Lathe Side-tool for Facing Ends of Shafts, etc.Fig. 6. Lathe Side-tool for Facing Ends of Shafts, etc.
Lathe Side-tool for Facing Ends of Shafts, etc.
Fig. 6. Lathe Side-tool for Facing Ends of Shafts, etc.
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 inFig. 6; this is known as a side-tool. It has a cutting edgeewhich shaves off the metal as indicated in the end view by the dotted lines. The sidefis 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, sidefisground 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 inChapter IIwe shall for the present consider the tool's use rather than its form.
Facing End with Side-tool and Turning Work CylindricalFig. 7. Facing End with Side-tool and Turning Work Cylindrical
Facing End with Side-tool and Turning Work Cylindrical
Fig. 7. Facing End with Side-tool and Turning Work Cylindrical
For facing the end, the side tool is clamped in the toolpost by tightening the screwu,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 atA,Fig. 7. The tool-point is then fed against the end by handled,Fig. 5, until a light chip is being turned off, and then it is moved outward by handlee(as indicated by the arrow atB,Fig. 7), the carriage remaining stationary. As the movement of the tool-point is guided by the cross-slideD, 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 only1/8inch too long so that this end need only be made true.
After taking a cut as described, the surface, if left rough bythe 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 atC,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 141/2inches 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.
Tool used for Cylindrical TurningFig. 8. Tool used for Cylindrical Turning
Tool used for Cylindrical Turning
Fig. 8. Tool used for Cylindrical Turning
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 edgeEof most tools used for plain cylindrical turning being curved as shown inFig. 8. Atool 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 arrowa, the top of the tool is ground to slope back from this part to give it keenness. The endF, 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.
Setting Calipers by Scale—Setting by Gage—Fixed GageFig. 9. Setting Calipers by Scale—Setting by Gage—Fixed Gage
Setting Calipers by Scale—Setting by Gage—Fixed Gage
Fig. 9. Setting Calipers by Scale—Setting by Gage—Fixed Gage
Before beginning to turn, a pair of outside calipers or a micrometer should be set to 21/4inches, which, in this case, is the finished diameter of the work. Calipers are sometimes set by using a graduated scale as atA,Fig. 9, or they can be adjusted to fit a standard cylindrical gage of the required size as atB. Very often fixed caliper gagesCare 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.
Views showing how the Cross-slide and Carriage are Manipulated by Hand when Starting a CutFig. 10. Views showing how the Cross-slide and Carriage are Manipulatedby Hand when Starting a Cut—View to Left, Feeding Tool Laterally;View to Right, Feeding Tool in a Lengthwise Direction
Fig. 10. Views showing how the Cross-slide and Carriage are Manipulatedby Hand when Starting a Cut—View to Left, Feeding Tool Laterally;View to Right, Feeding Tool in a Lengthwise Direction
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 inFig. 10, in order to have the right hand free for calipering. A short space is first turned by hand feeding, as atD,Fig. 7, and when the calipers show that the diameter is slightly greater than the finished size (to allow for a light finishingcut, 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 atE. Evidently, if the movement is along a lineb—b, parallel with the axisa—a, the diameterdwill be the same at all points, and a true cylindrical piece will be turned. On the other hand, if the axisa—ais inclined one way or the other, the work will be made tapering; in fact, the tailstock centerh1can 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.
If the relative positions of the lathe centers are not known, the work should be calipered as the cut progresses to see if the diameterdis 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 diametergradually diminishes, the adjustment would, of course, be made in the opposite direction. The diameter is tested by attempting to pass the calipers over the work. When the measuring points just touch the work as they are gently passed across it, the diameter being turned is evidently the same as the size to which the calipers are set.
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 positionx,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 classes of cuts, known as “roughing†and “finishing†cuts. Roughing cuts arefor 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 reduced3/8inch. Ordinarily the roughing cut would be deep enough to leave the work about1/32or perhaps1/16inch 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 inChapter II.
Filing WorkFig. 11. Filing Work after Finishing Cut is taken
Fig. 11. Filing Work after Finishing Cut is taken
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 inFig. 11. The file is passed 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 anumber of revolutions for each stroke, which tends to keep it round, as practically the same amount of metal is removed from the entire circumference. 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.
