Читать книгу The Wood Turner's Handybook - A Practical Manual for Workers at the Lathe: Embracing Information on the Tools, Appliances and Processes Employed in Wood Turning - Paul N. Hasluck - Страница 14

THE wood-turner who works at the foot-lathe employs but few edged tools except chisels and gouges. By the aid of these alone nearly all external turning is done.

Economy of time is essential to the economic production of work, and it is, therefore, in some cases, advantageous to roughly round the material to be turned before putting it between the lathe centres, as there are ways of doing this at a less cost than by turning. In speaking of cost, it is here used in its abstract form, and means value of labour irrespective of the time employed; and if two unskilled hands can do more work than one skilled workman receiving double the wages, it would of course be cheaper to employ the two hands. Balks of wood may be trimmed up to a cylindrical shape with a hatchet, a paring knife, or a draw knife; and large chips of superfluous material are hewed off by these means quicker than could be done on the lathe, with a less expenditure of technical skill, and with appliances far less costly. Thus there are many reasons for getting material roughly to shape before mounting it in the lathe. For the same reasons the centres of such balks should be marked, and so obviate delay in finding the centre when chucked, though a general and very good plan is to finally centre the work as the turning progresses, for it is only when the rough, irregular figure is turned that its hollow places can be determined with certainty; and when the turning has reached the stage at which this shows itself, the work is driven, by blows of a mallet on its full side, to run so that the hollow will come up to the tool.

The cutting edges of tools used for turning soft wood are found to act best when ground to an angle of about twenty-five to thirty degrees. This gives a keen edge, capable of withstanding the ordinary usage of such work. To work with tools which are ground less acutely is like cutting wood with a cold chisel—and here it is as well to explain that a cold chisel is the name by which chisels used for cutting cold metal are known.

There is a great deal in keeping the tools in a proper condition for work; and the turner has to learn this, even before he begins to learn turning itself. New tools require to be ground and set before they can be used.

Cylindrical work, both plain and ornamental, is first roughly turned to shape with the gouge, and then the chisel is brought into requisition to smooth and finish the work. The same description of tools are employed for turning flat surfaces, both plankways and endways of the wood. Hollows and internal recesses are also turned with gouges and chisels whenever there is sufficient space for their introduction.

The correct position for the cutting edge of the tool is at a tangent to the circle which it is turning—that is, the circle left after the passage of the tool; and it is only necessary to thoroughly understand the meaning of this to be able to place the tool with precision and certainty in the best possible position for cutting. A tangent is a line which touches, but does not cut into the circumference of a circle, and the chamfer ground on the convex side of the gouge has to form a tangent to the circumference of the diameter being turned. It must be perfectly understood that a tangent may be formed at any point of the circumference, and equally well at the highest or lowest point. It is generally said that the height of a tool should be exactly of the same height as the lathe centres, and though this is perfectly correct in the case of slide-rest tools as usually ground, yet the axiom has no bearing if applied to hand tools, though in turning metal the same rule is advisable, as it affords a rigid rest at the most convenient place. No matter at what height the rest is fixed, the tool can be placed at a tangent to the work by elevating or depressing the handle as may be required.

To make the action of turning tools clear, so far as their cutting edges are concerned, I will borrow from “Lathework” a diagram which shows two tools correctly applied for cutting both soft and hard material. By this it is seen that the slide-rest tool, with a strong cutting edge suited for operating upon highly cohesive metal, such as steel, and the acute wood-turning chisel, suited for the softest material, have each the lower face-angle placed in the same position with regard to the work. The upper face, which wedges back the shavings, curling or breaking them according to the nature of the material operated upon, is the only one in which any difference is observable agreeable to this latter condition.

In Fig. 13 the line of centres is shown by a, b. It is precisely to this height that all slide-rest tools should be set. With hand tools it is of little importance whether they be applied above, below, or on the line of centres. The edge of the metal-turning tool is formed by the meeting of the faces a, x, and d, x; d, x is three degrees from the perpendicular, and this gives the angle of the edge as eighty-seven degrees. The edge of the soft wood chisel is formed by the meeting of the faces c, x and d, x enclosing twenty-five degrees, still keeping the lower face in the same line, that is a tangent to the circle.

Suppose the tool is laid on to a true cylinder, so that its bevel forms a tangent to the cylinder, it cannot cut the material; but directly the handle is raised the cutting edge is depressed into the cylinder, and all that material lying outside of the diameter of the edge is removed. The position of the cutting edge is best determined by the sense of touch, the tool being laid on the T rest with its end overhanging the work considerably. The lathe being in motion, the tool is then drawn gradually towards the operator, all the while keeping it resting on the cylinder, till the edge reaches the point at which it forms the tangent, and then it commences to cut. By slightly tilting the tool, the edge is brought to act on a smaller diameter, and turnings are removed under the most advantageous circumstances.

