Open Segment Involuted Table Lamp
Friday 27 January 2012
Sue Harker uses open segmenting and involuting to produce this table lamp from sycamore and purpleheart
For some time now I've wanted to bring together open segmented turning and involuted turning. I have already designed and turned a lamp where an open segment 'bulb' is sat in an open-ended involuted section, which can be viewed on my website.
For this lamp I thicknessed and squared up all the required timber as I would for any involuted project. Instead of gluing the four solid blocks together with the newspaper and Titebond glue joint, I cut the timber into slices and reconstructed them with open segments using Titebond Moulding & Trim wood glue, which sets fast and dries clear. For the positioning and gluing of the open segments, I designed a simple homemade jig.
I had to glue a purpleheart and sycamore segment to a slice of sycamore one at a time so I made two jigs to speed up the process.
With all the segmented sections glued I could then construct the four identical blocks that would form the involuted section. To the top and bottom of these blocks I glued 63mm (2 1/2in) long blocks of sacrificial sycamore. I glued the four blocks together with newspaper and Titebond Original glue and allowed to cure for 24 hours.
The first profile was quite straightforward, but I found that the second was a little tricky. I reinforced the weakest points with some turned dowel cut in half lengthwise, screwed them into the end blocks and secured them to the first row of segments with Cyanoacrylate adhesive - this enabled me to safely remove the bulk of waste timber from the profile.
This project is definitely not for the faint hearted. If you do decide to have a go, take care and enjoy the experience.
Tools used: 6mm (1/4in) parting tool, 12mm (1/2in) fingernail-profile spindle gouge and 10mm (3/8in) standard-grind bowl gouge
Prepare the timber so all pieces are totally square and of the same dimensions. For the segments, cut a length of purpleheart and a length of sycamore 75 x 75 x 305mm (3 x 3 x 12in) in half lengthways on a bandsaw with the bed tilted at 45 degrees
Set up a chop saw and ensure the blade and bed are totally square. Attach a piece of timber as a stop at a distance of 8mm (5/16in) from the blade. Cut 36 x 8mm (5/16in) thick sycamore segments; 36 x 8mm (5/16in) thick purpleheart segments; 32 x 8mm (5/16in) thick sycamore squares and 8 x 12mm (1/2in) thickness purpleheart squares, and for the end pieces four sycamore blocks that measure approximately 63-75mm (2 1/2-3in) long
Make a gluing jig and glue one purpleheart and one sycamore segment to a square slice of sycamore, using suitable glue
Clamp the timber ensuring the square slice of sycamore is butted up to the edges of the jig and the segments are pushed in as far as possible. Leave clamped for 30 minutes then remove any excess glue. Glue the remaining segments to the remaining 8mm (5/16in) thick sycamore squares and four of the 12mm (1/2in) thick purpleheart squares, alternating the segments to form the pattern required
When all the segments have been glued construct the four blocks for the involuted section by gluing together one 75 x 75 x 63mm (3 x 3 x 2 1/2in) block of sycamore to a 12mm (1/2in) thickness slice of purpleheart with a segment row attached: eight sycamore segmented sections, another 12mm (1/2in) thickness purpleheart slice and another 75 x 75 x 63mm (3 x 3 x 2 1/2in) sycamore block. These four blocks should be the same length. Glue them together using Titebond Original and strips of newspaper. The end sycamore blocks will be glued together on the final stage without newspaper so they need to be free from glue
Number the blocks and draw a diamond shape around the centre. This will help with the positioning of the pieces for the second and third stages. Using offcuts of plywood, clamp the blocks together ensuring everything is as square as possible. Leave under clamped pressure for at least 30 minutes
When the glue has cured cut two pieces of plywood the exact size of the ends. Drill pilot holes and screw the plywood to the top and bottom
Mark the centres and mount on the lathe. The top and bottom blocks of sycamore need to be left in the square. Position the toolrest so these square sections cannot knock your hands. Using a standard-grind bowl gouge, gently turn the centre section to round and start to form the shape. Use a 6mm (1/4in) parting tool to reduce the diameter of the 12mm (1/2in) thick purpleheart at the top and bottom to the required size. Refine the shape using a 12mm (1/2in) fingernail-profile spindle gouge. Check for any gaps in the glue joint and fill with CA and sycamore dust before sanding. Apply two coats of sanding sealer and remove from the lathe