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 bastard†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 passed across the work. When this occurs, the teeth should be cleaned by usinga 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.
If a polish is desired, this can be obtained by holding a piece of emery cloth tightly around the work as it revolves. The coarseness of emery cloth is indicated by letters and numbers corresponding to the grain number of loose emery. The letters and numbers for grits ranging from fine to coarse are as follows:FF,F, 120, 100, 90, 80, 70, 60, 54, 46, 40. For large work roughly filed, use coarse cloth such as Nos. 46 or 54, and then finer grades to obtain the required polish. If the work has been carefully filed, a good polish can be obtained with Nos. 60 and 90 cloth, and a brilliant polish by finishing with No. 120 and flour-emery.
Most cylindrical parts can be finished more quickly and accurately in the grinder than in the lathe, and many classes of work are, at the present time, simply rough-turned in the lathe and then ground to size in a cylindrical grinding machine.
Two Methods of Aligning Centers for Cylindrical TurningFig. 12. Two Methods of Aligning Centers for Cylindrical Turning
Two Methods of Aligning Centers for Cylindrical Turning
Fig. 12. Two Methods of Aligning Centers for Cylindrical Turning
Aligning Centers for Cylindrical Turning.—Whena rod or shaft must be turned cylindrical or to the same diameter throughout its entire length, it is good practice to test the alignment of the centers, before inserting the work. The position of the tailstock center for cylindrical turning may be indicated by the coincidence of graduation marks on the base, but if accuracy is necessary, the relative position of the two centers should be determined in a more positive way. A very simple and convenient method of testing the alignment is shown atAinFig. 12. The work is first turned for a short distance, near the dogged end, as shown, and the tool is left as set for this cut; then the tailstock center is withdrawn and the work is moved sufficiently to permit running the tool back to the tailstock end without changing its original setting. A short cut is then taken at this end and the diametersdandd1are carefully compared. In case there is any variation, the tailstock center is adjusted laterally, other trial cuts are taken, and the test repeated.
Another method is illustrated atB, which requires the use of a test-bart. This bar should have accurately made centers and the ends finished to exactly the same diameter. The lathe centers are aligned by placing the bar between them and then testing the position of the ends. This can be done by comparing each end with a tool held in the toolpost and moved from one to the other by shifting the carriage, but a better method is to clamp a test indicatoriin the toolpost and bring it in contact with first one end of the bar and then the other. If the dial does not register the same at each end, it shows that the lathe centers are not in line. Even when centers are correctly set, lathes that have been in use a long time do not always turn cylindrical or straight, because if the ways that guide the carriage are worn unevenly, the tool as it moves along does not remain in the same plane and this causes a variation in the diameter of the part being turned.
Dog that is too Short for Faceplate and Straight Driving DogFig. 13. (A) Dog that is too Short for Faceplate.(B) Straight Driving Dog
Dog that is too Short for Faceplate and Straight Driving Dog
Fig. 13. (A) Dog that is too Short for Faceplate.(B) Straight Driving Dog
Application of Drivers or Dogs.—Work that is turned between centers is sometimes driven by a dog which is so short for the faceplate that the bent driving end bears against the bottomaof the faceplate slot, as shown atA,Fig. 13. If thedog is nearly the right length, it may allow the headstock center to enter the center in the work part way, with the result that the turned surface is not true with the centers. When a driving dog of this type is used, care should be taken to see that it moves freely in the faceplate slot and does not bind against the bottom. By using a straight dog (B), which is driven by a pinbbolted to the faceplate, all danger from this source is eliminated. The straight dog, however, is used more particularly to do away with the leveragelof a bent dog, as this leverage tends to spring a flexible part when a cut is being taken.
Straight dogs are also made with two driving ends which engage pins on opposite sides of the faceplate. This type is preferable because it applies the power required for turning, evenly to the work, which still further reduces the tendency to spring it out of shape. The principal objection to the double-ended type lies in the difficulty of adjusting the driving pins so that each bears with equal pressure against the dog. The double-ended driver is often used for large work especially if deep roughing cuts are necessary.
