Fig. 1849Fig. 1849.
Fig. 1849.
Fig. 1850Fig. 1850.
Fig. 1850.
Fig. 1851Fig. 1851.
Fig. 1851.
As regards the number of cutting points, suppose that there were a certain number, as three, shown inFig. 1849, all taking an equal cut; then, when the position indicated by the dotted lineswas reached, where cutterhruns out, the entire duty would be only two-thirds as much as it was, and the die would shift laterally in the direction of the arrow enough to equalize this smaller amount of duty on all three, or makeh,e, anddeach cut two-thirds as much as at first. With four as shown inFig. 1850whenhreached the depression where its cut would run out, the entire duty would be three-fourths of what it was at first, and the die would travel laterally in the direction of the arrow sufficiently to equalise the pressure uponhandf, and uponeandg. With five, as shown inFig. 1851, in similar position the entire duty would be four-fifths as much; with six, five-sixths, and so on. Thus it can be seen that the variation between the least amount to be cut and the full amount is relatively less, the greater the number of cutting points that it is divided between, and hence the lateral movement would be less; therefore the general tendency of an increase in the number of cutting points would be to promote true work.
Hence, from these considerations it appears that it is not material whether the number is odd or even merely on that account; so four would be preferable to three only on account of being one more, and, in turn, five would be better than four, and six better than five, and so on. It is found, however, that bar iron usually inclines to the elliptical form, and that an even number is, therefore, preferable.
Thus far the cutting edges of the die have been assumed to be points equidistant about a circle—that is, it has been supposed to have absolute clearance, so that its movements would be regulated entirely by the depth of cut taken, in order to ascertain the inherent tendency to untruth caused by an odd or an even, a greater or a less, number of cutters. This tendency is, of course, modified in each case by the amount of clearance.
Fig. 1852Fig. 1852.
Fig. 1852.
Fig. 1853Fig. 1854Fig. 1853.Fig. 1854.
Fig. 1853.Fig. 1854.
The position of the dies in the head and with relation to the work is, in bolt cutting machines, a matter of great importance, and in all cases the dies should be held in the same position when being hobbed (that is, having their teeth cut by the hob or master tap) as they will stand in when put to work, and the diameter of the hob must be governed by the position of the dies in the head. If they are placed as inFig. 1852the diameter of the hob must be1⁄32inch larger than the diameter of bolt the dies are intended to thread, so that the point or cutting edge may meet the work first and the heel may have clearance, it being borne in mind that the clearance is less at the tops than it is at the bottoms of the teeth, because of their difference in curvature. In this position the teeth are keen and yet retain their strength, acting somewhat as a chaser. If placed in the position shown inFig. 1853the hob or master tap must be1⁄32inch smaller than the diameter of bolt they are to thread, so as to give the teeth clearance. In this case the dies are somewhat harder to feed into their cut and do not cut quite so freely, but on the other hand they work more steadily as the bolt is better guided, while left-hand dies may be used in the same head. If placed as inFig. 1854they must be cut with a hob1⁄32inch larger in diameter than the bolt they are to thread, so that the teeth will have less curvature than the work, and will, therefore, have clearance. In this position the dies do not cut so freely as inFig. 1852.
The dies should be broad enough to contain at least as many teeth as there are in a length of bolt equal to its diameter, and should be thick enough to withstand the pressure of the cut without perceptible spring or deflection.
Fig. 1855Fig. 1855.
Fig. 1855.
The cutting edges of dies may be brought in their best cutting position and the dies placed in radial slots in the head by forming the dies as inFig. 1855. Facexis at an angle of 18° to the leading or front face of the die steel, and the heel is filed off at an angle of 45° and extends to the centre line of the die. This gives a strong and a keen die, and by using a hob1⁄32inch smaller than the diameter of bolt to be cut, the clearance is sufficiently maintained.
Fig. 1856Fig. 1856.
Fig. 1856.
The heel of the die should not when the cutting edge is in front extend past the axis of the work, but should be cut off so as to terminate at the work axis as denoted by the dotted lineginFig. 1856.
