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Polychromatic Bowl
Thursday 10 February 2011
Dennis Keeling creates this unusual polychromatic offset bowl in plywood and Perspex
I attended one of Sarah Thirlwell's mixed recycled media demonstrations and was taken in by the stunning effects that could be obtained by integrating coloured Perspex (Plexiglas) into woodturning. It seemed straightforward but she was a bit cagey about the choice of glues.
Perspex is a hard plastic material - acrylic - that is supplied in flat sheets. It cuts well on a circular saw and bandsaw, but does not plane. Negative-rake scrapers and tungsten-tipped tools work well.
The equipment used for the project includes a circular saw, bandsaw and of course a lathe, as well as the turning tools pictured opposite.
Tools used: Tungsten-tipped shear scraper, 20mm (3/4in) negative-rake round scraper, negative-rake flat scraper, internal teardrop shear scraper, miniature parting off tool and conventional 25mm (1in) parting tool
Materials
Step 1
I had some old 12mm (1/2in) birch plywood and some Perspex off-cuts from a local display company and thought I would have a go. My first attempt was not a success. I had used quite a lot of clear Perspex and that showed up the many air bubbles in the glue and imperfections in the plywood. The plywood that I used was also not suitable as it had splintered badly at the feathered edges. The glue that I had used - polyurethane - had not stuck to the Perspex and the bowl collapsed under the turning load. I saw a 15kg pack of 3-7mm mixed coloured sheets of A4 sized Perspex for £10 on eBay, so I bought it and decided to have another go. I decided to invest in a sheet of good quality 18mm (3/4in) birch plywood and again I was not disappointed. But what about the glue? I searched the Internet and found a local glue guru, Alan Brown from Adhesive Brokers, who prescribed AB 121, a strong flexible epoxy resin glue that would glue Perspex to wood
Design
Step 2
I used the offset bowl design that I had earlier developed for my popular Corian pieces. You need to build a cube up and then cut diagonally to create two offset bowls. Cut squares of 21mm (8 1/4in) plywood and Perspex on the circular saw with a fine tungsten-tipped blade and gradually build up a cube of Perspex and plywood. Put Perspex on the top and bottom - which will be the two circles on the inside and outside - but be aware that plywood does not feather easily. You don't necessarily have to have a plan for this bowl. The bowl I made was a 210mm (8 1/4in) cube. Draw a 150mm (6in) circle on one face to show the internal diameter of the bowls
Step 3
You will use this to determine the centre sections to be cut away before assembly. It's easier to cut away the waste now, before gluing up, than trying to remove the waste later. These centre segments can be saved for future projects. Next, individually mount the centre sections in a large button jaw chuck and cut out using a small parting tool. The large button jaw chuck will take both square and round work
Step 4
Be careful when cutting the plywood with the parting tool, as it splinters. It's better to cut halfway through and then turn the square round and cut the other half from the back
Step 5
Stack the levels up to ensure the cut-outs match the round internal shape
Gluing Perspex
Step 6
Perspex will accept instant glues - such as Cyanoacrylate adhesives - but unfortunately you can see the join line. Bostik makes a Perspex adhesive for gluing Perspex sheets together, Tensol 12; other than the occasional air bubble, it is completely invisible but it will not glue Perspex to wood. Cyanoacrylate glues are brittle and are not really suitable for joining a hard material to wood. The flexible two-part glue prescribed by Alan Brown, AB 121, is flexible and very strong in use. One part is coloured blue and the other red, but when mixed together they turn into a light grey film
Step 7
Mix the two-part glue together using a palette knife, making sure there is plenty of glue on the surfaces. Apply the Tensol to both sides of the Perspex surfaces
Step 8
The Perspex sheet has a vinyl protective covering which has to be removed before gluing. Whilst you should experience no problems with the Tensol joints, when I made my bowl a few of the Perspex-to-wood joints failed even with AB 121. I then remembered what the plasterers used to say: you have to have a key for the plaster to stick. In this case it is a good idea to sandpaper the Perspex and reapply the glue
Step 9
Be careful when using these glues - always wear protective gloves and have adequate ventilation - as they give off irritating fumes. When the glue has cured - after about four hours - remove it from the jig and cut the cube diagonally on the bandsaw. Remove as much of the waste material before mounting on the lathe. Mark the rough shape on the outer rim of the bowl with pencil and cut off the corners on the bandsaw. Take slow cuts, but don't push too hard as it will overheat the tool
Step 10
Clean up the face and glue to a square of 18mm (3/4in) MDF, which in turn will be glued to a wooden faceplate on the chuck. You will also need to glue a wooden faceplate onto the base of the bowl so that it can be reversed later
Step 11
Using the tungsten-tipped tool, remove the sharp corners from the sides of the block and begin to shape the base of the bowl
Step 12
Ensure to wear gloves and a visor for this step as the cuttings that come off are very sharp. Perspex is non-toxic at normal temperatures so normal dust extraction will work effectively here. Now you can gradually begin to form the finished outside shape
Step 13
Reverse the bowl on the lathe to prepare to cut out the inside. Firstly, remove the MDF faceplate with a parting tool and then use a tungsten-tipped shear scraper to start removing the sharp edges form the inside
Step 14
Use a teardrop shear scraper to refine the shape and eliminate the tool marks
Step 15
As the shape gets closer to the finished design, you need to test it with a simple plastic template made to fit the inner radius. With an offset design, the inside has to be quite precisely cut otherwise inaccuracies will show up in the circular features of the bowl
Step 16
Now you have to reverse the bowl on the lathe to support the rim while the outside is brought down to the finished shape. It was at this stage that I had two major disasters as the Perspex-to-wood joints failed and the bowl broke up into flying boomerangs around my workshop. To eliminate the expansion strain on the bowl, cut a ridge in the MDF to hold the re-glued bowl centrally but do not apply any lateral stress on it
Step 17
Rough the outside to almost the finished shape with the tungsten-tipped ring tool then use a negative-rake flat scraper to even out the ridges
Step 18
Take the bowl off the lathe and check the wall thickness - fingers are more accurate than callipers here. When the inside shape has been finalised, remount it on its base and start sanding. I recommend sanding with Mirka Abranet discs - these have a coarse texture which allows the dust to escape without clogging the sanding disc. Start at 80 and work up to 400 grit. Very often I have to go back a few grits to eliminate any scratches
Step 19
Once the inside is finished, the base needs to be parted off. It is then remounted on the MDF jam chuck and the base centred on the tailstock using a small steb-centre, which will not mark the base
