Modern Machine-Shop Practice Part 64

[Ill.u.s.tration: Fig. 889.]

It is obvious that the pin may be pa.s.sed through one of the radial slots in the chuck, and set the required distance from the centre, but in this case the pin would be liable to become moved in its position in the slot.

Side plates to prevent the link from moving should of course be applied as at D, D in the figure.

The whole process of the second chucking will thus consist of fastening the links on the pin, and setting the free end to the circle made to mark its location. This is done as shown in Fig. 890, which represents the free end of a link, D is the circle marked to set the link by, and P a pointed tool held firmly in the slide rest tool post. The link is obviously set true when the dotted circle on its end face runs true, the pointer merely serving to test the dotted circle.

[Ill.u.s.tration: Fig. 890.]

When, however, one or two links only require to be turned it will not pay to make the pins shown in Fig. 888, especially if the holes of the different links vary in diameter, hence the work must be set by lines.

In the promiscuous practice of the general workshop, where it may and often does happen that two pieces of work are rarely of the same shape and size, lines whereby to set the work are an absolute necessity, not only to set the work by in chucking it, but also to denote the quant.i.ty of metal requiring to be taken off one face in order to bring its distance correct with relation to other faces. An example of this kind is given in Fig. 891, which represents a lever to be bored and faced at the two ends, the radial faces standing at different distances from the centre of the lever stem as denoted by the lines (defined by centre punch dots) E, F, G, H, I, J, K, L. It will be noted that at H, I, F, and E there is but little metal to be taken off, while there is ample at L. Suppose then that the face L were the first one turned, and it was only just trued up, then when F or H were turned there would be no metal to turn, for they may be too near the plane of L already.

[Ill.u.s.tration: Fig. 891.]

The necessity for these lines now being shown, we may proceed to show how they should be located and their services in setting the work. The line A is called the centre line, it pa.s.sing through the centre of the thickness of the link body on both edges of the link. From it all the other lines, as J, F, L, G, E, K, and H, I, are marked.

The first question that arises in the chucking is, which of the holes B, C, or D, shall be bored first. Now the faces K and L are those that project farthest from the centre line A, hence if the hole at that end be bored and the faces K, L, be turned first, we may bolt those faces against the chuck plate, and thus insure that all three holes shall stand axially true one with the other. If the holes B or C were bored first, L projecting beyond J and F (which are the faces of holes B, C) would prevent the radial face first turned from serving as a guide in the subsequent chuckings, unless a parallel piece were placed between the face and the chuck. In this case, however, there is not only the extra trouble of using the parallel piece, but there would obviously be more liability of error, as from the parallel piece not being dead true and the amount of the error multiplying in the length of the lever, and so on.

The hole D is the one, therefore, to be bored first, the chucking proceeding as follows:--Two parallel pieces of sufficient thickness to keep L clear of the chuck plate should be placed one on each side of the hub E, and bolts and plates placed directly over them. The work must be set so that the line A on each side of the link stands exactly parallel with the face of the chuck, the parallelism being tried at each end of the line, because any error that may be made in setting the work by the full length of the line will have a less effect upon the work than the same amount of error in a shorter length of line. For this reason the centre line should always be marked as long as possible and used to set by, unless there is a longer line running parallel to it and marked on both sides of the link, as would be the case if the dotted line at J and that at L were equidistant from A, in which event they may preferably be used.

The work is set true to the lines by a scribing block, or surface gauge, but as that instrument is more used in setting work with chuck dogs its application will be shown in connection with chucking by dogs; hence to proceed: To set the work true to the line A it may be necessary to place a thickness of paper, a piece of sheet tin, or the equivalent, beneath one of the parallel pieces to bring A parallel with the chuck plate surface. This being done, however, and the circle D being set to run true, the hole may be bored and the radial face L turned off so as to just split the dotted line at L, and this radial face may be used instead of the line A for all subsequent chuckings, so as to avoid the errors that might occur in referring to the line, and from the alterations that might occur in the form of the work from removing the surface metal.

[Ill.u.s.tration: Fig. 892.]

Fig. 892 represents a view of the end L as held for the second chucking.

C is a section of the chuck plate, and O O represents the line of centres of the lathe, and it is obvious that the radial face of the lever end (which is here represented by L) being used for all but the first chucking, the holes will all stand axially true one with the other, no matter how many chuckings and holes there may be, hence it becomes obvious that the face that will meet the chuck plate is the one that should be turned at the first chucking. It is of no consequence in the case of a single lever whether the pin fits the hole in the end of L, Fig. 892, or not, because the dotted circles at B, C, D in Fig. 891 form the guides whereby to set the holes for distance apart, and any bolt may be used to clamp the work.

It is usual in an example of this kind to turn the stem of the lever to its proper thickness for a short distance from the hubs, so as to have the stem true with the bores, and form a guide whereby to set the lever in the planer or shaper when cutting down the lever stem to size. The rules of chucking and the balance weighting described with reference to chucking a crank, of course also apply to this example.

It will now be observed that in all cases in which work is chucked by bolts and plates, the whole of the faces cannot be turned at one chucking unless the shape of the work is such that it will permit the plates and the bolts to pa.s.s or be below the level of the work surface.

