Modern Machine-Shop Practice Part 78

Lathe Number 2.--Turning tool steel 2 inches long and 1/2 inch diameter, reducing diameter 1/8 inch. Revolutions of work 100 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 3.--Turning tool steel 4 inches long and 7/8 inch in diameter, reducing the diameter 1/8 inch. Revolutions of work 40 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 4.--Turning tool steel 6 to 8 inches long and 1-3/16 diameter, reducing work 1/8 inch in diameter. Revolutions of work 35 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 5.--Turning tool steel 8 to 10 inches long, and 2 inches in diameter, reducing diameter 1/8 inch. Lathe revolutions 30 per minute.

Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 6.--Turning tool steel 5 inches long and 3-1/2 inches diameter, reducing diameter 3/16. Lathe revolutions 19 per minute. Feed 200 lathe revolutions per inch of tool travel.

The power required to drive the work under a given depth of cut varies greatly with the following elements:--

1st. The diameter of the work, all other conditions being equal.

2nd. The degree of hardness of the metal, all other conditions being equal.

3rd. Upon the shape of the cutting tool; and--

4th. Upon the quality of the steel composing the cutting tool, and the degree of its hardness.

That the diameter of the work is an important element in small work may be shown as follows:--

[Ill.u.s.tration: Fig. 1154.]

[Ill.u.s.tration: Fig. 1155.]

In Fig. 1153 let W represent a piece of work having a cut taken off it, and the line of detachment of the metal from the body of the work will be represented by the part of the dotted line pa.s.sing through the depth of the cut (denoted by C). Let Fig. 1154 represent a similar tool with the same depth of cut on a piece of work of larger diameter, and it will be observed that the dotted line of severance is much longer, involving the expenditure of more power.

In boring these effects are magnified: thus in Fig. 1155 let W represent a washer to be bored with the tool T, and let the same depth of cut be taken by the tool, the diameter of the work being simply increased. It is manifest that the cutting would require to be bent considerably more in the case of the small diameter of work than in that of the large, and would thus require more power for an equal depth of cut.

Again, from a reference to Figs. 950 and 952, it will be observed that the height of the tool will make a difference in the power required to drive a given depth of cut, the shaving being bent more when the tool is above the centre in the case of boring tools, and below the centre in the case of outside tools. But when the diameter of the work exceeds about 6 inches, it has little effect in the respects here enumerated.

The following, however, are the general rules applicable when considering the power required for the cutting of metal with lathe or planer tools. The harder the metal, the more power required to cut off a given weight of metal. The deeper the cut the less power required to cut off a given weight of metal. The quicker the feed the less power required to cut off a given weight of metal. The smaller the diameter of outside work, and the larger the diameter of inside or bored work, the less power required.

Copper requires less power than bra.s.s; yellow, and other bra.s.s containing zinc, less than bra.s.s containing a greater proportion of tin.

Bra.s.s containing lead requires less power than that not containing it.

Cast iron requires more power than bra.s.s, but less than wrought iron; steel requires more power than wrought iron.

CHAPTER XII.--EXAMPLES IN LATHE WORK.

TECHNICAL TERMS USED WITH REFERENCE TO LATHE WORK.--Work held between the lathe centres is said to run true, when a fixed point set to touch its perimeter will have an equal degree of contact all around the circ.u.mference, and at any part of the length of the same when the work is cylindrical and is rotated. When such a fixed point has contact at one part more than at another of the work circ.u.mference, it is said to run "out of true," "out of truth," or not to run true.

Radial or side faces (as they are sometimes called) also run true when a fixed point has equal contact (at all parts of the revolution) with the work surface.

Work that is held in chucks is said to be set true when it is adjusted in the intended position.

To true up is to take off the work a cut of sufficient depth to cause a fixed point to touch the work surface equally at each point in the revolution.

To clean work up is to take off it a cut sufficiently deep to cause it to run true, and at the same time removes the rough surface or scale from the metal.

Roughing out work is taking off a cut which reduces it to nearly the finis.h.i.+ng size, leaving sufficient metal to take a finis.h.i.+ng cut, and reduce it to the proper size.

_Facing_ a piece of work is taking a cut off its radial face.

When a radial face or surface is convex, it is said to be _rounding_ or _round_, and when it is concave it is said to be _hollow_.

When a radial face is at a right angle to a cylindrical parallel surface, it is said to be _square_; but in taper work, it is said to be _square_ when it is at a right angle to the axis of the taper.

_Outside work_ includes all operations performed on a piece of work except those executed within the bores of holes or recesses, which is termed inside or internal work.

_Jarring_ or _chattering_ is the term applied to a condition in which the tool does not cut the work smooth, but leaves a succession of elevations and depressions on it, these forming sometimes a regular pattern on the work. In this case the projections only will have contact with the measuring tools, or with the enveloped or enveloping work surface, when the two pieces are put together.

