Modern Machine-Shop Practice Part 65

[Ill.u.s.tration: Fig. 912.]

The inside faces of the cheeks are turned to the dotted lines shown in Fig. 909, and the outside faces being turned each to the proper thickness measured from the outside ones, the job will be complete and true in every direction.

[Ill.u.s.tration: _VOL. I._ =EXAMPLES IN ANGLE-PLATE CHUCKING.= _PLATE XII._

Fig. 907.

Fig. 908.

Fig. 909.

Fig. 910.

Fig. 911.]

An excellent example of angle plate chucking is shown in Fig. 912--the actual dimension of the piece, measuring, say, 24 inches in length. It is required to have the cylindrical stems A, B turned parallel to each other, of equal diameters, equidistant from the central hole C, and true with the hub D. A large piece of work of this kind would be marked off with lines defined by centre-punch dots, as shown. The ends of A, B, D would require dotted circles to set them by. Now, in all work of this kind it is advisable to turn that surface first that will afford the greatest length of finished surface, to serve as a guide for the subsequent chucking, which in this case is the hub D, and the face on that side as denoted by the dotted line which has to be cut to that line. The method of chucking would, for this purpose, be as in Fig. 913.

[Ill.u.s.tration: Fig. 913.]

[Ill.u.s.tration: Fig. 914.]

[Ill.u.s.tration: Fig. 915.]

The second chucking would be as in Fig. 914 to bore the hole at C, while, at the same time, the surface from F to G may be turned. Either inside calipers or a surface gauge may be employed to set E E parallel to the chuck plate surface. It is supposed that the location C is defined by a dotted circle, by which the work may be set for concentricity, as should be the case. At the next chucking it will simply be necessary to move the work on the angle plate to the position shown in Fig. 915, setting the circle on the end of A to run true, and the surface E parallel to the chuck surface as before. The third chucking is made by simply moving the work on the angle plate again, and setting as in the last instance.

CHAPTER X.--CUTTING TOOLS FOR LATHES.

The cutting tools for lathes are composed of a fine grain of cast steel termed "tool-steel," and are made hard, to enable them to cut, by heating them to a red heat and dipping them in water, and subsequently reheating them to temper them or lower their degree of hardness, which is necessary for weak tools.

These cutting tools may be divided into two princ.i.p.al cla.s.ses, viz., slide rest tools, or those held in the slide rest, and hand tools, which are held by hand.

The latter, however, have lost most of their former importance in the practice of the machine shop, by reason of the employment of self-acting lathes.

The proper shape for lathe slide rest tools depends upon--

1st. The kind of metal to be cut.

2nd. Upon the amount of metal to be cut off.

3rd. Upon the purpose of the cut, as whether to rough out or to finish the surface.

4th. Upon the degree of hardness of the metal to be cut.

5th. Upon the distance the tool edge is required to stand out from the tool clamp, or part that supports it.

Lathe tools are designated either from the nature of their duty, or from some characteristic peculiar to the tool itself.

The term "diamond point" is given because the face of the tool is diamond shaped; but in England and in some practice in the United States the same tool is termed a front tool, because it is employed on the front of external work.

A side tool is one intended for use on the side faces of the work, as the side of a collar or the face of a face plate. An outside tool is one for use on external surfaces, and an inside one for internal, as the walls or bores of holes, &c.

A spring tool is formed to spring or yield to excessive pressure rather than dig or jump into the work.

A boring tool is one used for boring purposes.

[Ill.u.s.tration: Fig. 916.]

[Ill.u.s.tration: Fig. 917.]

The princ.i.p.al forms of cutting tools for lathes are the diamond points or front tools, the side tools (right and left), and the cutting off or parting tool. The cutting edges of lathe tools are formed by grinding the upper surface, as _a_ in Fig. 916, and the bottom or side faces as _b_, so that the cutting edges _c_ and _d_ shall be brought to a clean and sharp edge, the figure representing a common form of front tool. The manner in which this tool is used to cut is shown in Fig. 917, in which the work is supposed to be revolved between the lathe centres in the manner already described with reference to driving work in the lathe.

The tool is firmly held in the tool post or tool clamp, as the case may be, and is fed into the work by the cross-feed screw taking a cut to reduce the work diameter and make it cylindrically true; the depth to which the tool enters the work is the depth of the cut. The tool is traversed, or fed, or moved parallel to the work axis, and the motion in that is termed the feed, or feed traverse.

[Ill.u.s.tration: Fig. 918.]

The cutting action of the tool depends upon the angles one to the other of faces B, D (Fig. 918), and the position in which they are presented to the work, and in discussing these elements the face D will be termed the top face, and its inclination or angle above an horizontal line, or in the direction of the arrow in Fig. 918, will be termed the rake, this angle being considered with relation to the top A A, or what is the same thing, the bottom E E of the tool steel. The angle of the bottom face B to the line C is the bottom rake, or more properly, the clearance.

[Ill.u.s.tration: Fig. 919.]

In the form of diamond point or front tool, shown in Fig. 916, there is an unnecessary amount of surface to grind at _b_, hence the form shown in Fig. 919 is also employed on light work, while it is in its main features also employed on large work, hence it will be here employed in preference to that shown in Fig. 916, the cutting action of the two being precisely alike so long as the angles of the faces are equal in the two tools.

The strength of the cutting edge is determined by the angles of the rake and clearance, but in this combination the clearance has the greater strength value. On the other hand the keenness of the tool though dependent in some degree upon the amount of clearance, is much more dependent upon the angle of the top face.

It follows therefore that for copper, tin, lead, and other metals that may be comparatively easily severed, a tool may be given a maximum of top rake, and it is found in practice that top rake can be employed to advantage upon steel, wrought iron, and cast iron, but the amount must be decreased in proportion as the nature of either of those metals is hard.

For the combinations of copper and tin which are generally termed bra.s.s or composition, either no top rake or negative top rake is employed according to the conditions.

[Ill.u.s.tration: Fig. 920.]

It may be pointed out, however, that in a given tool the cutting qualification is governed to a great extent by the position in which the tool is presented to the work, thus in Fig. 920, let C represent a piece of work and B, B, B, B, four tools having their top and bottom faces ground at the same angle to each other. In position 1, the top face of the tool is at an acute angle below the radial line A, hence the tool possesses top rake, the amount being about suitable for hard steel or hard cast iron.

In position 2 the top face is at an acute angle above the radial line A, hence the tool has negative top rake, the amount being about suitable for bra.s.s work under some conditions.

In position 3 the top face has no rake of any kind, and the tool is suitable (in this respect) for ordinary bra.s.s work.

In position 4 the tool possesses an amount of top rake about suitable for ordinary wrought-iron work.

If the tool was presented to bra.s.s work in positions 1 or 4 it would rip or tear the metal instead of cutting it, while if the tool was presented to iron or steel (of an ordinary degree of hardness) in positions 2 or 3, it would force rather than cut the metal.

Furthermore it will be readily perceived that though each tool may have its faces, whose junction forms the cutting edge, at the same angles, yet the strength of the cutting edge is varied by the position in which the tool is presented to the work, thus the edge in position 2, will be weaker than that in position 4.

We have now to consider another point bearing upon the proper presentment of top rake and the presentment of the tool to the work. It is obvious that the strain of the cut falls upon the top face of the tool, and therefore the direction in which this strain is exerted is the direction in which the tool will endeavour to move if the strain is sufficient to bend the tool and cause motion.

[Ill.u.s.tration: Fig. 921.]