Turning and Boring.
by Franklin D. Jones.
PREFACE
Specialization in machine-tool manufacture has been developed to such a degree that there is need also for treatises which specialize on different cla.s.ses of tools and their application in modern practice.
This book deals exclusively with the use of various types of turning and boring machines and their attachments, and is believed to be unusually complete. In addition to standard practice, it describes many special operations seldom or never presented in text-books. Very little s.p.a.ce is given to mere descriptions of different types of machine tools, the princ.i.p.al purpose being to explain the use of the machine and the practical problems connected with its operation, rather than the constructional details. No attempt has been made to describe every machine or tool which might properly be included, but rather to deal with the more important and useful operations, especially those which ill.u.s.trate general principles.
Readers of mechanical literature are familiar with MACHINERY"S 25-cent Reference Books, of which one hundred and twenty-five different t.i.tles have been published during the past six years. Many subjects, however, cannot be adequately covered in all their phases in books of this size, and in response to a demand for more comprehensive and detailed treatments on the more important mechanical subjects, it has been deemed advisable to bring out a number of larger volumes, of which this is one.
This work includes much of the material published in MACHINERY"S Reference Books Nos. 91, 92 and 95, together with a great amount of additional information on modern boring and turning methods.
It is a pleasure to acknowledge our indebtedness to the manufacturers who generously supplied ill.u.s.trations and data, including many interesting operations from actual practice. Much valuable information was also obtained from MACHINERY.
F. D. J.
NEW YORK, _May, 1914_.
CHAPTER I
THE ENGINE LATHE--TURNING AND BORING OPERATIONS
The standard "engine" lathe, which is the type commonly used by machinists for doing general work, is one of the most important tools in a machine shop, because it is adapted to a great variety of operations, such as turning all sorts of cylindrical and taper parts, boring holes, cutting threads, etc. The ill.u.s.tration Fig. 1 shows a lathe which, in many respects, represents a typical design, and while some of the parts are arranged differently on other makes, the general construction is practically the same as on the machine ill.u.s.trated.
The princ.i.p.al parts are the bed _B_, the headstock _H_, the tailstock _T_, and the carriage _C_. The headstock contains a spindle which is rotated by a belt that pa.s.ses over the cone-pulley _P_, and this spindle rotates the work, which is usually held between pointed or conical centers _h_ and _h_{1}_ in the headstock and tailstock, or in a chuck screwed onto the spindle instead of the faceplate _F_. The carriage _C_ can be moved lengthwise along the bed by turning handle _d_, and it can also be moved by power, the movement being transmitted from the headstock spindle either through gears _a_, _b_, _c_, and lead-screw _S_, or by a belt operating on pulleys _p_ and _p_{1}_, which drive the feed-rod _R_. The lead-screw _S_ is used when cutting threads, and the feed-rod _R_ for ordinary turning operations; in this way the wear on the lead-screw is reduced and its accuracy is preserved.
[Ill.u.s.tration: Fig. 1. Bradford Belt-driven Lathe--View of Front or Operating Side]
On the carriage, there is a cross-slide _D_ which can be moved at right angles to the lathe bed by handle _e_, and on _D_ there is an upper or compound slide _E_ which can be swiveled to different positions. The tool _t_, that does the turning, is clamped to the upper slide, as shown, and it can be moved with relation to the work by the movement of the carriage _C_ along the bed, or by moving slide _D_ crosswise. The lengthwise movement is used to feed the tool along the work when turning, boring or cutting a screw, and the crosswise movement for facing the ends of shafts, etc., or for radial turning. When the tool is to be fed at an angle, other than at right angles to the bed, slide _E_, which can be set to the required angle, is used. The lengthwise and crosswise feeding movements can be effected by power, the lengthwise feed being engaged by tightening k.n.o.b _k_, and the cross-feed by tightening k.n.o.b _l_. The direction of either of these movements can also be reversed by shifting lever _r_. Ordinarily the carriage and slide are adjusted by hand to bring the tool into the proper position for turning to the required diameter, and then the power feed (operating in the desired direction) is engaged. The tailstock _T_ can be clamped in different positions along the bed, to suit the length of the work, and its center _h_{1}_ can be moved in or out for a short distance, when adjusting it to the work, by turning handle _n_.