Using a pillar drill and a Forstner bit, drill a hole in the top and bottom of the timber to a depth of 75-90mm (3-3 1/2in). This will be used to insert the centre cable holder
Remove the plywood ends then use a chisel and hammer to part the paper glue joints
Rotate each piece 180 degrees until the lines that formed the diamond are on the outer edges and the outside shape is facing inwards
Using Titebond Original and newspaper, glue the four pieces together and clamp
Whilst waiting for the glue joints to cure, mount a piece of purpleheart that measures 25 x 25 x 200mm (1 x 1 x 8in) long between centres and turn into a cylinder 180-200mm (7-8in) in diameter. Using a 10mm (3/8in) bead former, cut a series of beads leaving approximately 25mm (1in) at the top and bottom for gluing into the 22mm (7/8in) hole drilled through the centre of the involuted section. Using a long hole boring auger, you need to bore a hole down the centre to receive the cable
Re-attach the plywood end blocks to the involuted section - fresh holes will need to be pre-drilled. Turn a sacrificial chucking spigot and glue it to the bottom piece of plywood. To add extra support and strength to the weakest points of the partially turned glued blocks, secure some 22mm (7/8in) diameter turned dowel which have been cut in half lengthwise. Use small screws to secure them into the solid wood and glue onto the segmented sections using CA adhesive
Mount on the lathe using the sacrificial chucking spigot and bring up the taildrive for support. Very gently form the shape to match that of the outside. Keep stopping the lathe to check for gaps in the glue joints. Fill with CA and wood dust if needed. When an even wall thickness and, more importantly, even width of spines is achieved, sand by hand through the grits and apply two coats of sanding sealer
Remove the plywood end plates and with extreme care, part the four blocks. The pieces are now at their most fragile
Rotate each piece to the starting position, with the diamond drawn in the middle and glue, this time without newspaper. You can glue in the cable holding centre at the same time
When the glue is dry it should look something like this
Attach the end plates again and mount on the lathe. This time secure the screws towards the centre to enable you to remove as much of the end blocks as possible
Mount on the lathe, wrap masking tape and/or duct tape around the involuted section for support and turn the end blocks into a cylinder. Mark with black marker pen the depth of the screws and using a 12mm (1/2in) fingernail-profile spindle gouge, form curves at the top and bottom. These will flow into the top piece and base. The shaping should finish at the end of the 12mm (1/2in) thick purpleheart sections. Cut a spigot approximately 50mm (2in) diameter at the base end and 38mm (1 1/2in) diameter at the top. Sand, seal and apply a finish, with the lathe stationary, before carefully sawing off the excess timber
For the top section mount a piece of sycamore 75 x 75 x 75mm (3 x 3 x 3in) on the lathe and drill a 38mm (1 1/2in) diameter hole to receive the involuted section. Drill a 10mm (3/8in) hole the full depth to receive the cable. Turn the required shape and try for fit before sanding, sealing and applying a finish
The base is turned from a piece of sycamore measuring 140 x 75mm (5 1/2 x 3in) thick. Drill a Forstner bit hole the correct size for your chuck and mount on the lathe. Drill a 50mm (2in) diameter hole to receive the spigot on the bottom of the involuted section and roughly turn to the required shape
Using a precision drilling jig mounted in your toolpost, drill a 6mm (1/4in) hole, at an angle, towards the centre of the base. This should break through into the hole already drilled down the middle. Take a refining cut to remove any torn out fibres caused by the drill. Try for fit, then sand, seal and apply a finish. The pieces are now ready to fit together. Thread the cord through the base, up the centre section and through the top of the lamp. Attach to a brass lamp fitting and secure this to the top of the lamp with brass screws. Finally, you can now glue the top and base to the involuted section