It will further be noticed that if one face of the work is held against the chuck surface it cannot be turned at the same chucking that the other face is turned at. Now it may be very desirable that a part or the whole of the back face as well as the front one be turned at the same chucking as that at which the hole is bored, so as to have the hole and those two faces true without incurring the errors that might arise from a second chucking. Again, the diameter of the work may be equal to that of the chuck so as to preclude the possibility of using bolts and plates outside of the circ.u.mference, and though there be cavities or slots running through the work through which the bolts might be pa.s.sed, yet the presence of the plates would prevent the face from being turned.

[Ill.u.s.tration: Fig. 893.]

To meet these and many other requirements that might be named, chucking by the aid of chucking dogs is resorted to, one of these dogs being shown in Fig. 893. B represents a section of the chuck plate with a piece broken out to show the stem A of the dog, which is squared to prevent its revolving when the nut D, which holds the dog to the chuck plate, is tightened, the holes of the chuck, of course, being square also; E is the set-screw which holds the work, its end at E being turned down below the thread, and the head squared to receive a wrench.

[Ill.u.s.tration: Fig. 894.]

Fig. 894 represents an example of chucking by dogs, it being required to face the work off to the dotted line F F. Three of the four dogs used are shown at D, D, D. To set the work the scribing block shown in the figure is employed, the point of the needle being set to the line at any one spot, and the scribing block or surface gauge carried around the work rested with its base against the chuck plate and the needle point tried for coincidence with the line at various points in the work's circ.u.mference. The work is not at first held too firmly by the dogs, so that light blows will suffice to so move the work that the surface gauge needle point applied as shown and at any point around the work will coincide with the line. It will here be observed that using the dogs obviates the necessity for parallel pieces, when the work has projections at the back face as shown in the cut.

[Ill.u.s.tration: Fig. 895.]

Fig. 895 represents another example in chucking by dogs. It is required to surface the whole of the surfaces shown, to bore the hole C and to face a face similar to A, but on the other side or chuck side of the work. Then the work is placed so that its outer face will project beyond the extreme surface of the dogs, and the whole of the operations can be performed at one chucking. It will be observed that in this case the surface of the chuck plate does not automatically serve to guide the work in the chucking, because there is no contact between the two, but the chuck surface can be used as a guide whereby to chuck the work as has just been shown. Or suppose the work to require to be set as true as can be to its exposed face, then the work end of the surface gauge is applied as shown in Fig. 896 at E.

[Ill.u.s.tration: Fig. 896.]

[Ill.u.s.tration: Fig. 897.]

[Ill.u.s.tration: Fig. 898.]

The surface gauge may indeed be dispensed with if the work is sufficiently light that the lathe can be swung around by pulling the chuck plate with the hand, and the work merely requires to be set to run true on its exposed radial face. A pointer held in the slide rest, and applied as in Fig. 890, will denote the setting of the work, which must be tapped until the pointer touches it equally on four equidistant points of the surface; but if it is essential to take as little as possible off the face while truing it up, the tool point should be held stationary, while the work should be so set that the four most distant points (in that circle on the work which is equivalent in radius to the radius to which the tool point stands from the chuck centre) are equidistant as measured by a rule from the tool point. The philosophy of this will be understood from a reference to Fig. 894 and the remarks thereon, this being a parallel case, but applied to a radial face instead of to a circ.u.mference.

Now suppose we have the piece of work shown in Fig. 897, which requires to have its surfaces A and B parallel and at a right angle to C and D, the end faces E and F parallel to each other, and at a right angle to both A, B, C, and D, the hole at G is to be axially true with the surfaces A, B, C, and D, as well as with the pin at I, and the hole at H at a dead right angle to that at G.

We may put a plug in G and turn up the surfaces E and F, and turn the pin I; this, however, would leave the hole G unbored, whereas it should be bored when the surface E is turned; again, after these surfaces are turned they are of no advantage as guides in the subsequent chuckings.

We may grip the surfaces E and F in a jaw chuck to turn the surfaces A, B, C and D, but depending upon the face jaws of the dogs to set the work surface true by; but this would not be apt to produce true work on account of the spring of the jaws, as explained in the remarks upon jaw chucks; furthermore, the work, supposing it to be a foot long, could not be held in a dog chuck sufficiently firmly to enable the turning of the end face E or the pin I, and this brings us to that most excellent adjunct to a general chucking lathe, the angle plate shown in Fig. 898.

It is simply a plate of the form shown in the figure, having two flat and true surfaces, one at a right angle to the other; one of these surfaces bolts to the chuck plate, while the other is to fasten the work on. The slots shown are to pa.s.s the bolts through to fasten the angle plate to the chuck plate, and the work surface of the plate contains similar slots and holes to receive the bolts used to fasten the work.

[Ill.u.s.tration: Fig. 899.]