[Ill.u.s.tration: Fig. 2. Plan View of Lathe Headstock showing Back-gears]
[Ill.u.s.tration: Fig. 3. Feed Mechanism of Lathe Ap.r.o.n]
As some metals are much harder than others, and as the diameters of parts to be turned also vary considerably, speed changes are necessary, because if the speed is excessive, the turning tool will become dull in too short a time. These speed changes (with a belt-driven lathe) are obtained by placing the driving belt on different steps of cone-pulley _P_, and also by the use of back-gears. The cone-pulley can be connected directly with the spindle or be disengaged from it by means of bolt _m_.
When the pulley and spindle are connected, five speeds (with this particular lathe) are obtained by simply shifting the driving belt to different steps of the cone. When a slower speed is required than can be obtained with the belt on the largest step of the cone, the latter is disconnected from the spindle, and the back-gears _G_ and _G_{1}_ (shown in the plan view Fig. 2) are moved forward into mesh by turning handle _O_; the drive is then from cone-pulley _P_ and gear _L_ to gear _G_, and from gear _G_{1}_ to the large gear _J_ on the spindle. When driving through the back-gears, five more speed changes are obtained by shifting the position of the driving belt, as before. The fastest speed with the back-gears in mesh is somewhat slower than the slowest speed when driving direct or with the back-gears out of mesh; hence, with this particular lathe, a series of ten gradually increasing speeds is obtained. Changes of feed for the turning tool are also required, and these are obtained by shifting the belt operating on pulleys _p_ and _p_{1}_ to different-sized steps. On some lathes these feed changes are obtained through gears which can be shifted to give different ratios.
Many lathes also have gears in the headstock for changing the speeds.
[Ill.u.s.tration: Fig. 4. Rear View of Lathe Ap.r.o.n]
Front and rear views of the carriage ap.r.o.n, which contains the feeding mechanism, are shown in Figs. 3 and 4, to indicate how the feeds are engaged and reversed. The feed-rod _R_ (Fig. 1) drives the small bevel gears _A_ and _A_{1}_ (Figs. 3 and 4), which are mounted on a slide _S_ that can be moved by lever _r_ to bring either bevel gear into mesh with gear _B_. Gear _B_ is attached to pinion _b_ (see Fig. 3) meshing with gear _C_, which, when k.n.o.b _k_ (Fig. 1) is tightened, is locked by a friction clutch to pinion _c_. The latter pinion drives gear _D_ which rotates shaft _E_. A pinion cut on the end of shaft _E_ engages rack _K_ (Fig. 1) attached to the bed, so that the rotation of _E_ (which is controlled by k.n.o.b _k_) moves the carriage along the bed. To reverse the direction of the movement, it is only necessary to throw gear _A_ into mesh and gear _A_{1}_ out, or _vice versa_, by operating lever _r_. When the carriage is traversed by hand, shaft _E_ and gear _D_ are rotated by pinion _d_{1}_ connected with handle _d_ (Fig. 1).
The drive for the cross-feed is from gear _C_ to gear _F_ which can be engaged through a friction clutch (operated by k.n.o.b _l_, Fig. 1) with gear _G_ meshing with a pinion _H_. The latter rotates the cross-feed screw, which pa.s.ses through a nut attached to slide _D_ (Fig. 1), thus moving the latter at right angles to the ways of the bed. The cross-feed is also reversed by means of lever _r_. As previously explained, lead-screw _S_ is only used for feeding the carriage when cutting threads. The carriage is engaged with this screw by means of two half-nuts _N_ (Fig. 4) that are free to slide vertically and are closed around the screw by operating lever _u_. These half-nuts can only be closed when lever _r_ is in a central or neutral position, so that the screw feed and the regular turning feed cannot be engaged at the same time. As previously mentioned, lead-screw _S_, Fig. 1, is rotated from the lathe spindle, through gears _a_, _b_ and _c_, called change gears.
