Modern Machine-Shop Practice Part 66

In Fig. 921 let W represent the work having a cut C being taken off by the tool T; let E represent the slide rest, and F the extreme point at which the tool is supported; then the pressure placed by C on the top face of the tool will be at a right angle to the plane of that top face, or in the direction of the arrow B; to whatever amount therefore the tool sprung under the cut pressure (its motion being in an arc of a circle, of which F is the centre) it would enter the work deeper, and as a result, the rough work not being cylindrically true, the tool will dip farthest beyond its proper line of work where the cut is deepest, and therefore will not cut the work cylindrically true; as this, however, naturally leads to a variation in the direction of the top rake, and as the cutting action of the point of such a tool differs from that of the side edge, which also leads to a variation in the direction of the top rake, it becomes necessary to consider just what the cutting action is both at the point and on the side of the tool.

Suppose, then, that the tool carries so fine a cut that it cuts at the point only, and the pressure will be as denoted by the arrow B in Fig.

921.

[Ill.u.s.tration: Fig. 922.]

If the tool be given no traverse, but be merely moved in towards the centre of the work, the cut will move outward and in a line with the body of the tool, the cutting coming off as shown in Fig. 922.

So soon, however, as the tool is fed to its feed traverse the form of the cutting alters to the special form shown in Fig. 917, and moves to one side of the tool, as well as outwards from the work.

[Ill.u.s.tration: Fig. 923.]

Fig. 923 is a top view of a tool and piece of work, and the arrow A denotes the direction of the resistance of the work to the cut, being at a right angle to plane of the cutting edge.

Now the duty of the side edge is simply to remove metal, while that of the point is to finish the surface, and it is obvious that for finishing purposes the most important part of the tool edge is the point, and this it is that requires to be kept sharp, hence the angle or rake should be in the direction of the point. But when the object is to remove metal and prepare the work for the finishing cut the duty falls heavily on the side edge of the tool, and the angle of the top face and the direction of its rake may be varied with a view to increase the efficiency of the side edge, and at the same time to diminish the amount of power necessary to pull the tool along to its feed traverse. This may be accomplished by altering the top rake from front to side rake, which is done in varying degrees according to the nature of the work.

[Ill.u.s.tration: Fig. 924.]

In Fig. 924 the angle of the top face in the direction of A is the front, and that in the direction of B is the side rake.

In small work where the cuts are not great, and where but one roughing cut is taken, it is an object to have the roughing cut leave the work with as smooth a surface as possible, and the amount of side rake may be small as in Fig. 924. For heavy deep cuts, however, a maximum of side rake may be used.

[Ill.u.s.tration: Fig. 925.]

Thus in Fig. 925 is an engraving of a tool used for roughing in the Morgan Iron Works, its top rake being all side rake.

When a tool has side rake, its cutting capacity is obviously increased on one side only, hence it should be fed to cut on that side only. It is for this reason that no side rake is given to tools for very small and short work, because it is then more convenient to traverse the tool to cut in either direction at will.

In long and large work, however, where the motion of the slide rest is slow, tools having right and left-hand side rake are used. The tools in Figs. 924 and 925 are right-hand tools, their direction of feed travel being to the left.

[Ill.u.s.tration: Fig. 926.]

In Fig. 926 is a left-hand tool, its direction of feed traverse being from left to right; hence edge G is the cutting one, edge F being dulled by the side angle B.

[Ill.u.s.tration: Fig. 927.]

It is obvious that various combinations of side rake and front rake may be given to produce the same degree of keenness to the tool. For example, a tool may have its keenness from side rake alone, or it may have the same degree of keenness by using less side rake and some front rake. The principles governing the selections of these combinations are as follows:--

Suppose that in addition to say 20 degrees of side rake a tool is given a certain amount of front rake as denoted in Fig. 927 by E E, and suppose that the tool is moved in to its cut by the cross feed screw.