Suppose, then, we fasten the piece of work to the angle plate as shown in Fig. 899, and face off the surface C, and bore the hole H, the work being set true with its surface, or to a line, by the aid of a surface gauge, as may be required. We then turn surface C down to meet the surface of the angle plate, fasten it to the same with bolts and plates and setting it as before, and on turning its surface A we shall have the two surfaces A and C at a right angle to one another. We then turn the surface A down upon the angle plate and bolt it again as before. But we have now to set it so that the surface C shall be quite parallel with the surface of the chuck plate. This we may do by placing one or more parallel strips behind it, as at S S, in the plan view, Fig. 900, setting the work so that it binds the parallel strips tight against the chuck plate along their full lengths; or we may measure the distance of C from the chuck plate surface with a pair of inside calipers; or we may turn the bent end of a surface-gauge needle outwards and gauge the work as shown in the plan view, trying the work all along. On turning the surface D, Fig. 897, we shall have three of the surfaces done at right angles and with C and D parallel.

[Ill.u.s.tration: Fig. 900.]

It is obvious that the surface D may be turned down on the angle plate and bolted as before, the surface A being set parallel to the chuck plate surface as before, and all four of these surfaces will be finished true as required. Next come the two end surfaces and the pin I. For F and the pin I we chuck the work on the angle plate, as shown in the plan view, Fig. 901, P, P representing the clamping-plates. The angle plate will here again serve to hold the work true one way, and all we have to do to set it true the other way is to fasten a pointer in the tool post and bring it up to just touch the corners of the work at the outer end, as at K. Now run the carriage up so as to bring the pointer to position L, and when the work is so set that all four corners just touch the pointer, tried in their two positions, _without touching the cross-feed screw_, the work is true, and the end surface E and hole G may be turned; E will then be at a true right angle to the four faces, A, B, C, D, while G will be axially true with them.

[Ill.u.s.tration: Fig. 901.]

We may, instead of using the pointer at K and L, or in addition to so using it, apply a square against the chuck plate and bring the blade against the work, as shown at R.

We have now to turn the pin I and end face, and to do this we simply reverse the work, end for end, and bolt it as before. But we may now employ the trued surface E as an aid in setting by causing it to abut against the chuck plate surface, and, as an aid to finding that it abuts fair, we may put two strips of the same piece of paper behind it, one on each side of the square, and, after the work is bolted, see that both are held firm; but it is necessary to test with the pointer as before, as well as with the square.

It is obvious that the angle plate requires counterbalancing, which is done by means of the weight W. (Fig. 900).

An excellent example of angle plate chucking is furnished in a pipe bend requiring both f.l.a.n.g.es to be turned up. The method of chucking is shown in Figs. 902 and 903, the f.l.a.n.g.es being simply bolted to the angle plate. The work may be set true to the body of the bend close to the neck of the f.l.a.n.g.e or by the circ.u.mference of the f.l.a.n.g.e. The face of the f.l.a.n.g.e will be held true one way by the face on the angle plate, but must be set true the other way. The truest f.l.a.n.g.e should be the one first bolted to the angle plate.

[Ill.u.s.tration: Fig. 902.]

[Ill.u.s.tration: Fig. 903.]

[Ill.u.s.tration: Fig. 904.]

A common but good example of angle plate chucking is shown in Fig. 904, which represents a cross head requiring to have its two holes bored one at a right angle to the other, the jaws faced inside and outside, and the hub or boss turned.

[Ill.u.s.tration: Fig. 905.]

It would be proper to mark the cross-head out by lines, giving dotted circles to set the work by, and dotted lines to give the thickness of the jaws. In thus marking out two centre lines A A and B B in Fig. 905 would be used to locate the centres of the holes; and the thickness of the jaws would be marked from the line B B. In marking these lines the cross head should be rested upon a table or plate as in Fig. 905, and the line A A should be made with the jaws of the cross head lying flat on the table, that is without the interposition of any packing or paper between them and the plate, so that the edges of the jaws on that side will be true with the line A A, and will therefore serve to apply a square against when chucking to bore the hole through the jaws. If the jaw edges are not sufficiently true to permit of their lying on the table, they should be made so by filing a flat place on them, so that when a square is applied to them as in Fig. 906, the edges C, C will be parallel with the axis A A of the holes in the chucks or jaws. The first chucking should be as in Fig. 907, the cross head being bolted to an angle plate set true by the circle on the end face of its hub D, and a square being applied to the centre line A, as in Fig. 908, and to the dotted lines on the jaws as shown in Fig. 909. A balance weight W, Fig.

907, is necessary to counterbalance the weight of the angle plate.

[Ill.u.s.tration: Fig. 906.]

The second chucking to bore the cheeks and face them inside and out to the required thickness would be as in Fig. 910, a single plate and two bolts being used to hold the cross head to the angle plate. To set the cross head true in one direction, the outer circle shown marked upon the face of the cheek is used.

It remains to so set the face of the cheeks that the hole through them shall be central with that already bored through the hub D and all that is necessary to accomplish this is to set the edge true as shown in the top view in Fig. 911, in which S is a square rested against the face of the chuck and applied to the edges of the cheeks, these edges being those that were rested on the plate when marking the line A A in Fig.

905, or that were filed square if it was found necessary as already mentioned.