Modern Machine-Shop Practice Part 127

[Ill.u.s.tration: Fig. 1846.]

[Ill.u.s.tration: Fig. 1847.]

The three-cutter die would in this case cut the perfectly circular form of Fig. 1847.

Now, suppose both of the dies to have been made or set to some certain diameter--in fact, presume them to be made by taking a ring of steel having a round hole of the required diameter, say 1 inch, and removing the metal shown by the dotted lines, Fig. 1848, and leaving only the four cutting points in one case (and the three in the other). Then it is evident that our dies are both of the same diameter, and likewise both of the a.s.sumed diameter, or 1 inch; then it is fair to presume that the plugs or sections just cut by either one of the dies should enter a round hole of the same diameter as the dies; but it is obvious that only two, Figs. 1840 and 1847, will do so, all the rest being considerably too large, from their irregularity of form, notwithstanding the fact that the diameter of any of those cut by four cutters is never more than that of the die, while any one of the equal radii, taken at equal distances on any of the forms cut by the three-cutter die, will not exceed the radius of the die. Now, six of the pieces being too large when referred to the standard of a round hole of the size of the die, while two are of the correct size, it is obvious that if the four-die, for example, which cut Fig. 1846, were reduced enough to make Fig. 1843 just enter the standard, that, Fig. 1840, which is now just correct in size and form, would, when cut, be altogether too small. The same would be the case also with the three-cutter die.

Now let us consider the two productions (Figs. 1840 and 1847) that answer the requirements, the two different sections (Figs. 1839 and 1845) from which they were cut, and also the other two pieces (Figs.

1841 and 1846) that were cut from the same bars at the same time. The general shape of Fig. 1839, is oval or four-sided, and while the four cutters operated upon it to produce perfectly circular work, the three cutters reproduced the general shape started with, only somewhat modified, as Fig. 1841 plainly shows. Upon the blank, Fig. 1845, the general shape of which is triangular, the very opposite is the case, for the three cutters now produce a perfect circle, while the four modify only the figure that they commenced to operate upon.

[Ill.u.s.tration: Fig. 1848.]

Considering that every irregular form may be approximated by a square, an equilateral triangle, or in general by either a parallelogram or a regular polygon, it will be found that from a flat, oval, or square piece of metal the four cutters will produce a true circle; from a triangular piece the three; from a heptagon neither will do so, while from a hexagon both the three and four cutters are calculated to do so.

Following in the same manner, and increasing the sides, it will be found that the four cutters will produce a true circle from every parallelogram, whether all the sides are equal or not, while the three cutters will produce a true circle also from every regular polygon the number of sides of which is a multiple of three--that is, four cutters would operate correctly upon a figure having 4, 6, 8, 10, 12, &c., parallel sides, while the three would do so upon a figure having 3, 6, 9, 12, 15, &c., equal sides. Thus, for regular forms varying between these two series neither one would be adapted. Hence, if the general form of the work is represented by the first series, the four cutters are the best; if the general and average form of the material to be operated upon corresponds to the second series, then the three dies are the best adapted, so far as their two principles of action, mentioned at the outset, are concerned; hence, if it is considered that the material or bars of metal to be wrought vary from a circular form indifferently, then there is no choice between an even and an odd number merely on that account.

Placing the same dies that cut these six irregular figures upon their respective productions would not serve to correct their form; as, for instance, if the die that cut Fig. 1846 were revolved around it--even if set up or reduced in diameter to take a cut--it would remove an equal amount all round and leave the same figure still. Similarly with, say, Fig. 1841, cut by the three; but if the three were run over Fig. 1846, cut by the four, it would tend to correct the errors, and likewise if the four were run over Fig. 1841, the tendency would be to modify the discrepancies left by the three that cut it.

[Ill.u.s.tration: Fig. 1849.]

As regards the number of cutting points, suppose that there were a certain number, as three, shown in Fig. 1849, all taking an equal cut; then, when the position indicated by the dotted lines was reached, where cutter H runs 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 make H, E, and D each cut two-thirds as much as at first. With four as shown in Fig. 1850 when H reached 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 upon H and F, and upon E and G.

With five, as shown in Fig. 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.

[Ill.u.s.tration: Fig. 1850.]

[Ill.u.s.tration: Fig. 1851.]

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 a.s.sumed 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.

[Ill.u.s.tration: Fig. 1852.]

[Ill.u.s.tration: Fig. 1853.]

[Ill.u.s.tration: 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 in Fig. 1852 the diameter of the hob must be 1/32 inch 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 in Fig. 1853 the hob or master tap must be 1/32 inch 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 in Fig. 1854 they must be cut with a hob 1/32 inch 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 in Fig. 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.

[Ill.u.s.tration: 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 in Fig. 1855. Face X is 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 hob 1/32 inch smaller than the diameter of bolt to be cut, the clearance is sufficiently maintained.

[Ill.u.s.tration: 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 line G in Fig. 1856.

