The recent SOT Seminar was attended by 12 students and 7 instructors. The theme was napkin rings and designed to show many different set-ups and techniques but still allowing the students to produce a finished article. What came out of this process were the wrinkles, the difficulties and the short cuts - a true learning experience. It was felt that these experiences were worth recording and hence these notes and photographs.
The form of the holder was two taper shoulders of boxwood on a piece of steel studding half inch diameter with washers and lock-nuts at each end (fig 1). The shoulders were at about 45 degrees to give an end force for clamping. In the case of the rose-engine work John Edwards preferred to use a slow taper on the shoulder with an almost matching taper on the workpiece to maximise grip. Some of the workpieces ended up with only a small section so a clamping force was to be avoided. Some of the early prototypes did in fact split due to too high a force, so a balance had to be struck.
The metal work-pieces were cut from tube and were initially clamped with boxwood shoulders on the work-holding studs.
The use of wood for the clamping shoulders proved to be the best way as often the cuts finished on the shoulders. An example of the sacrificial nature of this is shown in the photograph taken after the event (fig 2)
For all the set-ups a series of blanks was prepared and the count made on the instructors' set-up day was 140 – a lot of work in its own right. The wooden ones were prepared as cylinders with holes of a nominal 1.25 - 1.5 inch diameter squared off in a lathe to length. The metal ones were cut from brass or copper tube in a bandsaw and then squared off in a metal-cutting lathe to length. Thin section tube is a little tricky to square off as the piece has a tendency to snatch and pull out of the chuck.
The set-up was unusual in OT terms since it requires a broad sweeping cut. Several ways were explored including the use of an Evans universal cutting frame that has its own vertical feed (fig 3) or a vertical slide commonly seen on later metal-cutting lathes. In the event the instructor used a dome chuck with the work-holding fixture screwed on. For indexing, the chuck was used upside down as the engraving of the dial could only be seen from underneath (fig 4). The dome chuck was set vertically using a set square on the lathe bed and then clamped in this position using 2 pins in the Segment Plate (fig 5). The Birch Universal Cutting Frame was used and fed along the length of the cross-slide and the ring indexed by the dome chuck (fig 6). Care had to be taken to ensure that the bottom of the cut did not break through into the inside diameter. The first trial piece was actually so close one could see light through the wood. The balance was, however, to machine enough away to make sure the adjoining curvilinear facets meet each other all the way along.
Having made all the cuts, the piece was then taken to a metal cutting lathe to finish the bead off at both ends, rather than re-setting the ornamental lathe. The end result was a pleasing waisted shape (fig 7).
In this case the set-up was to use a double indexing technique on the Birch lathe (fig 8). The main indexing used either the index plate marked out with 12 divisions (30 degrees) using Tippex correcting fluid (as formerly used by typists), or to use the gear-driven index on the segment apparatus. The work was mounted between centres with a dividing chuck as a secondary index in place. The double indexing method can be very useful to ensure accuracy of dividing but it can also lead to trying to read the recording index upside down if insufficient thought is given to the initial position. The students commented to their instructor on what they thought of the two approaches, some for and some against.
The first 12 cuts were made with the vertical cutting frame using the main indexing device. The ornamental slide was then moved to the next position along ensuring that there was the slightest overlapping of cuts. The secondary index was moved by 6 degrees and the main indexing done for the next 12 cuts. This was repeated 5 times and then reversed to give a chevron pattern (fig 9).
As an alternative some students chose to have a straightforward basket-weave effect and used 12 divisions of 30 degrees on the main and then 15 degrees on the secondary (fig 10).
In this case a hexagonal napkin ring was produced on the Rose Engine with each face having a rose pattern cut on it.
The set-up again used a dome chuck to present each of the six sides in turn to the horizontal cutting frame. The dome chuck was mounted on a Morris double-eccentric chuck which ensured that the pattern was centred in its true position on the workpiece (fig 11).
The handwheel was turned slowly and steadily to give a good pattern on each face (fig 12). The workpiece was boxwood and one of the the finished product was later dyed red to give a most pleasing effect
Some interesting swash work was tried by John with some students. Swash work is where the mandrel oscillates axially once per revolution using the rubber on the swash device. It produces an ‘interesting’ end results (fig 13).
