Part 5 (1/2)
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.
[Ill.u.s.tration: Fig. 1. Taper Turning by the Offset-center Method]
[Ill.u.s.tration: Fig. 2. Examples of Taper Work]
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.
[Ill.u.s.tration: Fig. 3. Detail View of Lathe Tailstock]
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 a.s.sume 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.
[Ill.u.s.tration: Fig. 4. Taper Plug and Gage]
To adjust the tailstock, the nuts _N_ (Fig. 3) are first loosened and then the upper part _A_ is s.h.i.+fted 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 s.h.i.+fted 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.
[Ill.u.s.tration: Fig. 5. Setting Work for Taper Turning by use of Caliper Gage]
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.
[Ill.u.s.tration: Fig. 6. Side View showing Relative Positions of Gage and Work]
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 ill.u.s.trated 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.
[Ill.u.s.tration: Fig. 7. Obtaining Tailstock Center Adjustment by use of Square]
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.
[Ill.u.s.tration: Fig. 8. Second Step in Adjusting Tailstock Center by use of Square]
=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 s.h.i.+fted 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 pa.s.sing 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 s.h.i.+fted 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.
[Ill.u.s.tration: Fig. 9. A Lathe Taper Attachment]
=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.