Thursday 16 October 2008
Paul Richardson makes a table that conceals its ability to extend
The phrase: 'necessity is the mother of invention' is never truer than when designing and making furniture. I would go further, and say that it is most difficult to come up with an idea when there are no constraints of necessity to be worked within; an entirely open brief is the hardest to fulfil.
Having said this, sometimes a client can ask for a piece which, on first consideration, can't be made, due to apparent contradictions in its form, or appearance, and function - the job it has to do. The ensuing process of research and problem-solving can be frustrating, but will also teach us a lot about furniture design and construction - more, certainly, than if we simply knock out another version of a piece that we already know how to make.
ProblemThis table is a good example. The client asked for a piece to be used primarily as a centre table for reading, writing and the comfortable accommodation of four diners, and occasional use as an 8-to-10-seater dining table. This is a reasonable requirement and presents no unusual problems, until the desired appearance is considered: when in its more commonly used four-seater form, it was not to look like a dining table at all, having a pedestal base.
These pedestal centre tables were popular throughout the late Georgian, Regency and Victorian periods, most often being circular and supported by a single column, in turn usually sitting on a tripartite platform with feet and castors. An attractive class of table which lends itself well to the snap-top approach - the table top hinging to the vertical in order that it might be placed against a wall when not in use - but not offering much scope for extending.
Some extending pedestal tables were made during the Victorian period, with great heavy bases which split to be drawn apart, providing the necessary support to the ends of the top as leaves are inserted. The inherent heaviness of this approach ruled it out. Others allowed for the insertion of a single, small leaf, but this would not provide anywhere near enough room for 10 place settings.
SolutionThis doesn't mean that history provides no solution, just that period elements have to be combined in a new way. The first step is to provide support as close to the four corners of the un-extended top as possible. This can be achieved by exploiting the platform base used quite widely on sofa, side and centre tables by late Georgian and Regency makers. In this style, good-sized legs start almost below the top's corners, arch upwards and inwards to be fixed on a platform near the vertical mid-point of the base, and the table itself sits on scrolled supports or turned columns fixed to the platform.
This arrangement results in a centre table which is as stable as if it has a straightforward leg at each corner and, if built strongly, will bear the leverage imposed by an extended top.
Now that the decision has been made not to extend the base, all that is required is a means of extending the top. This is done with a runner pack based on the sliding dovetail slip principle, in which two five-section telescopic runners are made; the centre sections fixed to the base, the outer sections to the main table top, and the intervening sections floating.
Runner slipsA word of advice - do not attempt an extending table of any complexity without making full-sized drawings in plan and both elevations.
Starting with the runner pack is usual, as all else is straightforward and must accommodate the moving parts. Whenever I have a piece to make which requires great strength and stability, I use well-dried rock maple (Acer saccharum). The runner pack is made entirely from this hard-wearing stuff, in sections deep enough to withstand deflection under heavy load.
I can't remember where I first saw this type of runner used, but it has served me well more than once. The principle is simple - dovetail-section slips run in dovetail slots machined in the runners' sides, thus allowing a telescopic action while maintaining close contact of the sliding faces. Each slip is fixed to one runner side, so acting as a stop. On a smaller table, one slip per side is adequate, but I wasn't taking any chances with this one so I used two - if you're into calculating shear and torsion stresses you'll know that this more than doubles the strength of the assembly.
Once the drawing is made, preparing the runner pack is simplicity itself - but cut the maple generously over-length as the first and last couple of inches won't machine accurately, so must be considered sacrificial and trimmed after routing.
Rout the dovetail slots using a suitable cutter and a router table, being careful to cut both from the same datum edge to ensure parallel, then use the same cutter to machine lengths of maple slip.
Spend some time adjusting the set-up to make the slips exactly right - they can be adjusted afterwards but it's easier to get it bang-on at the machining stage.
Pack assemblyThe two centre, fixed runners are linked by two cross-pieces, forming a frame which is that main structural component of the table.
I thought of using wedged tenons for this joint, but decided to let gravity help rather than hinder so went for dovetails again. To allow enough material at this joint without unnecessary visual bulk, the centre runners are dressed away on their inner face, see photo.
Before assembling the centre section run an ovolo moulding on the upper edges, just to tidy it up.
Assembling the runners has to be done in the right order. First fix one pair of slips to the inner face and end of an intermediate runner. Slide this onto a centre runner, and overshoot to leave room to fix another pair of slips to the outer face and end of the centre runner. Slide the intermediate runner back over these, which leaves room to fit the opposite intermediate runner in the same way. Slips can then be fixed to the outer face and edge of the intermediate runner, and the inner face and edge of the outer runner, which are slid together last of all.
