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The String 'n Ring Puzzle

by Temple Blackwood
Castine, ME

Note: Click on any picture to see a larger version.

Several years ago I was casting about for a "hands-on" activity to promote primary research skills for the engineers in my Technical Communications classes at the Maine Maritime Academy. This would lead to an assignment on writing brief, accurate, clear instructions. One extremely successful activity resulted from my introducing them to a wood-turned puzzle, which was first introduced to me in the mid-1990's from my highly skilled woodturning friend, Al Hockenbery (recently honored by the AAW). The puzzle is amazingly simple to solve once you know the secret procedure, but it does contain a complexity of parts that will confound and frustrate someone new to it for a satisfying period of time (sometimes even longer).

The puzzle consists of a center post with a slot, a trapped ring that must be freed, and two similar rings fastened with a length of string that runs through two beads. The object of the puzzle is to first free the trapped ring (the dark ring) and then replace it to its trapped position (this ensures it will not be lost or stepped on between puzzlers who are working on it).

The measured sketch offers optimal sizes for the puzzle. While the post may be any length and include a longer handle, a design element that frees the turner's creative vision, there are a number of critical elements necessary to the puzzle:

  • All three rings plus a thickness of string must all be able to pass through the slot.
  • All three rings must also be able to slide over the upper (slotted) end of the post.
  • The two "string" rings must be firmly fashioned to the string.
  • Below the slot, the post must have a shoulder larger than the inner diameter of the rings.
  • The beads must float on the string and must fit through the rings, but must not fit through the slot.

Constructing the Puzzle

Having made quite a number of these puzzles over the years, I know of two handy ways to make the slot in the post.

The first method is to bandsaw the post blank in such a way as to then bandsaw out the ultimate slot either completely from one side (shown) or partially from both sides. This is best and most safely done while the unturned blank is still square. The important thing is to keep the slot narrow enough that the beads will not fit through it, but wide and long enough so each of the rings plus a thickness of string will fit.

Measure and cut carefully.

Make the saw cuts with a thin blade.

Saw out the slot and finish by gluing the blank back together. Let the glue dry fully before turning.

The other method is also best done while the post blank is still square. If the turner has access to a mortising set-up with a 1⁄4" mortise bit then this can fairly quickly produce a clean slot without creating the ultimate glue seam that the first method invariably cannot avoid.

Be sure the mortising bit is properly squared and sharp for a clean cut to avoid a time wasting clean-up for later.

This second method using no glue and producing a visible seam is the most efficient when doing a series of puzzles (I typically make twenty puzzles at a time), because the turner can set stops that keep the blank centered and properly sized.

Turning the Parts

Center the blank between the centers on the lathe for turning.

The mortise machine method seems better suited to highly figured wood like this cherry burl.

Sharp Gouges and handy Skew Chisels make short work of the turning portion of this project.

Lightly figured maple benefits from some mild detail. In this example, the "shoulder" or "collar" that keeps the freed ring from sliding too far down is a little too close to the slot in this blank.

The post may be sanded and finished on the lathe or off of it. Several coats of finishing oil such as Watco make it easy to put finish in the slot.

Turning the Beads

Once the post is turned and set aside to dry, the next step is to turn the beads. Depending on the diameter of the string you will ultimately use, select a larger drill bit and pre-drill the center hole.

Turn, sand, and finish the beads, being sure that they will not fit into or through the mortised slot. I typically finish the separation of the beads with a thin 1/8" Bandsaw Blade that I keep mounted on my Grandfather's 1940's Homecraft Saw. Then I hand sand the beads smooth before oiling.

As you might notice, selecting dark and light wood for the different parts will make the puzzle more interesting and will help the puzzler identify the terminology (part of the task for my struggling engineers).

For the rings

I typically select a dark ring for the "free" ring and both light-colored rings to attach to the string – or I reverse these. The contrast helps everyone, puzzler and teacher alike, keep the parts separate and instructions clear. For people new to the puzzle, it does not help to have each of the three rings and two beads a different color or all of them the same, nor does the end result improve the artistic look.

Using a 1-1/8" Forstner Bit , drill out the center of the rings.

Set the lathe on a slower speed for drilling.

Define, sand, and finish the rings. Cut them apart either on the lathe or using the bandsaw (as with the beads). Consider testing each ring for its slide-through clearance in the slot and run oversized rings briefly on the belt-sander to thin it appropriately. Given the slight inconsistency of hand-turned beads, fine tuning the size and fit at this point avoids the frustration of assembling the puzzle only to find it will not solve.

Shaping and finishing a series of rings with the 3/8" Sorby beading and parting tool makes the "manufacturing" process go fairly quickly.

For the rings attached to the string, use support and drill carefully with a sharp bit that is just the size of your string thickness.

Set up a sanding station for sanding the inner circle and hard edges. One way is to use the lathe with a chuck holding a dowel wrapped with 220 sandpaper.

Cut a length of string about 11" long. Put a drop of Medium CA Glue in the hole drilled into the ring and twist the string into the hole. A shot of accelerator will fashion that in place. Thread on a bead; run the string through the slot; thread the other bead; and repeat the glue process on the other string-ring.

If you are initially unsure about how to solve the puzzle, try putting the free-ring over the post before you assemble the string and its parts. The puzzle is typically easier to solve for the first time if you start with the "free ring" (dark) captured.

The puzzle is simple to solve – free the dark ring and then put it back into captivity – if the puzzler knows how to do it. For those new to the puzzle, the challenge can be frustrating and daunting. The engineers in my classes frequently, to their dismay, struggle unsuccessfully for fifteen or twenty minutes before one of them finally "gets it" and is allowed to teach the others.

This puzzle makes a great gift and offers a rich opportunity to explore a more creative design/implement approach for a woodturner.

While the solution instructions are not part of this article, turners are welcome to email me at temple@highlandswoodturning.com for a solution. Each semester my classes generate a large number of successful "instructions" for the puzzle, complete with terminology, step-by-step instruction on freeing the ring and recapturing it, and affirming the puzzler's success.

Having spent a great deal of time thinking about and designing other turned puzzles, I have not yet developed any better. I welcome photos of other folks' creations as well as suggestions for new designs of woodturned puzzle challenges.

Located in Castine, Highlands Woodturning gallery and shop offers woodturning classes and shop time, a gallery of woodturned art, custom woodturning for repairs, renovations, and architectural installations.

You can email Temple at temple@highlandswoodturning.com

Take a look at Temple's Website at http://www.highlandswoodturning.com/ .

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