An inside-out turning
Tim Killen demonstrates how inside-out turning can be duplicated using SketchUp.
Recently I was asked about an interesting technique called Inside-Out Turning, and how this would work in SketchUp. This lathe turning process is used for creating decorative objects and particularly tree ornaments. Here is a simple example: my design used to test the SketchUp process. This object is 8 in. tall with a 3-3/8-in. dia. That overall size set the dimension for the four glued blocks.
In this process, you combine four identical sized blocks and glue them with paper in the joints. This allows the blocks to be disassembled, rearranged, and turned again. There are two turnings: one to create the “inside” shape and a second turning for the outside shape. From what I understand about this process, the first turning is done to create the “inside” shape. This makes sense for the lathe work as it requires the blocks to be rearranged once.
It seemed to me that SketchUp would allow a different sequence of turning and enable a more determinate design process. To avid turners, I realize this design idea is out of place. Much of turning is accomplished without design, rather ad hoc shaping to fit the material and the eye. But my engineering background takes over, so that I go to the shop with all the details in hand.
My process in SketchUp is reversed; that is, I do the outside turning first. I want to know the final size and shape in the very beginning of the process. This sets the sizes of the blocks that will be used. It also facilitates the design of the second “inside” turning that must protrude through outside skin of the piece. That protrusion decides the size of the “windows” through which the hollow inside is exposed.
In doing the “inside” turning first, you are limited on shape, size, and look of the final object. This seems backward to me.
Shown below is the first turning using the profile shown and the Follow Me Tool. Note that I did an Intersection on this full shape on one quarter block. So that full shape below is actually four identical components. The later video shows this process in detail.
By having the overall outside shape and size, I could determine the profile shape and size required to make the second inside turning. This second turning determines the window size cut and the hollow shape. It also determines the thickness of material left in the final design, and thus its resistance to breakage.