Methods of Ovalturning

The essential differences between ovalturning and normal (circular) turning are the ellipse motion of the workpiece and the complicated tool guiding resulting from this motion. The cut should always be carried out in the central plane or lengthwise along the central line. When circular turning it does not matter whether the position of the cutting edge of the tool is above or below centre height as circles are always produced concentric to the spindle axis. When ovalturning the position of the cutting edge plays an essential roll.

Figure 5101 Result of tool positions at the circular cylinder and at an elliptical cylindrical workpiece

Figure 5101 right shows an ellipse cut by the centrally positioned edge and an ellipse cut by an edge positioned above the central line. The two ellipses penetrate each other. This means that the ellipse cut centrally is penetrated by the ellipse cut more highly. Two facing grooves arise on the surface. If such grooves appear at the cylindrical area of the oval turned workpiece, this shows that the tool edge was not guided centrally. Only by guiding the edge centrally can one achieve a clean surface. The tool should cut, theoretically, with a point of its edge in order to obtain an exact ellipse but this is practically impossible. Therefore the ovalturner uses pointed gouges. Scrapers, as they may be held horizontally at a precise height and position, enable the turner to always cut on a central line.

6.2 Lightline

In order to make the central line at the workpiece visible and to guide the cutting edge exactly along this line J. Volmer uses a lightline. It is generated using a projector with a special slide or using a laser. When the workpiece is rotating, the line will remain static allowing the tool to follow this correct path. An added important benefit is that this lightline clearly shows the turned profile as work progresses. The projector generates a lightplane, and is positioned so that the lightplane coincides with the central plane. A template is used to adjust the projector on its stand. It is placed on the lathe bed in front of the workpiece and has a marked horizontal line at centre height with which the projected lightline must coincide (see figure 5504).

Application of the lightline when turning the inside of an oval bowl. The projector is on a stand, about 1.5 m in front on right side, separate of the ovalturning lathe ODM30.

The lightline makes the internal profile of the finished bowl visible, it is standing still in spite of the bowl´s running.

The lightline shows the fine profile of a frame (here a rectangular frame) exactly visible while the workpiece is rotating. The tool has cut along the projected lightline.

6.3 Ovalturning Tools

The ovalturning tools are free hand guided gouges and scrapers and, in some cases, chisels. The gouges are ground to a lady-finger shape in order to have a short cutting area at the edge. The side parts of those edges are used for straight surfaces. The wedge angle depends on the type of wood to be cut. Scrapers have different edge profiles, e.g. straight edges for recesses and round for inside profiles of deep and shallow bowls.

For hard materials, like bone, ivory, horn and soft stones, one uses turning tools formed for non-ferrous metals. Those tools are fixed in a compound slide rest and fed into the workpiece slowly. Figure 5202 shows, as an example, ovalturning napkins rings from cattle bone.

Figure 5202
Ovalturning rings of bone at the ODM15 with the tool in the compound slide rest

When ovalturning a series of identical workpieces special devices are helpful to ensure uniformity. For this purpose the Piercer (Stecher) was designed following the example of the Bankfräser, common in the Ore Mountains (Erzgebirge). The piercer is used, for instance, to produce exactly the same diameter of the recess in all the frames of a series. The piercer consists of a swivelling arm, also movable in direction of the spindle axis. Upon the arm is fastened at a certain position a narrow pointed gouge . The arm lies on the tool rest or with an adjustable stopper on the lathe bed. The axis direction is also limited by an adjustable stop.

Figure 5203
Piercer at the ovalturning lathe ODM15 with its arm in working position

Marking the recess edge at a small rectangle picture frame by the piercer. The small pointed gouge is fastened upright and stings exactly on the center line

After roughly turning the recess using hand-held tools, to within one or two millimetres of the required size. Using the piercer the recess wall is now neatly cut

6.4 Ovalmilling

At the ovalturning lathes if rotating profile knives or routers are used, instead of hand-held woodturning tools, then Parallel Curves of the ellipse are cut. These curves are not ellipses. Woodturners who make workpieces in this way at the oval chuck, because it is an easier method of production, are often surprised with the results. The curves obtained by milling can visibly differ from that of the ellipse, they can even have cusps (figures 5206 and 5207).

Figure 5206
Routing-Motor with chucked Two-Knife-Ball-Cutter, fixed in a compound slide rest, milling the underside of a bowl at the Bollinger Ovalturning Lathe (BOTULA) [6.3]

The oval chuck or the ellipse mechanism of the ovalturning lathe guides the router-axis C relatively to the workpiece on the ellipse e (figure 5208a). A cylindrical router with radius r cuts the outer curve qo and the inner curve qi into the face of the workpiece. Both curves have the distance r to the ellipse e. They are called Equidistant or Parallel Curves. While qo is similar with the ellipse e the inner parallel curve qi shows visible deviations of the ellipse e. For small axes ratios qi has a cusp (figure 5208b), and for a smaller ratio arises undercut (figure 5208c). The mathematical relation for that is r/b => b/a (compare figure 2102). The Parallel Curves of the ellipse have been investigated in the 19th century, e.g. by the renown British mathematician Arthur Cayley (1821 - 1895). They are algebraic curves of eighth order and called Toroides.

