Modern Machine-Shop Practice Part 103

For marking upon one surface a line parallel to another surface, the scribing block or surface gauge shown in Fig. 1458 is employed. It consists of a foot piece or stand D, carrying a stem. In the form shown this stem contains a slot running centrally up it. Through this slot pa.s.ses a bolt whose diameter close to the head is larger than the width of the slot, so that it is necessary to file flat places on the side of the slot to permit the bolt to pa.s.s through it.

On the stem of the bolt close to the head, and between the bolt head and the stem of the stand, pa.s.ses the piece shown at F. This consists of a piece of bra.s.s having a full hole through which the bolt pa.s.ses clear up to the bolt head. On the edge view there is shown a slot, and on each side of the slot a section of a hole to receive a needle. A view of the bolt is given at E, the flat place to fit the slot in the stem being shown in dotted lines, and the s.p.a.ce between the flat place and the bolt head is where the piece of bra.s.s, shown in figure, pa.s.ses. This piece of bra.s.s being placed on the bolt, and the bolt being pa.s.sed through the slot in the stem, the needle is pa.s.sed through the split in the bra.s.s, and the thumb-nut is screwed on so that tightening up the thumb-nut causes the needle to be gripped in the bra.s.s split in any position in the length of the stem slot in which the bolt may be placed. The advantage of this form over all others is that the needle may be made of a simple piece of wire, and therefore very readily. Again, the piece of bra.s.s carrying the needle may be rotated upon the pin any number of consecutive rotations backwards and forwards, and there is no danger of slacking the thumb-nut, because the needle is on the opposite side of the stem to what the thumb-nut is, and the flat place prevents the bolt from rotating. Furthermore, the needle can be rotated on the bolt for adjustment for height without becoming loosened, whereas when the thumb-nut is screwed up firmly the needle is held very fast indeed, and finally all adjustments are made with a single thumb-nut.

The figure represents a view of this gauge from the bolt head and needle side of the stem, the thumb-nut being on the opposite side.

This tool finds its field of application upon lathe work, planer work, and, indeed, for one purpose or another upon all machine tools, and in vice work and erecting, examples of its employment being given in connection with all these operations.

[Ill.u.s.tration: Fig. 1459.]

Fig. 1459 represents a scribing block for marking the curves to which to cut the ends of a cylindrical body that joins another, as in the case of a [T]-pipe. It is much used by pattern-makers. In the figure A is a stem on a stand E. A loose sleeve B slides on A carrying an arm C, holding a pencil at D. A piece of truly surfaced wood or iron W, has marked on it the line J. Two [V]s, G, G, receive the work P. Now, if the centres of G, G and of the stand E all coincide with the line J then E will stand central to P, and D may be moved by the hand round P, being allowed to lift and fall so as to conform to the cylindrical surface of P, and a line will be marked showing where to cut away the wood on that side, and all that remains to do is to turn the work over and mark a similar line diametrically opposite, the second line being dotted in at K.

[Ill.u.s.tration: Fig. 1460.]

The try square, Fig. 1460, is composed of a rectangular back F, holding a blade, the edges of the two being at a right angle one to the other and as straight as it is possible to make them. The form shown in the figure is an [L]-square.

[Ill.u.s.tration: Fig. 1461.]

Fig. 1461 represents the [T]-square, whose blade is some distance from the end of the back and is sometimes placed in the middle. When the square edges are at a true right angle the square is said to be true or square, the latter being a technical term meaning at practically a true right angle.

The machinists' square is in fact a gauge whereby to test if one face stands at a right angle to another. It is applied by holding one edge firmly and fairly bedded against the work, while the other edge is brought to touch at some part against the face to be tested.

If in applying a square it be pressed firmly into the corner of the work, any error in the latter is apt to escape observation, because the square will tilt and the error be divided between the two surfaces tested. To avoid this the back should be pressed firmly against one surface of the work and the square edge then brought down or up to just touch the work, which it will do at one end only if the work surface is out of square or not at a right angle to the face to which the square back is applied.

[Ill.u.s.tration: Fig. 1462.]