Bushing mounted on Arbor for TurningFig. 14. Bushing mounted on Arbor for Turning
Bushing mounted on Arbor for Turning
Fig. 14. Bushing mounted on Arbor for Turning
Lathe Arbors or Mandrels.—When it is necessary to turn the outside of a part having a hole through it, centers cannot, of course, be drilled in the ends and other means must be resorted to. We shall assume that the bushingB,Fig. 14, has afinished hole through the center, and it is desired to turn the outside cylindrical and concentric with the hole. This could be done by forcing a tightly-fitted arborM, having accurately-centered ends, into the bushing and inserting the mandrel and work between the lathe centershandh1as shown. Evidently, if the arbor runs true on its centers, the hole in the bushing will also run true and the outside can be turned the same as though the arbor and bushing were a solid piece. From this it will be seen that an arbor simply forms a temporary support for parts that are bored and therefore cannot be centered.
Turning Pulley Held on an ArborFig. 15. Turning Pulley Held on an Arbor
Turning Pulley Held on an Arbor
Fig. 15. Turning Pulley Held on an Arbor
Another example of work that would be turned on an arbor is shown inFig. 15. This is a small cast-iron wheel having a finished hole through the hub, and the outer surface and sides of the rim are to be turned true with this hole. In this case, the casting would also be held by pressing a mandrel through the hub; as shown. This method, however, would only apply to comparatively small wheels because it would be difficult, if not impossible, to prevent a large wheel from turning on the arbor when taking a cut, and even if it could be driven, large work could be done to better advantage on another type of machine. (The vertical boring mill is used extensively for turning large wheels, as explained inChapter VI.) When turning the outside of the rim, a tool similar to that shown attshould be used, but for facing or turning the sides, it might be better, if not necessary, to use tools having bent ends as shown by the dotted lines; infact, turning tools of various kinds are made with the ends bent to the right or left, as this enables them to be used on surfaces that could not be reached very well with a straight tool. If a comparatively large pulley is mounted near the end of the arbor, it can be driven directly by pins attached to the faceplate and engaging the pulley arms. This method of driving is often employed when the diameter to be turned is large and the hole for the arbor is so small that there will not be sufficient friction for driving.
Different Types of Lathe ArborsFig. 16. Different Types of Lathe Arbors
Different Types of Lathe Arbors
Fig. 16. Different Types of Lathe Arbors
Different Types of Lathe Arbors.—Three different types of lathe arbors are shown inFig. 16. The kind shown atAis usually made of tool steel and the body is finished to a standard size. The ends are somewhat reduced and flat spots are milled, as shown, to give the clamping screw of the dog a good grip. The body of the arbor is usually tapered about 0.006 inch per foot. This taper makes it easier to insert the arbor in a close-fitting hole, and it also permits slight variations in the diameter of different holes. As to hardening, the practice at the present time among manufacturers is to harden arbors all over, but for extremely accurate work, an arbor having hardened ends and a soft body is generally considered superior, as there is less tendency of distortion from internal stresses. Hardened arbors are “seasoned†before finish-grinding to relieve these internal stresses.
The solid typeA,Fig. 16, is used very extensively, but inshops where a great variety of work is being done and there are many odd-sized holes, some form of expanding arborBcan be used to advantage. This type, instead of being solid, consists of a tapering inner arborMon which is placed a split bushing that can be expanded, within certain limits, by driving in the tapering member. The advantage of this type is that a comparatively small stock of arbors is required, as different-sized bushings can be used. This type can also be fitted to holes of odd sizes, whereas a solid arbor must be provided for each different size hole, unless the variation is very slight. The latter are, however, more accurate than the expanding type.
Another form of expanding arbor is shown atC. This type has a straight bodyNin which four tapering grooves are cut lengthwise, as shown, and there is a sleeveS, containing four slots that are located to correspond with the tapering grooves. Strips s are fitted into these slots, and as the partNis driven in, the strips are moved outward as they ascend the tapering grooves. By having different sets of these strips of various heights, one arbor of this type can be made to cover quite a range of sizes. It is not suited, however, to thin work, as the pressure, being concentrated in four places, would spring a flexible part out of shape.