During this motion and until the tool point meets the work surface the contact between the cross feed screw and feed nut will be on the sides of the threads facing the line of lathe centres, and all the play between those threads will be on their other sides, but so soon as the tool meets the cut it will jump forward and into the work to the amount that the play between the threads will allow it, and this is very apt to cause the tool to break. Furthermore the point of the tool is apt from its extreme keenness to become dulled quickly.

[Ill.u.s.tration: Fig. 928.]

The amount of side rake may, however, be considerably increased if the heel D, Fig. 928, be made higher than the point A in that figure, the plane of the middle being denoted by the arrow at A; a view of the other side of this tool is shown in Fig. 929, the plane of the cutting edge being denoted by the dotted line.

[Ill.u.s.tration: Fig. 929.]

A tool thus formed will require a slight cross feed screw pressure to force it to its cut, thus causing the cross feed nut to have contact with the sides of the thread in contact when winding the tool into its cut, hence the tendency to jump into the depth of cut is eliminated, and regulating the depth of the cut is much more easily accomplished.

In proportion as a tool is given side rake, it is more easily traversed to its cut, as will be perceived from the following:--

[Ill.u.s.tration: Fig. 930.]

[Ill.u.s.tration: Fig. 931.]

Fig. 930 represents a section of a tool T, whose feed traverse is in the direction of A. Now all the force that is expended in bending the cutting C out of the straight line, or in other words the pressure on the top face of the tool, acts to a great extent to force the tool to the left, and therefore traverse it to its feed. The more side rake a tool has the nearer the thickness of its cutting will accord to the thickness of the feed traverse. For example, if a tool having a side rake of say 35 degrees of angle feeds forward 1/32 inch per work revolution, the thickness of the cutting will but slightly exceed 1/32 inch, but if no top rake at all be given, as shown in Fig. 931, then the cutting will come off nearly straight, will be considerably thicker than 1/32 inch, and will be ragged and broken up, and it follows that the thickening and the bending of the cutting has required an expenditure of the driving power of the lathe, diminishing the depth of cut the lathe will be capable of driving. With such a tool the pressure of the cut will fall downwards as denoted by the arrow B.

[Ill.u.s.tration: Fig. 932.]

In the practice of many tool makers in the Eastern States the tool is ground to a point A, Fig. 932, that is, ground sharp and merely rounded off with an oil-stone. This may serve when the lathe has an exceedingly fine feed, and the strain being in that case very slight the tool point may be made to stand well above the level of the body of the steel, as in the figure, and thus save forging; but this is a slow method of procedure, and produces no better work than a tool which is rounded at the point, and therefore capable of producing smoother work with a much coa.r.s.er feed.

The diameter of the curls of the cutting, shaving, or chip produced by a turning and also the direction in which it moves after leaving the tool, depends upon the amount of the top rake and the direction in which it is provided. The greater the amount of rake, whether it be front or side rake, the larger the coils of the cutting, and, therefore, the less the amount of power expended in bending it. Furthermore, it may be remarked that the thickness of the cutting is always greater than is due to the amount of feed traverse, and it requires power to produce this thickening of the cutting. The larger the coils of the cutting the nearer the thickness accords with the rate of feed.

[Ill.u.s.tration: Fig. 933.]

In these considerations we have referred to the angle of the top face only, but if we consider the angle of the two faces one to the other we shall see that they form a wedge, and that all cutting tools are simply wedges which enter the material the more easily in proportion as the angles are more acute, providing always that they are presented to the work in the most desirable position, as was explained with reference to Fig. 920.

[Ill.u.s.tration: Fig. 934.]

We may now consider the degree of a bottom rake or clearance desirable for a tool, and this it can be shown depends entirely upon the conditions of work, diameter, and rate of tool traverse, and cannot, therefore, be made a constant degree of angle. This is shown in Fig.

934, in which a tool T is represented in three positions, marked respectively 1, 2, and 3. Line A A is at a right angle to the axis of the work W, and the side of the tool is given in each case 5 of angle from this line A A. In position 1 the tool has 3 of clearance from the side of the cut; in position 2 it has 2 clearance, but in position 3 it would require to have 2 more clearance given to it to enable the cutting-edge to meet the side of the cut, without even then having the clearance necessary to enable it to cut. This occurs because the side of the cut is not at a right angle to the work axis, but at an angle the degree of which depends upon the rate of feed.

