Chapter III: Taper Turning--Special Operations--Fitting (1)

It is often necessary, in connection with lathe work, to turn parts tapering instead of straight or cylindrical. If the work is mounted between the centers, one method of turning a taper is to set the tailstock center out of alignment with the headstock center. When both of these centers are in line, the movement of the tool is parallel to the axis of the work and, consequently, a cylindrical surface is produced; but if the tailstock _h_{1}_ is set out of alignment, as shown in Fig. 1, the work will then be turned tapering as the tool is traversed from _a_ to _b_, because the axis _x--x_ is at an angle with the movement of the tool. Furthermore the amount of taper or the difference between the diameters at the ends for a given length, will depend on how much center _h_{1}_ is set over from the central position.

The amount of taper is usually given on drawings in inches per foot, or the difference in the diameter at points twelve inches apart. For example, the taper of the piece shown at _A_, Fig. 2, is 1 inch per foot, as the length of the tapering surface is just twelve inches and the difference between the diameters at the ends is 1 inch. The conical roller shown at _B_ has a total length of 9 inches and a tapering surface 6 inches long, and in this case the taper per foot is also 1 inch, there being a difference of 1/2 inch in a length of 6 inches or 1 inch in twice that length. When the taper per foot is known, the amount that the tailstock center should be set over for turning that taper can easily be estimated, but it should be remembered that the setting obtained in this way is not absolutely correct, and is only intended to locate the center approximately. When a taper needs to be at all accurate, it is tested with a gage, or by other means, after taking a trial cut, as will be explained later, and the tailstock center is readjusted accordingly. There are also more accurate methods of setting the center, than by figuring the amount of offset, but as the latter is often convenient this will be referred to first.

=Setting Tailstock Center for Taper Turning.=--Suppose the tailstock center is to be set for turning part _C_, Fig. 2, to a taper of approximately 1 inch per foot. In this case the center would simply be moved toward the front of the machine 1/2 inch, or one-half the required taper per foot, because the total length of the work happens to be just 12 inches. This setting, however, would not be correct for all work requiring a taper of 1 inch per foot, as the adjustment depends not only on the _amount_ of the taper but on the _total length_ of the piece.

For example, the taper roller _B_ has a taper of 1 inch per foot, but the center, in this case, would be offset less than one-half the taper per foot, because the total length is only 9 inches. For lengths longer or shorter than twelve inches, the taper per inch should be found first; this is then multiplied by the _total_ length of the work (not the length of the taper) which gives the taper for that length, and one-half this taper is the amount to set over the center. For example, the taper per inch of part _B_ equals 1 inch divided by 12 = 1/12 inch. The total length of 9 inches multiplied by 1/12 inch = 3/4 inch, and 1/2 of 3/4 = 3/8, which is the distance that the tailstock center should be offset. In this example if the taper per foot were not known, and only the diameters of the large and small ends of the tapered part were given, the difference between these diameters should first be found (2-1/2-2 = 1/2); this difference should then be divided by the length of the taper (1/2 / 6 = 1/12 inch) to obtain the taper per inch. The taper per inch times the _total_ length represents what the taper would be if it extended throughout the entire length, and one-half of this equals the offset, which is 3/8 inch.

=Example of Taper Turning.=--As a practical example of taper turning let us assume that the piece A, Fig. 4, which has been centered and rough-turned as shown, is to be made into a taper plug, as indicated at _B_, to fit a ring gage as at _C_. If the required taper is 1-1/2 inch per foot and the total length is 8 inches, the tailstock center would be offset 1/2 inch.

To adjust the tailstock, the nuts _N_ (Fig. 3) are first loosened and then the upper part _A_ is shifted sidewise by turning screw _S_. Scales are provided on some tailstocks for measuring the amount of this adjustment; if there is no scale, draw a line across the movable and stationary parts _A_ and _B_, when the tailstock is set for straight turning. The movement of the upper line in relation to the lower will then show the offset, which can be measured with a scale.

When the adjustment has been made, nuts _N_ are tightened and the part to be turned, with a dog attached, is placed between the centers the same as for straight turning. The taper end is then reduced by turning, but before it is near the finished size, the work is removed and the taper tested by inserting it in the gage. If it is much out, this can be felt, as the end that is too small can be shaken in the hole. Suppose the plug did not taper enough and only the small end came into contact with the gage, as shown somewhat exaggerated at _D_; in that case the center would be shifted a little more towards the front, whereas if the taper were too steep, the adjustment would, of course, be in the opposite direction. A light cut would then be taken, to be followed by another test. If the plug should fit the gage so well that there was no perceptible shake, it could be tested more closely as follows: Draw three or four chalk lines along the tapering surface, place the work in the gage and turn it a few times. The chalk marks will then show whether the taper of the plug corresponds to that of the gage; for example, if the taper is too great, the marks will be rubbed out on the large end, but if the taper is correct, the lines throughout their length will be partially erased.