Cone Arbor, Nut Arbor, Pipe CenterFig. 17. (A) Cone Arbor.(B) Nut Arbor.(C) Pipe Center
Cone Arbor, Nut Arbor, Pipe Center
Fig. 17. (A) Cone Arbor.(B) Nut Arbor.(C) Pipe Center
The cone arbor or mandrel shown atA, inFig. 17, is convenientfor holding parts having comparatively large holes, as it can be adjusted for quite a range of diameters. The work is gripped between the two conescandc1which are forced together by nutn. The cones are prevented from turning upon the arbor by keys. This style of arbor should not be used for accurate work. The threaded arborBis used for facing the sides of nuts square with the tapped hole. When a nut is first put upon the arbor, the rough side comes against an equalizing washerw. This washer rests against a spherical seat so that it can shift to provide a uniform bearing for the rough side of the nut, even though it is not square with the tapped hole. This feature prevents the nut from being canted on the arbor and insures an accurately faced nut. The revolving conical center shown atCis often used for holding a pipe or tube while turning the outside. The cone is adjusted to fit into the hole of the pipe, by means of the tailstock spindle, and the opposite end is usually held in a chuck.
Particular care should be taken to preserve the accuracy ofthe centers of lathe arbors by keeping them clean and well-oiled while in use.
Press for Forcing Arbors into WorkFig. 18. Press for Forcing Arbors into Work
Fig. 18. Press for Forcing Arbors into Work
Mandrel or Arbor Press.—The best method of inserting an arbor of the solid type in a hole is by using a press,Fig. 18, designed for that purpose, but if such a press is not available and it is necessary to drive the mandrel in, a “soft†hammer, made of copper, lead or other soft material, should be used to protect the centered end of the arbor. In either case, the arbor should not be forced in too tightly, for if it fits properly, this will not be necessary in order to hold the work securely. On the other hand, the work might easily be broken by attempting to force the arbor in as far and as tightly as possible. In using the arbor press, the work is placed on the baseBwith the hole in a vertical position, and the arbor (which should be oiled slightly) is forced down into it by ramR, operated by leverL. Slots are provided in the base, as shown, so that the end of the arbor can come through at the bottom of the hole. The lever of this particular press is counter-weighted so that it rises to a vertical position when released. The ram can then be adjusted quickly to any required height by the handwheel seen at the left.
Some shops are equipped with power-driven mandrel orarbor presses. This type is particularly desirable for large work, owing to the greater pressure required for inserting mandrels that are comparatively large in diameter. One well-known type of power press is driven by a belt, and the downward pressure of the ram is controlled by a handwheel. The ram is raised or lowered by turning this handwheel in one direction or the other, and a gage shows how much pressure is being applied. This type of press can also be used for other purposes, such as forcing bushings or pins into or out of holes, bending or straightening parts, or for similar work.
Steadyrest and Follow-rest for Supporting Flexible PartsFig. 19. Steadyrest and Follow-rest for Supporting Flexible Parts
Steadyrest and Follow-rest for Supporting Flexible Parts
Fig. 19. Steadyrest and Follow-rest for Supporting Flexible Parts
Steadyrest for Supporting Flexible Parts.—Occasionally long slender shafts, rods, etc., which have to be turned, are so flexible that it is necessary to support them at some point between the lathe centers. An attachment for the lathe known as a steadyrest is often used for this purpose. A steadyrest is composed of a frame containing three jawsJ(Fig. 19), that can be adjusted in or out radially by turning screwsS. The frame is hinged ath, thus allowing the upper half to be swung back (as shown by the dotted lines) for inserting or removing the work. The bolt-clampcholds the hinged part in the closed position. The base of the frame has V-grooves in it that fit the ways of the lathe bed. When the steadyrest is in use, it issecured to the bed by clampC, and the jawsJare set in against the work, thus supporting or steadying it during the turning operation. The steadyrest must, of course, be located at a point where it will not interfere with the turning tool.
