----------------+------------------+----------------+----------------+
Thickness of bar|Thickness outside | Punched bars | Drilled bars |
in inches. |of hole in inches.|broke in pounds.|broke in pounds.|
----------------+------------------+----------------+----------------+
3/8 or 0.375 | 3/8 or 0.375 | 31,740 | 28,000 |
3/8 or 0.375 | 3/8 or 0.375 | 31,380 | 26,950 |
5/8 or 0.625 | 1/4 or 0.25 | 18,820 | 18,000 |
5/8 or 0.625 | 1/4 or 0.25 | 18,750 | 17,590 |
5/8 or 0.625 | 3/16 or 0.1875 | 14,590 | 13,230 |
5/8 or 0.625 | 3/16 or 0.1875 | 15,420 | 13,750 |
5/8 or 0.625 | 1/8 or 0.125 | 10,670 | 9,320 |
5/8 or 0.625 | 1/8 or 0.125 | 11,730 | 9,580 |
---------------------------------------------------------------------+
It will be seen from the above that the punched bars had the greatest
strength, indicating that punching had the effect of strengthening
instead of weakening the metal. These experiments have given results
just the reverse of similar experiments made on boiler plates; but the
material, such as above experimented upon, is what should be placed in
boilers, tough and ductile, and the manner of, and care taken in,
punching contribute to these results.
It is usual to have the rivet holes one-sixteenth of an inch in
diameter larger than the rivets, in order to allow for their expansion
when hot; it is evident, however, that the difference between the
diameters of the rivet hole and of the rivet should vary with the size
of the rivet.
The hole in the die is made larger than the punch; for ordinary work
the proportion of their respective diameters varies from 1:1.5 to 1:2.
As I have before stated, the best plate joint is that in which the
strength of the plate and the resistance of the rivet to shearing are
equal to each other.
In boilers as commercially made and sold the difference in quality of
the plates and rivets, together with the great uncertainty as to the
exact effect of punching the plates, have, so far, prevented anything
like the determination either by calculation or experiment of what
might be accepted as the best proportions of riveted joints.
In regard to steel plates for boilers Mr. F.W. Webb, of Crewe,
England, chief engineer of the London and Northwestern Railway, has
made over 10,000 tests of steel plates, but had only two plates fail
in actual work; these failures he thought were attributable solely to
the want of care on the part of the men who worked the plates up.
All their rivet holes for boilers were punched in a Jacquard machine,
the plates then annealed, and afterward bent in rolls; they only used
the reamer slightly when they had three thicknesses of plate to deal
with, as in butt joints with inside and outside covering strips. These
works turn out two locomotive boilers every three days.
The Baldwin Locomotive Works, which turn out on an average three
locomotives per day, punch all their rivet holes one sixteenth inch
less in diameter and ream them to driven rivet size when in place.
They also use rivets with a fillet formed under head made in solid
dies.
_Rivets._--Rivets of steel or iron should be made in solid dies.
Rivets made in open dies are liable to have a fin on the shank, which
prevents a close fit into the holes of the plates. The use of solid
dies in forming the rivet insures a round shank, and an accurate fit
in a round hole. In addition, there is secured by the use of solid
dies, a strong, clean fillet under the head, the point where strength
is most needed.
Commencing with a countersunk head as the strongest form of head, the
greater the fillet permissible under the head of a rivet, or bolt, the
greater the strength and the decrease in liability to fracture, as a
fillet is the life of the rivet.
If rivets are made of iron, the material should be strong, tough, and
ductile, of a tensile strength not exceeding 54,000 pounds per square
inch, and giving an elongation in _eight inches_ of not less than
twenty-five per cent. The rivet iron should be as ductile as the best
boiler plate when cold. Iron rivets should be annealed and the iron in
the bar should be sufficiently ductile to be bent cold to a right
angle without fracture. When heated it should be capable of being
flattened out to one-third its diameter without crack or flaw.
[Illustration: FIG. 15. Solid Die Rivet.]
[Illustration: FIG. 16. Open Die Rivet.]
If rivets are made of steel they must be low in carbon, otherwise they
will harden by chilling when the hot rivets are placed in the cold
plates. Therefore, the steel must be particularly a low grade or mild
steel. The material should show a tensile strength not greater than
54,000 pounds per square inch and an elongation in _eight inches_ of
thirty per cent. The United States government requirements are that
steel rivets shall flatten out cold under the hammer to the thickness
of one-half their diameter without showing cracks or flaws; shall
flatten out hot to one-third their diameter, and be capable of being
bent cold in the form of a hook with parallel sides without cracks or
flaws. These requirements were thought at first to be severe, but the
makers of steel now find no practical difficulty in meeting these
specifications.
