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Friday, August 7, 2015

Open Segmented Bowl - The Start

At the end of the Maloof Chair blog I had talked about another project I had in mind.  I did not mention what, but it was some low voltage exterior lighting fixtures.  I finished up the design, built a mockup and  was ready to go.  However, because of escalating project costs I reviewed my cost estimate to make sure I had everything and found I had left out one item, the nearly $200 of wire needed to connect all the lights together.  That pushed the project out of what we had budgeted.  At that point it was back to the drawing board where we found some quite nice solar LED lights.  They have a soft yellow color and provide enough light to meet my goal of highlighting the limits of the driveway for less than 20% of my final estimated cost.  Also, the idea of digging several hundred feet of trench by hand was not all the appealing.

This all led to the next project on my bucket list, a segmented bowl turned on the lathe made with gaps between the segments.  I have done several solid segmented wood turning pieces and  wanted to see if I could do one of these. 

 
I started out using SketchUp to make a general profile drawing of what I wanted the bowl to look like.  The next step is to take that profile turn it into a 3D rendering  and take a critical look at it.  It’s then back to the profile to edit as needed, create a 3D rendering, evaluate that and repeat the process until I get the final shape.  “Final” shape is a rough term as things can and often do change as I go through the building process.  With the aforementioned “final” shape done I made the decision that there would be a one piece top and bottom with 6 individual segmented layers between.  The top and bottom layers would be 3/8” thick and the segmented layers would be a ½” thick.

Before I go into the specifics of the math needed to size the individual segments here is the finished drawing of the piece.  I include it here to help out with the visualization of the following series of decisions I made .


My next step is to decide how many segments per layer.  I want the segments to be big enough to work with but not so few as to make the piece look clunky.  To get the outer face sizes meant I needed to create an Excel spreadsheet to run the geometry calculations.   I made one assumption in the calculations, and bear with me as I go through the logic.  Each piece in the open segment layer would be 2/3 the angle of a piece needed for a solid layer leaving a gap between segments equal to 1/3 the angle of a solid piece.  Got that?  To help here is an example, if there are 12 pieces in a solid ring each piece covers 30° (360° in a circle divided by 12 pieces).  This means each of the 12 pieces in the open segment piece would cover 2/3 of 30° or 20° and the gap between the segments would be 10 °.  The drawings below should help out, a solid ring on the left and the open segmented ring on the right.


I chose a 10° gap as a balance between openness and strength.  The bigger the gap the more delicate and airy the piece is but the less glue surface there is holding it all together.  Remember, when I get the ring layers all glued together I am going to mount it in the lathe and spin it at well over a thousand revolutions per minute.  Once at speed I will then shape it freehand using a lath chisel.  The piece needs to have enough strength to hold together and not fly apart due to centrifugal force or from the impact shock when that steel chisel starts cutting away.  After all, that little 5° glued overlap between pieces is all that is holding everything together and if it lets go pieces will fly!  Exciting, yes but not the kind of excitement I am after.

With that one assumption I built a spreadsheet to give me the outer face segment length for each layer based on two entries, the number of segments in the layer and the layer radius .  The radius for each layer comes from the final profile drawing  as does the segment width by layer which I will use later.

In reviewing the calculations it looked like a 12 segment ring would make the smallest piece just under an inch wide, which for this piece is a good size to work with.  Also, since the pieces are tapered this is the largest dimension, the inside face will be smaller.  With the number of segments set I had the calculated outer face length and from the drawing I got the segment width by layer.  Plugging the segment width by layer into the spreadsheet gives me the inner face length.  Using all the data gives me the needed board length for each layer  required to cut the segments.  With this I had everything needed to generate my cut sheet that showed everything I needed to build the piece.


It was using this data I created the final drawing shown above and here below.  Note the Material column is blank.  I have not yet decided what wood to make it out of although the stack of leftover scraps from the Maloof chair do come to mind....  While the drawing gives a good idea of the overall shape it does not show all the corners cleaned up and smoothed together.  However, it’s good enough to proceed on with the project.


Next up – The Jigs


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