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TECHNIQUES
O.T.
TECHNIQUES – THE ECCENTRIC CUTTING FRAME
Introduction. The Eccentric Cutting Frame is the most versatile of the ornamental cutting frames; it is similar to a boring head, having a cutter box that can be set to drill a hole on centre or set eccentrically to cut a circular groove of any radius within its range. It is used with a single point cutter either for individual plunge cuts or continuous swathe cuts. A plunge cut gives a circular indentation with a radius equal to the eccentricity of the instrument, which is adjustable from zero to around 1½ inches. Using a point tool; a series of cuts may be taken at regular intervals around the work to form a variety of patterns such as the barleycorn, the turk's head and others; or, by adjusting the radius of the cutter by regular intervals, shell patterns may be formed. Using a round-nosed cutter the tool may be used to take planing cuts to form facets or hemispheres.
Balancing. Running the ECF at a comfortable speed (say, around 300 r.p.m.) is facilitated by a balancing device which avoids undue vibration, thus giving a smooth cut. There are several methods described in early issues of the Bulletin of the Society of Ornamental Turners and also in “The Art & Craft of Ornamental Turning” by Rev. G.A.Grace. Advancing the cutter into the work. Any play in the bearings of the ECF should be eliminated so as to avoid the cutter snatching and spoiling the surface of the work. The tool-slide should be advanced smoothly towards the depth stop by the combined use of the guide screw and the hand-lever (or push it with the left thumb to take up any backlash); otherwise the cutter can ‘snatch’ and be pulled suddenly into the work thus spoiling the cut. To avoid the danger of the ECF striking the front of the slide-rest when withdrawing from the cut; drill two holes in a flat, thin rectangular strip of metal, so it will form a “bridge” over the hand-lever pins on the slide-rest so restricting the backward movement of the tool-slide. The ECF is set as far back in the tool-slide as is possible without it fouling the front of the slide-rest; the rear tool-clamp is then butted up against this bridge and tightened, thus stopping the tool-slide from being retracted any further. This bridge also serves another useful purpose with all types of cutting frame: if fixed in position when the cutter is just clear of the work, one can retract the tool between cuts until stopped by the bridge; then one can have confidence that the tool is sufficiently retracted without undue winding of the guide screw - saves too much wear on those arthritic wrists! Any backlash in the slide-rest leadscrew should be eliminated or, if this is not easily done, it may be held firmly in position by the fluting stops. The chuck should be sufficiently tight that it will not tighten further during cutting and the cutter must strike the work with clockwise force so as not to loosen the chuck from the mandrel (Many turners have found it hard to believe that the small force of a cutting frame can be sufficient to unscrew a chuck; that is, until they experience it for themselves. It can easily happen, especially when the cut is intermittent; it is like a lot of tiny taps by a light hammer and it can unscrew the chuck; it usually happens towards the end of a long series of cuts, thus spoiling the work and wasting a lot of time. If an ornamental chuck is used there should be little or no play in its slide or division wheel. Cutters should be honed and polished so as to give a fine finish to the cuts. “All-at-Centre” is the position from which each operation should start. The Eccentric Chuck, if used, should be fixed in the horizontal position; i.e. with its slide exactly parallel with the lathe bed, with the chuck wheel at the 96th division and with the index if possible adjusted to the 96th hole on the 96 scale of the division plate (or the 192nd hole of 192 or the 120th hole of 120; these being the most useful ranges). There are several ways of finding the “all-at-centre” position. One way is to use a Centre Finder* which may be mounted on the chuck nose. *(The Centre Finder may be a cup chuck, a faceplate or a workpiece whose face is perpendicular to and whose edge is concentric with the mandrel.) The ECF should then be set parallel to the mandrel by adjusting its radius so a dial test indicator (d.t.i.) held in the ECF toolbox runs evenly on the face of the Centre Finder throughout a complete rotation. An error in the horizontal plane is corrected by adjusting the right angle stop of the slide-rest. A vertical error may indicate dirt under the slide-rest base or in the tool-box or, more seriously, wear of a bearing surface which will need to be corrected by re-grinding or by shimming. Failure to start “all-at-centre” will mean the cutter will penetrate deeper on one half of its circle than on the other. Next the ECF should be set concentric with the mandrel by adjusting its radius so the d.t.i. runs evenly all around the edge of the Centre Finder. An error of height is corrected by raising or lowering the slide-rest and an error of concentricity may be corrected by loosening the cradle of the slide-rest and gently tapping the whole slide-rest away from or towards to operator. This concentricity adjustment should be carried out with the micrometer of the slide-rest main leadscrew set at zero, thus providing a convenient datum for the counting of distances from centre. An adjustable gauge may be applied to the slide-rest base to preserve this setting for future operations where “all-at-centre” is required: the facility to adjust this gauge is necessary to cater for different cutting frames or cutters with offset points. Another adjustable gauge may be made to quickly set the cutting frame at exact height of centre; a note being taken of the exact measurement for each cutting frame. If a d.t.i. is not available “all-at-centre” may be found with reasonable accuracy by facing a piece of wood (or the workpiece) with a slide-rest cutter and checking its truth with a straight edge; adjusting the right-angle stop as required until a perfectly flat surface results. A tiny pip is left at the centre of the face: the ECF is then placed in the toolbox with the point of the cutter adjusted to run exactly concentric with the ECF spindle, such that it will cut only a dot on the surface. Concentricity of the ECF with the mandrel is then achieved by adjusting the height of the slide-rest and the position of the slide-rest