Fig. 1857Fig. 1857.
Fig. 1857.
In hobbing the dies it is necessary that they be all of equal length so that the hob may cut an equal depth in each, and may, therefore, work steadily and hob them true. After the dies are hobbed their front ends should be reamed with a taper reamer as inFig. 1857, chamfering off not more than three threads, and the chamfered teeth must then be filed, just bringing the frontedges up to a cutting edge, but filing nothing off them, the reamed chamfer acting as a guide to file them by.
This will cause each tooth to take its proper share of the cut, thus preserving the teeth and causing the dies to cut steadily. Back from the cutting edge towards the heels of the teeth the clearance may gradually increase so that the heel will not meet the work and cause friction.
The chasers or dies are obviously changed for each diameter of bolt, and it follows that as the chasers all fit in the same slots in the head they must all be made of the same size of steel whatever diameter of bolt they are intended to cut, and this leads to the following considerations.
Suppose the capacity of the machine is for bolts between1⁄4inch and 11⁄4inches in diameter, and the size of the chaser or die will be 11⁄4inches wide and1⁄2inch thick.
The width of a die or chaser should never be less than the diameter of bolt it is to thread, so that it may contain as many threads as are contained in a length of bolt equal to the bolt diameter. Now the 11⁄4-inch chaser equals in width the diameter of bolt it is to cut, viz. 11⁄4inches; but if the chaser for1⁄4-inch bolts was threaded parallel and left its full width it would be five times as wide as the diameter of the bolt and the thread cut would be imperfect, because the chasers alter their pitches in the hardening process, as was explained with reference to taps, and it is found that the error induced in the hardening varies in amount and sometimes in direction: thus of the four chasers three may expand and become of coarser pitch, each varying in degree from the other two, and the other may remain true, or contract and become of finer pitch.
Fig. 1858Fig. 1858.
Fig. 1858.
As a rule the dies expand, but do not so equally. The more teeth there are in the die the more the pitch error from the hardening; or in other words, there is obviously more error in an inch than there is in half an inch of length. Suppose then that we have a die for 20 threads per inch, and as the chaser is 11⁄4inches wide, it will contain 25 teeth, and the amount of pitch error due to 11⁄4inches of length; and this amount not being equal in all the chasers, the result is that the dies cut the sides of the thread away, leaving it sharp at the top but widened at the bottom, as shown inFig. 1858, weakening it and impairing its durability while placing excessive duty on the dies and on the machine.
Fig. 1859Fig. 1859.
Fig. 1859.
Fig. 1860Fig. 1860.
Fig. 1860.
Fig. 1861Fig. 1861.
Fig. 1861.
A common method of avoiding this is to cut away all the teeth save for a width of die equal to the diameter of the bolt, as shown inFig. 1859. An equally effective and much simpler plan is to form the dies as inFig. 1860, the diameter at the backbbeing slightly larger than that at the moutha, so that the back teeth are relieved of cutting duty. This enables the dies to undergo more grindings and still retain sufficient teeth. For example, the chamfer atamay be ground farther towardsb, and still leave in action sufficient teeth to equal in width of chaser the diameter of the bolt. To enable the threading of dies in this manner the hobs or master taps employed to thread them are formed as inFig. 1861, the proportions of the master taps for the different sizes of bolts being as given in the followingtable:—
The cutting speeds for the dies and taps are as given in the following table, in which it will be seen that the speeds for bolt factories are greater than for machine shops. This occurs on account of the greater experience of the operators and the greater care taken in lubricating the dies and keeping themsharp:—
Fig. 1862Fig. 1862.
Fig. 1862.
InFig. 1862is represented a nut threading or tapping machine. The vertical spindles have spring sockets in which the taps are held, so that they can be inserted or removed without stopping the machine. The nuts are fed down the slots of the inclined plates shown on the upper face of the circular base, and the spindles are raised and lowered by the pivoted levers shown. The nuts lie in a dish that contains water up to the level of the bottom of the nuts, the object being to prevent the taps from getting hot and therefore expanding in diameter. Upon the top of the water floats a body of oil about1⁄2inch deep, which lubricates the cutting edges of the tap. These machines are also made with six instead of four spindles, which in both machines run at different speeds to suit different sizes of nuts, and which are balanced by weights hanging inside the central hollow column or frame.
