Chairs and Curves archive

Wednesday 19 November 2008

John Bullar introduces us to some essential chair-making techniques

Gallery

Chair frames need curved components and angled joints.

You might make a simple chair with the legs and the seat rails all straight and joined at right angles, but even then, the back must be raked at a small angle to prevent an awkward forward-leaning position for the sitter.

Mass-produced chairs can be astonishingly cheap because of the economies of scale provided by jigs and CNC machines, as well as replicating large numbers of parts from bulk materials. One-off chairs take a disproportionate amount of effort to make and for this reason, chair making tends to be treated as a specialist area that some bespoke furniture designer-makers avoid. However, producing chairs in small batches, such as for a dining set, can also provide some benefits of scale to the individual maker.

In this part of the series, we will look at some chair-making techniques used by furniture makers, before moving on to building a set of dining chairs next time. Windsor chairs, which depend largely on lathe turning, are a different branch of work that is not covered here.

Marking out

Components are marked on the stock timber around the template using a pencil or felt-tip pen ready for cutting on the bandsaw. It takes considerable time and care to coordinate the shapes with the direction of grain, both for aesthetic appeal and to ensure there is no risk of short-grain failure at points of high stress, such as around the rear leg joints.

Curved components are often 'nested' so that one is cut from inside another to avoid excess wastage. While the unusually shaped offcuts are inevitably numerous, it is often possible to mark chair components around faults or unwanted features, such as knots in the wood, in a way that may not be possible with rectangular furniture. All this takes time and the more complex the chair is, the longer it will take to mark the shapes and cut the parts.

Bandsawn components

The bandsaw is an invaluable machine for cutting out shaped components of chairs. Tenons for sturdy joints can also be cut on the bandsaw as part of the same process.

A narrow blade is necessary for negotiating tight curves; as a rule of thumb, the tightest curve you can cut has a radius of five times the blade width. In practice, this depends on how much set has been applied to the teeth and hence how wide the kerf is.

It is best to cut curves slowly with the kerf just meeting the outside edge of the line so as to minimise the excess to be trimmed off later. At the same time, the kerf should not cross the line, otherwise there will be a blemish to remove when the component is trimmed to the size of the template.

If a component is to be shaped on the bandsaw in two axes, it is essential to support the underside, normally by using the first offcut during the second cut. This will avoid snatching that can occur if a poorly supported piece of wood becomes trapped by the blade. While bandsaws are normally benign, even-tempered machines, it is surprising how aggressive they become if poorly supported wood is allowed to snatch and jar the full momentum of the wheels.

Trimming to size

Router cutters fitted with guide bearings above or below, can be used to trim curved components to match the exact outline of the template. The template is clamped against the rough-sawn component, which is very slightly larger because it was bandsawn to the outside edge of the line. The work is normally done on a heavy-duty router table or spindle moulder while the component and template are clamped to a sliding carriage or 'sledge'. The cutter height is arranged so it runs along the component while the bearing runs along the template. Care must be taken to avoid snatching at the start of a cut, generally by making the template longer than the component.

For hand-tool work, the compass plane has the same mechanism as a normal Bailey plane but the sole is made from a flexible steel strip, held in a curve by a large control screw. Having produced a rough-sawn shaped component on the bandsaw, you need to fair the curves smooth so hands can glide over them and clothes will not catch. The compass plane is an ideal tool for this, provided the radius of the sawn curve is large and constant. If the radius varies, you can plane it in small sections, adjusting the curvature as you move along. Traditional tool users also find that old wooden spokeshaves, when tuned-up, are very controllable and well suited to curved edge work. The relatively wide mouth and steep cutting angle of the forged blade, work surprisingly well inside tight curves, and changes of radius are no problem.

Bulk removal

Where large quantities of material need to be hollowed out, such as for producing shaped wooden seat tops, the Arbortech is a fairly aggressive woodcutting tool that can, with practice, be controlled to produce neat work. The device is in effect, a steel disc edged with chainsaw teeth that is fitted to an angle-grinder. It is provided with a guard that also limits the depth of cut, but for more precise work, you need to provide further means of regulating it.