The runner assembly is supported on four substantial turned mahogany (Swietania macrophylla) columns. Jointing is effected by turned spigots top and bottom - these are both glued and wedged for added security. Even so, it is important that the spigots are accurately turned to suit the drill that will be used for their mortices, so drill a test hole in a piece of scrap and use this rather than calipers to check the dimension while turning.
BaseAs previously mentioned, the leverage imposed on the base by the fully-extended top is considerable, and so no weak points should be allowed to creep in at the base, especially at the leg to platform joint.
Consequently, the platform itself is a non-structural, decorative superstructure concealing a substantial maple framework. Two pieces of maple are halved together to form an X, then the leg bearers, maple again, are glued and screwed into place - but not before a dovetail socket is routed or bandsawn into the end of each one to accept the legs' dovetails.
A suitably shaped piece of 12mm (1/2in) MDF is then fixed to the top of the framework, serving as the platform's top, then the curved sides are built up brick-fashion from bandsawn blocks of mahogany - although laminated sides would do just as well, since the whole platform is finally veneered with mahogany, I had a lot of small mahogany offcuts from which to make the bricks.
Veneering is straightforward enough - it can be seen from the photographs that the sides are crossbanded, which should help to reduce splintering when assaulted by diners' feet.
After veneering, holes for the columns' spigots can be drilled - for accuracy's sake, mark through the holes already drilled in the runner frame.
LegsThis dovetail joint is about as stressed as a dovetail can be, so make it big and a perfect fit. The dovetails should be formed on the legs, which are maple again, before they are profiled - make an oblique cut on the stock, then use this as a reference when cutting the long dovetail. To create the shoulder above the dovetail, simply glue on a piece at the top of the leg.
Working from a template indexed from the dovetail shoulder, mark out and bandsaw the profile of the leg. The resultant shape can be cleaned up by hand, with sanders, or using a long profile cutter in a router table, its bearing guided by the template. I used the latter method which, while quick and effective, was hair-raising due to the reluctance of maple's endgrain to be routed.
The visual bulk of the legs is mitigated by their being tapered in thickness towards the foot. This is easy to do with a tapered jig in a thickness planer - or rather two tapered jigs, as if the first side is thicknessed while sitting on a jig tapered to 5 degrees, the second face will require support at 10 degrees to give symmetry.
Next, veneer the legs in mahogany, crossbanding their leading edges, after which the castors' tenons are cut prior to assembling the legs, platform and columns. There is a school of thought which recommends French polishing these components before assembly, by the way - I leave the decision up to you.
Table topThe top itself represents some acreage, much of which is unrestrained - consequently it is prudent to use a stable man-made board, lipped and veneered.
Prepare five pieces of MDF, these being for each fixed side of the top and three leaves, by applying a solid mahogany lipping to all edges.
This lipping should be mitred at what will be the outer corners of the ends, and continuous along what will be the moulded edges.
As a lipping has been used, a counter veneer under the face veneer is necessary to prevent telegraphing through - a synthetic fibre is a good idea given the large area.
This double veneering must be matched on the underside of the top, which altogether makes for a very large area if using Scotch glue - not impossible, but certainly taxing.
This is a case where discretion is the better part of valour, and either use of a vacuum press or a trip to a local hot-press is recommended.
After veneering, radius the two outer corners of each fixed leaf. Before any further trimming, though, plane the meeting edges and mount them on the outer runners of the pack. Check that they are exactly perpendicular to the runners and that tracking is consistent, then mark and fit locating dowels or biscuits to the meeting edges.
When this is done, close the leaves together and trim the sides. Next, plane the long edges of the loose leaves, then fit each one in turn.
When you are happy with the fit of each leaf, set up the table with all in place and rout the thumb-moulding to the edge.
Frieze and drawersNice as a maple runner pack is, the closed table is intended to conceal its extendibility and so a mahogany frieze is fitted under the leaves. This is a straightforward mahogany affair, crossbanded and biscuited in place.
The removable leaves do not have a frieze; at the client's request they were left plain to present less of a storage problem - fitting a frieze to them would be simple enough should they be required.
You will notice that this table also has frieze drawers at each end. These are optional, and are merely suspended from runners which are fixed to the underside of the top; their fronts are constructed to form an uninterrupted frieze when closed.