The forming of the head of rivets, whether of steel or iron, and
whether the heads are conical or semi-spherical, should not be changed
by the process of riveting. The form of the head is intended to be
permanent, and this permanent form can only be retained by the use of
a "hold fast," which conforms to the shape of the head. In the use of
the flat hold fast (in general use in a majority of boiler shops) the
form of the head is changed, and if the rivet, by inadequate heating,
requires severe hammering, there is danger that the head of the rivet
may be "punched" off. By the use of a hold fast made to the shape of
the rivet head, this danger is avoided and the original form of the
head is retained. This feature of the use of proper rivet tools in
boiler shops has not received the attention it deserves. Practical use
of the above named hold fast would soon convince the consumers of
rivets of its value and efficiency.
The practice of driving rivets into a punched rivet hole from which
the fin or cold drag, caused by the movement of the punch, has not
been removed by reaming with a countersunk reamer, or better still a
countersunk set, should be condemned, as by driving the hot rivet head
down against the fin around the hole in the cold plate caused by the
action of punching the countersunk fillet is not only destroyed, but
it is liable to be driven into the head of the rivet, partially
cutting the head from the shank. If the rivet is driven into a hole
that has been punched with a sharp punch and sharp die, the result is
that the fillet is cut off under the head, and the riveted end is also
cut, and does not give the clinch or hold desired. That is to say,
rivet holes in plates to be riveted should have the burr or sharp edge
taken off, either by countersinking, by reamer, or set.
_Heating of Rivets._--Iron rivets are generally heated in an ordinary
blacksmith's or rivet fire having a forced blast; they are inserted
with the points down into the fire, so that the heads are kept
practically cool.
Steel rivets should be heated in the hearth of a reverberatory furnace
so arranged that the flame shall play over the top of the rivets, and
should be heated uniformly throughout the entire length of the rivet
to a cherry red. Particular attention must be given to the thickness
of the fire in which they are heated.
Steel, of whatever kind, should never be heated in a thin fire,
especially in one having a forced blast, such as an ordinary
blacksmith's or iron rivet furnace fire. The reason for this is that
more air passes through the fire than is needed for combustion, and in
consequence there is a considerable quantity of free oxygen in the
fire which will oxidize the steel, or in other words, burn it. If free
oxygen is excluded steel cannot burn; if the temperature is high
enough it can be melted and will run down through the fire, but
burning is impossible in a thick fire with a moderate draught.
This is an important matter in using steel rivets and should not be
overlooked; the same principle applies to the heating of steel plates
for flanging.
_Riveting._--There are four descriptions of riveting, namely:
(1) Hammered or hand riveting.
(2) Snapped or set.
(3) Countersunk.
(4) Machine.
For good, sound work, machine riveting is the best.
Snapped riveting is next in quality to machine riveting.
Countersunk riveting is generally tighter than snapped, because
countersinking the hole is really facing it; and the countersunk rivet
is, in point of fact, made on a face joint. But countersinking the
hole also weakens the plate, inasmuch as it takes away a portion of
the metal, and should only be resorted to where necessary, such as
around the front of furnaces, steam chests or an odd hole here and
there to clear a flange, or something of that sort.
Hammered riveting is much more expensive than machine or snapped
riveting, and has a tendency to crystallize the iron in the rivets,
causing brittleness.
In the present state of the arts all the best machine riveters do
their work by pressure, and not by impact or blow.
The best machines are those of the hydraulic riveting system, which
combines all of the advantages and avoids all the difficulties which
have characterized previous machine systems; that is to say, the
machine compresses without a blow, and with a uniform pressure at
will; each rivet is driven with a single progressive movement,
controlled at will. The pressure upon the rivet after it is driven is
maintained, or the die is retracted at will.
[Illustration: FIG. 17.]
Hydraulic riveting has demonstrated not only that the work could be as
well done without a blow, but that it could be _better done without a
blow_, and that the riveted material was stronger when so secured than
when subjected to the more severe treatment under impact.
What is manifestly required in perfect riveting is that the metal of
the rivet while hot and plastic shall be made to flow into all the
irregularities of the rivet holes in the boiler sheets; that the
surplus metal be formed into heads as large as need be, and that the
pressure used to produce these results should not be in excess of what
the metal forming the boiler shall be capable of resisting.
