Making the Most of It:
Maurice Tebbutt and Keith Drew talk us through the maths and making of a segmented bowl.
Maurice Tebbutt and Keith Drew talk us through the maths and making of a segmented bowl
A ‘surprise package’ arrived in the post. Given its size – about 20 x 15 x 5cm, it seemed surprisingly heavy and inspection showed that it was a block of something like teak (Tectonia grandis) or oak (Quercus robur). The letter inside the package revealed that it was a gift from a friend, who lives on the Falkland Islands and knew of my interest in woodturning. She indicated that the timber was not just ‘any old block of wood’, but a part of an old sailing ship, called the Vicar of Bray, which had been rotting away there, more or less steadily, aided by the frequent storms to which the Falklands are subject, until she finally disappeared in 2015.
A bit of internet research showed that the ship was launched in 1841 and had plied her trade for some time in various parts of the world including involvement in the ‘Gold Rush’ before foundering on the Falklands
in about the 1880s. All of this reinforced my intrigue with the block of wood and I resolved to try to do something with it in keeping with its venerable history. Here is the original block that was contaminated with tar and showing signs of rot, which was not altogether surprising for timber which was about 170 years old and had potentially spent much of that time in salt water.
I thought that a 150mm shallow dish or even a plaque didn’t fit the bill and also involved turning away and losing a substantial part of the timber in the block. Ideally what I wanted was to preserve as much of the block as possible; to find some means of ‘bulking it up’ and also present it attractively. Eventually it occurred to me that segmenting could meet my requirements. It can, and often does, involve combinations of a number of different colours of timber, hence meeting the ‘attractiveness’ requirement, while coincidentally achieving the desired bulking up. This would seem to solve the problem, except that I had little idea how to do it. Fortunately, there are a number of members of my woodturning club, Worcestershire Woodturners, who are skilled in the techniques required, one of whom is Keith Drew.
The block of wood we worked with
Plans, equipment and materials
Tools
• Bandsaw
• Hotmelt glue gun
• Adhesive
• Bowl gauge
• Scraper
• Clamping system
Materials
• Contrasting timber to the block
• Plywood
The maths
The main characteristic of Keith’s suggested bowl design was a simple shape, which led to ease of construction while also minimising the wastage of material. The diagram also shows that the circular edges of the slices are at 45° to the horizontal (and vertical, of course!). It may also be apparent that the combination of the thickness of the slices and the angle of cut (in this example 45°) means that the rings can be cut in succession from one board, and that they will sit on top of each other to form the shape shown. This unusual method of constructing the bowl opened the way to being able to determine whether it would be feasible to construct the bowl from the block of wood which we had available. The greatest volume of available timber, which we can call ‘A’, was 20 x 15 x 5cm or 1500 cubic centimetres. So, A = 1500.
We decided to make the initial board, i.e. the top of the bowl, about 20cm square. Making a board t cm thick would require 20 x 20 x t cubic centimetres of timber. We called this ‘R’, so R = 20 x 20 x t. If we assumed that we would just use the timber from the block and could perform the redistribution without any waste, the two volumes would be the same. i.e. R = A or 1500 = 20 x 20 x t (1) and t = 15/4 or 3.75cm.
So, if our assumptions were correct then the task would be eminently feasible, producing an initial board, and hence also the subsequent layers, almost 4cm thick. However, we knew that the whole of this volume would not be available for one of two possible reasons; either because of wastage during the conversion process, which would happen in any case, or because of the effect of the rot on the block.
Keith with an example of the kind of bowl he suggested (a design originated by Malcolm Tibbetts)
The direct effect and indirect effect of the rot in our block meant that only about one third of the original block (which we can call the ‘conversion fraction’) would be available for use, this would give a volume of A/3 or about 500 cubic cm. Equation (1) now becomes 500 = 20 x 20 x t (2) So t = 500/400cm, or about 1.3cm).
So, if it turns out that only a third of the block is usable, it would be possible to produce a board with a reasonable thickness, so that the whole fabrication process would be feasible. However, it will be clear that this is not the whole ‘story’ since the ‘contrast’ timber, which we have ignored so far, would also contribute to the amount of available timber, and we did not have a reliable method of determining the conversion fraction.
Although this may appear to be very complicated, Keith’s solution was eminently simple – basically to start at the end!
He decided that he wanted the ‘bowl’ to be about 20cm square (as quoted in the calculation above) and to sit about 9cm high. It was to consist of the top, together with four supporting layers (five in all), which are cut in succession it from it so each layer would be 9/5 or 1.8cm thick. It will be clear that the bowl is constructed mainly from rings of timber, with a circular base and a topsquare with a circular hole.
1. Construct the layers from the ‘block timber’, together with strips of contrasting timber – in this case, ash (Franxinus excelsior) – which are destined to emphasise the pattern in the finished bowl, glued along the edges of the pieces of dark timber, using cramps to ensure good adhesion. Here you can see some evidence of rot remaining in spite of the careful selection process. Note that the available pieces of timber need to be utilised in such a way as to make two similar, rectangular boards, each one made as described above
2. Glue a sheet of ash to one of the large faces of each board. You can see the ash face on the lower section in contact with the worktop
3. Although the two composite boards we used were similar, they were not identical so when they were put together the central point did not lie on the joint between the two, as yours might not. This needs to be determined by measurement
4. The circular bottom section, the circular rings and the square top section can now be drawn in
5. It might be thought that the required 45° angle of cut can be achieved by setting a bandsaw blade at this angle and sawing ‘freehand’ along the marked lines. However, this is very difficult to do in practice and Keith found that the task was easier to perform and more successful by using a jig similar to that used by Tibbett. This effectively replaces the saw table of the bandsaw and allows it to be set at the required 45°
Top tip: When laminating and using various types of adhesives, you may find that placing a thin sheet of plastic or shrink wrap underneath will help minimise the risk of any excess adhesive squeeze out bonding to other hard surfaces on which your laminations sit.
6. If you have cut things correctly you will end up with a group of segments which are produced by the process described above
7. Now, the segments can be re-assembled in a number of ways, one of which reproduces the arrangement before the rings were cut
8. Alternatively, each additional ring can be rotated through an angle before being glued. You can see the effect of rotating each ring 90°, for example. We used the arrangement in step 7
9. To re-assemble the rings successfully, attach the top of the bowl first by ‘tacks’ of hot-melt glue to a thick hardboard disc which can be mounted on the lathe. The ‘tacks’ allow the fixing to be firm, and yet capable of being easily removed by melting the glue when the bowl needs to be removed. The disc becomes the basis of a clamping system in order to exert pressure on the remaining glue joints
10. Once the glue has cured, the final stage is to clean up and finish the bowl surfaces. Treat the outside by mounting the circular disc referred to above, on the lathe using a faceplate ring or equivalent. To reduce the risk of a catch, clean it up using a negative-rake, slightly modified half-round scraper rather than a gouge, and keep the scraper sharp
11a. Once the outside has been cleaned up, treat with the usual sequence of abrasives and a number of coats of finishing oil. Initially, this process produces a gloss finish, which can be sanded using fine abrasive to produce a ‘satin’ finish. Use the same technique to clean up the inside of the bowl, with two exceptions; another jig was required to hold the bowl firmly during this process.
11b. The flat base inside the bowl was cleaned up using a square-ended negative rake scraper, while the flat top of the bowl was cleaned up using abrasive, glued to a flat ‘sanding board’. Treat the inside of the bowl with coats of finishing oil, and sand like the outside
12. The finished piece