Another and more accurate method of testing tapers is to apply a thin coat of Prussian-blue to one-half of the tapering surface, in a lengthwise direction. The work is then inserted in the hole or gage and turned to mark the bearing. If the taper is correct, the bearing marks will be evenly distributed, whereas if the taper is incorrect, they will appear at one end. Tapering pieces that have to be driven tightly into a hole, such as a piston-rod, can be tested by the location of the bearing marks produced by actual contact.

After the taper is found to be correct, the plug is reduced in size until it just enters the gage as at _C_. The final cut should leave it slightly above the required size, so that a smooth surface can be obtained by filing. It should be mentioned that on work of this kind, especially if great accuracy is required, the final finish is often obtained by grinding in a regular grinding machine, instead of by filing. When this method is employed, a lathe is used merely to rough-turn the part close to size.

When the amount that the tailstock center should be offset is determined by calculating, as in the foregoing example, it is usually necessary to make slight changes afterward, and the work should be tested before it is too near the finished size so that in case one or more trial cuts are necessary, there will be material enough to permit this. When there are a number of tapered pieces to be turned to the same taper, the adjustment of the tailstock center will have to be changed unless the total length of each piece and the depth of the center holes are the same in each case.

=Setting the Tailstock Center with a Caliper Tool.=--Another method of setting the tailstock center for taper turning is illustrated in Fig. 5. The end of an engine piston-rod is to be made tapering as at A and to dimensions _a_, _b_, _c_ and _d_. It is first turned with the centers in line as at _B_. The end _d_ is reduced to diameter _b_ up to the beginning of the taper and it is then turned to diameter _a_ as far as the taper part _c_ extends. The tailstock center is next set over by guess and a caliper tool is clamped in the toolpost. This tool, a side view of which is shown in Fig. 6, has a pointer _p_ that is free to swing about pivot _r_, which should be set to about the same height as the center of the work. The tailstock center is adjusted until this pointer just touches the work when in the positions shown by the full and dotted lines at _C_, Fig. 5; that is, until the pointer makes contact at the beginning and end of the taper part. The travel of the carriage will then be parallel to a line _x--x_, representing the taper; consequently, if a tool is started at the small end, as shown by the dotted lines at _D_, with the nose just grazing the work, it will also just graze it when fed to the extreme left as shown. Of course, if the taper were at all steep, more than one cut would be taken.

If these various operations are carefully performed, a fairly accurate taper can be produced. The straight end _d_ is reduced to size after the tail-center is set back to the central position. Some mechanics turn notches or grooves at the beginning and end of the tapering part, having diameters equal to the largest and smallest part of the taper; the work is then set by these grooves with a caliper tool. The advantage of the first method is that most of the metal is removed while the centers are in alignment.

=Setting the Tailstock Center with a Square.=--Still another method of adjusting the tailstock for taper turning, which is very simple and eliminates all figuring, is as follows: The part to be made tapering is first turned cylindrical or straight for 3 or 4 inches of its length, after the ends have been properly centered and faced square. The work is then removed and the tailstock is shifted along the bed until the distance _a--b_ between the extreme points of the centers is exactly 1 foot. The center is next offset a distance _b--c_ equal to one-half the required taper per foot, after which a parallel strip _D_, having true sides, is clamped in the toolpost. Part _D_ is then set at right angles to a line passing from one center point to the other. This can be done conveniently by holding a 1-foot square (preferably with a sliding head) against one side of _D_ and adjusting the latter in the toolpost until edge _E_ of the square blade is exactly in line with both center points. After part _D_ is set, it should be clamped carefully to prevent changing the position. The angle between the side of _D_ and an imaginary line which is perpendicular to axis _a--b_ is now equal to one-half the angle of the required taper.

The axis of the part to be turned should be set parallel with line _E_, which can be done by setting the cylindrical surface which was previously finished, at right angles to the side of _D_. In order to do this the work is first placed between centers, the tailstock being shifted along the bed if necessary; the tail-center is then adjusted laterally until the finished cylindrical surface is square with the side of _D_. A small try-square can be used for testing the position of the work, as indicated in Fig. 8. If the length of the work is less than 1 foot, it will be necessary to move the center toward the rear of the machine, and if the length is greater than 1 foot, the adjustment is, of course, in the opposite direction.

=The Taper Attachment.=--Turning tapers by setting over the tailstock center has some objectionable features. When the lathe centers are not in alignment, as when set for taper turning, they bear unevenly in the work centers because the axis of the work is at an angle with them; this causes the work centers to wear unevenly and results in inaccuracy. Furthermore, the adjustment of the tailstock center must be changed when turning duplicate tapers, unless the length of each piece and the depth of the center holes are the same. To overcome these objections, many modern lathes are equipped with a special device for turning tapers, known as a taper attachment, which permits the lathe centers to be kept in alignment, as for cylindrical turning, and enables more accurate work to be done.