An application of the [T]-square is shown in Fig. 1462, in which W is a piece of work requiring to have the face A of the jaw C at a right angle to the face B C. Sometimes the [L]-square is employed in conjunction with a straight-edge in place of the [T]-square. This is usually done in cases where the faces against which the square rests are so far apart as to require a larger [T]-square than is at hand. It is obvious that if the face A of the work is the one to be tested, the edge B is the part pressed to the work; or per contra, if B C is the face to be tested, the edge of the blade is pressed to the work.

[Ill.u.s.tration: Fig. 1463.]

The plane of the edges of a square should, both on the blade and on the back, stand at a right angle to the side faces of the body or stock, and the side of the blade should be parallel to the sides of the back and not at an angle to either side, nor should it be curved or bent, because if under these conditions the plane of the square edge is not applied parallel with the surface of the work the square will not test the work properly. This is shown in Fig. 1463, in which W is a piece of work, and S a square having its blade bent or curved and applied slightly out of the vertical, so that presuming the plane of the blade edge to be a right angle to the stock or back of the square the plane of the blade edge will not be parallel with the plane of the work, hence it touches the work at the ends A B only, whereas if placed vertically the blade edge would coincide with the work surface all the way along. It is obvious then that by making the edge of the blade at a right angle, crossways as well as in its length, to the stock, the latter will serve as a guide to the eye in adjusting the surface of the blade edge parallel to that of the work by placing the stock at a right angle to the same.

[Ill.u.s.tration: Fig. 1464.]

There are three methods of testing the angle of a square blade to the square back. The first is shown in Fig. 1464, in which A is a surface plate having its edge a true plane. The square S is placed in the position shown by full lines pressed firmly to the edge of the surface plate and a fine line is drawn with a needle point on the face of the surface plate, using the edge of the square blade as denoted by the arrow C as a guide. The square is then turned over as denoted by the dotted lines and the edge is again brought up to the line and the parallelism of the edge with the line denotes the truth, for whatever amount the blade may be out of true will be doubled in the want of coincidence of the blade edge with the line.

[Ill.u.s.tration: Fig. 1465.]

A better plan is shown in Fig. 1465, in which A is the surface plate, B a cylindrical piece of iron turned true and parallel in the lathe and having its end face true and cupped as denoted by the dotted lines so as to insure that it shall stand steadily and true. The surface of A and the vertical outline of B forming a true right angle we have nothing to do but make the square S true to them when placed in the position shown.

[Ill.u.s.tration: Fig. 1466.]

[Ill.u.s.tration: Fig. 1467.]

[Ill.u.s.tration: Fig. 1468.]

If we have two squares that are trued and have their edges parallel, we may test them for being at a right angle by trying them together as in Figs. 1466 and 1467, in which A, B, are the two squares which, having their back edges pressed firmly together (when quite clean), must coincide along the blade edges; this being so we may place them on a truly surfaced plate as shown in Fig. 1468, in which S is one square and S' the other, P being the surface plate. Any want of truth in the right angle will be shown doubled in amount by a want of coincidence of the blade edges.

[Ill.u.s.tration: Fig. 1469.]

For some purposes, as for marking out work on a surface plate, it is better that the square be formed of a single piece having the back and blade of equal thickness, as shown in Fig. 1469, which represents a side and edge view of an [L]- and [T]-square respectively.

[Ill.u.s.tration: Fig. 1470.]

For angles other than a right angle we have the bevel or bevel square (as it is sometimes called), shown in Fig. 1470, A representing the stock or back, and B the blade, the latter being provided with a slot so that it may be extended to any required distance (within its scope) on either side of the stock. C is the rivet, which is made sufficiently tight to permit of the movement by hand of the blade, and yet it must hold firmly enough to be used without moving in the stock. Instead of the rivet C, however, a thumb-screw and nut may be employed, in which case, after the blade is set to the required angle, it may be locked in the stock by the thumb-screw.

Fig. 1471 represents a Brown and Sharpe bevel protractor, with a pivot and thumb-nut in the middle of the back with a half-circle struck from the centre of the pivot and marked to angular degrees. The pointer for denoting the degrees of angle has also a thumb-screw and nut so that the blade may, by loosening the pivot and pointer, be moved to project to the required distance on either side of the back.

[Ill.u.s.tration: Fig. 1471.]

[Ill.u.s.tration: Fig. 1472.]

