Modern Machine-Shop Practice Part 202

[Ill.u.s.tration: Fig. 2707.]

[Ill.u.s.tration: Fig. 2708.]

It is evident, therefore, that to obtain a uniform degree of shrinkage throughout the length of a piece of timber, it should be sawn as near as possible parallel with the grain of the log. Thus in Figs. 2708 and 2709 we have a side and an end view of a log, the saw cuts at A being from logs that have been squared, the upper slab B being waste material, and the planks being parallel to the squared sides of the log.

[Ill.u.s.tration: Fig. 2709.]

The lines from A to C on the lower half of Fig. 2709 represent planks that are what is termed flitched, the saw cuts following the taper of the tree, and it is plain that the shrinkage would be more uniform; thus the outside plank is near the bark from end to end, while at the top of the figure the outside plank is near the outside at the small end only of the log, and would therefore shrink most at the right hand end.

Furthermore as the planks at A cross the grain of the log at its large end, they are therefore weaker and more liable to split at that end.

BENDING TIMBER.--By bending a piece of timber to bring it as near as possible the required shape the strength of the work is increased, because the grain of the wood runs parallel with the shape of the work, and, furthermore, the cutting tools act on this account to better advantage. In bending a piece of timber it is obvious that either the convex side must stretch, or the concave one compress, or if no extraneous pressure is brought to bear upon the piece, both of these actions may occur, and as the side of the piece that was nearest to the heart of the tree is the hardest and strongest, it will stretch less if made the convex side, or compress less if made the concave side of the timber, but the bent piece will maintain its shape better if the heart is the convex or outside of the curve.

The modern method of bending wood is to fasten on the outside, or convex side of the piece, a strap that will prevent it from stretching. And it is found that wood thus bent is stronger, stiffer, and heavier than before it was bent, because the fibres become interwoven, and it is found that the wood is harder to split than before.

[Ill.u.s.tration: Fig. 2710.]

[Ill.u.s.tration: Fig. 2711.]

[Ill.u.s.tration: Fig. 2712.]

Suppose we require to bend a piece to a half circle, and after it has been boiled, steamed, or heated with a dry heat it is placed in an iron strap, such as shown in Fig. 2710, having an eye at each end in which a hook may be inserted to hold the piece in shape (after it is bent) until it is dry again. The piece with this strap on its outside or uppermost surface is laid on the _former_ or forming piece shown in Fig. 2711, which has a projection at A, fitting into the recess A of the bending block in Fig. 2712. On the outside of the piece is then placed the strap, shown in Fig. 2713, its blocks of wood fitting to the ends of the piece to be bent.

The winch of the bending block is provided with a rope, whose ends have two hooks which are engaged in the eyes of the straps, shown in Fig.

2714, and by operating the winch the piece is bent to shape, as shown in Fig. 2714. While in this position a hook is placed through the eyes of the band that is around the bent piece of work, so that when removed from the forming block or stand it appears as in Fig. 2715.

[Ill.u.s.tration: Fig. 2713.]

[Ill.u.s.tration: Fig. 2714.]

[Ill.u.s.tration: Fig. 2715.]

[Ill.u.s.tration: Fig. 2716.]

[Ill.u.s.tration: Fig. 2717.]

[Ill.u.s.tration: Fig. 2718.]

When, however, the piece requires to be bent to more than one sweep or bend, the process requires to be changed somewhat. Thus, suppose the middle is to be bent circular and the two ends left straight, and the strap on the piece to be bent is provided with a piece, such as in Fig.

2716, the ends B engaging in eyes in the strap, and the screw A ab.u.t.ting against the end of the piece to bind the strap firmly upon the ends, as in Fig. 2717, in which the piece is shown within the strap. After it has been bent to the former it is held there by straps and wedges, as shown in Fig. 2718.

[Ill.u.s.tration: Fig. 2719.]

The next operation is to lock the curve, as shown in Fig. 2719, between an inside and outside former by means of straps A A and wedges C, when the ends D of the piece may be bent up to the dotted lines and locked to the ends of the top former by straps and wedges.

The length of time a piece should be boiled or steamed for the bending process depends upon the size of the piece and the kind of wood, hard wood requiring longer boiling or steaming. A piece of ash, say 2 by 4 inches in cross section, would require about six hours' steaming with a low pressure of moist or wet steam, but it would not suffer damage if it were steamed for a day. Pieces not over half an inch thick may be bent after steaming them about half an hour.

If the wood is steamed too much it loses its elasticity and will pucker on the inside surface of the bend when in the former or bending block.

