Monday 9 July 2018
I recently watched Sarah Thirlwell demonstrate mixing plastics, composites and wood together to form unusual turned objects and was fascinated by the idea of using the mixed media technique.
In this project, brown tortoiseshell plastic made from acrylic resin has been used as a feature ring in the bud vase which is made from a South American variety of lignum vitae (Bulnesia Sarmientoi). A plastic bud-vase insert has been fitted in the top of the vase to enable real flowers and water to be used, to make the vase more functional.
Glues are a problem when mixing media like this. Some adhesives are ideal for gluing plastics together and others for gluing wood together; but most of these are not suitable for gluing plastic to wood. The problem here is that wood moves and plastics don't. After seeking expert advice, a two-part epoxy resin glue was used to glue the plastic segment rings together and Polyurethane Glue (PU) was used to glue segment rings to the wood.
Tools used: Bowl gouge, fingernail spindle gouge, teardrop scraper, negative-rake scraper and thin parting tool
The tortoiseshell plastic was supplied as four pieces of material, which were originally designed for pen turning. To transform them into segments, square the ends on a disc sander and glue together to form a long bar. Use an F-clamp to hold the glued pieces together squarely and flat
Take care to match the grain pattern – the tortoiseshell has a layered pattern which needs to be utilised in the final design. Machine the bar down to the correct thickness for segmenting – 15mm (5/8in). Ideally a drum sander should be used here. Assemble six segments of scrap wood without glue to check the mitre angle using a straight edge. Once the angle is accurate, cut the glued length of plastic into segments using the bandsaw…
…and the faces need to be cleaned up on the disc sander. Care must be taken to not include the square joints that were made to join the pieces together in the segments, as these would be seen
Since the glue used for plastics does not grab as Titebond II does, use a different technique to glue the segments together. The technique was demonstrated to me by the eminent segmenter Charles Faucher at the AAW Symposium in Albuquerque, 2009. He used a waxed string to keep the ring of segments taught anchored by two pegs inserted in the bench top. Small dowels are placed between the two rings to allow for differences in the mitre angle. The two tapered pegs are then turned to tighten up the string
Glue the segments using a two-part epoxy glue and leave for eight hours until the glue is cured. When dry, these segments need to be faced on the disc sander and glued together as circular rings
After eight hours, the glue will have cured and the rings then faced on the drum sander before you glue them to the wood
The unusual piece of wood used for this project had a dark brown centre and a light cream outer sapwood. Lignum vitae is the hardest and heaviest of woods. It turns well, but the tools have to be sharp. Before starting, square both ends off on the lathe and glue dummy wooden faceplates onto each end using a paper joint, for mounting the blocks in the chuck. This avoids wasting any valuable material during parting off. Divide the block into two pieces – about one-third of the length – and part off with a fine parting tool, before finishing with a handsaw on the lathe
Lignum vitae is very heavy so each end of the block of wood then needs to be hollowed out and the top end drilled through with a 15mm (5/8in) Forstner drill-bit to take the plastic vase insert. The plastic splinters with the cut from a bowl gouge, so use a shear scraper instead. Use a negative-rake scraper to obtain the final finish
The faces then need to be squared off to take the plastic feature ring
The plastic rings are then glued to each part of the block using PU glue – be careful to wear vinyl gloves to avoid the glue getting on your hands – it's difficult to get off afterwards. Position each ring centrally using a custom-made MDF backplate supported by the tailstock. Keep the rings in compression for a couple of hours until the PU adhesive has set. Then leave for a further six hours to ensure the glue cures completely
You then need to bring the inside of the feature ring down to the internal size, using a shear scraper. The final finish is achieved using a negative-rake scraper
Sand the inside of the plastic feature ring and finally polish using a Carnauba wax stick. The outsides are not shaped at this stage – this is to ensure maximum rigidity for gluing up and final finishing
Square the faces off and sand flat ready to be glued together using a 120 grit sanding sheet sandwiched between the ring and the backplate of the tailstock. Once the two faces are square and flat they are glued together on the lathe supported by the tailstock, using the epoxy two-part glue
Take care when lining up the segments brick-work fashion. Do not apply too much pressure otherwise the glue is forced out of the joint. Leave two hours for the glue to set before removing from the lathe – they need a further six hours for the glue to fully cure
The composite is now complete and the outside can be turned to the finished design. Remove the faceplate from the top by splitting away the paper joint with the edge of a knife
It is now easy to support the base in the chuck using the faceplate on the headstock and the top in a wooden jam-chuck positioned on the tailstock. Rough shape the piece with a spindle roughing gouge…
…then refine the shape with a fingernail -profile gouge and finish the shaping off using a negative-rake scraper. The negative-rake scraper calms the cutting process down and on very dense, close grained timbers like lignum the finish produced off a honed edge can be superb
Bring the plastic rings down to size using a shear scraper and finally finish using a negative-rake scraper. Leave the base in its basic shape until removed from the faceplate and reverse-chucked. Remove the vase from the lathe and split the vase from the base using a knife edge in the paper-joint
Reverse chuck the vase with the neck supported on a jam chuck at the headstock and the base supported by a small steb centre chuck on the tailstock. It takes a few goes at positioning the base to ensure it is centred. The steb centre hardly marks the base. Now, turn the lower section to the finished shape of the bottom of the vase. Slightly hollow the base to ensure it sits correctly. The pip around the steb centre will be removed after finishing. The next step is to sand the vase using a rotary sander, from 120 to 400 grit. Lignum vitae is not easy to sand and several attempts may need to be made before all the sanding marks are eliminated. Give the vase a coat of sanding sealer – this is to raise the grain to allow for the final finishing. Once the sanding sealer has dried, after about two hours, reposition the vase in the chuck, sand with 400 grit and then fine Webrax to give a silk sheen. Remove the vase from the lathe and cut away the pip in the centre of the base with a chisel, before sanding the base. Finish the vase with three coats of Chestnut acrylic lacquer – silk finish – allowing four hours between each coat and polish with fine Webrax between coats