Tuesday 10 March 2009
Rob Stoakley makes this distinctive piece with secret mitred dovetails
This casket uses secret mitred dovetails but in addition, I wanted to make a piece with a sculpted feel to it - something quite tactile with soft, smooth contours that would be pleasant to hold and to open.
I put quite a lot of thought into the design but ultimately ended up with a straightforward rectangular box on a plinth where the lid conformed exactly to the Golden Ratio (1:1.618). Making the front face conform to the same ratio made the casket look too heavy so the depth was reduced to a more practical distance. The top part of the casket would be sculpted with a delicate handle inset and contoured to fit. There would be a rectangular panel for the lid with a concave moulding around the outside.
TimberI had a well figured board of teak (Tectona grandis) and decided to use this for the main construction, and I also found a small piece with a pronounced grain that I could use for the top panel. The plinth would be made from ebony (Diospyros ebenum) to give an interesting colour contrast whilst the lining would be made from some English walnut (Juglans regia).
Secret mitre dovetailsSecret mitre dovetails have an almost mystical reputation as being difficult to cut accurately. This is very true, but not quite so difficult if certain basic, fundamental workshop practices are strictly adhered to. I use the Techniques of Furniture Making, by Ernest Joyce as a standard reference and this does in fact contain a very good piece on making this joint.
Begin by preparing the material to size, ensuring that an allowance is included for the lid and making sure that the ends on each are shot absolutely dead square. In my workshop I use my old wooden jack plane with a deeply curved blade which makes short work of the stock removal, with the final shavings removed with a Norris fore plane down to the required dimension.
The next task is to mark the shoulder lines and the corner mitres, which again need to be very accurately done. These are the datum lines that edge tools will eventually work to, so any slight discrepancy on these knife lines will result in a joint that doesn't quite pull together as well as it ought to. The rebate is machined next on the router table and the corner of the rebate needs to just touch the mitre at both sides of the piece. General practice is to leave about a quarter of the thickness of the wood for the mitre but on this casket, the sides are sculpted, so be a little more generous with the timber here as much of it will be removed in the shaping process. Whilst still on the router table, a 4mm (5/32in) groove can be made for the lid and also a rebate, slightly over deep, for the bottom.
Decide next where the lid will be sawn off and use a pencil gauge on the outside to mark this, and then transfer the lines to the inside of the material. I used an allowance of about 5mm (3/16in) for the saw cut and it's imperative that these marks are always visible as when the joint goes together, it could become a trifle embarrassing later on if you don't know where to saw off the lid!
PinsOnce all these preliminary processes have been done, the actual joint can start to be marked out. On this type of joint the pins must be marked out first, as it is impossible to cut the tails and then mark and make the pins, as is the normal practice. Some considerable attention needs to be paid at this point to make sure that pins are located in the correct place, not forgetting that a mitre needs to be included where the lid is separated, so constant reference needs to be made to the text to ensure that everything is in the correct place. I now use a thin black Bic biro for marking out as I find it easier to see the lines compared to a 4H pencil. The pins are chopped out in the normal way and then the tails are marked from the pins using a sharp awl to mark the timber. This is a little awkward to do but is easy enough with some care and once done, the tails are again chopped out carefully.
MitresMitres can be chiselled at the corners and lid joint freehand but it's much easier to make a wide block from an oddment of hardwood that has been very accurately planed to 45 degrees across the end. If the block is left at about 40mm (1 5/8in) thick this will give a very wide surface to rest the chisel on and accurate mitres ought to be the result. Most of the waste for the mitres can be removed with a skew chisel before the same block is cramped to a casket side and used as a guide for the shoulder plane when the main part of the mitre is produced. Great care needs to be exercised here as the intention is to produce a flat and straight mitre that will link up with the three shorter mitres, already cut to form one continuous surface with a knife edge at the corner.
Once the joints have been cut and the mitres planed, they can be dry fitted together. It is normal practice with through dovetails to partially fit once to ensure that they will pull up acceptably, however this is not the case with a secret mitred dovetail. Unless you are extraordinarily confident in your joint cutting abilities and are supremely accurate with hand tools, these joints always seem to need a little bit of adjustment here and there to bring the mitres together. Mine were no exception and needed two or three careful observational and trimming sessions before they would close satisfactorily, after which the pieces are separated again.
The moulding around the top of the casket is done by removing most of the waste with passes from a router and then completing the profile with a scratch stock and cutter ground from a piece of HSS machine hack-saw blade. The initial shaping of the corners can now be done with a small gouge before the casket is glued together.