Fig. 13. CUTTING ANGLES OF TOOLS.

Apertures which, in comparison with their depth, are of small diameter necessitate the use of tools of special form, which are, generally speaking, hook-shaped. Tangs of various forms and dimensions have the ends bent to form gouges and chisels, so as to be available for work where the ordinary straight gouge or chisel could not enter. To attain efficient mastery over these hook tools requires considerable practice. In the hands of inexperienced persons their application frequently leads to mishaps. A strong wrist and, above all, practice in using the tool are required, in order to counteract the powerful leverage exerted by the work, which tends to twist the tool and wrench it from the grasp.

The length of handle is an item of importance in all hand tools, as reflection will show. They are all, when in use, governed by the laws which act on a lever, the fulcrum being the point of support on the rest, the cutting edge then terminates the short arm of the lever, and the power over it is proportionate to the length of the handle end as compared with the length of the short end. The control over the tool is therefore greatest when the rest, and fulcrum point, is nearest to the work, and the hand holding the handle is nearest to the end. When the handle is not sufficiently long, the edge of the tool is liable, when caught by the work, to be drawn into cutting too deeply, through the hand not having complete control over it, and, from the same cause, with greater force, the tool will be twisted out of the grasp, or the work forced from the chuck. In order to give greater freedom in the movement of the cutting edge, tools used for wood-turning, which are generally required to do a long range of surface, they are used with the rest placed at some distance from the work, and this, in giving longer motion to the cutting edge, gives a much greater power on the leverage: and it is for this reason that such tools require to have long handles.

Gouges for turning are made in all sizes, having cutting edges from one-eighth to two inches. The sectional form varies from crescent-shaped tools, with a thick back tapering to a thin edge, to circular ones, in which back and edge are of equal thickness. The curvature of the tools varies also greatly: from those in which it is so slight that they may almost be considered straight-edged to others having a semicircular edge. The length of gouges is partly governed by their size, one inch gouges measuring generally from ten to fifteen inches long.

The ordinary gouge and chisel used by the wood-turner must not be confounded with those tools having the same names but used in cabinet making and joinery. The carpenter’s and the turner’s gouges are quite distinct tools, though they have a general similarity; and the same may be said of the chisels used in the respective trades. The turner’s gouge is a much stronger tool, having more metal in it than its namesake; it is ground to a different shape, and has a handle from eight to sixteen inches in length, according to the size of the gouge. Fig. 14 illustrates a gouge handled complete. Both tools are ground on the outside of the curvature, but the turner’s gouge terminates in a rounding edge, without corners, instead of being square with the shaft of the tool. This round form of edge is necessary in order to obliterate the sharp corners, which would be liable to catch in the work. It also allows the most prominent parts of the edge to be used in grooves which are comparatively narrow. Very small curves and mouldings can thus be easily operated upon by small gouges.

Fig. 14. A TURNER’S GOUGE.

Several sizes are always provided for use, and whilst the larger ones take off large shavings and rough the material to shape quickly, the small ones are available for more minute work, and may be used for turning in shoulders to very nearly an acute angle.

Chisels for turning are made in sizes to correspond with the gouges, but larger ones can also be procured. They resemble ordinary paring chisels, but have no shoulder to the tang, the edge being also formed very differently. The blade of a turner’s chisel is bevelled equally from each side, leaving the cutting edge in the centre of the thickness and at an oblique angle with the sides. This angle is usually about seventy degrees and one hundred and ten degrees, instead of being square across like a paring chisel. Turning chisels are ground obliquely, for the sake of greater convenience in use. One of ordinary shape performs the work equally well, but, as that tool has to be applied at an angle to the work, which is often inconvenient, an oblique edge is preferable, allowing, as it does, a cylinder to be turned with the tool at right angles to the axis of rotation.

The method is the same for applying both gouges and chisels. The tool is grasped firmly near the cutting edge by the left hand, the knuckles being uppermost. The right hand holds the handle near the end, and, to afford greater steadiness, it usually rests against the side of the body. Thus held, the tool, if a gouge, is laid on the T-rest with its convex side downwards. The edge is brought sufficiently near to the revolving cylinder to touch it in the position of a tangent; that is to say, a straight line, drawn in continuation of the ground bevel of the tool, will touch, but not cut into, the cylinder. In this position the gouge will not cut, but by raising the end of the handle with the right hand, the edge of the tool is depressed, and it then comes in the position of a tangent to a smaller circle. When the work is rotated, all the material outside of that diameter will be shaved off by the tool. During this process the pressure on the edge of the gouge tends to force the tool deeper into the work; the right hand must, therefore, hold down the handle till the work has been reduced all round to the new diameter.

