Thermed And Involuted Lamp stand
Tuesday 14 September 2010
For this article I decided to make a table lamp with a difference, using some techniques which you may be familiar with already. The base of the lamp is made using a technique called 'therming,' and the stem is produced using the 'inside out' or involuted turning method. The transition from the base to the stem uses a small bowl-like design, upside down, thin turned and then pierced.
Therming and involuted turning are both dying techniques while piercing only became popular in the last 20 years or so. Therming and involuted turning were both established centuries ago, and both were popular with furniture makers because they allowed certain features to be mass-produced, for which there were no dedicated tools and easy technology available at the time. Items produced by therming can be replicated today by using machines such as shapers, routers, bandsaws and moulding tools. Whereas thin-turned, pierced items only became popular with the availability of the Dremel tool, the ultra high-speed dental drill and, more recently, lasers. And, in my opinion, nothing illustrates that product development directly depends on the availability of techniques and tools better than this.
Tools used 10mm (3/8in) bowl bouge and 12mm (1/2in) spindle gouge
Therming through the ages
When therming, usually several pieces are turned at the same time on a specialised holder. One type of 'specialised holder' was a pair of metal discs about 800mm in diameter, over 1m apart with a central shaft connecting the two discs. Therming was originally used only in spindle turning. The main applications were producing flat sided chair backs and chair legs. The contradiction in this is obvious: you cannot produce anything by spindle turning on a lathe that is flat. But by producing the 'thermed' surfaces on a very large diameter - 800mm or even larger - the curvature of these surfaces are practically flat. Therming the two sides of a chair back spindle involves two operations: the back and the front of the spindles. But therming can be used to produce a 'flat' surface on the sides of all polygons. As many therming operations, as many sides to the polygon. The drum based design from the early 1800s evolved into numerous methods to produce thermed items. Perfume bottles, salt shakers, pepper mills and boxes of all shapes and sizes were produced by therming with three, four or even more sides. The long spindles of the chair backs were only the starting point leading to short, stubby boxes. The segments produced for the base of the table lamp are an extreme form of therming: only about 25mm (1in) long and thermed on one side only
The lamp base will be a scalloped edge circle, cut out using a bandsaw. Start with a circle divided into six equal segments - equal sided triangles - with each corner measuring 60°. To achieve the 'scalloped' effect divide all six segments into two halves. If you draw an arc (part of a circle) with its centre on one of the dotted lines and of smaller diameter than the original circle, you then obtain the scalloping effect. The degree or depth of the scalloping depends on the diameter of the new circle. If you want to save wood, make the six segments with 60 degree sides from smaller pieces of timber
You can use a strip or small pieces of wood to produce alternating triangles. The important thing is to get the corners all 60°
To reproduce the scalloped shape by therming, you can make a jig. The jig is mounted on a faceplate and is a 25mm (1in) thick disc with a 60 degrees 'V' cut, of the same diameter as the circles on the scalloped edge of the triangles - 150mm (5in). The jig will hold one triangle at a time
The triangles are held in position in the jig by a screw
Using a 10mm (3/8in) bowl gouge, turn away the projecting straight bit of the triangle to match the curve - arc - of the jig. Then, gently round off the face of the triangle. This side will become the top of the base
Sand the turned part. The six segments are then ready for ebonising, as shown here
The decoration on the stem of the lamp is produced by involuted or 'inside out' turning. You usually start with four identical square pieces of timber which after both - inside and outside - turnings are completed, are glued together for final finishing. If the final turning will be polished and the grain will show, the four pieces should be cut from a larger square. This way, after gluing together the grain can be matched. Now, fix together the stack created from the four pieces of timber, using either tape, Jubilee clips, purpose made square holders or the very special chucking arrangement, inspired by Tom Puckley and designed and made by the late John Lovatt
Use the front part of the drilling jig to drill eight holes at both ends of the stack. The jig is designed for 33mm (1 1/4in) timber and for this I am using 50mm (2in) square timber. Drill into each piece of timber at two diagonal corners: one set of holes for the first stage of turning, and one set of holes after inversion
The other two items in step 7 are two Multistar chucks: one for the headstock and one for the tailstock. The chuck for the tailstock end has left-hand thread and it is mounted on a 'live centre'. Both chucks have screw inserts welded into the jaws. The holes drilled with the drilling jig line up with the screw inserts. When chucks are tightened they pull the stack together and keep the stack of timber tight. You can now mount this stack of four pieces on the special jaws with a rough outline of the shape to be turned
The finished outside - of the part that will be 'involuted' - can be seen here. Once happy with the outside shape you can reassemble the stack 'inside out'
Now you have to change your approach to looking at the turned shape. At this stage of involuted turning it is not the shape of the turning, it is the missing shape you have turned away that is significant. Here you can see the 'window' or 'transverse opening' being turned. You only see the shape of the window if you look at the outline of the missing shape - in this case half a little leaf. When the window is completed, re-assemble the four pieces of timber in the original order. If everything is correct, you can glue and clamp the assembly
When the glue is dry, it is time to finalise the outside shape
From a little spigot at the base end to centralise the stem you have assembled
After ebonising and polishing, the lamp is ready for assembly. From both the top end and from the bottom end drill a 7mm (9/32in) hole for the wire to feed through
Between the base and stem
The transition from the scalloped base to the stem will be made through a small upside-down bowl, pierced and ebonised. Turn a recess on top of the base to position/centralise the small pierced bowl. You can see the bowl positioned on the scalloped base in this recess with a couple of telltale holes to show the thickness of turning. You can pierce this if you choose, if so, it needs to be about 2mm (5/64in) thick. The hole in the centre of the top will accommodate the spigot you turned at the bottom end of the stem