Monday 9 July 2018
In this, the concluding article in the series on routers and their application to fine furniture-making, we will explore the possibilities made available by the use of beam trammels and guide bushes. Essentially, a beam trammel allows us to rout accurate circles and parts of circles without the need for any kind of template, while guide bushes, used with a template, permit us to shape components to their final profile, and carry out various grooving and inlaying operations, no matter what the shape – curved or straight.
Beaming with pride
A beam trammel works in exactly the same way as a beam compass. It consists of a rigid bar or rod, with a locating point at one end and the router fixed to the other. The position of the locating point can be moved along the bar to give circles of any number of radii – the maximum radius being limited to the overall length of the bar. Ready-made trammels, such as the one available from Trend, have a point or spike which is pushed into the workpiece to form the centre, or fulcrum, of the arc or circle to be described. The beam, or rod, locates into the same holes in the router baseplate which accommodate the side fence rods.
These proprietory trammels work well enough for many purposes but – in the interests of robustness and accuracy – I devised my own beam trammel, which I had made for me by an engineer friend. This consists of two 600mm lengths of 15mm diameter bright steel rod, with a threaded spigot at one end and a corresponding tapped hole at the other. For radii of up to 600mm (approx 24 1/2in) one rod is used on its own, while for larger radii, both rods can be screwed together to form one beam 1200mm (over 47in) long – rather like a chimney sweep's extending brush! One of the threaded spigots then fits into one of the baseplate side fence rod holes. A steel boss is turned, through which the beam passes and is locked with a hexagonal headed bolt. Instead of having a spike to form the fulcrum, a 25mm spigot locates into a hole of the same diameter bored into either the centre of the workpiece or a separate piece of chipboard or MDF, depending on the job.
Having a 25mm (1in) hole in the centre of a table top may not sound like a great idea, but in practice it can easily be plugged and veneered over to form a decorative element in the design, as in my spiral tables. If this method is inappropriate, you can attach a separate board, complete with its 25mm (1in) hole, to the workpiece, but the router base must be packed up accordingly to keep it parallel. With hindsight, the spigot on the boss need not have been so large a diameter and a 10mm (1/4in) hole would be just as effective.
I became concerned that in certain circumstances the single point, off-centre fixing of the router to the beam may result in flexing, so to overcome this I have devised a 'Mark 2' version of this trammel. This uses all the components of my 'Mark 1' device, with the addition of a U-shaped assembly, made of mild steel bar, threaded rod and nuts. These bars fit into both the fence rod holes in the baseplate and add more rigidity to the whole assembly and prevent any possible flexing.
Like many of the best ideas, the guide bush is brilliantly simple; just a concentric ring which locates onto the baseplate of the router in seconds with two screws. They are manufactured by Trend, with 15 sizes available with outside diameters ranging from 10mm to 40mm. Special sub-bases, for mounting on the underside of the baseplate, are available, which allow these guide bushes to be fitted to just about any make of router even if it will not accept Trend guide bushes as standard.
Guide bushes are designed to follow a template and since they are circular, will work with a template of any shape, whether straight, convex or concave. They are invaluable for much edge profiling work, using both straight and shaped cutters, and invaluable for the accurate cutting of grooves for inlaid stringing, and the like. The beauty of the way they work is that if you use the right combination of guide bushes and cutters, you only need one template to achieve more than one shaping operation.
Working in practice
The following actual example, will, it is hoped, illustrate the benefits which the router equipped with a beam trammel and guide bushes can bring to our work.
In 2000 a regular client commissioned a small low table, of bandsawn English walnut (Juglans regia) veneers with sycamore (Acer pseudoplatanus) stringing, on a core of MDF. The design made use of various radii – I named it the 'arcs' table – and since these were each constant, its construction made it an ideal candidate for the router.
I shaped the top after veneering, by first making a template of 6mm thick MDF of the required diameter, using the method mentioned above to calculate this. I made the template quickly and accurately by using the beam trammel and sharp 6mm straight cutter. Then I carefully aligned the template and secured it to the table top, and then cut it slowly to shape with multiple passes, using a new cutter.
The next stage involved gluing a 3mm (1/8in) thick rim all around the edge of the top to form a lipping. This was glued onto the curved ends using a concave former which was likewise accurately cut to the precise radius using the trammel. Running all around the edge of the table top, set in 3mm (1/8in), is an inlaid string of sycamore. By using the appropriate guide bush and a 3mm cutter, it was possible to achieve a concentric inlay using the same template that had produced the outer profile of the top. For profiling the outer edge of the top, I used a 9mm cutter with a 30mm guide bush and for cutting a groove for the inlay, I used a 3mm cutter with a 12mm guide bush. I now regret that I didn't take any photographs of this during construction, but hope the photographs of the finished table and mock-ups in this article will make things clear.
Well, that just about wraps up this series! To finish where I began: machines and power tools – even the router – can never entirely take the place of hand skills. But, if you have mastered hand work and understand the nature of wood as a constructional and decorative material, the router can add an extra dimension to your creativity and greatly speed up the making process.