Large Segmented Bowl - #1 The Beginning

In a recent trip to Santa Fe, NM I saw some large segmented bowls in various Native American motifs.  I have made several segmented pieces (below) but nothing larger than about 11” in diameter. 

Intrigued I wondered what making a bowl that pushed the limits of my lathe would entail and what kind of obstacles I would encounter.  Since my lathe has a maximum turning diameter of 20” I decided to shoot for a finished diameter of about 19”.  Working in SketchUp I created my finished shape paying no attention to the size but concentrated on what the profile should look like.  Most of the pieces I have done have had a fairly small opening.  In this piece I wanted to go with a design that is reminiscence of a wide mouth storage container.  When I got something that I was happy with I scaled it to be 19” in diameter.  With that done I measured the height and found it to be about 12”.  Here is the piece in profile and looking down  slightly.

Next is to create a section as though one had sawn the bowl in half vertically .

With that done and deciding to use ¾” thick rings except for the bottom I could lay out each layer’s  diameter and segment width.

The next decision is to decide how many segments to make each ring out of.  To give me some help in this I turn to a spreadsheet I made.  Once I enter the radius of each ring it shows the Outer Face Length for each segment by ring in a variety of segments per ring.  For this piece I chose to use 18 segments  per ring.

Taking this information, I can use another set of calculations to get the particulars of each piece in each ring plus the total blank size I will need to cut the ring segments from.

 The headings like "Outer face length" in the above calculations are based on the drawing below.

To get an idea of what the piece would look like when done I built each ring using cherry for the body  and walnut for the feature pieces.  Here are a couple of views on what that looks like.

The unturned rings give a pretty good idea what the finished piece will look like but I wanted to get a better representation as I had two different designs I was considering.  That required me to intersect the unturned rings with the smoothed profile.  I have to say for the computer to calculate  the intersection of all those facets it did take some time.  I probably could have gotten by without doing this but I could so I did.

Once I had decided on the feature pieces size it is back to the spreadsheet to set up the last two  columns to calculate the blank length needed for the cherry and walnut pieces.  While it looks like everything is set in stone that is not really the case.  As I build the bowl adding the rings and turning if things do not look right I can and will make changes.

I usually do a double check on the Outer Face Length by taking the drawing one step further to see what it comes up with.  Below is a copy of that drawing.  If you compare it to the spreadsheet there is a difference of a few hundredths of an inch but that does not really worry me as for making the rings is it within tolerances.  If there is a big error then I need to take a look and see why.  It is my guess that when  SketchUp gets into that fine a measurement then it may not be as precise as the spreadsheet which carries out the calculations to a whole slew of decimals.  It’s 15 places if you are interested.

One more detail needs to be addressed and that is where all the pieces come together in the bottom layer.  There are 18 pieces so that’s 36 cuts I have to make and have them all come at an exact center point.  Now I am pretty meticulous when it comes to setting things up but if I am off by even 3 thousands of an inch then when the pieces all go together then there is going to be a gap of just over a tenth of an inch.  I could cut or drill a circle in the center then put a plug in but have had some problems with that.  Since wood moves around with humidity changes what usually happens is the piece cracks or ends up with a hairline gap.  This is going to be a pretty big piece and as such the wood movement is going to be more so I decided to take a page from cabinet making and float the center piece in a slot.  It is similar to a raised panel door where the panel floats in a frame.  Here is what a section through the bottom would look like with the slotted pieces and disk in place.

Next up – Making the Base