An a.s.sortment of these gears, of various sizes, is provided with the lathe, for cutting screws of different pitch. The gears to use for any pitch within the range of the lathe are given on the plate _I_.
=Example of Cylindrical Turning.=--Having now considered the princ.i.p.al features of what might be called a standard lathe, the method of using it in the production of machine parts will be explained. To begin with a simple example of work, suppose a steel shaft is to be turned to a diameter of 2-1/4 inches and a length of 14-1/2 inches, these being the finished dimensions. We will a.s.sume that the rough stock is cut off to a length of 14-5/8 inches and has a diameter of 2-5/8 inches. The first step in this operation is to form conically shaped center-holes in each end of the piece as indicated at _c_ in Fig. 5. As all work of this kind is held, while being turned, between the centers _h_ and _h_{1}_, holes corresponding in shape to these centers are necessary to keep the work in place. There are several methods of forming these center-holes, as explained later.
After the work is centered, a dog _A_ is clamped to one end by tightening screw _s_; it is then placed between the centers of the lathe. The dog has a projecting end or "tail," as it is commonly called, which enters a slot in the faceplate _F_ and thereby drives or rotates the work, when power is applied to the lathe spindle onto which the faceplate is screwed. The tailstock center _h_1_, after being oiled, should be set up just tight enough to eliminate all play, without interfering with a free rotary movement of the work. This is done by turning handle _n_, and when the center is properly adjusted, the tailstock spindle containing the center is locked by tightening handle _p_. (Ordinary machine oil is commonly used for lubricating lathe centers, but a lubricant having more "body" should be used, especially when turning heavy parts. The following mixtures are recommended: 1. Dry or powdered red lead mixed with a good grade of mineral oil to the consistency of cream. 2. White lead mixed with sperm oil with enough graphite added to give the mixture a dark lead color.)
[Ill.u.s.tration: Fig. 5. Plan View showing Work Mounted between Centers of Lathe]
=Facing the Ends Square with a Side-tool.=--Everything is now ready for the turning operation. The ends of the piece should be faced square before turning the body to size, and the tool for this squaring operation is shown in Fig. 6; this is known as a side-tool. It has a cutting edge _e_ which shaves off the metal as indicated in the end view by the dotted lines. The side _f_ is ground to an angle so that when the tool is moved in the direction shown by the arrow, the cutting edge will come in contact with the part to be turned; in other words, side _f_ is ground so as to provide clearance for the cutting edge. In addition, the top surface against which the chip bears, is beveled to give the tool keenness so that it will cut easily. As the principles of tool grinding are treated separately in Chapter II we shall for the present consider the tool"s use rather than its form.
[Ill.u.s.tration: Fig. 6. Lathe Side-tool for Facing Ends of Shafts, etc.]
For facing the end, the side tool is clamped in the toolpost by tightening the screw _u_, Fig. 5, and it should be set with the cutting edge slightly inclined from a right-angled position, the point being in advance so that it will first come into contact with the work. The cutting edge should also be about the same height as the center of the work. When the tool is set, the lathe (if belt-driven) is started by shifting an overhead belt and the tool is then moved in until the point is in the position shown at _A_, Fig. 7. The tool-point is then fed against the end by handle _d_, Fig. 5, until a light chip is being turned off, and then it is moved outward by handle _e_ (as indicated by the arrow at _B_, Fig. 7), the carriage remaining stationary. As the movement of the tool-point is guided by the cross-slide _D_, which is at right angles with the axis of the work, the end will be faced square.
For short turning operations of this kind, the power feeds ordinarily are not used as they are intended for comparatively long cuts. If it were necessary to remove much metal from the end, a number of cuts would be taken across it; in this case, however, the rough stock is only 1/8 inch too long so that this end need only be made true.