base in its cradle until the cutter runs central to the pip. It is important to make a note of the reading on the ECF micrometer when the cutter is in the “all-at-centre position because this setting may not necessarily be exactly at zero and it will be needed as the datum from which all changes in radius of this cutter will be counted (different settings are required for cutters having their points in non-central positions; e.g. angled to the left or to the right ). Before clamping the cutter in its toolbox always push it with the thumb towards the right, thus ensuring any slackness in the toolbox and any backlash in the adjusting screw is always taken up in the same direction; otherwise it will be impossible to re-set the radius of the cutter with consistent accuracy. Surface patterns may take many forms. For patterns which are to meet in the centre, merely start “all-at-centre”; set the radius of the cutter and wind the slide-rest out (to the left of centre) so that its eccentricity equals the radius of the cutter. For patterns which are to fall short of the centre: set the slide-rest eccentricity to a margin greater than the radius of the cutter. For patterns which are to cross over the centre: set the slide-rest eccentricity to a margin less than the radius of the cutter. Always ensure that the material is of sufficient size to accommodate the slide-rest eccentricity plus the radius of the cutter. Surface patterns are usually cut with a shallow cutter (e.g. one honed to an obtuse angle of around 110° or 120°) which gives good light reflection from the facets. Progressive patterns are made by adjusting the slide-rest eccentricity between cuts, plus or minus the change, if any, in the cutter radius, to achieve the desired progression. Interrupted patterns are made by missing one or more cuts at regular intervals in the pattern sequence.
Combination patterns are made by repeating a pattern or cutting a series of patterns in different positions on a surface by using one or more of the ornamental chucks in conjunction with the ECF. Compound patterns may be effected by “double-counting” with the Eccentric Chuck nose-wheel being advanced by one division and the division plate being moved by a different number of divisions in the same or opposite direction to the chuck nose-wheel. When the ratio of movements is two-to-one a two-looped figure or an ellipse results; a ratio of three-to-one gives a three-looped figure or a triangle; a ratio of four-to-one gives a four-looped figure or a square, and so on. This system was devised by Captain James Ash and is described in detail in his book “The Art of Double Counting on the Lathe” 1857.
Compound eccentric patterns may also be effected without an Eccentric Chuck using a method described as “dual counting” where series of cuts are made at pre-determined points on the division plate, the number of points dictating the number of loops required in the pattern. The eccentricity of the slide-rest is adjusted for each further series of cuts from a table of division plate points. By this means figures with any number of loops, ellipses, straight-line patterns, hollow and solid polygons may be cut. This system was devised by H W Elphinstone and is fully described in his book “Patterns for Turning” 1872. Cylindrical or Conical patterns are cut with the slide-rest parallel with or, at a taper angle to the bed and their intervals are regulated by the division plate. Such patterns are very prone to chipping where the grain is short, so care must be exercised. A Circle Chart is mentioned in some publications as a useful aid for adjusting the radius of the ECF to cut a series of touching circles. Such a chart may be scratched with a point tool from the centre to the edge of a disc or plain face-plate at such angles as are required to indicate the diameter of circle to be cut according to its distance from the centre of the workpiece and the number of touching circles to be cut around it (See Fig.2). A 45° Cone Chart, similarly marked on a cone, can also indicate positions for touching circles on a cone or sphere. Such Charts provide only a general guide and they are unwieldy, it being necessary to remove the work in order to use the chart. A much simpler and more effective method is to make light scratch cuts on a trial and error basis; if the scratches don’t quite meet, the radius of the cutter may be increased or the eccentricity of the slide-rest decreased until the scratches meet exactly; if the scratches overlap the adjustment options are reversed. Dimensions for touching circles around a base circle may be calculated using the Table for Equidistant Contact Circles (Holtz Vol.V p.222). These calculations will be found to be quite reliable, however, taking a few initial scratch cuts is still recommended as the most reliable method. Barleycorn patterns are often cut too shallow so that flat spots are left on the work, or they are too widely spaced so there are gaps between them, or crossing over so they cut into each other. True barleycorns are cut so that the corns have sharp crests with no flat spots, no mutilation of their sides, and the cuts between them should meet; neither crossing over nor falling short. It should be noted that the required ECF radius for cuts into cylinders, cones and spheres is the radius at the bottom of the cut and not on the surface, (e.g. it is like cutting through one layer of an onion to the surface of the next) so the radius of the cutter must be adjusted according to the circumference of the work at the bottom of the cuts and this inner circumference (or true base circle) will be reduced if the cuts are made deeper. This is easily remedied by ensuring the initial scratch marks don’t quite meet, then when the barleycorns are cut almost to their full depth, check them, if necessary with a magnifying glass and/or an inspection mirror and adjust the radius of the cutter as necessary to perfect the corns with a finishing cut. A finishing cut is often desirable because, not only does it give the opportunity to check the work for perfect alignment and re-sharpen the cutter if necessary, but also, being a very light cut with virtually no pressure on the cutter, the finish will be as perfect as the tool can give. As a general rule a barleycorn comprising four times as many cuts as the number of touching circles (i.e. 4 corns within a circle) will be well-proportioned: a lesser or greater number of cuts produces corns which appear fatter or thinner and these may be used according to the effect desired.