Fig. 1863Fig. 1863.
Fig. 1863.
Fig. 1863represents the socket for driving the tap, so devised that when the tap is strung for its intended length with nuts, the top nut releases the tap of itself, the construction being as follows:sis the socket that fits into the driving spindle of the machine; its bore, which fits the stem of the tap easily, receives two headless screwsb, a pinp, which is a sliding fit, and the screwa.ris a ring or sleeve fitting easily to the socket, and is prevented from falling off by screwa. The tap is provided with an annular grooveg. The flattened end of the tap passes up between and is driven by the ends of screwsb, the weight of the collar ring or sleeverforcing pinpinto the grooveg, thus holding the tap up. When the tap is full of nuts the top nut meets facevof ringr, lifting this ring upon the socket and relieving pinpof the weight ofr, the weight of the tap and the nuts then causes the tap to be released. By this construction the tap can be inserted or removed while the machine is in motion.
InFig. 1864is represented a rotary nut tapper, and inFig. 1865, is also represented a sectional view of the same machine.
The tap driving spindles are driven from a central vertical shafts, driven by bevel-gearb. The horizontal driving shaft operates a wormc, to drive a worm-wheel in a vertical shaft, which drives a piniona, driving a spur wheelwin the base of the spindle head, by which means this head is revolved so as to bring the successive spindles in front of the operator. A trough is provided attto cool the tap with oil and water after it has passed through the nut.
Fig. 1866represents a nut tapping machine designed for light work, the spindles are raised after each nut is tapped by the foot levers and rods shown, the latter connecting to a shoe fitting into a groove in a collar directly beneath the driving pulleys of the spindles.
Fig. 1867represents a three-spindle nut tapping machine, in which the spindles are horizontal and the nuts are held in three separate heads or horizontal slideways and are traversed by the ball levers shown, and a self-acting pump supplies them with oil. The three spindles are driven by a cone pulley having four changes of speed to suit different diameters of taps.
Fig. 1868Fig. 1868.
Fig. 1868.
Fig. 1869Fig. 1869.
Fig. 1869.
Pipe Threading Machinery.—InFig. 1868is represented a machine for threading and cutting off pipe of large diameter. This machine consists of a driving head corresponding to the headstock of a lathe, but having a hollow spindle through which the pipe may pass. The pipe is driven by a three-jawed chuck, and the threading and cutting off tools are carried on a carriage which has a threading head for ordinary lengths of pipe, and one for short pieces such as nipples, the latter swinging out of the way when not in use. Between these two is a pair of steadying jaws for the pipe. A side view of the front of the carriage is shown inFig. 1869,h h, &c., representing the threading dies used for nipples. It is movable along a slidewayeand pivoted upon its slider. The dies are carried in a chuckg, and are opened or closed by the levern; atlis the handle for the screw that operates the guide jawsa a.
Fig. 1870Fig. 1870.
Fig. 1870.
Fig. 1871Fig. 1871.
Fig. 1871.
The threading head ath(right-hand end ofFig. 1868), is represented inFig. 1870, being pivoted so that it also can be swung out of the way to permit of the removal of the pipe. The diescare opened or closed by the hand wheelb, operating a worm meshing into a segment of a worm-wheel upon the body of the head, the amount of motion being regulated by the stop screw atf, which therefore regulates the size to which the dies can be closed, and therefore the diameter of thread the dies will cut. The construction of the cutting-off head is shown inFig. 1871,trepresenting the cutting tool which is operated by the hand wheelk. The carriage is fed or traversed by means of two pinions operated by the six-handled wheel shown atw,Fig. 1868; these two pinions engaging racks beneath the carriage, and near theinside edges of the bed, one of them being seen at the extreme right-hand end ofFig. 1868.
Fig. 1872Fig. 1872.