The first cut of the gouge is usually made at a short distance from the right-hand end of the rough balk, and when a groove is turned the tool is inclined towards the left, so as to remove the material between it and the end. A new cut is then made towards the left, and this is made continuous with the previous one by inclining the point of the gouge as before; thus by a continual shifting of the tool, and turning a small distance at a time, the entire cylinder is brought roughly to form. The size is gauged by means of callipers, and if much in excess of what is required a further application of the gouge is the best way of reducing it; and the cylinder is made as straight and even as possible by this tool before employing the chisel. In this levelling operation the gouge is slid along the top of the T-rest, guided by the left hand, and tracing on the work a regular spiral path. An expert hand can by this means produce a very tolerable smooth surface to the work, the gouge being traversed from both ends alternately, and the parallelism is checked by callipering.

The chisel used for smoothing the work is applied similarly to the gouge, and all that has been said applies to both tools, subject to any modifications rendered necessary by their different forms. The chisel is always applied so that its edge lies obliquely across the surface of the cylinder, the handle being also slightly inclined to place the edge of the chisel—which is, as has been already stated, ground obliquely—at a slightly greater angle with the line of centres than that it makes with the chisel blade. The tool is land on the rest, with the blade resting on one corner of the obtuse angle. The chisel is tilted up sufficiently to bring the central part of the cutting edge against the work. Only the central part touches, both corners being free, the edge lying obliquely across the cylinder. If either corner were allowed to act on the work it would be extremely difficult to guide the tool, which would have a tendency to “catch in.” This tool is held with the left hand grasping the blade close to its cutting end, with the knuckles above; the right hand grasps the handle near to its end, holding it near to the right hip, and the chisel blade rests on the T, one of its lower corners only in contact. The tool is slid along with the obtuse angle leading, and may be used from either right or left. To reverse the direction it is only necessary to turn over the tool.

Through the tool being presented with its edge obliquely, only the central part cuts, the two corners not coming in contact with the work, and the extreme central part cuts deepest, the shaving cut by the chisel being thickest in the centre and tapering off on each side to a feather-edge. By carefully considering this, the necessity of correct tangential position will be better understood. If the chisel is laid on to cut with the entire breadth of its edge, the tool becomes unmanageable from the quantity of material it has to cut. The production of straight, level work will depend on maintaining, during the entire longitudinal traverse of the tool, a perfectly equal amount of tilting from the T, and the same relative position of the handle held in the right hand. It must be remembered that the chisel lying obliquely across the surface of the cylinder, and forming a tangent to it, will not cut at all, acting just as a gouge under the same conditions; but directly the handle is raised, and the edge penetrates the diameter, then the tool lies at a tangent to a circle of smaller diameter, and all material outside of that circle will be cut off. The cut is taken from either end, as most convenient, by simply turning over the tool. The principles which govern the cutting action of the gouge are equally applicable to the chisel, and it is by tilting up the handle that the tool is fed into the cut.

Fig. 15 represents a gouge of the form in general use; it measures nearly half-an-inch on the edge. The section shows much less metal than is embodied in Fig. 17, and this kind of gouge is much lighter and handier in many respects than the one shown at Fig. 17. The grinding is more easily accomplished. The substance of the tool is quite adequate to the requirements of all ordinary wood-turning, and it is with gouges of this form that most work is done. Larger sizes, from one inch to one-and-a-half inches, are used for roughing-down purposes. The section of this tool is crescent-shaped; the centres of the two circles, whose arcs form the concave and convex sides, are about one-sixteenth of an inch apart. The points of the horns are rounded off, so that all sharp corners are avoided. The convex surface has a radius of a quarter of an inch, the concave being only one-thirty-second less. From these measurements, taken from a gouge by the aid of which I have made many a bushel of shavings, the precise form of the tool may be easily understood. The bevel ground around the convex part very much resembles that of the following figure. The elliptical form of the edge is more circular, owing to the larger channel in this tool. After what has been said in connection with the chisel, it is scarcely necessary to mention that the angle enclosed by the bevel and the inner surface should be about twenty-five degrees. Tools for soft wood turning are seldom more than thirty degrees on the edge.

Fig. 15. A HALF-INCH GOUGE.