Application of Steadyrest to a Flexible RodFig. 20. Application of Steadyrest to a Flexible Rod
Application of Steadyrest to a Flexible Rod
Fig. 20. Application of Steadyrest to a Flexible Rod
Fig. 20shows the application of the steadyrest to a long forged rod, having one small end, which makes it too flexible to be turned without support. As this forging is rough, a true surfacena little wider than the jawsJ(Fig. 19) is first turned as a bearing for the jaws. This should be done very carefully to prevent the work from mounting the tool. A sharp pointed tool should be used and very light cuts taken. The steadyrest is next clamped to the lathe bed opposite the turned surface, and the jaws are adjusted in against this surface, thus forming a bearing. Care should be taken not to set up the jaws too tightly, as the work should turn freely but without play. The large part of the rod and central collar are then turned to size, this half being machined while the small part is in the rough and as stiff as possible. The rod is then reversed and the steadyrest is applied to the part just finished, as shown atB, thus supporting the work while the small end is being turned. That part against which the jaws bear should be kept well oiled, and if the surface is finished it should be protected by placing a strip of emery cloth beneath the jaws with the emery side out;a strip of belt leather is also used for this purpose, the object in each case being to prevent the jaws from scratching and marring the finished surface, as they tend to do, especially if at all rough.
If the work were too flexible to permit turning a spot atn, this could be done by first “spotting†it at some pointo, and placing the steadyrest at that point while turning another spot atn.
Cat-head which is sometimes used as Bearing for SteadyrestFig. 21. Cat-head which is sometimes used as Bearing for Steadyrest
Cat-head which is sometimes used as Bearing for Steadyrest
Fig. 21. Cat-head which is sometimes used as Bearing for Steadyrest
Sometimes it is desirable to apply a steadyrest to a surface that does not run true and one which is not to be turned; in such a case a device called a “cat-head†is used. This is simply a sleeveS(Fig. 21) which is placed over the untrue surface to serve as a bearing for the steadyrest. The sleeve is made to run true by adjusting the four set-screws at each end, and the jaws of the steadyrest are set against it, thus supporting the work.
Shaft supported by Steadyrest for Drilling and Boring EndFig. 22. Shaft supported by Steadyrest for Drilling and Boring End
Shaft supported by Steadyrest for Drilling and Boring End
Fig. 22. Shaft supported by Steadyrest for Drilling and Boring End
Application of Steadyrest when Boring.—Another example illustrating the use of the steadyrest is shown inFig. 22. The rodRis turned on the outside and a hole is to be bored in the end (as shown by dotted lines) true with the outer surface. If the centers used for turning the rod are still in the ends, as they would be ordinarily, this work could be done very accurately by the following method: The rod is first placed between the centers as for turning, with a driving dogDattached, and the steadyrest jawsJare set against it near the outer end, as shown.
Hold-back used when Outer End of Work is held in SteadyrestFig. 23. Hold-back used when Outer End of Work is held in Steadyrest
Hold-back used when Outer End of Work is held in Steadyrest
Fig. 23. Hold-back used when Outer End of Work is held in Steadyrest
Before any machine work is done, means must be provided for holding the rod back against the headstock centerh, because, for an operation of this kind, the outer end cannot be supportedby the tailstock center; consequently the work tends to shift to the right. One method of accomplishing this is shown in the illustration. A hardwood piecew, having a hole somewhat larger than the work, is clamped against the dog, in a crosswise position, by the swinging bolts and thumb-screws shown. If the dog is not square with the work, the wood piece should be canted so that the bearing will not be all on one side. For large heavy parts a similar “bridle†or “hold-backâ€â€”as this is commonly called—is made by using steel instead of wood for the partw. Another very common method which requires no special equipment is illustrated inFig. 23. An ordinary leather belt lacingLis attached to the work and faceplate while the latter is screwed off a few turns as shown. Then the lacing is drawn up by hand and tied, and the faceplate is screwed onto the spindle, thus tightening the lacing and drawing the work against the headstock center. The method of applying the lacing is quite clearly indicated in the illustration. If a small driving faceplate is used, it may be necessary to drill holes for the belt lacing, as shown.