Taper attachments, like lathes, vary some in their construction, but all operate on the same principle. An improved form of taper attachment is illustrated in Figs. 9 and 10. Fig. 9 shows a plan view of a lathe carriage with an attachment fitted to it, and Fig. 10 a sectional view. This attachment has an arm _A_ on which is mounted a slide _S_ that can be turned about a central pivot by adjusting screw _D_. The arm _A_ is supported by, and is free to slide on, a bracket _B_ (see also sectional view) that is fastened to the carriage, and on one end of the arm there is a clamp _C_ that is attached to the lathe bed when turning tapers. On the slide _S_ there is a shoe _F_ that is connected to bar _E_ which passes beneath the toolslide. The rear end of the cross-feed screw is connected to this bar, and the latter is clamped to the toolslide when the attachment is in use.

When a taper is to be turned, the carriage is moved opposite the taper part and clamp _C_ is fastened to the bed; this holds arm _A_ and slide _S_ stationary so that the carriage, with bracket _B_ and shoe _F_, can be moved with relation to the slide. If this slide _S_ is set at an angle, as shown, the shoe as it moves along causes the toolslide and tool to move in or out, but if the slide is set parallel to the carriage travel, the toolslide remains stationary. Now if the tool, as it feeds lengthwise of the work, is also gradually moved crosswise, it will turn a taper, and as this crosswise movement is caused by the angularity of slide _S_, different tapers are obtained by setting the slide to different positions.

By means of a graduated scale _G_ at the end of slide _S_, the taper that will be obtained for any angular position of the slide is shown. On some attachments there are two sets of graduations, one giving the taper in inches per foot and the other in degrees. While tapers are ordinarily given in inches per foot on drawings, sometimes the taper is given in degrees instead. The attachment is set for turning tapers by adjusting slide _S_ until pointer _p_ is opposite the division or fractional part of a division representing the taper. The whole divisions on the scale represent taper in inches per foot, and by means of the sub-divisions, the slide can be set for turning fractional parts of an inch per foot. When slide _S_ is properly set, it is clamped to arm _A_ by the nuts _N_. Bar _E_ is also clamped to the toolslide by bolt _H_, as previously stated. The attachment is disconnected for straight turning by simply loosening clamp _C_ and the bolt _H_.

=Application of Taper Attachment.=--Practical examples of lathe work, which illustrate the use of the taper attachment, are shown in Figs, 11 and 12. Fig. 11 shows how a taper hole is bored in an engine piston-head, preparatory to reaming. The casting must be held either in a chuck _C_ or on a faceplate if too large for the chuck. The side of the casting (after it has been "chucked") should run true, and also the circumference, unless the cored hole for the rod is considerably out of center, in which case the work should be shifted to divide the error. The side of the casting for a short space around the hole is faced true with a round nose turning tool, after which the rough-cored hole is bored with an ordinary boring tool _t_, and then it is finished with a reamer to exactly the right size and taper.

This particular taper attachment is set to whatever taper is given on the drawing, by loosening nuts _N_ and turning slide _S_ until pointer _P_ is opposite that division on the scale which represents the taper. The attachment is then ready, after bolt _H_ and nuts _N_ are tightened, and clamp _C_ is fastened to the lathe bed. The hole is bored just as though it were straight, and as the carriage advances, the tool is gradually moved inward by the attachment. If the lathe did not have a taper attachment, the taper hole could be bored by using the compound rest.

The hole should be bored slightly less than the finish size to allow for reaming. When a reamer is used in the lathe, the outer end is supported by the tailstock center and should have a deep center-hole. The lathe is run very slowly for reaming and the reamer is fed into the work by feeding out the tailstock spindle. The reamer can be kept from revolving, either by attaching a heavy dog to the end or, if the end is squared, by the use of a wrench long enough to rest against the lathe carriage. A common method is to clamp a dog to the reamer shank, and then place the tool-rest beneath it to prevent rotation. If the shank of a tool is clamped to the toolpost so that the dog rests against it, the reamer will be prevented from slipping off the center as it tends to do; with this arrangement, the carriage is gradually moved along as the tailstock spindle is fed outward. Some reamers are provided with stop-collars which come against the finished side of the casting when the hole has been reamed to size.

After the reaming operation, the casting is removed from the chuck and a taper mandrel is driven into the hole for turning the outside of the piston. This mandrel should run true on its centers, as otherwise the outside surface of the piston will not be true with the bored hole. The driving dog, especially for large work of this kind, should be heavy and stiff, because light flexible clamps or dogs vibrate and frequently cause chattering. For such heavy work it is also preferable to drive at two points on opposite sides of the faceplate, but the driving pins should be carefully adjusted to secure a uniform bearing on both sides.

The foregoing method of machining a piston is one that would ordinarily be followed when using a standard engine lathe, and it would, perhaps, be as economical as any if only one piston were being made; but where such work is done in large quantities, time could be saved by proceeding in a different way. For example, the boring and reaming operation could be performed much faster in a turret lathe, which is a type designed for just such work, but a turret lathe cannot be used for as great a variety of turning operations as a lathe of the regular type. There are also many other classes of work that can be turned more quickly in special types of machines, but as more or less time is required for arranging these special machines and often special tools have to be made, the ordinary lathe is frequently indispensable when only a few parts are needed; in addition, it is better adapted to some turning operations than any other machine.

