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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