The period during which the piece should be held to its bent shape before being released varies from twelve hours for thin pieces to twenty-four hours for thick ones, and it is found that pieces which have been bent in a strap so as to prevent the outside from stretching, will, in drying, increase their bend or curvature, while those not confined at their ends straighten out.

[Ill.u.s.tration: Fig. 2720.]

[Ill.u.s.tration: Fig. 2721.]

[Ill.u.s.tration: Fig. 2722.]

The cracks that are found in timber are termed _shakes_; thus in Fig.

2720 the black lines represent what are called heart shakes, while those in Fig. 2721, being wider, are termed star shakes. When the shakes are circular, as in Fig. 2722, they are called cup shakes.

Many of the tools used by the pattern-maker have been described in connection with hand turning, hand boring tools, lathe tools, &c., and therefore need no further reference.

PLANES.--For roughing out the work the jack plane is employed, varying in size from 14 inches long with a cutter knife or blade 2 inches wide, to 27 long with a blade 2-1/4 inches wide, and as its purpose is to make a flat surface, it is preferable that it be as long as the work will conveniently permit. The jack plane is followed by the fore plane, the truing, or trying plane, which varies in size from about 18 inches long with a blade 2-1/8 inches wide, to 20 inches long with a cutter or blade 2-3/8 inches wide. When the fore plane is made longer, as for planing long joints, it is termed a jointer plane, the length being as much as 30 inches and the blade 2-5/8 inches wide.

The smoothing plane varies from about 5 inches long with a blade 1-1/2 inches wide, to 10 inches long with a blade 2-3/8 inches wide. Smoothing planes are, as the name implies, used to simply smoothen the work surface after it has been trued.

The angle of the plane blade to the sole of the plane is for ordinarily soft wood 45, but 50 or 55 may be used for very hard woods.

[Ill.u.s.tration: Fig. 2723.]

[Ill.u.s.tration: Fig. 2724.]

To break the shaving the blade is attached to what is termed a cover, which is shown in Fig. 2723, B representing the blade and A the cover.

The cover is curved to insure that it shall bed against the blade at its very end, and, therefore, as near to the cutting edge as a maximum distance 1/16 inch for rough and 1/32 inch for finis.h.i.+ng cuts. The blade of a jack plane is most efficient when it is ground well away towards the corners, as at A B in Fig. 2724, thus producing an edge curved in its length.

When the blade is in position in the stock for cutting off the maximum of stuff, its blade should project nearly 1/16 through the sole of the stock, while the corners A B are about level with the face of the stock.

The bevelled face should stand at about an angle of 25 to the flat face. In grinding it care should be taken to grind it as level as possible, rounding off the corners as shown above. The grindstone should be kept true and liberally supplied with water; the straight face should not be ground away, nor indeed touched upon the stone, except to remove the burr which will sometimes turn over. The pressure with which the blade is held against the grindstone should be slight at and toward the finis.h.i.+ng part of the grinding process, so as not to leave a long ragged burr on the end of the blade, as is sure to be the case if much pressure is applied, and it will occur to a slight extent even with the greatest of care. The blade should not be held still upon the grindstone, no matter how true, flat, or smooth the latter may be; but it should be moved back and forth across the width of the stone, which will not only grind the blade bevel even and level, but will also tend to keep the grindstone in good order.

In oilstoning a plane blade, the straight face should be held quite level with the face of the oilstone, so that the cutting edge may not be bevelled off. Not much application to the oilstone is necessary to the straight face, because that face is not ground upon the grindstone, and it only requires to have the wire edge or burr removed, leaving an oilstone polish all along the cutting edge. The oilstoning should be performed alternately on the flat and bevelled faces, the blade being pressed very lightly on the oilstone toward the last part of the operation, so as to leave as fine a wire edge as possible. The wire is the edge or burr which bends or turns over at the extreme edge of the tool, in consequence of that extreme edge giving way to the pressure of the abrading tool, be it a grindstone or an oilstone. This wire edge is reduced to a minimum by the oilstone, and is then so fine that it is practically of but little account; to remove it, however, the plane blade or iron may be buffed backwards and forwards on the palm of the hand.