Fig. 12 illustrates how a taper attachment would be used for turning the taper fitting for the crosshead end of an engine piston-rod. Even though this taper corresponds to the taper of the hole in the piston, slide _S_ would have to be reset to the corresponding division on the opposite side of the central zero mark, because the taper of the hole decreased in size during the boring operation, whereas the rod is smallest at the beginning of the cut, so that the tool must move outward rather than inward as it advances. The taper part is turned practically the same as a cylindrical part; that is, the power feed is used and, as the carriage moves along the bed, the tool is gradually moved outward by the taper attachment.

If the rod is being fitted directly to the crosshead (as is usually the case), the approximate size of the small end of the taper could be determined by calipering, the calipers being set to the size of the hole at a distance from the shoulder or face side of the crosshead, equal to the length of the taper fitting on the rod. If the crosshead were bored originally to fit a standard plug gage, the taper on the rod could be turned with reference to this gage, but, whatever the method, the taper should be tested before turning too close to the finished size. The test is made by removing the rod from the lathe and driving it tightly into the crosshead. This shows how near the taper is to size, and when the rod is driven out, the bearing marks show whether the taper is exactly right or not. If the rod could be driven in until the shoulder is, say, 1/8 inch from the crosshead face, it would then be near enough to finish to size by filing. When filing, the lathe is run much faster than for turning, and most of the filing should be done where the bearing marks are the heaviest, to distribute the bearing throughout the length of the taper. Care should be taken when driving the rod in or out, to protect the center-holes in the ends by using a "soft" hammer or holding a piece of soft metal against the driving end.

After the crosshead end is finished, the rod is reversed in the lathe for turning the piston end. The dog is clamped to the finished end, preferably over a piece of sheet copper to prevent the surface from being marred. When turning this end, either the piston reamer or the finished hole in the piston can be calipered. The size and angle of the taper are tested by driving the rod into the piston, and the end should be fitted so that by driving tightly, the shoulder will just come up against the finished face of the piston. When the taper is finished, the attachment is disengaged and a finishing cut is taken over the body of the rod, unless it is to be finished by grinding, which is the modern and most economical method.

=Height of Tool when Turning Tapers.=--The cutting edge of the tool, when turning tapers, should be at the same height as the center or axis of the work, whether an attachment is used or not. The importance of this will be apparent by referring to Fig. 13. To turn the taper shown, the tool _T_ would be moved back a distance _x_ (assuming that an attachment is used) while traversing the length _l_. As an illustration, if the tool could be placed as high as point _a_, the setting of the attachment remaining as before, the tool would again move back a distance _x_, while traversing a distance _l_, but the large end would be under-sized (as shown by the dotted line) if the diameters of the small ends were the same in each case. Of course, if the tool point were only slightly above or below the center, the resulting error would also be small. The tool can easily be set central by comparing the height of the cutting edge at the point of the tool with one of the lathe centers before placing the work in the lathe.

=Taper Turning with the Compound Rest.=--The amount of taper that can be turned by setting over the tailstock center and by the taper attachment is limited, as the centers can only be offset a certain distance, and the slide _S_ (Fig. 9) of the attachment cannot be swiveled beyond a certain position. For steep tapers, the compound rest _E_ is swiveled to the required angle and used as indicated in Fig. 14, which shows a plan view of a rest set for turning the valve _V_. This compound rest is an upper slide mounted on the lower or main cross-slide _D_, and it can be turned to any angular position so that the tool, which ordinarily is moved either lengthwise or crosswise of the bed, can be fed at an angle. The base of the compound rest is graduated in degrees and the position of these graduations shows to what angle the upper slide is set. Suppose the seat of valve _V_ is to be turned to an angle of 45 degrees with the axis or center, as shown on the drawing at _A_, Fig. 15. To set the compound rest, nuts _n_ on either side, which hold it rigidly to the lower slide, are first loosened and the slide is then turned until the 45-degree graduation is exactly opposite the zero line; the slide is then tightened in this position. A cut is next taken across the valve by operating handle _w_ and feeding the tool in the direction of the arrow.

In this particular instance the compound rest is set to the same angle given on the drawing, but this is not always the case. If the draftsman had given the included angle of 90 degrees, as shown at _B_, which would be another way of expressing it, the setting of the compound rest would, of course, be the same as before, or to 45 degrees, but the number of degrees marked on the drawing does not correspond with the angle to which the rest must be set. As another illustration, suppose the valve were to be turned to an angle of 30 degrees with the axis as shown at _C_. In this case the compound rest would not be set to 30 degrees but to 60 degrees, because in order to turn the work to an angle of 30 degrees, the rest must be 60 degrees from its zero position, as shown. From this it will be seen that the number of degrees marked on the drawing does not necessarily correspond to the angle to which the rest must be set, as the graduations on the rest show the number of degrees that it is moved from its zero position, which corresponds to the line _a--b_. The angle to which the compound rest should be set can be found, when the drawing is marked as at _A_ or _C_, by subtracting the angle given from 90 degrees. When the included angle is given, as at _B_, subtract one-half the included angle from 90 degrees to obtain the required setting. Of course, when using a compound rest, the lathe centers are set in line as for straight turning, as otherwise the angle will be incorrect.