[Ill.u.s.tration: 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 in Fig. 1857, chamfering off not more than three threads, and the chamfered teeth must then be filed, just bringing the front edges 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 between 1/4 inch and 1-1/4 inches in diameter, and the size of the chaser or die will be 1-1/4 inches wide and 1/2 inch 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 1-1/4-inch chaser equals in width the diameter of bolt it is to cut, viz. 1-1/4 inches; but if the chaser for 1/4-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 coa.r.s.er pitch, each varying in degree from the other two, and the other may remain true, or contract and become of finer pitch.

[Ill.u.s.tration: 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 1-1/4 inches wide, it will contain 25 teeth, and the amount of pitch error due to 1-1/4 inches 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 in Fig. 1858, weakening it and impairing its durability while placing excessive duty on the dies and on the machine.

[Ill.u.s.tration: Fig. 1859.]

[Ill.u.s.tration: Fig. 1860.]

[Ill.u.s.tration: 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 in Fig. 1859.

An equally effective and much simpler plan is to form the dies as in Fig. 1860, the diameter at the back B being slightly larger than that at the mouth A, 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 at A may be ground farther towards B, 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 in Fig.

1861, the proportions of the master taps for the different sizes of bolts being as given in the following table:--

------+-----------------------+---------------+----------------------------- Dia- | | | meter| | | of | | | Length bolt.| ---- | ---- |at A.|at B.|at C.|at D.|at E.

------+-----------------------+---------------+-----+-----+-----+-----+----- 1/4 |Dia. from G to H 15/64 |At J 7/32 | 1/2|1 |1 |1-1/2| 1/2 5/16| " " 19/64 | " 9/32 | 1/2|1 |1 |1-1/2| 1/2 3/8 | " " 23/64 | " 11/32 | 1/2|1 |1 |1-1/2| 1/2 7/16| " " 27/64 | " 13/32 | 1/2|1 |1 |1-1/2| 1/2 1/2 | " " 31/64 | " 15/32 | 1/2|1-1/2|1-1/2|1-1/2| 3/4 5/8 | " " 39/64 | " 19/32 | 1/2|1-1/2|1-1/2|1-1/2| 3/4 3/4 | " " 47/64 | " 23/32 | 1/2|1-1/2|2 |1-1/2| 3/4 7/8 | " " 55/64 | " 27/32 | 1/2|1-1/2|2 |1-1/2| 3/4 1 | Dia. at G 31/32 |At J 1/100 less| 1/2|4 |4 |1-1/2|1 1-1/8 | " 1-3/32 | ---- |1 |4 |4 |1-1/2|1 1-1/4 | " 1-7/32 | ---- |1 |4 |4 |1-1/2|1 1-3/8 | " 1-11/32| ---- |1 |4 |4 |1-1/2|1 1-1/2 | " 1-15/32| ---- |1 |4 |4 |1-1/2|1-1/4 1-5/8 | " 1-19/32| ---- |1 |5 |5 |2 |1-1/2 1-3/4 | " 1-23/32| ---- |1 |5 |5 |2 |1-1/2 1-7/8 | " 1-27/32| ---- |1 |6 |6 |2 |1-3/4 2 | " 1-31/32| ---- |1 |6 |6 |2 |1-3/4 ------+-----------------------+---------------+-----+-----+-----+-----+-----

All over 2 in. same length as the 2 in. Shanks J turned to bottom of last thread.

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 them sharp:--

--------+-----------+-----------+--------+-----------+------------ Diameter|Revolutions|Revolutions|Diameter|Revolutions|Revolutions of bolt.|of dies for|of dies for|of bolt.|of dies for|of dies for | machine | bolt | | machine | bolt | shops. | factories.| | shops. | factories.

--------+-----------+-----------+--------+-----------+------------ inch. | | | inch. | | 1/8 | 450 | 600 | 1-5/8 | 33 | 48 1/4 | 230 | 300 | 1-3/4 | 30 | 45 3/8 | 150 | 200 | 1-7/8 | 28 | 40 1/2 | 100 | 150 | 2 | 25 | 38 5/8 | 75 | 125 | 2-1/8 | 23 | 36 3/4 | 65 | 100 | 2-1/4 | 22 | 34 7/8 | 55 | 85 | 2-3/8 | 21 | 32 1 | 45 | 75 | 2-1/2 | 20 | 30 1-1/8 | 42 | 65 | 2-5/8 | 18 | 25 1-1/4 | 40 | 60 | 2-3/4 | 15 | 20 1-3/8 | 38 | 55 | 2-7/8 | 12 | 18 1-1/2 | 35 | 50 | 3 | 10 | 15 --------+-----------+-----------+--------+-----------+------------

Taps same speed as dies.

[Ill.u.s.tration: _VOL. I._ =NUT-TAPPING MACHINERY.= _PLATE XXIII._

Fig. 1864.

Fig. 1865.

Fig. 1866.

Fig. 1867.]

[Ill.u.s.tration: Fig. 1862.]