Turning and Boring / A specialized treatise for machinists, students in the industrial and engineering schools, and apprentices, on turning and boring methods, including modern practice with engine lathes, turret lathes, vertical and horizontal boring machines

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

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 A₁, which are in contact with disks B and B₁. The angle α 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.

The angle whose tangent is 0.03125 equals 1 degree 47.4 minutes;

To Find Center Distance for a Given Angle.—When straight-edges must be set to a given angle α, to determine center distance C between disks of known diameter. Rule: Find the sine of half the angle α in a table of sines; divide the difference between the disk diameters by double this sine.

To Find Angle for Given Taper per Foot.—When the taper in inches per foot is known, and the corresponding angle α 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.

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 α. 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 α.

The angle whose sine is 0.05 equals 2 degrees 52 minutes; then, 2 deg. 52 min. × 2 = 5 deg. 44 min. = angle α.

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 A₁, 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 A₁ 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 A₁ 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 C₁ and the ends are turned to fit closely the center-arms A and A₁. After the sides B and B₁ 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 D₁ (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 B₁ 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.

Turning with Front and Rear Tools.—In ordinary engine lathe practice, one tool is used at a time, but some lathes are equipped with tool-holders at the front and rear of the carriage so that two tools can be used simultaneously. Fig. 31 shows a detail view of a lathe in which front and rear tools are being used. These tools are of the inserted cutter type and the one at the rear is inverted, as the rotary movement of the work is, of course, upward on the rear side. This particular lathe was designed for taking heavy roughing cuts and has considerable driving power.

The part shown in this illustration is a chrome-nickel steel bar which is being roughed out to form a milling machine spindle. It is necessary to reduce the diameter of the bar from 5⁷/₁₆ inches to 3³/₄ inches for a length of 27 inches, because of a collar on one end. This reduction is made in one passage of the two tools, with a feed of ¹/₃₂ inch per revolution and a speed of 60 revolutions per minute. The use of two tools for such heavy roughing cuts is desirable, especially when the parts are required in large quantities, because the thrust of the cut on one side, which tends to deflect the work, is counteracted by the thrust on the opposite side.

Sometimes special tool-holders are made for the lathe, so that more than one tool can be used for turning different surfaces or diameters at the same time, the tools being set in the proper relation to each other. The advantage of this method has resulted in the design of a special lathe for multiple-tool turning.

A Multiple-tool Lathe.—The lathe shown in Fig. 32 (which is built by the Fitchburg Machine Works and is known as the Lo-swing) is designed especially for turning shafts, pins and forgings not exceeding 3¹/₂ inches in diameter. It has two carriages A and B which, in conjunction with special tool-holders, make it possible to turn several different diameters simultaneously. At the front of this lathe there is an automatic stop-rod C for disengaging the feed when the tools have turned a surface to the required length. This stop-rod carries adjustable stops D which are set to correspond with shoulders, etc., on the work. The rod itself is also adjustable axially, so that the tools, which are usually arranged in groups of two or more (depending upon the nature of the work), can be disengaged at a point nearer or farther from the headstock as may be required, owing to a variation in the depth of center holes. For example, if it were necessary to feed a group of tools farther toward the headstock after they had been automatically disengaged, the entire rod with its stops would be adjusted the required amount in that direction.

The gage G, which is attached to a swinging arm, is used to set the stop bar with reference to a shoulder near the end of the work, when it is necessary to finish other parts to a given distance from such a shoulder or other surface. The use of this gage will be explained more fully later. Cooling lubricant for the tools is supplied through the tubes E. The lathe shown in the illustration is arranged for turning Krupp steel bars. A rough bar and also one that has been turned may be seen to the right. The plain cylindrical bar is turned to five different diameters, by groups of tools held on both carriages.

Examples of Multiple Turning.—Figs. 33 and 34 show how a Lo-swing lathe is used for turning the steering knuckle of an automobile. Four tools are used in this case, three cylindrical surfaces and one tapering surface being turned at the same time. For this job, the four tools are mounted on one carriage. The taper part is turned by the second tool from the headstock, which is caused to feed outward as the carriage advances by a taper attachment. This tool is held in a special holder and bears against a templet at the rear, which is tapered to correspond with the taper to be turned. This templet is attached to a bar which, in turn, is fastened to a stationary bracket seen to the extreme left in Fig. 33. This part is finished in two operations, the tool setting being identical for each operation, except for diameter adjustments. As the illustrations show, three of the four tools employed are used for straight turning on different diameters, while the fourth finishes the taper.