Rules for Figuring Tapers

+---------------------+---------------------+--------------------------+ | Given | To Find | Rule | +---------------------+---------------------+--------------------------+ |The taper per foot. |The taper per inch. |Divide the taper per foot | | | | by 12. | | | | | |The taper per inch. |The taper per foot. |Multiply the taper per | | | | inch by 12. | | | | | |End diameters and |The taper per foot. |Subtract small diameter | | length of taper in | | from large; divide by | | inches. | | length of taper, and | | | | multiply quotient by 12.| | | | | |Large diameter and |Diameter at small |Divide taper per foot by | | length of taper in | end in inches. | 12; multiply by length | | inches and taper | | of length of taper, and | | per foot. | | subtract result from | | | | large diameter. | | | | | |Small diameter and |Diameter at large |Divide taper per foot by | | length of taper in | end in inches. | 12; multiply by length | | inches, and taper | | of taper, and add result| | per foot. | | to small diameter. | | | | | |The taper per foot |Distance between | Subtract small diameter | | and two diameters | two given diameters| from large; divide re- | | in inches. | in inches. | mainder by taper per | | | | foot, and multiply | | | | quotient by 12. | | | | | |The taper per foot. |Amount of taper in | Divide taper per foot by | | | a certain length | 12; multiply by given | | | given in inches. | length of tapered part.| +---------------------+---------------------+--------------------------+

=Accurate Measurement of Angles and Tapers.=--When great accuracy is required in the measurement of angles, or when originating tapers, disks are commonly used. The principle of the disk method of taper measurement is that if two disks of unequal diameters are placed either in contact or a certain distance apart, lines tangent to their peripheries will represent an angle or taper, the degree of which depends upon the diameters of the two disks and the distance between them. The gage shown in Fig. 16, which is a form commonly used for originating tapers or measuring angles accurately, is set by means of disks. This gage consists of two adjustable straight-edges _A_ and _A_{1}_, which are in contact with disks _B_ and _B_{1}_. The angle [alpha] or the taper between the straight-edges depends, of course, upon the diameters of the disks and the center distance _C_, and as these three dimensions can be measured accurately, it is possible to set the gage to a given angle within very close limits. Moreover, if a record of the three dimensions is kept, the exact setting of the gage can be reproduced quickly at any time. The following rules may be used for adjusting a gage of this type.

=To Find Center Distance for a Given Taper.=--When the taper, in inches per foot, is given, to determine center distance _C_. _Rule:_ Divide the taper by 24 and find the angle corresponding to the quotient in a table of tangents; then find the sine corresponding to this angle and divide the difference between the disk diameters by twice the sine.

_Example:_ Gage is to be set to 3/4 inch per foot, and disk diameters are 1.25 and 1.5 inch, respectively. Find the required center distance for the disks.

0.75
---- = 0.03125.
24

The angle whose tangent is 0.03125 equals 1 degree 47.4 minutes; sin 1 deg. 47.4' = 0.03123; 1.50 - 1.25 = 0.25 inch;

0.25
----------- = 4.002 inches = center distance C.
2 x 0.03123

=To Find Center Distance for a Given Angle.=--When straight-edges must be set to a given angle [alpha], to determine center distance _C_ between disks of known diameter. _Rule:_ Find the sine of half the angle [alpha] in a table of sines; divide the difference between the disk diameters by double this sine.

_Example:_ If an angle [alpha] of 20 degrees is required, and the disks are 1 and 3 inches in diameter, respectively, find the required center distance _C_.

20
---- = 10 degrees; sin 10 deg. = 0.17365;
2

3 - 1
----------- = 5.759 inches = center distance _C_.
2 x 0.17365

=To Find Angle for Given Taper per Foot.=--When the taper in inches per foot is known, and the corresponding angle [alpha] is required. _Rule:_ Divide the taper in inches per foot by 24; find the angle corresponding to the quotient, in a table of tangents, and double this angle.

_Example:_ What angle [alpha] is equivalent to a taper of 1-1/2 inch per foot?

1.5
--- = 0.0625.
24

The angle whose tangent is 0.0625 equals 3 degrees 35 minutes, nearly; then, 3 deg. 35 min. x 2 = 7 deg. 10 min.

=To Find Angle for Given Disk Dimensions.=--When the diameters of the large and small disks and the center distance are given, to determine the angle [alpha]. _Rule:_ Divide the difference between the disk diameters by twice the center distance; find the angle corresponding to the quotient, in a table of sines, and double the angle.

_Example:_ If the disk diameters are 1 and 1.5 inch, respectively, and the center distance is 5 inches, find the included angle [alpha].

1.5 - 1
------- = 0.05.
2 x 5

The angle whose sine is 0.05 equals 2 degrees 52 minutes; then, 2 deg. 52 min. x 2 = 5 deg. 44 min. = angle [alpha].