It is well known that metals, when subjected, either cold or hot, to
sufficient pressure, will obey almost exactly the same laws as fluids
under similar conditions, and will flow into and fill all the crevices
of the chamber or cavity in which they are contained. If, therefore, a
hot rivet is inserted into the holes made in a boiler to receive it,
and is then subjected to a sufficient pressure, it will fill every
irregularity of the holes, and thus fulfill one of the conditions of
perfect riveting. This result it is impossible to accomplish with
perfection or certainty by ordinary hand riveting, in doing which the
intermittent blows of an ordinary hammer are used to force the metal
into the holes. With a hydraulic riveting machine, however, an
absolutely uniform and continuous pressure can be imparted to each
rivet, so as to force the hot metal of the rivet into all the
irregularities of the holes in the same way as a hydraulic ram will
cause water to fill any cavity, however irregular.
[Illustration: FIG. 18.]
In order to illustrate the relative advantages of machine over hand
riveting, two plates were riveted together, the holes of which were
purposely made so as not to match perfectly. These plates were then
planed through the center of the rivets, so as to expose a section of
both the plates and rivets. From this an impression was taken with
printer's ink on paper and then transferred to a wooden block, from
which Figs. 17 and 18 were made.
The machine-driven rivet is marked _a_, and _b_ represents the
hammered rivet.
It will be observed that the machine rivet fills the hole completely,
while the hand rivet is very imperfect. This experiment was tried
several times, with similar results each time.
The hand rivet, it will be observed, filled up the hole very well
immediately under the head formed by the hammer; but sufficient
pressure could not be given to the metal--or at least it could not be
transferred far enough--to affect the metal at some distance from the
driven head. So great is this difficulty that in hand riveting much
shorter rivets must be used, because it is impossible to work
effectively so large a mass of metal with hammers as with a machine.
The heads of the machine rivets are, therefore, larger and stronger,
and will hold the plates together more firmly than the smaller
hammered heads.
To drive rivets by hand, two strikers and one helper are needed in the
gang, besides the boy who heats and passes the rivets; to drive each
five-eighths inch rivet, an average of 250 blows of the hammer is
needed, and the work is but imperfectly done. With a machine, two men
handle the boiler, and one man works the machine; thus, with the same
number of men as is required in riveting by hand, five rivets are
driven each minute.
The superior quality of the work done by the machine would alone make
its use advantageous; but to this is added greatly increased amount of
work done.
The difference in favor of the riveting machine over hand riveting is
at least _ten_ to _one_.
In a large establishment a record of the number of rivets driven by
the hand-driving gang, also by the gang at the steam-riveting machine
for a long period of time, in both cases making no allowances of any
kind of delays, the rivets driven per month by each was--for the hand
driven rivets at the rate of twelve rivets per hour, and for the
machine driven rivets, 120 per hour. In the case of the hand driven
rivets the boiler remains stationary and the men move about it, while
the machine driven rivets require the whole boiler to be hoisted and
moved about at the riveting machine to bring each hole to the position
required for the dies. Notwithstanding the trouble involved in
handling and moving the boiler, it shows that it is possible to do ten
times as much work, and with less skilled labor, by the employment of
the riveting machine.
_Calking._--One great source of danger in boiler making is excessive
joint calking--both inside and out--where a sharp nosed tool is
employed, and for the reason that it must be used so close to the
inner edge of plate as to indent, and in many cases actually cut
through the skin of the lower plate. This style of calking puts a
positive strain upon the rivets, commencing distortion and putting
excessive stress upon rivets--already in high tension before the
boiler is put in actual use. It is, I hope, rapidly becoming a thing
of the past.
With a proper proportion of diameter and pitch of rivet, all that is
required is the use of a light "fuller tool" or the round-nosed tool
used in what is known to the trade as the "Connery system."
There is but little need of calking if means are taken to secure a
clean metal-to-metal face at the joint surfaces. When the plates are
put together in ordinary course of manufacture, a portion of the mill
scale is left on, and this is reduced to powder or shaken loose in the
course of riveting and left between the plates, thus offering a
tempting opening for the steam to work through, and is really cause of
the heavy calking that puts so unnecessary a pressure on both plate
and rivet. A clean metallic joint can be secured by passing over the
two surfaces a sponge wet with a weak solution of sal-ammoniac and hot
water, an operation certainly cheap enough both as to materials and
labor required.
[Illustration: FIG. 19]
The above cut, Fig. 19, gives an illustration of calking done by
sharp-nosed and round nosed tools, respectively. It will be seen by
Fig. 20 that the effect of a round-nosed tool is to divide the plate
calked, and as the part divided is well driven toward the rivets, a
bearing is formed at _a_, from one-half to three-fourths of an inch,
which increases the strength of joint, and will in no way cut or
injure the surface of the under plate. A perfect joint is thus
secured.
[Illustration: Fig. 20.]
* * * * *