design in several formats so it could be built using a laser cutter, robo cutter, or 3D printer.

I built it on the SD300 by starting with ivory/white material for most of the build and then substituting red material for the last layer.  That resulted in neat, sharply-defined letters in deep relief.

I liked the joke so I took it a step further, designing a door hanger and sharing it as a derivative design.  I flattened the design a little, but kept some depth because the relief makes the graphics look attractive.

Thursday, October 20, 2011

"Screwy" Screw

 A Thingiverse user saw my Wrong Way Nut video on YouTube, which showed two nuts traveling in opposite directions on the same screw thread.  He was inspired to create his own version of it, which he named Screwy Screw.  Full marks for that name!

It was interesting to see how he came up with a similar solution, yet made some vastly different aesthetic choices.  Most conspicuously, his Screwy Screw is much larger than my own bolt model.
Here's a quick video comparison of Screwy Screw against samples of Wrong Way Nut built on my SD300 and another sample built by Bradley Rigdon courtesy of
Here's a closeup of the FDM samples of Wrong Way Nut, built by Bradley Rigdon.  Notice the funny patterns on the side walls?  See below.

Below is a closeup comparison of a nut built on the SD300 (red) and one built on a Dimension by Stratasys (blue).  Both build processes produce these wave-like patterns, often referred to as "chatter", as a result of very subtle vibrations that occur as the machine changes direction at the corners.  The marks don't affect the model's tolerances--in fact they're hard to see except by examining the model's sheen under a bright light source as here.
STL files for Wrong Way Nut are downloadable from Thingiverse here.

Sunday, October 16, 2011

Gear O'Clock

Last week Bradley Rigdon shared Gear O'Clock, a novel clock that indicates the current time using a large geared ring with numbered tabs around its rim.  This week I posted a set of Accessory Parts to adapt or customize Gear O'Clock.

Bradley used a clock movement salvaged from an inexpensive wall clock he bought at Wal-Mart.  I set out to adapt the parts for use with an ArtMinds clock movement from my local Michaels craft store.

For the benefit of Thingiverse users, here's a detailed explanation of my accessory parts:

Clock Drive Gear slotted NO SUPPORT.stl has a slotted opening that fits over the threaded hour-hand shank on the craft-store clock movement.   It took some effort to avoid excessive overhangs so the part retains its buildability on hobby 3D printers.  I added a wide opening on the back side of the gear because some of the craft store movements have big shanks (like this one).

The slotted-shape of the shank keeps the Drive Gear synchronized with the shaft, just as a conventional hour hand would be.  The clock movement kit includes a threaded brass ring that screws over the Drive Gear to secure it in position.

Some of the clock movement kits have short shanks and others have really long shanks like the one illustrated here, so I added a small fence inside the deep gear teeth so the driven clock gear won't tend to walk out of the back side of the Drive Gear.

Bradley's Clock Base ring is 10 inches in diameter so it can't be built in one piece on the SD300, nor on most hobby 3D printers.

Clock Base div4.stl divides the Clock Base into four parts, which can be spliced using the number tabs at the 3, 6, 9, and 12 o'clock positions.
 The number tabs are keyed into holes in the Base Ring, so I glued one number tab to the end of each of the four parts.
 Then join each piece to the next by hooking it into the keyed holes and injecting glue between the surfaces.  Notice how the diagonal join ensures a gradual transition from one section to the next so the gear won't snag when it turns in the clock.  Be sure to attach the numbers counterclockwise relative to an ordinary clock!

My revised Drive Gear exposes a hole for the clock's second hand, so I designed Clock Seconds Gear.stl to cover the hole and add a little animation to the clock.

This gear has a hole and a slot in the back to accommodate the original second hand from the clock movement kit.  The original second had can be left intact or clipped off.

After the Clock Seconds Gear is attached to the second hand parts, it pushes into the exposed opening in the clock shank.

The craft-store clock movement has a built-in wall hook so it wouldn't fit in Bradley's enclosed Clock Mechanism Mount.  Clock Mechanism Mount cutout.stl adds a cutout to accommodate the movement's wall hook.

Some clock movement kits feature a moving pendulum, which adds even more animation to the clock.  So I built the even-larger Clock Mechanism Mount Pendulum.stl mount which has another cutout on the bottom for the pendulum mechanism.
Clock Gear Pendulum.stl is a gear-themed pendulum for use with the pendulum-type clock movement kit.

For further reference:
  • The original Gear O'Clock files are located at Thingiverse
  • My accesory files are located at Thingiverse here
  • Bradley's video shows how the clock should be assembled at YouTube
  • Here's a video showing my clocks with moving second hands and pendulum.
  • An online shop that offers all sorts of clock kits at