On oval bowls or plates the effects of ovalmilling mentioned above may not disturb. Problems can appear at fits of two oval milled parts, e.g. when matching a lid with its box. In figure 5209 the router is guided for cutting the outer contour q1 of the box along the ellipse e1, for cutting the inner contour q2 in the lid along the ellpise e2. Both ellipses have the same axes difference, i.e. both contours are milled with the same adjustment of the oval chuck.

Figure 5208
Parallel curves qi und qo of ellipse e with different axes ratios b/a and router-radius-ratio r/b
a) b/a = 0,67 r/b = 0,38
b) b/a = 0,50 r/b = 0,50 (cusp at qi)
c) b/a = 0,33 r/b = 0,75 (undercut)

Figure 5209
Ovalmilling of outer and inner contour with same adjustment of the oval chuck

6.5 Ovalturning of Facework (Cross Grain)

Oval bowls, plates, platters, frames and many more of the oval objects mentioned in the list of oval turned items for use in general are turned as facework, i.e. of cross grain blanks. In ovalturning the flatter the object the more simple is the turning process. Ovalturning of deep objects, bowls or vases for instance, is more complicated. In the old literature there is no description of the way tool are used to produce the cleanest cuts and smoothest surfaces. Using scrapers, shavings can easily be made but producing a smooth surface on wooden pieces is not so easy. Spannagel, the author of the German woodturner bible „Das Drechslerwerk“, published in 1940 [1.1.2], wrote, in the chapter Ovalturning, that the only way to get acquainted with the practice of ovalturning, especially handling the tools, is to watch an efficient and competent master. Unfortunately such a master is not be found easily today. On the other hand, the masters hardly understood the geometric relations because any books of the time presented, at the most, a scant outline showing that the tool has to cut in spindle height. Scrapers have, historically, been used in preference to gouges. It is easier to lay scrapers on a plane tool rest enabling it to be kept at centre height.

6.6 Ovalturning of Frames

The first book that was dedicated exclusively to ovalturning of frames was the German "Handbuch der Ovaldreherei" (Manual of Ovalturning)[1.1.1]. It was written in 1920 by Hugo Knoppe, meritorious teacher at the Technical College for Turners, Carvers and Sculptors in Leipzig / Saxony.

Oval frames were produced in large quantities in frame-making factories. The frame blanks were made up of 4 or 6 parts. Each factory had its own technique for joining the frame blank parts, predominant were mortise and tenon joints as seen preserved in the oval frame factory, The Old Schwamb Mill in the USA [6.2]. In Germany there was a factory that offered horseshoe-like bent wood arcs. The arcs were glued together using a scarf joint. These frame blanks had the advantage that the fibres ran lengthways with the oval, and that assisted in yielding a good surface finish.

The frame factories delivered frames of all sizes, all profiles and ornaments and all manners of surfaces, e.g. painted, gilded, with spun metal inlays and also natural. The skills shown in the production of these forms were excellent. Today those same skills are less easily found. However, the oval frame form still compliments many a picture format. The ovalturning of picture and mirror frames is still of current interest and provides a special challenge. 5401 Ovalframe of rectangle board

The simplest form and therefore the simplest task is the production of a rectangular frame with an oval profile and an oval recess [2.2.11]. Up to a certain size – about 14 by 19 centimetre - the rectangle can be made from a solid board. Hard woods are preferred. In order to avoid warping, the grain of the frame blank boards should run as shown in figure 5401a, but not as in figure 5401b. Larger rectangles should be made up of 4 parts (figure 5402).

The picture-ellipse should have an axis ratio matching with the picture, e.g. for portraits b/a not smaller than 0,75 (figure 5403).

It always pays to make a series of the same pieces, and it is profitable to use the piercer to produce recesses exactly equal in size for all frames of the series (figure 5406). The recess and the profile are cut using narrow pointed gouges. The bottom is flattened using a straight edged scraper. Take care to leave sufficient thickness TH at the back of the frame. The profile is sanded to a fine finish using folded sandpaper (figure 5408), the front is cleaned up using a sanding block. The distances of the profile from the opposing rectangle sides should be equal (figure 5409).

Figure 5407
Turning the profile and the recess

Figure 5408 Sanding the profile near the central line

It is preferably to turn the profile P within the rectangular border S (picture 5410a and b), because a protruding profile (figure 5410c) requires to turn over the rest of the rectangle area S. This is difficult to turn cleanly because of the interrupted cut. The picture B can be put into the frame from the front side (figure 5410a) or into a recess in the rear (figure 5410b and c). The picture B, an inlay or a passepartout and the glass G are held by a rattan clamp-ring C (figure 5411). On the rear recess two curved wires CW keep the cover CR down [2.2.11]. Glass, passepartout, inlays and cover are cut by means of an ellipsograph provided with a cutting wheel or knife

Figure 5413
Holes on the rear for hanging the frame on small nails vertical or crossways

The methods described above are also used when making larger oval frames, but at the start the blank is chucked in the claw chuck for turning the rear and the recess W, the flank FL of the picture ellipse and the chamfer F of the outer contour (figure 5414 above). The workpiece is then turned over and chucked on prepared hardwood jaws of the four-jaw chuck. The last steps are turning and sanding the profile and outer contour (figure 5414 below).

