Multiaxis Turnings

                                                              TRIPLE HOLLOW ECCENTRIC BOWLS

 

1. Planning:       The ratio of eccentric circle diameter to hollowing circle diameter determines shape. The smaller the hollowing circle, the more accentuated the three hole shape; as the ratio gets larger, the pattern becomes less pronounced.

 

1 to .866    If the hollowing  circles are smaller than about 85% of the eccentric circle, the hollows will not intersect at all & you will have three separate hollows.  At a ratio of  1 : 0.866 the hollows just touch (circles tangent).

 

1 : 1          If the eccentric circle is the same size as the hollowing circles (3 to 3) there will be a spike at the center.

(For these, you must not drill a center hole in the open face -  (ash example).

 

                      1  :  1.3     A ratio of 3 : 4  (spalted maple example)

 

1 :1.5        A ratio of 3 to 4.5  (cherry example).

 

1 : 1.7       A ratio of 3 to 5 makes hollows  form a pattern of three semicircles.  (maple example)

 

1 : 3          Walnut bowl is about 3 to 9.

 

2. Sketch           Sketch pattern so it will fit the blank and the lathe.  Swing = blank diameter plus eccentric circle diameter.   The outside arcs can be chosen once hollowing circles are laid out.

 

        3. Sequence   Turning can be done in several ways; all are finished on vacuum chuck or similar         method.

 

                    a.          All eccentric turning with bowl mounted on holes in glue block. This avoids                                                                            drilling any holes in open face.  Holes may interfere with pattern (center                                                                                 hole in a 1 : 1 example),  may  make hollowing more difficult,  or may be too deep                                                                 in   small bowls.

 

                                      b.         Outside arcs turned with bowl mounted on open face; hollow with bowl mounted                                                                  on glue block. (Demo)

 

c.          Outside and bottom turned with bowl mounted on open face; then add glue block; &  hollow with bowl mounted on glue block.  This allows both the outside and the bottom to be eccentric. (Maple example)

 

4. Wood:      Use dry wood; these are difficult to “twice turn.”  Avoid  “chippy” woods: oak, burbinga, osage.

Note any poor wood in the blank that you want to cut away from the edge or hollow out.

A glue block must be thicker than the length of the chucking screw, and its diameter bigger than the sum of eccentric circle and screw chuck diameters.

 

5. Layout:     Careful layout will help in producing a symmetric pattern.  Slightly misplacing the centers or varying sizes of the hollowing circles results in an unbalanced and unattractive result.  Like a natural edge bowl, if it is to be asymmetric, make it very asymmetric.  It is best to have a pattern for the outside and hollowing profiles or make one after you have made the first cut.  

 

a.          Locate and mark center of blank;  draw eccentric circle.

 

b.         Pick a starting point OPPOSITE rim wood to be cut away or ON TOP of central wood to be hollowed out and using same radius, mark out six chords.  Three of these points will be centers of hollowing circles and centers of the outside arcs.

 

                            6. Turning:    Adjust lathe to SLOW speed (300 - 400 rpm.)   Speed can be increased as                                                            unbalanced wood is cut away.

 

a.          Mount blank on one eccentric hole and turn first outside arc.   Drill matching hole in other face.   Remount blank to turn other two outside arcs trying to match the pattern of first arc.   You will be able to see the “shadow” of  previous cuts, but they will be offset inward. (The lines where these arcs join will be straight radial lines if you get it right.  Minor corrections can be made when sanding.)

 

b.         Mount blank on eccentric hole in glue block and cut out first hollow.  Remount blank to cut other two hollows, again trying to match the pattern of first.   (Again, the lines where the hollows join will be straight radial lines if you get it right.  Minor corrections can be made when sanding.)

 

c.          Using whatever method you use when finishing a normal bowl to remount bowl, centered, with open face toward headstock and turn away glue block.  Finish foot and bottom of bowl.  (I usually use a vacuum chuck, with a face plate and gasket.)

 

i.          Sanding is mostly done with the old Armstrong Reciprocating Sander (ie. by hand.)

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