Part 16 (2/2)
The next operation is that of finis.h.i.+ng the hole through the hub. The hard scale is first removed by a roughing cutter _r_ (sketch _C_), which is followed by a ”sizing” cutter _s_. The hole is then finished smooth and to the right diameter by reamer _f_. The bars carrying cutters _r_ and _s_ have extensions or ”pilots” which enter a close-fitting bus.h.i.+ng in the table, in order to steady the bar and hold it in alignment.
When the hole is finished, the wheel is turned over, so that the lower side of the rim and hub can be faced. The method of holding the casting for the final operation is shown at _D_. The chuck jaws are removed, and the finished side of the rim is clamped against parallels _p_ resting on the table. The wheel is centrally located for turning this side by a plug _e_ which is inserted in a hole in the table and fits the bore of the hub. The wheel is held by clamps which bear against the spokes.
Roughing and finis.h.i.+ng cuts are next taken over the top surface of the rim and hub and the corners are rounded, which completes the machining operations. If the rim needs to be a certain width, about the same amount of metal should be removed from each side, unless sandy spots or ”blow-holes” in the casting make it necessary to take more from one side than from the other. That side of the rim which was up in the mold when the casting was made should be turned first, because the porous, spongy spots usually form on the ”cope” or top side of a casting.
=Convex Turning Attachment for Boring Mills.=--Fig. 12 shows a vertical boring mill arranged for turning pulleys having convex rims; that is, the rim, instead of being cylindrical, is rounded somewhat so that it slopes from the center toward either side. (The reason for turning a pulley rim convex is to prevent the belt from running off at one side, as it sometimes tends to do when a cylindrical pulley is used.) The convex surface is produced by a special attachment which causes the turning tool to gradually move outward as it feeds down, until the center of the rim is reached, after which the movement is inward.
[Ill.u.s.tration: Fig. 12. Gisholt Mill equipped with Convex Turning Attachment]
The particular attachment shown in Fig. 12 consists of a special box-shaped tool-head _F_ containing a sliding holder _G_, in which the tool is clamped by set-screws pa.s.sing through elongated slots in the front of the tool-head. In addition, there is a radius link _L_ which swivels on a stud at the rear of the tool-head and is attached to vertical link _H_. Link _L_ is so connected to the sliding tool-block that any downward movement of the tool-bar _I_ causes the tool to move outward until the link is in a horizontal position, after which the movement is reversed. When the attachment is first set up, the turning tool is placed at the center of the rim and then link _L_ is clamped to the vertical link while in a horizontal position. The cut is started at the top edge of the rim, and the tool is fed downward by power, the same as when turning a cylindrical surface. The amount of curvature or convexity of a rim can be varied by inserting the clamp bolt _J_ in different holes in link _L_.
[Ill.u.s.tration: Fig. 13. Turning a Taper or Conical Surface]
The tools for machining the hub and sides of the rim are held in a turret mounted on the left-hand head, as shown. The special tool-holder _A_ contains two bent tools for turning the upper and lower edges of the pulley rim at the same time as the tool-head is fed horizontally.
Roughing and finis.h.i.+ng tools _B_ are for facing the hub, and the tools _C_, _D_, and _E_ rough bore, finish bore, and ream the hole for the shaft.
=Turning Taper or Conical Surfaces.=--Conical or taper surfaces are turned in a vertical boring mill by swiveling the tool-bar to the proper angle as shown in Fig. 13. When the taper is given in degrees, the tool-bar can be set by graduations on the edge of the circular base _B_, which show the angle _a_ to which the bar is swiveled from a vertical position. The base turns on a central stud and is secured to the saddle _S_ by the bolts shown, which should be tightened after the tool-bar is set. The vertical power feed can be used for taper turning the same as for cylindrical work.