=Use of the Center Indicator.--=The center test indicator is used for setting a center-punch mark, the position of which corresponds with the center or axis of the hole to be bored, in alignment with the axis of the lathe spindle. To illustrate, if two holes are to be bored, say 5 inches apart, small punch marks having that center-to-center distance would be laid out as accurately as possible. One of these marks would then be set central with the lathe spindle by using a center test indicator as shown in Fig. 17. This indicator has a pointer _A_ the end of which is conical and enters the punch mark. The pointer is held by shank _B_ which is fastened in the toolpost. The joint _C_ by means of which the pointer is held to the shank is universal; that is, it allows the pointer to move in any direction. Now when the part being tested is rotated by running the lathe, if the center-punch mark is not in line with the axes of the lathe spindle, obviously the outer end of pointer _A_ will vibrate, and as joint _C_ is quite close to the inner end, a very slight error in the location of the center-punch mark will cause a perceptible movement of the outer end, as indicated by the dotted lines. When the work has been adjusted until the pointer remains practically stationary, the punch mark is central, and the hole is bored. The other center-punch mark is then set in the same way for boring the second hole. The accuracy of this method depends, of course, upon the location of the center-punch marks. A still more accurate way of setting parts for boring holes to a given center-to-center distance is described in the following:

=Locating Work by the Button Method.=--Among the different methods employed by machinists and toolmakers for accurately locating work such as jigs, etc., on the faceplate of a lathe, the one most commonly used is known as the button method. This scheme is so named because cylindrical bushings or buttons are attached to the work in positions corresponding to the holes to be bored, after which they are used in locating the work. These buttons, which are ordinarily about 1/2 inch in diameter, are ground and lapped to the same size and the ends squared. The diameter should, preferably, be such that the radius can be determined easily, and the hole through the center should be about 1/8 inch larger than the retaining screw, so that the button can be shifted.

As an illustration of the practical application of the button method, we shall consider, briefly, the way the holes would be accurately machined in the jig-plate in Fig. 18. First the centers of the seven holes should be laid off approximately correct by the usual methods, after which small holes should be drilled and tapped for the clamping screws _S_. After the buttons _B_ are clamped lightly in place, they are all set in correct relation with each other and with the jig-plate. The proper location of the buttons is very important as their positions largely determine the accuracy of the work. A definite method of procedure that would be applicable in all cases cannot, of course, be given, as the nature of the work as well as the tools available make it necessary to employ different methods.

In this particular case, the three buttons _a_, _b_ and _c_ should be set first, beginning with the one in the center. As this central hole must be 2.30 and 2.65 inches from the finished sides _A_ and _A_{1}_, respectively, the work is first placed on an accurate surface-plate as shown; by resting it first on one of these sides and then on the other, and measuring with a vernier height gage, the central button can be accurately set. The buttons _a_ and _c_ are also set to the correct height from side _A_{1}_ by using the height gage, and in proper relation to the central button by using a micrometer or a vernier caliper and measuring the over-all dimension _x_. When measuring in this way, the diameter of one button would be deducted to obtain the correct center-to-center distance. After buttons _a_, _b_ and _c_ are set equidistant from side A_{1} and in proper relation to each other, the remaining buttons should be set radially from the central button _b_ and the right distance apart. By having two micrometers or gages, one set for the radial dimension _x_ and the other for the chordal distance _y_, the work may be done in a comparatively short time.

After the buttons have been tightened, all measurements should be carefully checked; the work is then mounted on the faceplate of the lathe, and one of the buttons, say _b_, is set true by the use of a test indicator as shown in Fig. 19. When the end of this indicator (which is one of a number of types on the market) is brought into contact with the revolving button, the vibration of the pointer _I_ shows how much the button runs out of true. When the pointer remains practically stationary, thus showing that the button runs true, the latter should be removed. The hole is then drilled nearly to the required size, after which it is bored to the finish diameter. In a similar manner the other buttons are indicated and the holes bored, one at a time. It is evident that if each button is correctly located and set perfectly true in the lathe, the various holes will be located at the required center-to-center dimensions within very close limits.

Fig. 20 shows how one of the buttons attached to a plate in which three holes are to be bored is set true or concentric. The particular indicator illustrated is of the dial type, any error in the location of the button being shown by a hand over a dial having graduations representing thousandths of an inch. Fig. 21 shows how the hole is drilled after the button is removed. It will be noted that the drill is held in a chuck, the taper shank of which fits into the tailstock spindle, this being the method of holding small drills. After drilling, the hole is bored as shown in Fig. 22. The boring tool should have a keen edge to avoid springing, and if the work when clamped in position, throws the faceplate out of balance, it is advisable to restore the balance, before boring, by the use of a counter-weight, because the lathe can be rotated quite rapidly when boring such a small hole.

