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It was a real "cut and shut" job and the intention was to build a circular replacement after Xmas, and nearly 4 years on I'm getting round to it. I'm going to attempt a veneered sunburst table, with an American Cherry inner circle and a Birdseye Maple outer disc, separated with an ebony and sycamore string, like this one I made in miniature last year.
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I tested the concept with some and learned loads. However, these were only 30cms across, and the new table will be 140cms across, so there could be problems of scale. I'm using a substrate of 25mm ply. Weighs a ton!
The underside also needs a balancing veneer, and although this could be a simple straight veneer strip, I've decided to "sunburst" the underside as well as I need the practise on the full size table. The inner circle is 100cms in diameter, so I've cut some 2.5mm veneers from cherry using a brand new 5/8" Super Tuff Fastcut blade from Tuffsaws on my Startrite 352E. I tuned the saw using the Alex Snodgrass method (Youtube) and that combined with the new blade leaves a finish that would be ready for gluing if it wasn't for the fact that I will be bookmatching some of the veneers. The width of the cherry plank dictates that there should be 20 segments, and there needs to be an even number.
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I've cut a template for the segments from some scrap 9mm mrmdf, and here it is on the table substrate, which I've cut to 150cms so that it is 5cms oversize all the way round. It will be cut to its' final size of 140cms with a router and trammel once the veneering is complete.
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I've used the offcuts to frame the segments to get the grain looking right ....
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.... and then used the segment I've just cut to match the grain in the adjacent piece of veneer.
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The central angle is 18 deg (and a smidgeon) and I've cut them to the approximate shape with a japanese pull saw. 18 deg (+) means that 10 segments will come to just over 180 degrees.
The outer ring is from some maple, and I couldn't resist placing them roughly in position to get a preview of what it might look like!
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The segments now need to be trimmed to precisely butt against each other. Normally I would use a veneer shooting board, but this time I thought I would try something different. I turned the "hold down" board from my veneer shooting board upside down and placed it on an 18mm ply base. The shooting board has a strip of 180 grit aluminium oxide paper glued to one edge to prevent the veneer slipping. A veneer segment is then placed on the board ...
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.... and a 1400mm Festool rail placed on top. Then I shoot the edge of the veneer with a palm router mounted on a Makita track adaptor, which fits the Festool rail straight out of the box. I'm using a 6mm Wealden spiral downcutter which leaves a beautiful clean, and straight, edge. Works a treat!
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When the trimmed segments are assembled, they produce 2 semi circles (+) which come to about 185 degrees each.
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Once these have been glued up, I place the semi circle in the shooting jig and rout a straight edge across the diameter so that the 2 semi circles should then fit together perfectly. What can possibly go wrong? The assembled disk is near perfect, and completely flat. I'm well chuffed and go to bed!
The following morning I come down to see that the disk has split along the diameter join, and there is now a gap of about 5mm between the segments at the circumference.
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The cherry has been in a back bedroom for the last 5 years, and was only kept in the garage for a couple of days whilst it was being cut into veneers. However, the movement is significant in just 12 hours. The circumference of this inner circle is 314cms, and it has shrunk by 5mm overnight. Obviously the veneer needs to acclimatise longer. However, if I leave it to acclimatise longer, what effect will the absorption of water from the glue have? I'm planning to use Cascamite as usual in my veneering projects.
There is only one way to find out. Time for another test piece. This time I make a disk 60cms in diameter, which will be 188cms around the circumference, as I have some spare ply that size, and I can use the offcuts of cherry from the veneers I have just cut. I am careful to make sure the joins are good, and by now the cherry should have acclimatised. The assembled disk was near perfect, even though I say it myself.
However, after gluing to the substrate using Cascamite as planned in a vacuum bag, the result is as I anticipated. The veneers have absorbed water from the glue, expanded, and as expansion is around the circumference, the veneer has nowhere to go, it has pushed the veneers apart in the centre before the glue has set. In one place it has split the veneer rather than pull a joint apart.
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So what now? I have several ideas, but would welcome any other suggestions.
1. Use an epoxy resin as the glue - something like West Systems Epoxy. (Unibond 800 isn't available in the UK). I believe that this will not make the veneers expand before it sets in the same way a glue with a water base will, such as Cascamite or Titebond 2.
2. Cut the veneers thinner - say 1mm, or even thinner - as the 2.5mm thick veneers exert excessive expansion forces, behaving a bit like solid timber. I would hope that the reduced forces in a thinner veneer might compress the veneer rather than force the disk apart.
3. Both the above!?
4. Insert some sort of motif - a compass rose perhaps - in the centre which appears to be where the damage takes place.
5. Glue the disk in segments - say 3 segments at a time. Expansion should not then be a problem as they can expand sideways, and when the last segment goes in the glue either side will have cured and should contain the veneer. Not sure whether I could do this sufficiently accurately though, and is my least favoured idea. Would definitely need trialling in a test piece.
6. Something I've not yet thought of!
More test pieces are obviously needed. I'm going to use West Systems Epoxy on 2mm beech veneers first.
I've cut a batch of 2mm beech veneers, and also some that I have taken down to 1mm.
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These are now "in stick" on the dining room table where they will stay acclimatising over the weekend whilst I'm away. Next week I'll stick both these down to ply in the vacuum bag using the epoxy.
This certainly vindicates my decision to try a sunburst on the underside first. I know that straight veneers are not a problem and the expansion issues would only have become apparent when I used some prized stock on the top. So far, no real harm done, and the learning curve is near vertical! At least I'm learning what doesn't work, so I'm nearer to finding out what does. All good fun!
To be continued ...........