Jarring or chattering more commonly occurs in the bores of holes or upon radial surfaces, than upon plain cylindrical surfaces, unless the latter be very long and slender. It occurs more also upon bra.s.s than upon iron work, and more upon cast than upon wrought iron or steel. It is caused mainly by vibrations of either the work or the tool.

It is induced by weakness (or want of support) in the work, by weakness in the tool, or by its being improperly formed for the duty. Thus, if a tool have too broad a cutting surface it will jar; if it be held out far from the tool post it may jar; if it have too keen a top face for the conditions it will jar.

Jarring may almost always be remedied on bra.s.s work by reducing the keenness of the top face, giving it if necessary negative rake, as shown in Fig. 964. On iron or steel work it may be avoided by using as stiff a cutting tool as possible, holding its cutting edge as close to the tool post as convenient, and reducing the length of cutting edge to a minimum.

It may be prevented sometimes by simply placing the finger or a weight upon the tool, or by applying oil to the work, but if this be done it should be supplied continuously throughout the cut, as a tool will cut to a different depth when dry from what it will when lubricated.

In using hand tools such as sc.r.a.pers, too thin a tool may cause jarring, which may be obviated by keeping the tool rest as close to the work as possible, and placing a piece of leather between the work and the rest.

EXAMPLES IN LATHE WORK.--The simplest cla.s.s of lathe work is that cut from rods or short lengths of rod metal, which may be turned by being held in a small chuck, or between the lathe centres.

Such work is usually of small diameter and short length, and is therefore difficult to get at if turned between the lathe centres, because the dog that drives it, the lathe face plate, and the dead centre are in the way; such work may be more conveniently driven by a small chuck.

It is usually made of round wire or rod, cut into lengths to suit the conditions; thus if the lathe have a hollow spindle, the rod lengths may be so long as to pa.s.s entirely through the spindle, otherwise the lengths may be pa.s.sed through the chuck, and as far as possible into the live spindle centre hole.

In any event it is desirable to let the rod project so far out from the chuck as to enable its being finished and cut off, without removal from or moving it in the chuck, because such chucks are apt in course of time to wear, so that the jaws do not grip the work quite concentric to the line of centres; hence, if the work be moved in the chuck after having been turned, it is apt to run out of true.

Sometimes, however, the existence of a collar on the work prevents it from being trued for fit at both ends without being cut off from the rod, in which case, if it requires correction after being cut off, it must be rechucked, and it may be necessary at this rechucking to grip it in several successive positions (partly rotating it in the chuck at each trial) before it will run true.

Sometimes the length of work that may advantageously be driven by such a chuck is so great as to render the use of the dead centre to support one end necessary, in which case the rod should be removed from the chuck before each piece is turned, so as to centre drill the dead centre end.

There is one special advantage in driving small work in a chuck of this kind, inasmuch as the work can be tried for fit without removing it from the lathe, while in some cases operations can be performed on it which would otherwise require its removal to the vice; suppose, for example, a thread of very small diameter and pitch requires to be cut on the work end, then a pair of dies or a screw plate may be placed on it, and the lathe pulled round by the belt; after the dies have commenced to start the thread, they may be released and allowed to rotate with the lathe, which will show if they are starting the thread true upon the work.

In cases also where the end of the work requires fitting to a seat, or where it requires turning to a conical point, there is the advantage that the work can be tried to the seat, or turned to the point without taking from the lathe, or without any subsequent operations, whereas in the case of a conical point, the existence of a work centre would necessitate turning the cone some distance from the end, and cutting off the work centre.

As the size of the work increases, the form of the chuck is varied to make it more powerful and strong to resist the strains, but when the size of the chuck becomes so large that it is as much in the way as the face place would be, it is better to turn the work between the lathe centres.

For work to be turned between the lathe centres, it is essential that those centres run true, and be axially in line, and that both centres be turned to the same degree of angle or cone, which is usually for small lathes an angle of 60, and for lathes of about 30 inches swing and over an angle of about 70. Both centres should be of an equal angle, for the following reasons.

It is obvious that the work centres wear to fit the dead centre, because of the friction between the two. Now in order to turn a piece of work from end to end, it is necessary to reverse it in the lathe, because at the first turning one end is covered by the carrier or driver driving it. At the first turning one work centre only will have worn to fit the lathe centre; hence when at the second, the other work centre wears to fit the dead centre and in the process of such wearing moves (as it always does to some degree) its location, the part first turned will no longer run true. To obviate this difficulty it is proper at the first turning to cut the work down to nearly the finished size, and then reverse it in the lathe and turn up the other end. At this second turning the work will have had both work centres worn to fit the dead centre, hence if it be of the same angle as the live centre, the work will properly bed to both centres, otherwise it will plainly not bed well to the live centre, and in consequence will be apt to run in some degree out of true at the live centre end.

The lathe centres should, for parallel work, stand axially true one with the other, and this can only be the case when the live centre runs true.