A part-Sphere may be cut from a cylinder with a round-nosed cutter in the ECF. The slide-rest is set at an angle to the bed, say 25°- 45°, the tangent screw of the headstock is engaged (or a slow hand motion drive may be used) and the cutter is advanced very carefully. The cut is progressed around the work by winding the tangent screw (or the hand motion). The slide-rest angle and the cutter radius are adjusted as necessary and the depth of cut is increased progressively until a part-sphere of the required size emerges. When difficult grain is encountered it may be necessary to take a cut either, slightly above or, slightly below centre to avoid tearing the surface. To allow for such eventuality, have two shims prepared (say 0.010” thick) to raise the ECF in the tool-slide. Put one shim in before adjusting to the height of centre: thus by adding the second shim the cutter will be raised above centre and will only cut on the underside of the work (i.e. from right to left) and by removing both shims the cutter will run below centre and cut only on the topside of the work (from left to right). Similarly, by winding the slide-rest leadscrew a whisker to the left the cutter will cut only on the end of the part-sphere and by winding a whisker to the right the cutter will cut only on the neck. By this means the part-sphere may be cut all over in the direction of the grain although some finishing with a fine abrasive will be necessary to smooth off the slight inequalities between the cuts. A Spherical Barleycorn or Pineapple pattern may be cut on the surface of a prepared part-sphere using a cutter angled to the right by about 75° and with a sharpened bevel under the right side (which increases the facet-cutting angle by about 5°). This gives a better finish to the facets. Different effects may be produced by using different cutters with different angles, by varying the radius of the cutter, the slide-rest angle, the number of divisions used and according to whether the original surface is to be merely penetrated or entirely removed. Great care is needed with spherical cutting as the drag on the cutter and the effect of adverse grain are greater than with cuts on a plain surface and any slight snatching of the cutter will spoil the work. Flutes may be cut with the ECF (instead of the more frequently used Drilling Spindle); across a surface, along a cylinder or, around a part-sphere (the latter using a Dome Chuck or Spherical slide-rest). Facets may be cut on a cone or a cylinder by winding the slide-rest leadscrew so as to take planing cuts across the work. A series of facets may be cut at regular intervals governed by the division plate. Polygons may be cut by mounting the work on a Dome Chuck. The cutting angle is determined by engaging the headstock tangent screw and turning it until the required segment angle is reached. A series of facets may be cut around the work by adjusting the Chuck dividing-wheel. A facet may be ornamented subsequently by taking a series of cuts across it or around it at intervals governed by movements of the slide-rest leadscrew or the division plate. Swash Circles (or Crescents) may be cut on the surface by setting the slide-rest at a very slightly different angle to that of the face into which the pattern is to be cut. Crescents on the cylinder may be cut by setting the ECF above centre height. Undercut crescents can be produced using a cutter with a sharpened concavity on the inner side. The “swash” effect on surface cuts can be used in those patterns having many cuts which cross the centre and where deep cutting would remove the whole surface; (this is particularly useful when preparing a printing block where the cuts at the “busy” centre of the pattern need to be shallower than those more widely spaced on the periphery). An example of such patterns is the Elliptical Barleycorn when the cuts are numerous and sufficiently large at the minor axes to overlap across the centre.
Rose Engine work. The ECF may be used on the Rose Engine to cut pseudo barleycorns: as the rose engine rocks or pumps the ECF cuts deeper and shallower thus forming slightly rounded and not very crisp barleycorns. However, if a slightly rounded cutter with sharp flanks is used some interesting effects may be produced by setting the ECF to cut above centre. Inlay cutting may be effected with the ECF using a square-end
bevel-sided cutter. A series of
circular recesses is cut into a prepared surface. (If touching or nearly-touching circles are intended only
alternate ones should be cut and these must be inlaid before the intermediate
circles may be attempted: this is to avoid breaking away at the junction of two
circles.) The material for inlays
is turned very slightly taper and carefully fitted to each individual recess
before being parted off, glued and gently tapped home.
When the glue is fully hardened the raised portions of the inlays are
faced-off with a sharp round-nosed cutter, taking great care to avoid chipping
away the edges and, if possible, without penetrating the original surface: this
is because numerous series of circles may need to be excavated and inlaid before
the pattern is complete. By complex
overlaying of inlays made from a variety of materials, including inlays of the
same material as the original surface, some very delicate and pleasing patterns
are possible. The final facing off
must be done carefully with a very sharp cutter with which a very fine layer
will be shaved off the entire surface (for more details refer to “The Art and
Craft of Ornamental Turning” by Rev. G A Grace).
©
John Edwards 2008 |
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Last modified: August 30, 2010 |