The set-up called for a slow spiral of 3 inches pitch (fig 14). The first blank was set up in the lathe with the appropriate gears to give this pitch and a felt-tip pen used to mark the cutter path (fig 15). This was taken as a guide for angling the universal cutting frame to cut in the correct direction (fig 16). It could be calculated but this was a quick practical alternative to getting out the sine and cosine tables.
The pattern was a 16-start spiral and the indexing was done by an indexing chuck on the back of the mandrel. To give a good view of the readings on the back of the dial, an LED lamp was mounted nearby (fig 17). The engravings on traditional OT machines were always small and a number of people generally found their near-focus spectacles were not adequate.
The curvilinear effect was given by the template mounted on the curvilinear apparatus. Since the work was convex there was movement of the slide both in and out under the control of the student's left hand. The roller engaging with the template was very useful in giving a smooth transition from uphill to downhill movements. Students found controlling the rate of feed from the hand-wheel on the right hand end of the slide was at times difficult. There was a tendency to be too ‘greedy’ on this which did cause the work-piece to slip in its clamps on occasion.
One student worked so quickly in producing a spiral in one direction that an extra idle gear was added to the gear-train to reverse the process to produce a pineapple shape (fig 18).
Rather than disturbing the set-up the shoulders were then finished off on a taper wooden spigot mounted in another lathe to produce the finished article.
The Holtzapffel was set up with a spherical slide-rest with initially a fixed tool with a straight end but sharpened on the shoulders as well.
The choice of wood in this case was rosewood as the figure of this wood is very marked. The design of the napkin ring was to show this figure on the spherical part complemented with a limited amount of ornamental decoration on the beads. The wood is not as easy to work as boxwood or blackwood.
The instructor explained that the clamping had to be very firm with lock-nuts at both ends because of the breadth of the tool threatening to jam the work and move it and the nuts bodily along the studding. He also explained that running OT lathes required a minimum of pressure from the tailstock as the front bearings were susceptible to seizure.
Sweeps were done to the right and left to leave a shoulder at each end (Fig 19). The finish of the spherical surface was enhanced with some very fine sandpaper (flashes of lightening and thunder from the old school!).
The spherical slide-rest was brought to a position normal to the axis of the lathe. A vertical cutting frame was put in the tool-slide and the decoration done on the shoulders to finish (fig 20).
The Lienhard Brocade machine was set up with a pattern cylinder which was basically twice as broad as the finished napkin ring. The pantograph was set to give this reduction but in order to retain the aspect ratio of the original pattern the cylinder had to be run at twice the speed of finished napkin ring (fig 21). Normally brocade work has a background pattern but in this case it was found that a plain background produced the most satisfactory pattern (Fig 22).
Another type of napkin ring was produced on the Gudel automatic rose-engine. This is a production machine which basically produces a rose-pattern in the form of a spiral. It achieves this by having the bank of rosettes loose on the mandrel but slowly rotating relative to it and continually adding a few degrees per revolution (fig 23). The result looks like traditional ‘barley-corn’ pattern. In addition the front part of the mandrel is slowly advanced and retarded under the control of a cam to give a rippled effect. In this case a two-lobe cam produces a most pleasing pattern which the instructor was able to demonstrate on both brass and copper. (fig 24)
The pencil chuck of the Plant Straight-Line Machine was equipped with wooden clamps to hold napkin ring blanks for patterns in the axial direction (fig 25). The idea was to produce a napkin ring showing a mix of polished brass intersected with area of pattern In the event the students wished to experiment with all sorts of patterns from the pattern bar as they became engrossed with the possibilities of this machine (fig 26)
At the end of the 2-day Seminar students commented on the sense of immersion in the subject. The scope of what had been done and the realisation of what could be done had broadened people's imagination. There was also a general consensus that this was not just ‘twiddling’ the knobs to produce a result but that attention to detail and technique was important – the very foundations of skill development. The time to finish the individual rings was challenging as 2 students shared a 75 minute period for each set-up and the questions for the instructors were coming thick and fast.
The work of one proud student is shown (fig 27) but he wishes to remain anonymous. Some of the ‘horrors’ produced during the course of the Seminar are also shown (fig 28) and curiously the perpetrators also wish to remain anonymous! It is all part of the learning experience for the attendees.