Large Segmented Bowl - #2 Making the Base

Because there are a lot of cuts that have to be precisely made I started out by running a check of the table saw blade to make sure it was cutting square to the table.  First step is to cut a couple of scrap pieces that have parallel long edges then stand them on their cut edges like below.  The green tape is my cutting reference mark set against the crosscut fence.  Everything looks good on the left image but if you rotate the left board 180 degrees then the out of square becomes visible as shown on the right image.

Looking closer the below left image shows how much the blade is out-of-square.  Some minor adjustment of the blade, in this case rotating the blade to the right brings the pieces together and all is square.

A trip into my wood storage gave me a good group of cherry and walnut pieces to start with.

In the last post I decided to float a center panel in the bottom bowl layer.  The top image below shows a section through the bottom.  For reference the center panel is just under 3/16" thick.  To make the slot I made one pass on the table saw then moved the fence slightly to get the slot width I needed and made a second pass.  The bottom photo shows that setup.  The yellow and black thing is my magnetic fence.  I set it up to apply a slight pressure to keep the board pressed against the rip fence on the right.

Once the slot is cut I can start on cutting the wedge-shaped pieces that will get glued together to make the bottom layer.  Here is the jig with the slotted bottom blank in place.  I will use the jig to cut 18 pieces for each of the 16 rings that will make up the bowl.

This is the first cut which gives me one side of the wedge and a triangular waste piece.

Before making the second cut I flip the piece 180 degrees so the slot that was facing me now is away from me.  The piece is then slid to the right and clamped in place at the Outer Face Length I obtained from the spreadsheet. 

A stop that has a matching angle is then clamped in place so I can make all 18 pieces the same size.

 

The cut is made and I have one piece the right size.  For those of you with sharp eyes this first piece is not from the same place in the board as the previous photo.  That’s because when I checked the piece’s size it was a couple hundredths of an inch small.  I must have bumped the stop or something else.  Anyway, I reset the stop, recut the first angled side and made the cut you see here to get the right sized piece number 1.

Now it’s on to the second piece.  Because of the slot there is a specific inside and outside to the pieces.  This means that I have to flip the board 180 degrees back to its original position, cut a new first taper in the top view below then in the bottom view flip the board 180 degrees, push it up to the stop and make the second cut to get the second completed piece.  Two down and 16 to go for this ring.

In the image below, I have the first four pieces cut and arranged along with the waste triangular pieces.  The other layers without a defined inside and outside will be cut without flipping thus eliminating the waste triangle resulting in less waste. 

With all 18 pieces cut I made the floating circular panel and did a test fit.

To make the center disk float in the slot it could not be glued in-place and that presented a problem.  Gluing up the rings is a really messy process and glue gets EVERYWHERE.  There is no way I could glue the ring up and not get any in the slot which would glue the disk permanently in place losing its floating attribute.  I thought about doing a two-step gluing process where I would only glue up half the ring at a time then insert the disk and very carefully glue the two halves together.  Sounds good but if the slots are not aligned dead on then there would be no way to get the disk in place plus I would probably still have glue to clean out of the slot so the disk would fit.  A couple of other ways all had their problems too.  What I finally ended up doing was to finish the disk and the slots before the gluing all the pieces together.  Here you can see the disk and ring sections prefinished with shellac where I do not want the glue to stick.  Now when I do the actual turning I think that much of the finish will be removed but the slot to disk surfaces should not be permanently glued together.  At least that’s the theory.

Gluing up this many pieces, getting them aligned and clamped can best be described as semi-controlled chaos or a mad race against time before the glue sets.  According to the time stamp on the photos it took me right at four minutes from the time I started applying the glue until everything was together and the glue mess cleaned up.  I have to say that a couple of dry runs beforehand did not hurt my time either.

Here the first ring, which is the base with the glue-up finished and ready to set overnight before proceeding.

The next day the clamps are removed and several light passes made through the thickness sander to obtain the needed flat parallel faces.

Next up – Face Plate Sandwich, Making/Attaching Layers & Turning

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