[Ill.u.s.tration: Fig. 7. Facing End with Side-tool and Turning Work Cylindrical]
After taking a cut as described, the surface, if left rough by the tool-point, should be made smooth by a second or finishing cut. If the tool is ground slightly round at the point and the cutting edge is set almost square, as at _C_, Fig. 7, a smooth finish can be obtained; the cut, however, should be light and the outward feed uniform. The work is next reversed in the centers and the driving dog is placed on the end just finished; the other end is then faced, enough metal being removed to make the piece 14-1/2 inches long, as required in this particular case. This completes the facing operation. If the end of the work does not need to be perfectly square, the facing operation can be performed by setting the tool in a right-angled position and then feeding it sidewise, thus removing a chip equal to the width of one side. Evidently this method is confined to comparatively small diameters and the squareness of the turned end will be determined by the position of the tool"s cutting edge.
=Turning Tool--Turning Work Cylindrical.=--The tool used to turn the body to the required diameter is shaped differently from the side-tool, the cutting edge _E_ of most tools used for plain cylindrical turning being curved as shown in Fig. 8. A tool of this shape can be used for a variety of cylindrical turning operations. As most of the work is done by that part of the edge marked by arrow _a_, the top of the tool is ground to slope back from this part to give it keenness. The end _F_, or the flank, is also ground to an angle to provide clearance for the cutting edge. If the tool did not have this clearance, the flank would rub against the work and prevent the cutting edge from entering the metal. This type of tool is placed about square with the work, for turning, and with the cutting end a little above the center.
[Ill.u.s.tration: Fig. 8. Tool used for Cylindrical Turning]
Before beginning to turn, a pair of outside calipers or a micrometer should be set to 2-1/4 inches, which, in this case, is the finished diameter of the work. Calipers are sometimes set by using a graduated scale as at _A_, Fig. 9, or they can be adjusted to fit a standard cylindrical gage of the required size as at _B_. Very often fixed caliper gages _C_ are used instead of the adjustable spring calipers.
These fixed gages, sometimes called "snap" gages, are accurately made to different sizes, and they are particularly useful when a number of pieces have to be turned to exactly the same size.
The turning tool is started at the right-hand end of the work and the tool should be adjusted with the left hand when beginning a cut, as shown in Fig. 10, in order to have the right hand free for calipering. A short s.p.a.ce is first turned by hand feeding, as at _D_, Fig. 7, and when the calipers show that the diameter is slightly greater than the finished size (to allow for a light finishing cut, either in the lathe or grinding machine) the power feed for the carriage is engaged; the tool then moves along the work, reducing it as at _E_. Evidently, if the movement is along a line _b--b_, parallel with the axis _a--a_, the diameter _d_ will be the same at all points, and a true cylindrical piece will be turned. On the other hand, if the axis _a--a_ is inclined one way or the other, the work will be made tapering; in fact, the tailstock center _h_1_ can be adjusted laterally for turning tapers, but for straight turning, both centers must be in alignment with the carriage travel. Most lathes have lines on the stationary and movable parts of the tailstock base which show when the centers are set for straight turning. These lines, however, may not be absolutely correct, and it is good practice to test the alignment of the centers before beginning to turn. This can be done by taking trial cuts, at each end of the work (without disturbing the tool"s crosswise position), and then comparing the diameters, or by testing the carriage travel with a true cylindrical piece held between the centers as explained later.
[Ill.u.s.tration: Fig. 9. Setting Calipers by Scale--Setting by Gage--Fixed Gage]
If the relative positions of the lathe centers are not known, the work should be calipered as the cut progresses to see if the diameter _d_ is the same at all points. In case the diameter gradually increases, the tailstock center should be shifted slightly to the rear before taking the next cut, but if the diameter gradually diminishes, the adjustment would, of course, be made in the opposite direction. The diameter is tested by attempting to pa.s.s the calipers over the work. When the measuring points just touch the work as they are gently pa.s.sed across it, the diameter being turned is evidently the same as the size to which the calipers are set.