When doing precision work of this kind, the degree of accuracy will depend upon the instruments used, the judgment and skill of the workman and the care exercised. A good general rule to follow when locating bushings or buttons is to use the method which is the most direct and which requires the least number of measurements. As an illustration of how errors may accumulate, let us assume that seven holes are to be bored in the jig-plate shown in Fig. 23, so that they are the same distance from each other and in a straight line. The buttons may be brought into alignment by the use of a straight-edge, and to simplify matters, it will be taken for granted that they have been ground and lapped to the same size. If the diameter of the buttons is first determined by measuring with a micrometer, and then this diameter is deducted from the center distance _x_, the difference will be the distance _y_ between adjacent buttons. Now if a temporary gage is made to length _y_, all the buttons can be set practically the same distance apart, the error between any two adjacent ones being very slight. If, however, the total length _z_ over the end buttons is measured by some accurate means, the chances are that this distance will not equal six times dimension _x_ plus the diameter of one button, as it should, because even a very slight error in the gage for distance _y_ would gradually accumulate as each button was set. If a micrometer were available that would span two of the buttons, the measurements could be taken direct and greater accuracy would doubtless be obtained. On work of this kind where there are a number of holes that need to have accurate over-all dimensions, the long measurements should first be taken when setting the buttons, providing, of course, there are proper facilities for so doing, and then the short ones. For example, the end buttons in this case should first be set, then the central one and finally those for the sub-divisions.

=Eccentric Turning.=--When one cylindrical surface must be turned eccentric to another, as when turning the eccentric of a steam engine, an arbor having two sets of centers is commonly used, as shown in Fig. 24. The distance _x_ between the centers must equal one-half the total "throw" or stroke of the eccentric. The hub of the eccentric is turned upon the centers _a--a_, and the tongued eccentric surface, upon the offset centers, as indicated by the illustration. Sometimes eccentrics are turned while held upon special fixtures attached to the faceplate.

When making an eccentric arbor, the offset center in each end should be laid out upon radial lines which can be drawn across the arbor ends by means of a surface gage. Each center is then drilled and reamed to the same radius _x_ as near as possible. The uniformity of the distance _x_ at each end is then tested by placing the mandrel upon the offset centers and rotating it, by hand, with a dial indicator in contact at first one end and then the other. The amount of offset can also be tested either by measuring from the point of a tool held in the toolpost, or by setting the tool to just graze the mandrel at extreme inner and outer positions, and noting the movement of the cross-slide by referring to the dial gage of the cross-feed screw.

=Turning a Crankshaft in a Lathe.=--Another example of eccentric turning is shown in Fig. 25. The operation is that of turning the crank-pin of an engine crankshaft, in an ordinary lathe. The main shaft is first rough-turned while the forging revolves upon its centers _C_ and _C_{1}_ and the ends are turned to fit closely the center-arms _A_ and _A_{1}_. After the sides _B_ and _B_{1}_ of the crank webs have been rough-faced, the center-arms are attached to the ends of the shaft as shown in the illustration. These arms have centers at _D_ and _D_{1}_ (located at the required crank radius) which should be aligned with the rough pin, when attaching the arms, and it is advisable to insert braces _E_ between the arms and crank to take the thrust of the lathe centers. With the forging supported in this way, the crank-pin and inner sides of the webs are turned and faced, the work revolving about the axis of the pin. The turning tools must extend beyond the tool-holder far enough to allow the crank to clear as it swings around. Owing to this overhang, the tool should be as heavy as possible to make it rigid and it is necessary to take comparatively light cuts and proceed rather cautiously. After finishing the crank-pin and inside of the crank, the center-arms are removed and the main body of the shaft and the sides _B_ and _B_{1}_ are finished. This method of turning crankshafts is often used in general repair shops, etc., especially where new shafts do not have to be turned very often. It is slow and inefficient, however, and where crankshafts are frequently turned, special machines or attachments are used.

=Special Crankshaft Lathe.=--A lathe having special equipment for rough-turning gas engine crankshaft pins is shown in Fig. 26. This lathe is a heavy-duty type built by the R. K. LeBlond Machine Tool Co. It is equipped with special adjustable headstock and tailstock fixtures designed to take crankshafts having strokes up to about 6 inches. The tools are held in a three-tool turret type of toolpost and there are individual cross-stops for each tool. This lathe also has a roller steadyrest for supporting the crankshaft; automatic stops for the longitudinal feed, and a pump for supplying cutting lubricant. The headstock fixture is carried on a faceplate mounted on the spindle and so arranged as to be adjustable for cranks of different throw. When the proper adjustment for a given throw has been made, the slide is secured by four T-bolts. A graduated scale and adjusting screw permit of accurate adjustments.

The revolving fixture is accurately indexed for locating different crank-pins in line with the lathe centers, by a hardened steel plunger in the slide which engages with hardened bushings in the fixture. The index is so divided that the fixture may be rotated 120 or 180 degrees, making it adjustable for 2-, 4- and 6-throw cranks. After indexing, the fixture is clamped by two T-bolts which engage a circular T-slot. The revolving fixture is equipped with removable split bushings which can be replaced to fit the line bearings of different sized crankshafts. The work is driven by a V-shaped dovetail piece having a hand-nut adjustment, which also centers the pin by the cheek or web. The crank is held in position by a hinged clamp on the fixture. The tailstock fixture is also adjustable and it is mounted on a spindle which revolves in a bushing in the tailstock barrel. The adjustment is obtained in the same manner as on the headstock fixture, and removable split bushings as well as a hinged clamp are also employed.