The accompanying cut, Fig. 16, shows three views of the cutting end of a turner’s gouge, and are intended to illustrate the method of grinding. C is the front, or concave side, of the gouge, and shows the parabolic curve of the cutting-edge. B is the side, and shows the long sloping chamfer made by grinding the tool. The angle formed by this chamfer should be from twenty-five to thirty degrees. It is this chamfer that should form a tangent to the work when the gouge is in use. A shows the back, or convex side, of the gouge, and the chamfer will be seen to be of equal slope all round. In setting gouges, the turner has an ordinary oil-stone, with a number of different-sized, semi-circular groves worn into it. A groove the size of the gouge is selected, the back or chamfer part rubbed along it, at the same time turning the tool slightly, as when grinding. Then with a slip of oil-stone having a rounded edge, the wire-edge, which will be found all round the cutting edge in the hollow side, is removed. In doing this great care must be taken to rub the burr down only level with the surface on the hollow side, because tilting up the back end of the slip would produce a slight chamfer on the hollow side of the tool, and this is not desirable. The operation, however, turns the wire-edge backwards, when the tool must again be lightly applied to the hollow set-stone; and, finally, one or two more rubs on the inside with the slip, after which it is wiped with a piece of soft leather, or on the palm of the hand, which removes any remaining wire-edge, and the tool is now ready for work. All the gouges, large and small, are ground and set as described above, different-sized slips being used.

Fig. 16. GRINDING A GOUGE.

Fig. 17 shows a quarter-inch gouge. Top, side, and sectional views being given, the small semi-circle shows the size and form of the cutting-edge. This kind of gouge is particularly strong, as can be seen from the sectional view, there is only a small channel as compared with the substance of the tool. The bevel formed by grinding is shown in the side view; and the form of the point, seen from the top, illustrates the correct form for grinding. There are no sharp angles, the point ends in an elliptical edge, leaving the centre most prominent. The horns of the crescent-shaped section are thinned off on their outer sides, to save metal and leave less to be ground away. Both the inside and the outside curves are circular, excepting the part just alluded to; the centre of the smaller circle is, however, fully a sixteenth of an inch from the centre of the convex face.

Fig. 17. A QUARTER INCH GOUGE.

Fig. 18 represents a three-quarter inch chisel; side, top and end views are given. The two bevels, which meet at the cutting-edge, enclose an angle of twenty-five degrees. This, as will be seen by the diagram, Fig. 13, is the angle suited for wood-turning tools. The oblique edge forms at each corner angles of seventy degrees and one hundred and ten degrees respectively, as has been mentioned previously. A chisel of the size illustrated will measure from about eighteen to twenty inches from end to end, when handled ready for use, the blade being about half that length. The rectangular figure, facing the cutting-edge, is a view of the end.

Fig. 18. A THREE-QUARTER INCH CHISEL.

Fig. 19 illustrates a hook-tool used for turning interior hollows. The steel shaft is forged out thin and wide at the end, and curled into a circular form as shown. The outer part of the circle is ground square with the tang; the inner side is thick in the centre and bevelled towards both edges. This makes a sharp cutting-edge around the ring, as shown in the top view. The tang itself is square in section, with the corners bevelled off; near to where the hook commences it is nearly round. These hook-tools are carefully filed round the inside to form a smooth bevel; all the corners are rounded off, and the tool is then hardened and tempered for use. It may be applied to the work either side up, and considerable manual dexterity is required in its use to prevent accidents of a more or less serious nature.

Fig. 19. A HOOK TOOL.

The other necessary tools for ordinary wood-turning are but few, and consist of callipers, a pair of compasses, two or three slips of oil-stone for setting up the gouges, a side tool and a parting tool. Fig. 20 shows a large scale view of the side and edge of an ordinary parting tool. A grindstone and an oil-stone are also very useful, and, in practice, the use of these cannot well be dispensed with. There should be about half-a-dozen pairs of callipers of various sizes, the largest opening to about twelve inches to fourteen inches. Also one or two pairs of inside callipers, these being too well known to need illustration.

Fig. 20. PARTING TOOL.

The parting tool is in shape like Fig. 20, about five-eighths of an inch broad and one-eighth of an inch thick. It is used for cutting off pieces of finished work from the block, held in the cup chuck, or on a face plate, such as bosses, patellas, draught-men, &c. It works best when the grain of the wood is running at right angles to the lathe centres, that is plankways.

The above tools are about sufficient for most purposes. There are, however, a variety of other tools of peculiar forms required by turners who do a great variety of work. The shape of these suggest themselves from the nature of the work to be done. Old files of various forms are often utilised by grinding into the required form. In the grinding of all tools, see that the ground part, or chamfer, is a straight line from the heel to the cutting-edge; and in setting on the stone this surface should lie nearly close on the stone, the heel only slightly raised, so that the cutting-edge comes in contact with the stone. If the handle end is held much higher, a new chamfer is immediately formed at a more obtuse angle than the ground part, and the tool will not work satisfactorily until it is re-ground.

The first actual turning is done with the gouge, and large ones are used for large rough work. The correct angle for grinding the gouge, and the proportion of handle have been previously mentioned. The rest is placed just clear of the revolving work, and the T adjusted to a convenient height, so that when the cutting-edge is performing properly, and the tool resting on the top of the T, the handle will come to a convenient place at the right-hand side. The height of the rest is therefore generally to a certain extent governed by the stature and habit of the workman.