Top panelThis is a panel with a simple rebate all round. Ensure that it is a comfortable fit in the grooves on all sides and that there is just enough clearance in the width to allow for any movement. The panel is then polished on all surfaces, masking off a tiny piece of the rebate on the short end for gluing later.
Gluing togetherThe inside of the lid is polished and waxed prior to gluing together as this cannot be done when the lid is separated. Gluing a piece is always a fraught time in the workshop. I try and approach this in a methodical, reasonably organised way.
Teak presents some difficulty in gluing owing to its greasy nature - it's as if some malignant little workshop gnome had deliberately rubbed candle wax into it! However, it can be degreased quite well, immediately prior to gluing, with acetone scrubbed onto the surface with a stiff brush.
I decided not to use my normal PVA glue due to its short open-time and as the casket was glued in summer, I felt on this occasion, something with a much longer open-time was called for, so I opted for the original slow-setting Araldite which proved more than adequate, though somewhat expensive if a larger project were contemplated. Leave the glue to set for a full 24 hours and then clean off the now hardened glue on the inside of the box with a sharp blade.
The base can then be fitted and glued in. This is a piece of 6mm (1/4in) ply veneered in teak on the outside, and English walnut on the inside. Before gluing in place, the inside needs to be sanded and polished, as it is impossible to finish once it's glued into the rebate. A very small rebate is at this stage, machined onto the bottom of the casket to act as a shadow gap when the plinth is fitted.
lidThe lid is separated from the box using a tenon saw cutting down the router groove, reasonably easy to do providing some care is taken - much easier with a nice big bandsaw (which I unfortunately don't possess), and then the saw cut is cleaned up with a large hand plane so that a perfect fit results.
The lid is hinged to the box in the normal way initially, using steel screws.
I never seem to be able to exactly mate up the two parts after hinging, so some further work is needed with the curved sole plane, scraper and sandpaper to refine the shape. The corners of the casket can then be shaped with a file and sandpaper.
Hsndle detailDovetail and glue into the lid a block of teak which is then carved, scraped and sanded to a flowing teardrop shape. The top and lower surfaces of the handle are gently scalloped to form two concave depressions so that the finished effect is very tactile - the complete handle appears to be an integral part of the casket and not something added later.
PlinthThe material for the plinth is prepared 2mm (5/64in) oversize in length to allow for the curvature of the sides. The corners are mitred and internal slots are cut on the router table to accept some 4mm (5/32in) birch ply wafers. Internal surfaces are skimmed with a finely set smoother, the plinth is then glued together and cleaned up when set.
Make suitable templates for the shaping on the underside of the plinth and then construct two jigs to machine to shape. Ebony is a valuable material so any that can be saved in the solid rather than disappear as sawdust is worth keeping. With this in mind, I cut away most of the waste with a jigsaw to leave just a skim with a bottom bearing cutter in the router.
If all has gone to plan, the mitres on the plinth should line up exactly with the casket mitres and once this has been achieved, the two are temporarily cramped together and then steel screws used initially to join both together.
The plinth is then attached to the casket by counterbored screws in each corner and one countersunk screw on each long side. The curved sole plane can be used now to shape the lower half of the box so that the shaping on the plinth flows smoothly into the sides of the casket, which is then scraped and sanded to profile. The plinth is then removed and the curved shaping on the underside cleaned up and sanded through to 320grit. It's critical at this stage that some sort of location mark is used to indicate which corner of the plinth fits the relevant corner of the casket, as it's very easy to forget which way round it fits!
LiningThe lining is made from some English walnut. The long sides are shot exactly to the inside of the box and the small mitre is cut using the 45 degrees support block. The top profile is produced with a block plane and then sanded to its final shape. Polish and glue in place, not forgetting to degrease the teak interior just before the glue is applied.
The short sides are made in a similar way and are planed to exactly fit the width of the casket at each end before jointing and fitting.
Once all four pieces are in place, some minor attention with sandpaper may be needed to the profile at each corner to ensure that there are no little 'steps'
between adjacent sides.
FinishingIn a project such as this, certain parts need to be finished before assembly, the main one being the inside of the lid and panel just before gluing, as they cannot be done as an afterthought. In the same way, the lining and base must be given a suitable finish before they are glued.
Teak is a very easy timber to finish and will accept oil readily. I use three coats of Liberon Finishing Oil, leaving overnight between each coat - this is then cut back with some Alna Teak Wax and grey webrax pad to leave a lustrous finish when polished with a soft duster.
Both the brass hinges and all screws are mirror polished before the final assembly of the lid and plinth, replacing steel screws with their brass equivalents.