Disc sanders are also used by some chair makers, while others look on them with disapproval as tools of the motor-repair shop. However, they are easily controlled and quickly remove material on components or assembled chairs. Some well-regarded makers assemble chairs to quite a rough form, and then produce most of the detailed shaping after the frame is glued up.

Angled jigs

In the past, dowels were often used on mass-produced chairs because they are relatively easy to fit in the angled ends of components. However, dowel joints frequently fail in end-grain by breaking out of the side when they are subjected to heavy loading, such as in seat-rail joints. Small loose-fitted tenons have similar limitations. Large-diameter dowels can be turned on a lathe and used effectively as large-section loose tenons for heavy-duty joints.

Mortise-and-tenon joints can be cut at an angle either by hand or on the bandsaw, or by using a router together with a frame mortise-and-tenon jig. These jigs allow either the mortise, or more usefully the tenon, to be cut at a controlled angle. You can also cut tenons at a compound angle by sloping both the timber and the jig's clamping plate.

Pulling together

The chair is usually glued-up in stages, such as the back frame followed by the front, and then the side rails. Sash cramps are not very effective because they apply forces at right angles to the rails, and they are heavy. Both these factors tend to cause distortion in the shape of a frame that is based on angled and curved parts. Lightweight cramps can be successful on some small components such as back rails, if the joints are near to right angled.

However, band-cramps are ideal for pulling together chair frames. They are light in weight and pull joints at any angle while not imposing a distorted shape on the seat.

Finishing

Chairs are designed for touching. They are in contact with clothing which may be abrasive, and they are moved and handled a good deal, so they need to feel good to the fingertips. The surface must be smooth and free from sharp edges while the surface finish must be stable.

Chamfered edges are regarded as a styling feature associated with Arts & Crafts, however they also have a strong functional aspect. Chamfers help provide a smooth finish where any two surfaces meet at an arris, and they are particularly valuable on the edges of curved components - this is because there is inevitably one point along the curvature where it would be prone to forming a splinter if it were left as a sharp edge, and the chamfer helps to take away this tendency.

Having looked in detail at some of the techniques used for making chairs, in the next part of this series of the Apprentice, we will apply these to the project of making a set of oak dining chairs, as illustarted in the photograph on the right. To see the next part of this series, click here.


Woodworkers Institute

Tagged In:

John Bullar , Curves , Chairs , Chair

Glossary Rollover a term to view its definition

Bandsaw , Disc Sander , Router

"The surface must be smooth and free from sharp edges while the surface finish must be stable"


Make these chairs in part two of this series, using the techniques discussed in this article

Templates

If a chair is to be comfortable, its shape must take account of the human form. The design process is likely to be an evolutionary one with the maker building on previous successes and learning from previous problems. A 'rod' is a full-sized drawing marked out on a board, allowing the shapes of components and the angle of fit between them to be checked out. The shapes can then be transferred to another board for cutting out as templates. MDF or plywood are the most suitable materials of rods and templates. Using rods and templates allows the makers to repeat the shapes of previously made components and improve on them over time. Templates also ensure that matching parts of a chair, or a set of chairs, are the same shape. Makers build up sets of templates over the years, sometimes mixing components between sets to produce alternative designs and often varying the shape to derive new template designs.
Templates are shaped for each component
Marking around the template onto the wood

Marking Angled Joints

Chair joints are like any other furniture joint in principle but there are two specific factors to consider. Firstly, the joints often meet at an angle and secondly, they need to be particularly strong. The main joints between legs and seat rails can be severely stressed, especially if heavy sitters rock in their chairs or shuffle around in them, racking the joints. Chair legs and frames need to be exceptionally well-fitted, making best use of the component's dimensions.
Before cutting an angled joint, it is necessary to measure the angle and position, or transfer this information onto the component. This can be done by clamping the components in alignment before knife-marking the shoulder lines, however, when the joints are angled in two dimensions, direct marking becomes particularly difficult. It is not possible to align the shoulders of the uncut component in the way you would with conventional furniture.
Small protractor gauges are useful for transferring lengths and angles. I have joined two of these together at the stems to make a double-ended instrument that I often use for marking the joints on rails.
Compound angled rail joints can be positioned with the style joints disengaged
A double-ended angle gauge is valuable for measuring rail joint angles