Commentary and photos provided by Andrew DiLorenzo
For this meeting, Denny presented his own special tips and techniques for producing segmented turnings. As expected, segmented turnings can be quite complex, so Denny broke down his thinking into separate sections for this meeting.
He began with recommending a favorite book of his, “Segmented Wood Turning,” by William Smith.
Notably, he pointed out some charts and angles in the back of the book. Angles can be confusing, so remember there are 360° in a full circle. Tonight’s project has 24 segments, so some simple math has that 360° divided by 24 segments yields 15° each. What is not apparent at first glance is that the 15° for each segment gets divided in half, so each side of each segment gets 7.5°. Keeping that in mind for now, Denny goes on to point out that expansion and contraction in the bottom of any bowl can be a problem, since the wood grain in the bottom generally runs in one direction while the grain of the side segments runs around the bowl with a differing angle for each segment.
Next, Denny brought out his laptop and showed us how his favorite turning software, “Wood Turner’s Studio,” which Denny says is simple and fast to use. The software was originally developed by Craft Supplies USA, on the web at: http://www.woodturnerscatalog.com/b/39/Craft-Supplies-USA.
Interestingly, the software is no longer available there, but other sources exist. A screen shot is shown below.
A web search shows a site called “ software.informer” has the software available on this site: http://woodturner-studio.software.informer.com/1.0/ (sorry, neither I nor the club can endorse any software or online download, but if you have success with this, you can let the members know.)
As Denny was demonstrating the drag and shift nature of this software, he informed us of another detail, called padding. Since the software computes segment sizes, and their glue up can be critical, it is to the advantage of the turner to allow some “fudge factor” or extra amount of wood to be turned off in the bowl building process. Speaking of a fudge factor is a good segue to Denny’s next topic of jigs to control the cutting and building up process.
To illustrate, if on a closed segmented bowl with 24 segments, each with 7.5° cuts at each end, suppose that each cut was off by only 1/10° each. As the error multiplies around each layer of the bowl, 24 segments would sum to 2.4°. That means that either the bowl would have gaps in it, or that the segments would be too long. Predictably, Denny next showed off some of the jigs used to make his cuts.
For an open segmented bowl, small angle differences become invisible in the gaps as long as the gaps are uniform. Here, he is satisfied with his Inca miter gauge accurate to 1/10°. He groups several layers of the bowl to allow ripping with identical widths. Then he cuts the angles on the table saw with the miter gauge. The length of the 24 segment pieces is dialed in by small adjustments to his stop. He cut, measured, and adjusted the miter stop with an automotive feeler gauge. When satisfied, he makes 24 pieces, flipping the stock over after each cut. He allows a little extra thickness that is trimmed off with a jig mounted on his lathe at a later step. He lightly sands the pieces by hand to knock off the fuzz.
Next Denny demonstrated how he gets each layer to be the exact final thickness required in the layer he is working on. For this, he worked on a layer previously glued up on the work piece that had sufficient time for the glue to dry. Here he mounted a hand held router in a wooden strap mounted in turn to a
cross slide vice. The router had a straight trimmer bit, and the entire assembly was mounted on the lathe bed. The lathe was turned in the reverse direction to the router bit’s direction and a low rpm was selected. This jig quickly removed the extra stock down to the intended 1/8” thickness.
Denny’s next jig is a Plexiglas round set on the outboard side of the lathe. The Plexiglas has degree markings all around, and Denny used a vertical alignment piece that indicates then the lathe was turned to the desired angle, at which point he clamped the alignment piece in place and glued on the first of the next layers of segments. He likes Gorilla Glue in a one ounce dispenser bottle. One can find product details here: http://www.gorillatough.com/gorilla-wood-glue.
Denny says he can disregard the recommended clamp time of 15 to 20 minutes, and reduces that down to less than a minute for each piece. He says that once the glue wets the wood on both pieces, that it has high initial tack and generally stays puts. He showed us that if one piece comes loose, it can be easily refastened. As he glued up these segments, Denny demonstrated yet another jig, this one to set the segment’s penetration into the interior of the piece, and to hold the intended angle. After the glue dries, the same router jig is used to set the layer thickness according to plan. The balance of the layer glue up reads like shampoo directions, lather rinse repeat. After the layers are complete, and sometimes in between sections, the inside and outside of the work can be turned. That was left for another day, as the session ran out of time.