The method of chucking a four-throw crank is as follows: The two fixtures are brought into alignment by two locking pins. One of these is located in the head and enters a bushing in the large faceplate and the other is in the tailstock and engages the tailstock fixture. The crankshaft is delivered to the machine with the line bearings rough-turned and it is clamped by the hinged clamp previously referred to and centered by the V-shaped driver. The locking pins for both fixtures are then withdrawn and the machine is ready to turn two of the pins. After these have been machined, the fixtures are again aligned by the locking pins, the two T-bolts of the headstock fixture and the hinged clamp at the tailstock are released, the indexing plunger is withdrawn and the headstock fixture and crank are turned 180 degrees or until the index plunger drops into place. The crank is then clamped at the tailstock end and the revolving fixture is secured by the two T-bolts previously referred to. After the locking pins are withdrawn, the lathe is ready to turn the two opposite pins.

=Operation of Special Crankshaft Lathe.=--The total equipment of this machine (see Fig. 27) is carried on a three-tool turret tool-block. The method of turning a crankshaft is as follows: A round-nosed turning tool is first fed into a cross stop as illustrated in the plan view at _A_, which gives the proper diameter. The feed is then engaged and the tool feeds across the pin until the automatic stop lever engages the first stop, which throws out the feed automatically. The carriage is then moved against a positive stop by means of the handwheel. The roller back-rest is next adjusted against the work by the cross-feed handwheel operating through a telescopic screw, and the filleting tools are brought into position as at _B_. These are run in against a stop, removing the part left by the turning tool and giving the pin the proper width and fillets of the correct radius. If the crankshaft has straight webs which must be finished, two tools seen at _b_ are used for facing the webs to the correct width. During these last two operations, the crank is supported by the roller back-rest, thus eliminating any tendency of the work to spring.

After one pin is finished in the manner described, the back-rest is moved out of the way, the automatic stop lever raised, the carriage shifted to the next pin, and the operation repeated. The tools are held in position on the turret by studs, and they can be moved and other tools quickly substituted for pins of different widths. This machine is used for rough-turning the pins close to the required size, the finishing operation being done in a grinder. It should be mentioned, in passing, that many crankshafts, especially the lighter designs used in agricultural machinery, etc., are not turned at all but are ground from the rough.

=Spherical Turning.=--Occasionally it may be necessary to turn a spherical surface in the lathe. Sketch _A_, Fig. 28, shows how a small ball-shaped end can be turned on a piece held in a chuck. The lathe carriage is adjusted so that the pin around which the compound rest swivels is directly under the center a. The bolts which hold the swivel are slightly loosened to allow the top slide to be turned, as indicated by the dotted lines; this causes the tool point to move in an arc about center _a_, and a spherical surface is turned. Light cuts must be taken as otherwise it would be difficult to turn the slide around by hand.

Sketch _B_ illustrates how a concave surface can be turned. The cross-slide is adjusted until swivel pin is in line with the lathe centers, and the carriage is moved along the bed until the horizontal distance between center _b_ of the swivel, and the face of the work, equals the desired radius of the concave surface. The turning is then done by swinging the compound rest as indicated by the dotted lines. The slide can be turned more evenly by using the tailstock center to force it around. A projecting bar is clamped across the end of the slide at _d_, to act as a lever, and a centered bar is placed between this lever and the tailstock center; then by screwing out the tailstock spindle, the slide is turned about pivot _b_. The alignment between the swivel pin and the lathe centers can be tested by taking a trial cut; if the swivel pin is too far forward, the tool will not touch the turned surface if moved past center _c_, and if the pin is too far back, the tool will cut in on the rear side.

=Spherical Turning Attachments.=--When spherical turning must be done repeatedly, special attachments are sometimes used. Fig. 29 shows an attachment applied to a lathe for turning the spherical ends of ball-and-socket joints. The height or radius of the cutting tool and, consequently, the diameter of the turned ball, is regulated by adjusting screw _A_. The tool is swung around in an arc, by turning handle _B_ which revolves a worm meshing with an enclosed worm-wheel. As will be seen, the work is held in a special chuck, owing to its irregular shape.

Another spherical turning attachment is shown in Fig. 30. This is used for machining the ends of gasoline engine pistons. The cross-slide has bolted to it a bar _A_ carrying a roller which is pressed against a forming plate _B_ by a heavy spring _C_. The forming plate _B_, which is attached to a cross-piece fastened to the ways of the lathe bed, is curved to correspond with the radius required on the piston end, and when the tool is fed laterally by moving the cross-slide, it follows the curve of plate _B_. The piston is held in a special hollow chuck which locates it in a central position and holds it rigidly.

In connection with lathe work, special attachments and tools are often used, especially when considerable work of one class must be turned; however, if a certain part is required in large quantities, it is usually more economical to use some semi-automatic or automatic turning machine, especially designed for repetition work.