[Ill.u.s.tration: Fig. 14. Turning a Conical Surface by using the Combined Vertical and Horizontal Feeds]
Occasionally it is necessary to machine a conical surface which has such a large included angle that the tool-bar cannot be swiveled far enough around to permit turning by the method ill.u.s.trated in Fig. 13. Another method, which is sometimes resorted to for work of this cla.s.s, is to use the combined vertical and horizontal feeds. Suppose we want to turn the conical casting _W_ (Fig. 14), to an angle of 30 degrees, as shown, and that the tool-head of the boring mill moves horizontally 1/4 inch per turn of the feed-screw and has a vertical movement of 3/16 inch per turn of the upper feed-shaft. If the two feeds are used simultaneously, the tool will move a distance _h_ of say 8 inches, while it moves downward a distance _v_ of 6 inches, thus turning the surface to an angle _y_. This angle is greater (as measured from a horizontal plane) than the angle required, but, if the tool-bar is swiveled to an angle _x_, the tool, as it moves downward, will also be advanced horizontally, in addition to the regular horizontal movement. The result is that the angle _y_ is diminished and if the tool-bar is set over the right amount, the conical surface can be turned to an angle _a_ of 30 degrees. The problem, then, is to determine what the angle _x_ should be for turning to a given angle _a_.
[Ill.u.s.tration: Fig. 15. Diagram showing Method of Obtaining Angular Position of Tool-head when Turning Conical Surfaces by using Vertical and Horizontal Feeding Movements]
The way angle _x_ is calculated will be explained in connection with the enlarged diagram, Fig. 15, which shows one-half of the casting. The sine of the known angle _a_ is first found in a table of natural sines. Then the sine of angle _b_, between the taper surface and center-line of the tool-head, is determined as follows: sin_b_ = (sin_a_ _h_) _v_, in which _h_ represents the rate of horizontal feed and _v_ the rate of vertical feed. The angle corresponding to sine _b_ is next found in a table of sines. We now have angles _b_ and _a_, and by subtracting the sum of these angles from 90 degrees, the desired angle _x_ is obtained.
To ill.u.s.trate: The sine of 30 degrees is 0.5; then sin _b_ = (0.5 1/4) 3/16 = 0.6666; hence angle _b_ = 41 degrees 49 minutes, and _x_ = 90-(30 + 41 49') = 18 degrees 11 minutes. Hence to turn the casting to angle _a_ in a boring mill having the horizontal and vertical feeds given, the tool-head would be set over from the vertical 18 degrees and 11 minutes which is equivalent to about 18-1/6 degrees.
If the required angle _a_ were greater than angle _y_ obtained from the combined feeds with the tool-bar in a vertical position, it would then be necessary to swing the lower end of the bar to the left rather than to the right of a vertical plane. When the required angle _a_ exceeds angle _y_, the sum of angles _a_ and _b_ is greater than 90 degrees so that angle _x_ for the tool-head = (_a_ + _b_) - 90 degrees.
=Turret-lathe Type of Vertical Boring Mill.=--The machine ill.u.s.trated in Fig. 16 was designed to combine the advantages of the horizontal turret lathe and the vertical boring mill. It is known as a ”vertical turret lathe,” but resembles, in many respects, a vertical boring mill. This machine has a turret on the cross-rail the same as many vertical boring mills, and, in addition, a side-head _S_. The side-head has a vertical feeding movement, and the tool-bar _T_ can be fed horizontally. The tool-bar is also equipped with a four-sided turret for holding turning tools. This arrangement of the tool-heads makes it possible to use two tools simultaneously upon comparatively small work. When both heads are mounted on the cross-rail, as with a double-head boring mill, it is often impossible to machine certain parts to advantage, because one head interferes with the other.
The drive to the table (for the particular machine ill.u.s.trated) is from a belt pulley at the rear, and fifteen speed changes are available. Five changes are obtained by turning the pilot-wheel _A_ and this series of five speeds is compounded three times by turning lever _B_. Each spoke of pilot-wheel _A_ indicates a speed which is engaged only when the spoke is in a vertical position, and the three positions for _B_ are indicated, by slots in the disk shown. The number of table revolutions per minute for different positions of pilot-wheel _A_ and lever _B_ are shown by figures seen through whichever slot is at _C_. There are five rows of figures corresponding to the five spokes of the pilot-wheel and three figures in a row, and the speed is shown by arrows on the sides of the slots. The segment disk containing these figures also serves as an interlocking device which prevents moving more than one speed controlling lever at a time, in order to avoid damaging the driving mechanism.