Figure 5414
Sequence of work with big frames
W rim of picture recess, FL flank of picture ellipse, F part of rear contour, SS space for jigsaw

6.7 Ovalturning of bowls

Figure 5501

Saw oval (elliptical) blank on the band
saw using a template, mass 7kg, screw the blank with hexagon headed woodscrews from the rear of the faceplate, place between the work and the faceplate a protective cardboard disc.

Turn outer contour of the bowl using pointed 16mm gouge, on the base turn a 10mm deep spigot.

Screw a hardwood board to the faceplate, turn a recess to match the spigot, press the bowl spigot into the recess, do not hammer, apply pressure only to the middle of the bowl (5.7kg).

Figure 5506;

Wrap the bowl in newspaper or place it in a cloth bag, with shavings, to dry. Wind a steel band around to prevent cracking of the end grain. At intervals tighten the band using the screw lock. Once it is dry remove the spigot, sand the bowl roughly and oil it (1,6kg).

6.8 Ovalturning of Long Grain Work

Workpieces are described as long grain work when the grain runs in direction of the spindle ax*is. At circular turning long grain pieces – e.g. banisters – are turned between pins. At ovalturning this method is impossible due to the ellipse motion. The workpiece cannot be supported by the tailstock pin. Short long grain pieces can be chucked in a chuck with normal or extended jaws. Figure 5601 shows a box as example.

Figure 5601
Box of long grain bog oak in VICMARC Four-Jaw-Chuck on the ODM30

J. Volmer developed the special device ELLPIN for ovalturning of long workpieces. It consists of a screw chuck with an anti-twist device (see figure 4311), that via an universal joint on a flange is connected with the faceplate of the ovalturning lathe. Instead of the screw a cup chuck can be used for small workpieces. In the tailstock spindle is inserted a Morse taper #2 with a live centre but it has instead of the point a ball pan made of carbon material. In the pan sits a ball with a pin that has to be pressed into the long grain workpiece.

Figure 5602
ELLPIN:Device for ovalturning long
workpieces (Workshop J. Volmer,
type for ovalturning lathe ODM30)

The following figures show as examples of the application of the ELLPIN device the ovalturning of cutlery handles and of bashers or mallets. The blanks have to be prepared: they are turned round between pins and of wanted lengths, holes at the ends have to be drilled for the ball-pin and for the screw or a cone is to be turned matching with the ELLPIN cup chuck.

6.9 Cutlery Handles

Figure 5603

Knife and fork with tang and ferule,
Plastic handles are to be replaced with ebony handles

Figure 5604

Sawed ebony blanks

Figure 5605

After centre punching both ends the hole for the ball-pin is drilled

Figure 5606

Turning the conical spigot at the handle end using a template, white mark on the broad side

Figure 5607

Check the fit of the spigot in the conical hole in the ELLPIN-flange

Figure 5608

Press spigot into the hole of ELLPIN-flange, and press ball-pin into the blank. Notice the position of the white mark

Figure 5609

Adjust the ellipse mechanism to provide the correct size of the ellipse at the handle end, cut along the lightened central line, the round part at the end of the handle must fit the ferule..

Figure 5610

Sanding and polishing near the central line

Figure 5611

Finished handle

Figure 5612

Pull out the ball-pin using the extractor (workshop J. Volmer) and enlarge the hole to fit the tang, saw off the spigot and sand the elliptical end round.

Figure 5613

Assembling the cutlery: press ferules onto the handles and cement the tangs

6.10 Basher and Mallet

Basher for kitchen work as meat basher or as mallet for wood sculptors

Figure 5614

Ovalturning Lathe ODM30 with ELLPIN (Device for ovalturning long grain work) and chucked basher blank, right on the shelf finished bashers: Handle nearly round, basher part with oval cross section, left the extractor for the ball-pin

At the start the basher blanks are turned round between centres to the required length. The ends are slightly rounded over. The hole for the ball-pin at the handle end and the hole for the ELLPIN-screw-chuck at the other end are to be drilled coaxially. The blank is now screwed onto the ELLPIN-screw-chuck, and the prong ring – there are two diameters 20mm and 32mm – as anti-twist device must be pressed firmly into the face by revolving the wooden disc. The ball-pin is pressed into the handle end and the ball inserted in the ball pan at the tailstock.

The possibilities of the design of shapes and profiles of objects turned using the ELLPIN device are endless. Some experimental candle-holders have been produced.

Figure 5615
Experiments using ELLPIN at the Ovalturning Device ODV with wide cross and long grain pieces
(Workshop J. Volmer 1986)

6 Methods of Ovalturning

6.1 Guiding the Tool