How the window centre originated and finally dominated!
An article by Geoff Bassett, published in TRADA Timber Industry Yearbook 2001.
Windows and doors, with door frames, are among the most important items of joinery used by the construction industry and as they may be required in a variety of types, shapes and sizes, there has always been a need for flexibility in manufacture.
For some decades the principal demand has been for 'casement' windows which have the glass fitted in a hinged frame, rather than a 'sash' which moves vertically. The need to render windows both draught and weather proof has led to the casements and frames having matched profiles to exclude the elements and augmented by fitting elastomer seals to make the closure as storm proof as possible.
Making a complete window entails about ten operations starting with cutting the timber to size on a resaw, cutting to length and passing through a moulder to plane three sides and profile the fourth, cutting the glazing bead at the same time. The ends are tenoned and holes may be drilled for fittings and recesses cut for hinges. The frame is assembled in a cramp, glued and usually nailed with soft aluminium star pins after which the outer faces of the casements are profiled all round and grooved for the weather seal. The assembled parts are treated with preservative, the glass is fitted and the entire window with its hardware is assembled, wrapped and despatched. Depending on the size and type sills may be either formed during the initial machining or produced off line and fitted during assembly.
In small workshops the planing, profiling and tenoning were often done on very basic classical machines, using a spindle moulder and a single end tenoner. Long production runs were made on automated lines consisting of a multi-head moulder linked by a conveyor and right angle transfer to a double-end-tenoner. Equip both with some extra heads to take cutter blocks for different profiles and the system becomes quite flexible but is limited in its output by two aspects.
First there is a limit to the speed at which the moulder can be run. Joinery moulders are usually designed to run at a maximum speed of about 24 metres per minute but tend to be run at speeds lower than this to obtain the optimum 'single-knife' finish.
The second point is that the production runs for each component should be as long as possible to avoid loss of productive time for re-setting or changes of tooling, which means storing large quantities of parts until the matching parts are produced before assembly can begin. These stocks of work in progress can represent a vast sum of money awaiting processing. When the automobile industry identified this problem over fifty years ago, stock reduction was carefully planned and deliveries of raw materials were scheduled and timed to match the factory's demand. In the woodworking industry, makers of cabinet goods were the first to act on stock reduction and demanded flexible machines capable of making small batches of panels with quick tool changing and setting times. The effect was soon observed by joinery makers and the machinery makers embarked on the design of suitable plant.
The breakthrough came with the development of stacked tooling which began when machinery makers experimented with mounting a number of cutter blocks on a single long spindle and advancing them to the working position under computer control using a lead screw and rotary encoder to effect the mechanical positioning.
The application of stacked tooling with fully programmed electronic setting meant that a moulder equipped with a 620mm long spindle could be loaded with cutters to mould a number of different components. Changing from one to another took only a few seconds. The same method equips a tenoner to cut tenons of different shapes and dimensions. Now take a spindle moulder with a long guide fence, stacked tooling and an automatic feed and connect it at right angles to a similarly equipped single end tenoner and a window part can be profiled, the joints cut quickly and with minimal handling. With suitable clamping, the tenoner can machine two joints at a time: back the second piece with an automatic chip breaker and the joints will be perfect. When the sashes are assembled they can be storm proofed on the same machine.
A basic, low cost window centre of this type, can produce about 20 windows per shift and if you want a starting point for subsequent development, consider break out at the comers when storm proofing with a single head. Fit a counter rotating second head, programme it to move in and the first to move out and break out is eliminated.
From this basic concept, a wide range of window centres has been developed to produce larger outputs and perform other machining tasks including hinge recessing, etc. The largest, which show similarities in philosophy to the old moulder-DET lines, combine multi-head moulding with simultaneous tenoning of both ends of the workpiece, producing up to 150 windows per shift! The difference between old and new is the ability to produce batches of any size down to a single window with a minimum of manual intervention and thus confer dramatic saving in cost.