Fig. 1872.
InFig. 1872is represented a machine for threading or tapping the fittings for steam and gas pipe. The tap is carried in the end of the vertical spindle, and the work may be held in the vice upon the work table, or if too large the table may be swung out of the way.
The general design of the machine corresponds somewhat to that of a drilling machine.
Broaching Press.—Broaching consists in forcing cutters through keyways or apertures, to dress their sides to shape.
Fig. 1873Fig. 1873.
Fig. 1873.
InFig. 1873is represented a broaching press. Its driving gear which is within the box frame is so constructed that it may be started and stopped instantly, notwithstanding its heavy fly wheel.
Figs. 1874to1877represent the method of cutting out a keyway by broaching.
Fig. 1874Fig. 1874.
Fig. 1874.
InFig. 1874arepresents the end of a connecting rod having three holes,b,c, andd, pierced through it, their diameters nearly equalling the total finished width of keyway required. The punchd′is first forced through, thus making the three holes into one.
Fig. 1875Fig. 1875.
Fig. 1875.
TheV-shape of the end of the cutting punchd′tends to steady it while in operation, forces the cut outwards into the next hole, preventing them from jambing, and causes the strain upon the punch to begin and end gradually; thus it prevents violent action during the ingress and egress of the cutting punch. This roughing out process dispenses with the use of the hammer and chisel, and saves much time, since it is done at one stroke of the press. The next part of the process is the introduction of a series of broaches such as shown inFig. 1875, the principles involved being as follow: It is obvious that from the large amount of cutting edge possessed by a single tooth extending all around such a broach, it would be impracticable to take much of a cut at once; hence a succession of broaches is used, some of them performing duty on the sides only, others at the ends only, but the last andfinal broach is usually made to take a very fine cut all over. All these broaches are made slightly taper; that is to say, the breadth of the lower tooth atainFig. 1875is made less than that atb, the amount allowed varying according to the dimensions and depth of the keyway.
The smallest of the set of broaches is entered first and forced through until its end stands level with the upper face of the work. Each broach is provided with a conical teat at one end and a corresponding conical recess at the other, so that when the second broach is placed on top of the first, the teat fitting into the recess below it, will hold the two broaches central one to the other.
The head of each broach is made somewhat conical or tapered, and sets in a corresponding recess in the driving head in the machine, which, therefore, holds the broaches parallel one to the other. A succession of these broaches is used, each requiring one stroke of the press to force it within the keyway, and another to force it out.
Fig. 1876Fig. 1876.
Fig. 1876.
Fig. 1877Fig. 1877.
Fig. 1877.
The following is an example of broaching, relating to which, the dotted lines shown on the broaches,Fig. 1876, indicate the depths and shapes of the teeth. The small end of each broach corresponds to the large end of the one that preceded it, which is necessary in order to permit it to enter easily. Of the ten broaches used the first two operate to straighten the side walls of the hole, No. 3 being the first to operate upon the circular corners, which are not cut to the rectangle until No. 8 has passed through. But as the duty in cutting out the corners diminishes, the walls and ends of the hole are operated upon to finish them to size; thus broach No. 3 leaves the hole 11⁄8or 1.125 inches wide, and 2.7501 inches long, which No. 4 increases to 1.1354 inches wide and 2.7605 inches long. This increase of width and depth, or breadth, as it may more properly be termed, continues up to the last or tenth cutter, which is parallel and of the same dimensions as the large end of cutter No. 9.Fig. 1877gives two views of the No. 10 broach.
Broaches require a very free lubrication in order to prevent them from tearing the walls of the hole, and to enable them to cut easily and smoothly; hence it is found highly advantageous after the teeth are cut to cut out grooves or passages lengthways of the broach, and extending nearly to the bottom of the teeth, which eases the cut as well as affords the required lubrication; but it is obvious that the finishing cutter must not have such oil ways.
MODERNMACHINE-SHOPPRACTICE
VOL. II.MODERN MACHINE‑SHOP PRACTICE.FRONTISPIECE
Frontispiece-1 vol. II