The blade being sharpened, we may screw the cover on, adjusting it so that its edge stands a shade below the corners of the iron, and then s.c.r.e.w.i.n.g it tight; the blade or iron and the cover must now be placed in the mouth of the plane stock, and adjusted in the following manner:--

The plane iron should be pa.s.sed through the mouth of the stock until as much in depth of it is seen to protrude from the bottom face of the stock as is equal to the thickness of shaving it is intended to cut: to estimate which, place the back end of the plane upon the bench, holding the stock in the left hand with the thumb in the plane mouth, so as to retain the iron and wedge in position, the wedge being turned towards the workman. A glance down the face of the stock will be sufficient to inform the operator how much or how little the cutting edge of the iron protrudes from the face of the plane stock, and hence how thick his shaving will be. When the distance is adjusted as nearly as possible, the wedge may then be tightened by a few light hammer blows. If, after tightening the wedge, the blade is found to protrude too much, a light blow on the fore end on the top face of the plane will cause it to retire; while a similar blow upon the back end will cause it to advance.

In either case the wedge should be tightened by a light blow after it is finally adjusted.

In using a jack plane we commence each stroke by exerting a pressure mostly on the fore part of the plane, commencing at the end and towards the edge of the board, and taking off a shaving as long as the arms can conveniently reach. If the board is longer than can be reached without moving, we pa.s.s across the board, planing it all across at one standing; then we step sufficiently forward, and carry the planing forward, repeating this until the jack planing is completed. To try the level of the board, the edge or corner of the plane may be employed; and if the plane is moved back and forth on the corner or edge, it will indent and so point out the high places.

The fore plane (or truing plane, as it is sometimes called) is made large, so as to cover more surface, and therefore to cut more truly. It is ground and set in the same manner as the jack plane, with the exception that the corners of the iron or blade, for about one-eighth inch only, should be ground to a very little below the level of the rest of the cutting edge, the latter being made perfectly straight (or as near so as practically attainable) and square with the edge of the iron.

If the end edge of the cover is made square with the side edge, and the iron is ground with the cover on, the latter will form a guide whereby to grind the iron edge true and square; but in such case the cover should be set back so that there will be no danger of the grindstone touching it. The oilstoning should be performed in the manner described for the jack plane, bearing in mind that the object to be aimed at is to be able to take as broad and fine a shaving as possible without the corners of the plane iron digging into the work. The plane iron should be so set that its cutting edge can only just be seen projecting evenly through the stock. In using the fore or truing plane, it is usual, on the back stroke, to twist the body of the plane so that it will slide along the board on its edge, there being no contact between the cutting edge of the plane iron and the face of the board, which is done to preserve the cutting edge of the plane iron from abrasion by the wood: as it is obvious that such abrasion would be much more destructive to the edge than the cutting duty performed during the front stroke would be, because the strain during the latter tends mainly to compress the metal, but, during the former, the whole action tends to abrade the cutting edge. The face of the fore plane must be kept perfectly flat on the underside, which should be square with the sides of the plane. If the under side be hollow, the plane iron edge will have to protrude farther through the plane face to compensate for the hollowness of the latter; and in that case it will be impossible to take fine shavings off thin stuff, because the blade or iron will protrude too much, and as a consequence there will be an unnecessary amount of labor incurred in setting and resetting the plane iron. The reason that the under surface should be square, that is to say, at a right angle to the sides of the body of the plane, is because the plane is sometimes used on its side on a shooting board.

When the under surface of the plane is worn out of true, let the iron be wedged in the plane mouth, but let the cutting edge of the iron be well below the surface of the plane stock. Then, with another fore plane, freshly sharpened and set very fine, true up the surface, and be sure the surface does not wind, which may be ascertained by the application of a pair of winding strips, the manner of applying which will be explained hereafter. If the mouth of a fore plane wears too wide, as it is apt in time to do, short little shavings, tightly curled up, will fall half in and half out of the mouth, and prevent the iron from cutting, and will cause it to leave scores in the work, entailing a great loss of time in removing them at every few strokes. The smoothing plane is used for smoothing rather than truing work, and is made shorter than the truing plane so as to be handier in using. It is sometimes impracticable to make a surface as smooth as desirable with a truing plane, because of the direction of the grain of the wood.

[Ill.u.s.tration: Fig. 2725.]

Fig. 2725 represents an ordinary compa.s.s plane, which is a necessary and very useful tool for planing the surfaces of hollow sweeps. This tool is sometimes made adjustable by means of a piece dovetailed in the front end of the plane, which, by being lowered, alters the sweep and finally converts it from a convex to a concave.

[Ill.u.s.tration: Fig. 2726.]

In Fig. 2726 is shown a much superior form of circular or compa.s.s plane.

Its sole consists of a flexible steel blade, whose ends are attached to levers that are connected together by toothed segments. By means of the large hand-screw the levers are operated, causing the sole to bend to the required curvature, and by reason of the toothed segments the levers move equally, and therefore give the sole a uniform curve throughout its length.