_MACHINERY’S REFERENCE SERIES_
EACH NUMBER IS ONE UNIT IN A COMPLETE LIBRARY OF MACHINE DESIGN AND
SHOP PRACTICE REVISED AND REPUBLISHED FROM MACHINERY
NUMBER 57
METAL SPINNING
SECOND EDITION
CONTENTS
Principles of Metal Spinning, by C. TUELLS 3
Tools and Methods Used in Metal Spinning, by WILLIAM A. PAINTER 15
Copyright, 1912, The Industrial Press, Publishers of MACHINERY
49-55 Lafayette Street, New York City
CHAPTER I
PRINCIPLES OF METAL SPINNING[1]
Metal spinning, that process of sheet metal goods manufacturing which
deals with the forming of sheet metal into circular shapes of great
variety by means of the lathe, forms and hand-tools, is full of kinks
and schemes peculiar to itself. It is the purpose of this treatise to
give a description of spinning in general, and to outline some of the
methods and tools used in spinning for rapid production.
The products of metal spinning are used in a great many lines of
manufacture. Examples of this work are chandelier parts, cooking
utensils, silver and brittania hollow-ware, automobile lamps,
cane-heads and many other sheet metal specialties. Brass, copper,
zinc, aluminum, iron, soft steel, and, in fact nearly all metals yield
readily to the spinner’s skill. At best spinning is physically hard
work, and the softer the stock, the easier and quicker the spinner can
transform it into the required product.
There are but two practical ways of forming pieces of sheet metal into
hollow circular articles: by dies and by spinning. By far the cheapest
and best method of producing quantities of this class of work is by
the use of dies, but there are many cases where it is impractical or
impossible to follow this course. Dies are expensive and there is
constant danger of breakage, whereas spinning forms are easily and
cheaply made and are almost never damaged by use beyond a reasonable
amount of wear. Thus it will be seen that when the production is small,
it does not pay to make costly dies. Again, the styles or designs of
many articles that are spun are constantly being changed; if made
by dies each change would necessitate a new die, while in spinning
merely a new wooden form is required--and sometimes the old form can
be altered, costing practically nothing. Still other advantages of
spinning are that in working soft steel, a much cheaper grade may be
spun than can be drawn with dies; beads may be rolled at the edges of
shells at little expense; experimental pieces may be made quickly,
and, added to these features comes the fact that very difficult work
that cannot possibly be made with dies can be spun with comparative
ease. It must not be construed from the above that spinning is to be
preferred to die work in all or even in the majority of cases, because,
on the contrary, die work is a more economical method of manufacture,
and should always be used when possible on production work. The cases
already cited are merely given to point out some of the instances in
which, for economical reasons, spinning is to be preferred to die work.
The Spinning Lathe
The principal tool used in the operation of spinning is the spinning
lathe, shown in Fig. 1. While in many respects this machine is similar
to any other lathe, it is built without back-gears, carriage or
lead-screw, is very rigid in construction, and, on the whole, very
much resembles a speed lathe. Like other lathes, the spinning lathe is
fitted with a cone pulley (preferably of wood, because of its lightness
and gripping qualities), allowing the use of four or five different
speeds. Speed is an important factor in spinning. Arbitrary rules for
spinning speeds cannot be given, as the thicker the stock the slower
must be the speed; thus while ¹/₃₂-inch iron can be readily spun at
600 revolutions, ¹/₁₆-inch iron would necessitate reducing the speed
to 400 revolutions per minute. Zinc spins best at from 1,000 to 1,400
revolutions; copper works well at 800 to 1,000; brass and aluminum
require practically the same speed, from 800 to 1,200; while the
comparatively slow speed of 300 to 600 revolutions is effective on iron
and soft steel. Brittania and silver spin best at speeds from 800 to
1,000 revolutions.
[Illustration: Fig. 1 Spinning Lathe]
One of the essential parts of the spinning lathe is the T-rest. The
base of this rest is movable on the ways of the lathe, and it has at
the side nearest the operator, a stud about four inches in diameter
and six inches high, through which is swiveled the T-rest proper. As
the illustration shows, provision is made for raising and lowering
the rest, and the entire rest may be clamped in any desired position
by means of the hand-wheel shown beneath the ways. The rest proper
consists of an arm, 12 to 15 inches long, similar to a wood turner’s
rest, and through the face of this arm are from twelve to sixteen
closely spaced ⅜-inch holes. These holes are to receive the pin against
which the hand tools are held while spinning. The pin is three inches
long and of ¾-inch steel, turned down on one end to loosely fit the
holes in the rest.
Another important part of the spinning lathe is the tail-center. This
center is sometimes the ordinary dead center that is in general machine
shop use, but nearly all spinners use the revolving center, shown in
Fig. 2. The revolving center is ¾ inch diameter (without taper) and
about six inches long, and is fitted into the socket in which it runs;
this socket is, in turn, fitted to the taper hole in the tailstock. At
the bottom of the hole in the socket are two steel buttons, hardened
and ground convex on their faces. These buttons act as ball bearings
and reduce friction to a minimum.
