Yumi Build
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   I decided to try my hand at building Yumi since I've had an interest in how they are built for many years now.  The process of building a Yumi is a labor intensive process that involves many different woodworking skills to accomplish.  Also, to get the proper shape for a Yumi a method of tying and wedging was developed to hold the Yumi into shape while the glue dries.  Here in I will describe the procedure I used to build a Yumi.  Since there is very little information about the process is available (I tried to obtain Jaap Koppedrayer's video "All Tied Up in Bow Making" but the supplier was all out).  I did find an excellent write up on PrimitiveArcher.com's forum (http://www.primitivearcher.com/smf/index.php?topic=11083.0) and Paleoplanet69529.yuku.com's forum
(http://paleoplanet69529.yuku.com/topic/22774/t/Yumi.html?page=1) that explains what is on Jaap's video. 

   I decided to add what the forum's leave out and added a couple of gauges (poster board/wood patterns) that can be used to help get the various curves close to being right.  I also used the Yumi I bought a few years ago as my example, and guide, taking measurements of the thickness/width of the core, thickness/width of the bamboo, and traced its profile to get the proper curve needed.

 
     
  Materials and Tools Needed  
  Two (2) Flat Bamboo strips, 8' long by 1" wide
Core wood, I used Red Oak to practice with, but ended up using Cherry, 8' long by at least 1 1/4" wide
(core can have anywhere from a single piece to five pieces glued together, I'll explain later)
Two (2) Scrap pieces of the core material, 6" long by at least 1 1/4 " wide, used for the notch areas
Binding Cane, used just above the grip
Fine Cane, used at the ends near the string notches
Hide glue, 2 part Bow Building Glue, or a good woodworking glue (I used hide glue for the build) *UPDATE*  I now used 2 part Bow Building Glue because it is stronger, does not come unglued when wet, yet cleans up with water.
(last four items can be obtained at a craft store, or at a Woodcraft store)
1/4" twine or rope, used to tie up the glued bow blank, Hemp seems to work the best
Enough scrap wood, or more bamboo, to make approximately 100 wedges 6" long by 1" wide
Small hammer to set the wedges
Small hand plane (I used a small Japanese finishing plane), used to shape the finished yumi
Large hand plane (I used a large Japanese plane), used to plane down flat bamboo prior to belt sanding
(Japanese Planes can be obtained at Japan Woodworker)
Belt Sander, used to flatten the flat bamboo and core
 