[Ill.u.s.tration: Fig. 935.]

Thus in Fig. 935 the three tools have the same amount of clearance, and if they are supposed to be facing off the work they will maintain that clearance under all conditions of work, diameter, and rate of feed, but if they were traversed along instead of across the work the angle of the tool (both on the top and bottom face) to the cut will become changed, and will continue to change with every change of work diameter, so that the same tool stands at a different angle at each successive cut taken off the work, even though the lathe were used at or possessed but one rate of feed. But lathe tools are used at widely varying rates of feed, and we may therefore take an example in which a tool is at work taking a cut of the same diameter and depth at different rates of feed.

[Ill.u.s.tration: Fig. 936.]

This is shown in Fig. 936, tool 1 taking the coa.r.s.est, and 2 the finest feed, and it is seen that the finer the rate of feed the more clearance the tool has with a given degree of side clearance (for all the three tools have 7 of side angle). The only way to obtain an equal degree of clearance from the cut, therefore, clearly lies in giving to a tool a different angle for every variation, either in work diameter or in rate of feed traverse, and to show how much this will affect the shape of the tool, we have Fig. 937, in which the same rate of feed is used for all three cuts, and the tool is given in each position 5 of clearance from the cut. In position 1 the tool side stands at 8-1/2 of angle from line A, which is at a right angle to the work axis. In position 2 it stands at 10-1/2, and in position 3 at 15 of angle from line A, a variation of 6-1/2. Referring now to the top face of the tool, the variations occur to the same extent and from the same causes. It is in a fine degree of perception of these points that const.i.tutes the skill of expert workmen in grinding their lathe tools, varying the angle of the tool at every grinding to suit the varying requirements.

[Ill.u.s.tration: Fig. 937.]

It has been shown that for freedom of cutting and ease of driving a given cut, the direction of top rake as well as its degree needs to be a maximum that the nature of the material and its degree of hardness will admit; but this is not the only consideration, because in a finishing cut the surface requires to be left as smooth and clean cut as possible, and it remains to consider how this may best be accomplished. Now let it again be considered that it is that part of the cutting edge that lies at a right angle to the axial line of the work that removes the metal, while it is that part that lies parallel to the work axis (or in other words parallel to the finished work surface) that performs the finishing cutting duty.

[Ill.u.s.tration: Fig. 938.]

Now, in proportion as the length of the cutting edge is disposed parallel to the work axis, the tool has a tendency to spring (under an increase of cut) into the work, and also to dip into soft places or seams in the work, and the amount of its front rake must be decreased, because such rake causes a pressure pulling the tool deeper into its cut, as was explained with reference to Fig. 921. Round-nosed front tools, therefore, such as in Fig. 938, cannot be given so much front rake as ordinary ones, such as in the preceding figures.

[Ill.u.s.tration: Fig. 939.]

Round-nosed tools are used to cut out round corners, and the roughing tools are given a less curvature than that to be formed on the work, thus in Fig. 939 is an ordinary form of small round nose shown operating in what is termed a hollow corner, the directions of tool feed being marked by arrows. The tool may be fed by the feed traverse, and the tool gradually withdrawn, thus forming the work to the required curve.

The amount of cut a lathe will drive, the degree of hardness which the tool may be given, the length of time the tool will last without grinding, the speed at which the work may run, and the cleanness and truth of the cut, depend almost entirely upon the perfect adaptability of the tool to the conditions under which it is to be used. Upon the same kind of work, and using the same kind of tools, some workmen will give a tool from 20 to 30 more angle than others.

[Ill.u.s.tration: Fig. 940.]

It is a difficult matter to determine at just what point the utmost duty is being obtained from cutting tools, because the conditions of use are so variable; but one good general guide is the speed at which the tool cuts, and another is the appearance of the cuttings or chips.

[Ill.u.s.tration: Fig. 941.]