Swasey's improved protractor, however, is capable of direct and easy application to the work, forming a draughtsman's protractor, and at the same time a machinist's bevel or bevel square, while possessing the advantage that there is no protruding back or set-screw to prevent the close application of the blade to the work. This instrument is shown in Fig. 1472. The blade A is attached to the circular piece D, the latter being recessed into the square B B, and marked with the necessary degrees of angle, as shown, while the mark F upon the square B serves as an index point. The faces of A, B B, and D are all quite level, so that the edges will meet the lines upon the work and obviate any liability to error. The piece D is of the shape shown in section at G, which secures it in B B, the fit being sufficient to permit of its ready adjustment and retain it by friction in any required position. The dotted lines indicate the blade as it would appear when set to an angle, the point E being the centre of D, and hence that from which the blade A operates.

[Ill.u.s.tration: Fig. 1473.]

On account, however, of the numerous applications in machine work of the hexagon (as, for instance, on the sides of both heads and nuts), a special gauge for that angle is requisite, the usual form being shown in Fig. 1473. The edges A, B, form a hexagon gauge, and edges C, D, form a square, while the edge E serves as a straight-edge.

All these tools should be made of cast steel, the blades being made of straight saw blade, so that they will not be apt to permanently set from an ordinary accidental blow; while, on the other hand, if it becomes, as it does at times, necessary to bend the blade over to the work, it will resume its straightness and not remain bent.

For testing the straightness, in one direction only, of a surface the straight-edge is employed. It consists in the small sizes of a piece of steel whose edges are made straight and parallel one to the other. When used to test the straightness of a surface without reference to its alignment with another one, it is simply laid upon the work and sighted by the eye, or it may have its edge coated with red marking, and be moved upon the work so that its marking will be transferred to the high spots upon the work. The marking will look of the darkest colour in the places where the straight-edge bears the hardest. The most refined use of the straight-edge is that of testing the alignment of one surface to the other, and as this cla.s.s of work often requires straight-edges of great length, as six or ten feet, which if made of metal would bend of its own weight, therefore they are made of wood.

[Ill.u.s.tration: Fig. 1474.]

Fig. 1474 represents an example of the use of straight-edge for alignment purposes. It represents a fork and connecting rod, and it is required to find if the side faces of the end B are in line with the fork jaws. A straight-edge is held firmly against the side faces of B in the two positions S and S', and it is obvious that if they are in line the other end will be equidistant from the jaw faces, at the two measurements.

[Ill.u.s.tration: Fig. 1475.]

[Ill.u.s.tration: Fig. 1476.]

[Ill.u.s.tration: Fig. 1477.]

Figs. 1474, 1475, 1476, 1477, and 1478 represent the process of testing the alignment of a link with a straight-edge. First to test if the single eye E is in line with the double eye F at the other end, the straight-edge is pressed against the face of E, as in Fig. 1475, and the distance I is measured. The straight-edge is then applied on the other side of E, as in Fig. 1476, and the distance H is measured, and it is clear that if distances H and I are equal, then E is in line with the double eye. To test if the double eye F is in line with the single eye E, the straight-edge is pressed against the face of the double eye in the positions shown in Figs. 1477 and 1478, and when distances J and K measure equal the jaws of the double eye F are in line with those of the single eye E.

[Ill.u.s.tration: Fig. 1478.]

[Ill.u.s.tration: Fig. 1479.]

[Ill.u.s.tration: Fig. 1480.]

[Ill.u.s.tration: Fig. 1481.]

[Ill.u.s.tration: Fig. 1482.]

[Ill.u.s.tration: Fig. 1483.]

It is obvious, however, that we have here tested the alignment in one direction only. But to test in the other direction we may use a pair of straight-edges termed winding strips, applying them as in Fig. 1479, to test the stem, and as in Fig. 1480 to test the eye E, and finally placing the winding strip C on the eye of F while strip D remains upon E, as in Fig. 1480. The two strips are sighted together by the eye, as is shown in Fig. 1481, in which S and S" are the strips laid upon a connecting rod, their upper edges being level with the eye, hence if they are not in line the eye will readily detect the error. Fig. 1482 represents an application to a fork ended connecting rod. Pattern-makers let into their winding strips pieces of light-coloured wood as at C, C, C, C, in Fig. 1483, so that the eye may be a.s.sisted in sighting them.

It is obvious that in using winding strips they should be parallel one to the other; thus, for example, the ends A, B, in Fig. 1481, should be the same distance apart as ends C, D.