[Ill.u.s.tration: Fig. 16. Bullard Vertical Turret Lathe]
The feeding movement for each head is independent. Lever _D_ controls the engagement or disengagement of the vertical or cross feeds for the head on the cross-rail. The feed for the side-head is controlled by lever _E_. When this lever is pushed inward, the entire head feeds vertically, but when it is pulled out, the tool-bar feeds horizontally.
These two feeds can be disengaged by placing the lever in a neutral position. The direction of the feeding movement for either head can be reversed by lever _R_. The amount of feed is varied by feed-wheel _F_ and clutch-rod _G_. When lever _E_ is in the neutral position, the side-head or tool-bar can be adjusted by the hand-cranks _H_ and _I_, respectively. The cross-rail head and its turret slide have rapid power traverse movements for making quick adjustments. This rapid traverse is controlled by the key-handles _J_.
The feed-screws for the vertical head have micrometer dials _K_ for making accurate adjustments. There are also large dials at _L_ which indicate vertical movements of the side head and horizontal movements of the tool slide. All of these dials have small adjustable clips _c_ which are numbered to correspond to numbers on the faces of the respective turrets. These clips or ”observation stops” are used in the production of duplicate parts. For example, suppose a tool in face No. 1 for the main turret is set for a given diameter and height of shoulder on a part which is to be duplicated. To obtain the same setting of the tools for the next piece, clips No. 1, on both the vertical feed rod and screw dials, are placed opposite the graduations which are intersected by stationary pointers secured to the cross-rail. The clips are set in this way after the first part has been machined to the required size and before disturbing the final position of the tools. For turning a duplicate part, the tools are simply brought to the same position by turning the feed screws until the clips and stationary pointers again coincide. For setting tools on other faces of either turret, this operation is repeated, except that clips are used bearing numbers corresponding to the turret face in use.
The main turret of this machine has five holes in which are inserted the necessary boring and turning tools, drills or reamers, as may be required. By having all the tools mounted in the turret, they can be quickly and accurately set in the working position. When the turret is indexed from one face to the next, binder lever _N_ is first loosened.
The turret then moves forward, away from its seat, thus disengaging the indexing and registering pins which accurately locate it in any one of the five positions. The turret is revolved by turning crank _M_, one turn of this handle moving the turret 1/5 revolution or from one hole to the next. The side-head turret is turned by loosening lever _O_. The turret slide can be locked rigidly in any position by lever _P_ and its saddle is clamped to the cross-rail by lever _Q_. The binder levers for the saddle and toolslide of the side-head are located at _U_ and _V_, respectively. A slide that does not require feeding movements is locked in order to obtain greater rigidity. To ill.u.s.trate, if the main tool slide were to feed vertically and not horizontally, it might be advisable to lock the saddle to the cross-rail, while taking the vertical cut.
[Ill.u.s.tration: Fig. 17. Turning a Gear Blank on a Vertical Turret Lathe]
The vertical slide can be set at an angle for taper turning, and the turret is accurately located over the center of the table for boring or reaming, by a positive center stop. The machine is provided with a brake for stopping the work table quickly, which is operated by lifting the shaft of pilot-wheel _A_. The side-and cross-rails are a unit and are adjusted together to accommodate work of different heights. This adjustment is effected by power on the particular machine ill.u.s.trated, and it is controlled by a lever near the left end of the cross-rail.
Before making this adjustment, all binder bolts which normally hold the rails rigidly to the machine column must be released, and care should be taken to tighten them after the adjustment is made.
[Ill.u.s.tration: Fig. 18. Turning Gasoline Engine Flywheel on Vertical Turret Lathe--First Position]
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