Monday, October 24, 2011

Twisted box how-to

Laying out and numbering the centers for a 3 sided multi-axis pieces with a 120 degree twist.  The 2 diagrams represent opposite ends of the same blank.  Find and mark the center of each end of the blank.  Draw a line diagonally from corner to corner on each end of the blank.  This line should pass through the center mark and should be parallel on both ends.  This line is crucial as it will give us 1 of our 3 centerpoints and will give us an intermediate point that will allow us to find the other 2 points easily.  Using a compass draw a circle using the true center point as the pivot point.  For a 3" square blank I use a 1" diameter circle (1/2" radius).  Using a larger circle is possible but it will result in a slightly different shape (flatter sides) but will make it easier for the center points to be turned off by accident.  I always make the circles the same size on both ends.  I also do all of the layout and numbering before any cuts have been made. 
The diagonal line will intersect the circle at 2 points, once in the upper left, and again in the lower right.  When doing a lidded box I always number the top first and always number the point where the line intersects the circle in the upper left as point #1.  The intersection in the lower right will be used to find points 2 and 3.  Using a compass set to the radius as the original circle, place the pivot point on the intermediate point in the lower right corner.  Draw an arc that intersects the circle at 2 points and also passes through the original center point.  The 2 points where this new arc intersects the circle are the other 2 points that we need.

If we've done the layout correctly we should have 3 points on each end that are properly aligned to one another and are 120 degrees apart.  The next step is numbering the blank.  I always number the blanks in the same order and using the same method.  The top side is always side A and is always numbered first.

The intersection of the circle and the diagonal line in the upper left hand corner (opposite the intermediate point used to find points 2 and 3) is always labeled as point #1.  The remaining points on Side A are then always labeled going clockwise from point #1. I always number the points to the insde (interior) of the blank.  Numbering on the outside increases the chances that the numbers will be turned off.  This is an issue that can be overcame if it happens but it is easily avoidable by simply numbering on the inside of the circle.  

The layout for Side B can then be completed.  Point 1 should not be parallel to point 1 on Side A.  The other 2 points are then numbered going in a counterclockwise direction.  The direction of the twist can be controlled by putting point 1 opposite point 2 or opposite point 3 (I never taken the time to figure out which direction is which). 

After the layout is complete I start the turnin process.  Step 1 is to mount the blank between the true center points and turn a tenon on each end.  The lid portion can then be parted off (or cut off on a bandsaw, another reason to leave the blank square) and both halves hollowed, a tenon and recess formed to join the lid and body.  To turn the twisted exterior it's simply a matter of lining up point 1 on Side A with point 1 on Side B and turn that face.  Then line up points 2 and 2 and turn the face, and then points 3 and 3 and turn that face.  I usually end up turning each face 3 or 4 times to get the wall thickness I want and to get all the faces even.

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