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Lynn Jacobsen turns segmented bowls

Compiled by Dale Dallon

Lynn was introduced to woodturning four years ago by Dave Anthony, another member of Timp Woodturners Association. He currently uses a Nova DVR lathe and typically turns his pieces at speeds in the range of 500-1050 rpm.

Segmented bowls are laid up as successive rings. Each ring is comprised of a number of truncated-wedge shaped segments cut from thoroughly dried stock. The secret to successful segmented work is that the segments in each ring must mate with one another perfectly with no visible glue gaps or sanding burn lines. This demands great care and precision in cutting and sanding each segment to generate precise dimensions and angles with smooth facets. Lynn attributes his success in this effort to a good compound miter saw (7½" Makita) and a good disk sander with a table slide and appropriate home-made jigs for presenting the cut segments to the face of the sanding disk precisely and repeatably. A different jig is needed for each face angle and the face angle is dependent on the number of segments in the ring. He clamps glued segments into a ring with a large screw-driven hose clamp. Lynn recommends the book Wood Turning With Ray Allen by Dale Nish as a source of formulæ for calculating angles and dimensions for bowl layouts. Kurt Theobald's video is also recommended.

The basic formula Lynn uses for calculating the length of the outside facet of each segment is:

L   =     ( D + 0.25 ) × 3.142
N

where...
D = ring diameter in inches
L = length of outer facet of each segment
N = number of segments in each ring
 

Each segment is cut to the appropriate angles and facet length with the compound miter saw allowing a little margin for sanding. During the cutting, Lynn uses an L-shaped rod to hold the piece down while keeping fingers well away from the blade. Each edge to be glued is sanded carefully on the disk sander using the slide and the jig appropriate to the finished angle desired. Care must be taken not to burn the edges on the sander. Lynn generally uses 80 grit disks. The finished segments are then laid out in a dry-fit ring and clamped with a hose clamp to check for gaps or flaws.

The perfect dry ring is then disassembled and the segments glued together with carpenter's glue such as Tite Bond II™. Lynn prefers to create subassemblies of just two segments, then glue those subassemblies together in pairs to create two semi-circular half rings, then glue the two half rings together to complete the circle. The glued circle is then clamped in a hose clamp and excess glue wiped off with a damp cloth. The clamped ring can then be laid on a flat surface and the segments tapped down to ensure a flat and true bottom.

An alternative to gluing the segments in pairs is to secure a long piece of masking tape to the work bench adhesive side up then lay the dry segments edge to edge and outside-facet down on the adhesive along the length of tape. Glue can then be applied to the mating edges and the tape drawn around to close and secure the circle. The taped ring is then placed in a hose clamp for drying as described above.

When creating an accent ring, one containing contrasting patterns, special gang cutting techniques are helpful, if not essential. This involves laying up strips of contrasting woods into a rectangular bar such that the cross section of the bar presents the desired pattern. Segments of the desired thickness are then created by slicing across the bar. Those slices can then be sanded to the appropriate angles to create ring segments. The ring is then laid up with the desired sequence of patterned and un-patterned segments to create the intended design. A drawn plan is vital to this process.

The upper and lower faces of dried rings can be smoothed with a drum sander if available or by using an over-sized sanding disk. Lynn has created a sanding disk by attaching a faceplate to a large wood disk covered with sand paper. The sanding disk is mounted on the lathe spindle and the glued-ring face held flat against the turning disk.

When all rings have been prepared, assembly of the vessel can begin. Lynn lays the vessel up, and turns it in two parts, the bottom half and the top half. Each half is built beginning with a waste block that can be securely attached to a face plate or a scroll chuck. The bottom half is laid up on the waste block working from the bottom up and the top half is laid up on the waste block from the top down. Lynn uses a solid blank as the bottom of the vessel so it is the first piece to be glued to the waste block. Successive rings can then be added sequentially according to the design plan. The faceplate, or chuck, is mounted on the lathe spindle. The face of the base blank can be trued up and the first ring glued to it. That ring can be pressed against base blank with the tail stock. Lynn uses a special device sold by Kurt Theobald. It consists of a Morse tapered live center with a threaded adapter to receive either a face-plate/chuck or a large, shallow cone center. The large cone center serves well to apply pressure on a glued ring against preceding rings or the base blank. Once the freshly glued ring sets enough to be stable, Lynn removes the faceplate/chuck and its ring assembly from the lathe and places the entire assembly in a home-made vertical screw press to dry. When the glue has dried, the assembly is remounted on the lathe and the inside of the ring is turned down just enough to remove the corners. The exposed face of ring is then sanded to receive the next ring. Lynn does this with a flat rigid board covered with sand paper held against the turning face. This sequence is followed for each successive ring until the bottom half of the vessel is completely assembled. The bottom half can then be turned to near-finished profile both outside and in.

Assembly of the top half of the vessel proceeds in the same manner. The first piece to be glued to the waste block would be the lip ring of the vessel, either segmented or solid depending on the design. Using the same procedure described above, the rings are added until the upper half is fully assembled. It is then turned to near-finish profile.

With both halves finished the vessel is now ready for mating the halves together. By using the Theobald tailstock device it is possible to have both halves mounted on the lathe simultaneously, one half on the headstock and one half on the tailstock. With the mating faces trued and sanded, they can be pressed together on the lathe and the mating horizons turned to match one another. The halves can then be glued together. After drying, the exterior can be turned and sanded to final finish.

The top waste block is then turned off and the lip turned to finish. The bottom waste block can then be parted off. The final portion of the parting is best done with a saw.

To finish turning the bottom of the vessel, Lynn uses a mandrel assembly consisting of a rigid shaft with a mushroom block mounted on the outer end and centering cone mounted on the upper end. The mushroom block is faced with double-stick tape to contact the bottom of the interior of the vessel and the centering cone is faced with a foam gripping mat to contact the interior edge of the vessel lip. The centering cone is mounted on a bushing with a set screw so the position of the cone can be adjusted. The entire mandrel assembly mounts to the spindle via a Morse taper. The vessel is slid onto the mandrel so the bottom exterior can be freely turned and sanded. A jamb chuck can be used in the lip of the vessel in lieu of the mandrel.


 

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