   
Building Procedure
 
  1. The first thing that needs to be done is to cut out the core pieces.  The core for modern yumi are made up of (5) five pieces, (2) two being bamboo and the other (3) three out of wood.  The first Yumi I built, I used a (2) two piece core just for practice.  The core ended up working just fine and after some research I found that the first core to be used was a single piece of wood, followed by a two piece core, followed by a three piece core (which was either three pieces of wood or tow pieces of wood with a piece of bamboo in between), followed by the modern way of using a five piece core.
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  2. To cut out the core pieces (I will use the three piece all wood version for this build) first select wood that you'd like to use.  I decided to use red oak for my first attempt since it is readily available and relatively cheap to buy.  For the three piece core I used cherry.  When buying wood for the core, choose wood that have a fairly straight grain and seems heavy for it's size, typically a 1" x 1 1/2" x 8'  piece is needed.  You want to find wood that has wide growth rings.
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  3. To construct the core, the core wood needs to be ripped down to thin strips.  To do this, I first ripped the core wood to 1 1/2" wide x 1/2" thick strip.  I placed a sharpie mark on the end of the strip to show the width of the strip. The reason for the mark will be used during glue up.  I then ripped the 1 1/2" down into thirds, so that the finished width would be around 1 1/8" after gluing.
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  4. Next, glue the three strips together, turning the middle strip 90 to the two outside strips.  The reason for the turn is it adds a little more strength to the core, which in turn adds poundage of pull.  Winding the core pieces with cord will help the pieces to stay together while they dry.  I've clamped the whole thing to my workbench with many many clamps.  Set the core aside to dry completely.
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  5. Next I worked on the pieces of bamboo that will make up the back and belly of the Yumi.
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  6. The bamboo that I used on the first Yumi I built was called Chinese Bamboo.  It has a very dark skin to it, but looks like any other bamboo when sliced down into strips.  I've also used 1" wide x 8' long yellow bamboo strips that I found from an online supplier.  Both bamboos are good and work just as nicely as the other.  For a Yumi that looks like all the others, I would suggest using the yellow bamboo.
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  7. Which ever bamboo is used, the strips need to be at least 1" wide when finished.  To finish the bamboo, the inner part of the bamboo needs to be planed down in thickness so that the ends are thinner than the body.  I use a Japanese hand plane to accomplish this, but an electric bench joiner can be used as well.  The overall thickness of the body should be about 1/8" above the skin of the bamboo, and the ends need to be planed down to the skin.  As a side note, the overall thickness of the bamboo can stay the same for the entire length, but the ends will tend to be slow and lazy when shooting.
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  8. One of the bamboo strips needs to be shorter than the other.  The short one will be the back of the Yumi.  The shortness is for the nock blocks that will be added later to the ends of the Yumi.  At this point, put all the pieces together, like its going to be glued up, and mark the side of the Yumi with reference marks.  That way when the ends are cut off the back strip, everything will line up properly again during glue up.  To figure out how much shorter to make the back, place each nock block at each end of the back strip and mark a line. Then cut the end off. 
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  9. Mark one end, of the longer bamboo piece, top and the other end bottom in pencil.
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  10. Once the core has dried, it needs to be tapered along its length and towards the tips.  Meaning, the center should be thicker than the ends.
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  11. Now, the bamboo pieces can be glued to the core.
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  12. Now comes the fun part.  Fun meaning, totally frustrating the first time it is done.  I suggest taking a break, have a cup of coffee, use the bathroom, etc. before continuing on. 
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  13. Start by laying the back, the belly and the core next to one another on a workbench or floor (preferably a garage floor), and coat each piece from tip to tip with glue.  Now, sandwich all the pieces together and, starting at one end, begin winding the cord around the pieces to the other end, and then back again.  When done, there should be "X's" of cord on the back and belly of the Yumi.  If the X's are on the sides, it's done wrong and it needs to be done again.  One good thing is, the glue should not be quick curing so there is time to fix the cord wrapping.  Also, I typically use three squeeze clamps to hold everything together while wrapping.  The wrapping should be tight.
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  14. Now the fun part, adding the wedges. 
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  15. Starting at the bottom end, start inserting wedges into the X's for about 9".  It might take gently tapping each in with a hammer for them to stay.
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  16. The next procedure takes some getting used to, to get the bends right.  If a mistake is made, it is usually done with the next procedures.  Take your time, and if something does not seem right, it probably is not.
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  17. Now, gently bend the bottom end of the Yumi to create the lower nock area and tap in the wedges so the bend stays where you've bent it.  The bend should be a gentle bend of only a couple of inches, no more than 3" from straight.
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  18. Now go up, from the bottom, about 1/3 the length, and insert more wedges for about 9".  This will become the grip area.  Again, gently bend the Yumi a couple of inches from straight and tap in the wedges so the bend stays put.
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  19. Next, go to the top and start inserting wedges for about 18".  Again, gently bend the Yumi a couple of inches from straight, and tap the wedges so the bend stays.
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  20. Now comes an interesting procedure. 
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  21. You will see that there are two areas that do not have any wedges installed.  These two areas are the deflex areas and bend the opposite way of the other three.  I typically do this step after the other three since it evens out the other three bends.
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  22. Gently, bend the Yumi in the opposite direction starting at the top and then the bottom, and then insert wedges into the X's and tap so the bend stays.
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  23. Now set the whole thing aside for a couple of days for the glue to dry.
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References:
www.primitivearcher.com
www.bamboohabitat.com/index.htm
www.woodcraft.com
www.japanwoodworker.com