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Segmented bowl turning question, what am I doing wrong?

Discussion in 'Woodturning Discussion Forum' started by John Sisak, Aug 4, 2020.

  1. John Sisak

    John Sisak

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    May 7, 2020
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    Location (City & State):
    Howell, MI
    I am a relative beginner to turning and segmented bowl turning in particular. I have been making the bottom Of my bowls a segmented disc in 8, 12, or 16 segments. After some time I noticed the bottom of some bowls are cracking along one of the segment lines through the center l suppose to expansion or contraction of the wood. Is making the bottom with segments the wrong thing to do? Should I be turning a round disc out of a single piece of wood? My bottom segments have been typically 3” to 6” diameter for bowls up to 10” diameter so far.
     

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  2. john lucas

    john lucas AAW Forum Expert

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    Pie shaped segmented bottoms will almost always open up either in the middle or outer edges. It's all about wood movement. Wood moves differently with the grain, across the grain and side grain. So Everything is expanding or contracting from the center straight out more that it is with the grain. When there is a lot of moisture it will open up gaps along the outer rim. When it's really dry the center will open up. I get around this 2 ways but I really haven't done a test to see how it works on bottoms larger than 4". I take a solid piece and then resaw it into several thin pieces. I rotate each layer so the grain does not align and glue them back together. This gives me what looks like a solid piece of wood but it is basically plywood and doesn't expand or contract. I tried the same technique by glueing up my pie shaped bottom and then resawing it into 4 layers and gluing back together with the glue joints overlapping. It has worked for that piece (about 4 1/2") but I haven't tried this on larger bottoms to see if it will work. Hopefully all the segmented turners will get on here and give their technique. I used to do a lot 30 years ago and had a fair number of failures and tried to learn from them. In the last bunch of years I'm getting back into some segmented work and will be re learning how to do bottoms on larger pieces.
     
    Mike Adams likes this.
  3. brian horais

    brian horais

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    John, I took a segmented turning class from Curt Theobald a few years ago and he stressed many times that the base of a segmented bowl or vase should be solid to preclude separation of the base (if it were made from pie shaped segments). He also made a point about grain directions. Make sure that the base grain direction is the same as the segments. That means if you are using horizontal grain patterns on the segments, the base should have its grain oriented in a horizontal direction. This is to preclude separation at the joints between different grain directions. If your design requires you to integrate different grain directions, you can make a thin layer between the two grain directions by using model-maker's plywood. This relieves the stress between the grain patterns as the wood moves with different temperatures and humidities. Here's an example of a pineapple shaped segmented vase I made with horizontal grain patterns in the segments and vertical grain patterns in the twisted upper section. Note the narrow separating line between the upper and lower sections. This is a thin layer of the model-maker's plywood that I cut into a ring and glued into the stack while 'building' the turning.
     

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    Stan Semeniuk and Greg Norman like this.
  4. Charles Chaloupka

    Charles Chaloupka

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    Location (City & State):
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    John, I have been making segmented pieces for about five years. I learned my techniques from a turner in North Carolina. His name is Earl, (don’t know last name) and he demonstrates on video how to make a floating bottom for any base disc larger than five inches. I have never experienced a negative issue with solid wood discs up to five inches in diameter. All of the solid wood discs I have made have been end grain turnings. Probably why I have not had any failures. If you want to see a very good, well explained video tutorial, look for Earls small segment shop on YouTube. He has many other excellent videos. The floating bottom disc works really well and isn’t hard to do. You turn a disc the size you need, then turn a rabbit on face, then part it off in the center creating two halves. Then square the face of disc that is still mounted, make another rabbit.(both rabbits should be same depth and diameter). Remove and make a thin turning that fits the rabbits you created on the two halves. A dot of glue on the end grain sides of your thin disc holds it in place as you carefully glue the two halves back together. This sounds complicated, watch Earl’s video, it is not. Hope this helps
     
  5. John Sisak

    John Sisak

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    Thanks for all the great tips. John, when you resaw the bottom, how thin do you make the slices? Brian, will the bottom piece put strain in the cross-grain direction on the lowest segment ring causing cracking? The solid piece will have end grain on two sides of the first segment ring, is this a problem? I have never heard of “model-makers plywood “, where can I find it? Charles, thanks for your tips. I will look for his videos.
     
  6. Larry Copas

    Larry Copas

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    Funny ya brought the subject up. I just took a few pictures for our club zoom meeting. A mini demo of sorts "How to make a floating bottom". Hope my pictures explain the process.

    1-glue-up.JPG

    2-sand-flat.JPG

    3-chuck.JPG

    4-turn-id-round.JPG

    5-slot-cutter.JPG
    The router bit goes into a router table. Forgot to take a picture of that step.

    6-ready-to-assemble.JPG

    7-complete.JPG

    There several other ways to make floating bottoms. The first way I learned was in Malcom Tibbets great book. Its cool but time consuming, well worth learning.

    I once did a demo and made two bottoms like yours but with green wood. The first one I wrapped to keep wet. The second one I gently dried in the microwave and got cracks exactly as you show.
     
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  7. john lucas

    john lucas AAW Forum Expert

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    John I usually laminate my bottoms out of 3/4 or 1". out of 3/4 I can get 4 slices but I have to hand flatten them with sandpaper on a jointed 2x4. Usually do the same out of 1" although obviously each layer is thicker and I can true them up on the disc sander.
     
  8. brian horais

    brian horais

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    John, you can find the model makers plywood at most craft stores - usually near the plastic model kits at the big box craft stores. The solid bottom will have horizontal grain across the piece, even though you are seeing end grain on two sides. The horizontal grain will bond well with the horizontal grain of the segments. If you had vertical grain for a bottom piece, all of the grain is end grain and would not bond well with horizontal segment grains.
     
  9. Mike Adams

    Mike Adams

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