Saturday, November 09, 2013

Streptohedrons (Part II) - Here are 3 more

I have now turned three more streptohedrons, these based on some of the examples and plans in Woodturning Full Circle.

The first of these is a variation of a Four Branched Streptohedron, which has a kind of art deco shape.  It is turned as a series of right-angled "stairsteps" on each side, as indicated in the photos.  (Cherry, finished with Watco Danish Oil.)

The second streptohedron is a Twisted Pentagon.  Because of the additional side there is an additional axis available, giving the possibility of two variations in the final shape.  Here's what it looks like (Cherry; Watco Danish Oil finish):

The third streptohedron in this group is a Six Branched Streptohedron, with its center of rotation through the valleys.  The turning process here is rather more complex, and it is difficult (at my skill level, at least), to make the cuts precisely enough to get a perfect match-up when the two sections are rotated.  Here are some photos showing the overall approach:

Initially the blank (previously divided lengthwise and re-glued with a paper insert) is turned to the desired diameter; lines are drawn on the circumference to indicate the cutting locations;  and a tapered hole is turned in the end, using a template for size and shape:

Then the first of the shaped sections is turned; again a template is used to get the inside angle correct:

Here is the mostly turned blank.  The left (headstock) end is still fastened in the chuck.  The right end, where the tapered hole is shown in the first photo, is simply supported by the live center.

After the shaping is done, the piece is parted off at the headstock end.  It is then reversed into a jam chuck (a wooden block with a hole turned to hold the piece snugly), and a matching tapered hole turned in the other end.

The piece is then removed from the lathe, and re-split along the glue line into two halves.

The halves are then rotated so as to form the streptohedron and glued together.  The piece is then sanded and finished.  In this case I used several coats of black gesso as the finish - it is both dramatic and disguises minor defects pretty effectively.

The smooth outer rim of the resulting piece is continuous, having neither beginning nor end.  Pretty dramatic, I think.

Friday, November 08, 2013

Streptohedrons (Part I) - My first! And what in the world are they?

The cover article of the August 2013 issue of American Woodturner was titled "Behold, the Streptohedron" (by Bob Rollings, with David Heim), and had an elegant and intriguing photo:


I hadn't ever seen these forms before, and was fascinated.  I turned the form described in the article (shown below), and then acquired David Springett's amazing book, Woodturning Full Circle, which (among many things!) includes sections on how to turn a variety of streptohedrons of different configurations.

As defined by Springhett, who appears to have invented the term, a streptohedron is a "twisted polygon," that is, a polygon which has "rotational symmetry":  when split along an axis, rotated, and rejoined, it becomes a new shape with "compound-curved faces that seem to flow smoothly into each other." (Rollings)  The beginning polygon can have any number of sides.  To turn one you prepare a blank, split it down the middle, and then glue it back together with paper in the joint.  You then turn the specific shape you want, re-split the piece along the glue joint (and clean up the faces), rotate ("twist") one section in relation to the other, and then glue them back together.  If (!!) the turning has been done precisely, the edges will line up and the streptohedron emerges.  A little - or a lot - of careful sanding also can help.

This is the first streptohedron I turned, based on the AW article.  It has four "sides."  The wood is Claro walnut.

Here is the glued-up blank, turned to shape between centers on the lathe:

Then the turning is re-split along the glue line:

The two pieces are then turned 90 degrees and re-glued to create the new, complex form:

Once the glue is dry, the piece is completed - the extended ends removed, the piece sanded, and finish applied.  Here is the result (finished with Watco Danish Oil):

In the next entry I'll show you the others that I've done so far.

Back in Evanston

We moved back to Evanston IL last May, after an 18 year or so absence. Much has changed, much is remarkably the same. One major change is the appearance of high- and semi-high rise condo buildings in the downtown area. We are in one of the less tall and less voluminous ones - 15 stories. One result of our downsizing from house to condo is that I no longer have any home-based shop space. I've rented shop space before, in Madison, but haven't started that search yet here. For the time being I'm sharing the shop of a woodworker friend, who has kindly let me use some of his space for my lathe and some additional tools, though much is stored in his garage attic. My friend is a highly skilled furniture maker, and I took advantage of those skills to design and build a dining room table with him for our new place. So, in a bit of an excursion, the photos below don't depict any wood turning at all! The table is solid birch, with Panga Panga for the dark accents. The central part of the top (inside the darker strips) is butcher block. The legs also have Panga Panga inserts, and the tops of the legs are set in to create a reveal that suggests the top is floating.