[Ill.u.s.tration: Fig. 10. Views showing how the Cross-slide and Carriage are Manipulated by Hand when Starting a Cut--View to Left, Feeding Tool Laterally; View to Right, Feeding Tool in a Lengthwise Direction]
As the driving dog is on one end, the cut cannot be taken over the entire length, and when the tool has arrived at say position _x_, Fig.
5, it is returned to the starting point and the work is reversed in the centers, the dog being placed upon the other end. The unfinished part is then turned, and if the cross-slide is not moved, the tool will meet the first cut. It is not likely that the two cuts will be joined or blended together perfectly, however, and for this reason a cut should be continuous when this is possible.
=Roughing and Finishing Cuts.=--Ordinarily in lathe work, as well as in other machine work, there are two cla.s.ses of cuts, known as "roughing"
and "finishing" cuts. Roughing cuts are for reducing the work as quickly as possible almost to the required size, whereas finishing cuts, as the name implies, are intended to leave the part smooth and of the proper size. When the rough stock is only a little larger than the finished diameter, a single cut is sufficient, but if there is considerable metal to turn away, one or more deep roughing cuts would have to be taken, and, finally, a light cut for finishing. In this particular case, one roughing and one finishing cut would doubtless be taken, as the diameter has to be reduced 3/8 inch. Ordinarily the roughing cut would be deep enough to leave the work about 1/32 or perhaps 1/16 inch above the finished size. When there is considerable metal to remove and a number of roughing cuts have to be taken, the depth of each cut and the feed of the tool are governed largely by the pulling power of the lathe and the strength of the work to withstand the strain of a heavy cut. The depth of roughing cuts often has to be reduced considerably because the part being turned is so flexible that a heavy cut would spring the work and cause the tool to gouge in. Of course, just as few cuts as possible should be taken in order to save time. The speed of the work should also be as fast as the conditions will allow for the same reason, but as there are many things which govern the speed, the feed of the tool, and the depth of the cut, these important points are referred to separately in Chapter II.
=Filing and Finishing.=--In many cases the last or finishing cut does not leave as smooth a surface as is required and it is necessary to resort to other means. The method commonly employed for finishing in the lathe is by the use of a file and emery cloth. The work is rotated considerably faster for filing than for turning, and the entire surface is filed by a flat, single-cut file, held as shown in Fig. 11. The file is pa.s.sed across the work and advanced sidewise for each forward stroke, until the entire surface is finished. The file should be kept in contact with the work continually, but on the return stroke the pressure should be relieved. The movement of the file during the forward or cutting stroke should be much slower than when filing in a vise. By moving the file slowly, the work can make a number of revolutions for each stroke, which tends to keep it round, as practically the same amount of metal is removed from the entire circ.u.mference. On the other hand, short rapid strokes tend to produce flat spots, or at least an irregular surface, especially if the work can only make part of a revolution for each cutting stroke. The pressure on the file during the forward stroke should also be kept as nearly uniform as possible.
[Ill.u.s.tration: Fig. 11. Filing Work after Finishing Cut is taken]
It is very difficult to file a part smooth and at the same time to keep it round and cylindrical, and the more filing that has to be done, the greater the chance of error. For this reason, the amount left for filing should be very small; in fact, the metal removed by filing should be just enough to take out the tool marks and give a smooth finish. Very often a satisfactory finish can be obtained with a turning tool, and filing is not necessary at all. The file generally used for lathe work is a "single-cut b.a.s.t.a.r.d" of "mill" section, having a length of from 12 to 14 inches.
Sometimes particles of metal collect between the teeth of a file and make deep scratches as the file is pa.s.sed across the work. When this occurs, the teeth should be cleaned by using a wire brush or a file card, which is drawn across the file in the direction of the teeth. This forming of tiny particles between the teeth is known as "pinning" and it can sometimes be avoided by rubbing chalk on the file. Filing is not only done to obtain a smooth finish, but also to reduce the work to an exact diameter, as a very slight reduction can be made in this way.
[Ill.u.s.tration: Fig. 12. Two Methods of Aligning Centers for Cylindrical Turning]