Testing Work with Dial IndicatorFig. 24. Testing Work with Dial Indicator
Testing Work with Dial Indicator
Fig. 24. Testing Work with Dial Indicator
A hole is next drilled in the end of the rod by using a twist drill in the tailstock. If the hole is finished by boring, a depthmark should be made on the tool shank that will warn the workman of the cutting end's approach to the bottom. A chuck can also be used in connection with a steadyrest for doing work of this kind, as shown inFig. 24, the end of the rod being held and driven by the chuckC. If the piece is centered, it can be held on these centers while setting the steadyrest and adjusting the chuck, but if the ends are without centers, a very good way is to make light centers in the ends with a punch; after these are properly located they are used for holding the work until the steadyrest and chuck jaws have been adjusted. In case it is necessary to have the end hole very accurate with the outside of the finished rod, a test indicatorIshould be applied to the shaft as shown. This is an instrument which shows with great accuracy whether a rotating part runs true and it is also used for many other purposes in machine shops. The indicator is held in the lathe toolpost and the contact point beneath the dial is brought against the work. If the latter does not run true, the hand of the indicator vibrates and the graduations on the dial show how much the work is out in thousandths of an inch.
The Follow-rest.—When turning long slender parts, such as shafts, etc., a follow-rest is often used for supporting the work. The follow-rest differs from the steadyrest in that it is attachedto and travels with the lathe carriage. The type illustrated to the right inFig. 19has two adjustable jaws which are located nearly opposite the turning tool, thus providing support where it is most needed. In using this rest, a cut is started at the end and the jaws are adjusted to this turned part. The tool is then fed across the shaft, which cannot spring away from the cut because of the supporting jaws. Some follow-rests have, instead of jaws, a bushing bored to fit the diameter being turned, different bushings being used for different diameters. The bushing forms a bearing for the work and holds it rigidly. Whether a bushing or jaws are used, the turning tool is slightly in advance of the supporting member.
Centering End with Punch preparatory to DrillingFig. 25. Centering End with Punch preparatory to Drilling
Centering End with Punch preparatory to Drilling
Fig. 25. Centering End with Punch preparatory to Drilling
Centering Parts to be Turned.—As previously mentioned, there are a number of different methods of forming center-holes in the ends of parts that have to be turned while held between lathe centers. A method of centering light work, and one that requires few special tools, is first to locate a central point on the end and then drill and ream the center-hole by using the lathe itself. Hermaphrodite dividers are useful for finding the center, as illustrated atA,Fig. 25, but if the work is fairly round, a center-squareBis preferable. A line is scribed across the end and then another line at right angles to the first by changing the position of the square; the intersection of these two lines will be the center, which should be marked by strikinga pointed punchCwith a hammer. If a cup or bell center-punchDis available, it will not be necessary to first make center lines, as the conical part shown locates the punch in a central position. This style of punch should only be used on work which is fairly round.
After small centers have been located in both ends, their position can be tested by placing the work between the lathe centers and rotating it rapidly by drawing the hand quickly across it. By holding a piece of chalk close to the work as it spins around, a mark will be made on the “high†side if the centers are not accurate; the centers are then shifted toward these marks. If the work is close to the finished diameter, the centers should, of course, be located quite accurately in order that the entire surface of the work will be turned true when it is reduced to the finished size.
Drilling Centers in the LatheFig. 26. Drilling Centers in the Lathe
Drilling Centers in the Lathe
Fig. 26. Drilling Centers in the Lathe
One method of forming these center-holes is indicated inFig. 26. A chuckCis screwed onto the spindle in place of the faceplate, and a combination center drill and reamerRis gripped by the chuck jaws and set to run true. The center is then drilled and reamed at one end by pressing the work against the revolving drill with the tailstock spindle, which is fed out by turning handlen. The piece is then reversed for drilling the opposite end. The work may be kept from revolving while the centers are being drilled and reamed, by attaching a dog to it close to the tailstock end and then adjusting the cross-slideuntil the dog rests upon the slide. Many parts can be held by simply gripping them with one hand. From the foregoing it will be seen that the small centers made by punchC,Fig. 25, serve as a starting point for the drill and also as a support for the outer end of the work while the first hole is being drilled.
Centers of Incorrect and Correct FormFig. 27. Centers of Incorrect and Correct Form
Centers of Incorrect and Correct Form
Fig. 27. Centers of Incorrect and Correct Form
The form of center-hole produced by a combination drill and reamer is shown by the lower left-hand view inFig. 27. A small straight hole a in the bottom prevents the point of the lathe center from coming in contact with the work and insures a good bearing on the conical surfacec. The standard angle for lathe centers is sixty degrees, as the illustration shows, and the tapering part of all center-holes should be made to this angle.