[Illustration: Fig. 2. Revolving Center]
Forms and Chucks for Spinning
The shape of a shell made by spinning is dependent on the form or chuck
upon which the metal is spun. Forms are used for plain spinning where
the shape of the shell will permit of its being readily taken from the
form after the spinning has been completed; but when the shape of the
shell is such that it will not “draw,” as the molders say, it becomes
necessary to employ sectional chucks, similar to the one shown in Fig.
3. Generally speaking, spinning forms are made of kiln dried maple.
After being bored and threaded to fit the lathe spindle, the spinner
turns the maple block to agree with a templet shaped in outline to the
sample shell. When no sample is furnished, the templet must be laid
out from a sketch or drawing; in either case proper allowance is made
for the thickness of the stock. When large quantities of shells are to
be spun, all alike, the form is sometimes made of lignum vitæ. Another
method is to turn the maple form small enough so that one shell may be
spun and cemented to it and then this metal-cased form is used to spin
the balance of the shells. For continuous spinning, forms are made of
cast iron or steel, which of course makes a most satisfactory surface
to spin on and gives indefinite service.
[Illustration: Fig. 3. Sectional Spinning Chuck]
A sectional or “split” chuck, as it is sometimes called, is, as the
name implies, a spinning chuck or form which may be taken apart in
sections after the shell has been spun over it. As before stated, this
class of spinning chuck is only used when the finished shell could
not be removed from an ordinary form after spinning. After a shell
has been spun over a sectional chuck, the shell and the sections of
the chuck are together pulled lengthwise from the core of the chuck.
Then, starting with the key section, it is an easy matter to remove
each section from the inside of the shell. As the sections are removed,
they are replaced upon the core, slipped under the retaining flange
and the chuck is ready for spinning a new shell. The whole operation
of removing and replacing the sections of a chuck takes less time than
it does to tell it, and, as the sections are of different sizes, it is
easy to replace them in the proper order. Like other forms, sectional
chucks are made of wood or metal, according to the requirements of the
job. The core and retaining ring are first made from one piece and then
the sections are turned in a continuous ring and split with a fine saw.
In some cases it is necessary to add a small piece to the last section
to make up for the stock lost in splitting the sections.
Another kind of sectional chuck, known to the trade as a “plug” (shown
in Fig. 5) is used extensively in some shops in cases where the shell
must have projections or shoulders at both ends, and no bottom to the
shell is required. In making the plug, which is always in two parts,
the first half is turned to take the shell from one end to the center
of the smallest diameter. Into the end of this part is bored a hole to
which is fitted the end of the second part, which is afterwards turned
to fit the shell. Over this two-part plug the shell is spun; then the
bottom of the shell is cut out and the first half of the plug removed,
thus allowing the shell to be withdrawn. The first part is then
replaced and the plug is ready for use again. Fig. 4 shows a method of
spinning difficult shells that ordinarily would require a sectional
chuck. The shell shown at the left of Fig. 4 is first spun as far as
the bulged part on an ordinary form that ends at this point. Then
after annealing, it is replaced on the form and while another operator
holds the wooden arm, supported with a pin in the T-rest, the spinner
forms the metal around the bulge-shaped end of the arm. The arm, being
stationary on the inside of the shell, acts as a continuation of the
spinning form, and by this method as good a shell is obtained as could
be spun with a sectional chuck.
[Illustration: Fig. 4. Quick Method of Spinning Difficult Shell Without
Sectional Chuck]
[Illustration: Fig. 5. Spinning on Plugs]
For spinning operations upon tubing or press-drawn tubes, steel arbors
are generally used. Tubing may be readily spun upon an arbor and it can
be reduced or expanded to comply with the shape of shell required much
more quickly than the shell could be spun from the blank.
Followers
For holding the sheet metal blank to the spinning form, a block of wood
known as the follower, is used (see Fig. 6). Followers are made to
suit the shape of the work with which they are to be employed, always
being made with the largest possible bearing on the work; thus a shell
with a flat bottom twelve inches in diameter would be turned with the
aid of a follower having an 11¾-inch face, while a shell with a 4-inch
face would take a follower with a 3⅞-inch face. All shells do not have
flat bottoms, consequently, in spinning such as do not, it becomes
necessary to employ hollow followers. Hollow followers have their
bearing surfaces turned out to fit the ends of the forms with which
they are to be used. In practice, the blank is held against the end of
the spherical form with a small flat follower until enough of the shell
has been spun to admit of the hollow follower being used. All followers
are made with a large center hole in one end to receive the revolving
tail-center.
[Illustration: Fig. 6. Three Types of Followers]
In starting to spin a difficult shell it sometimes happens that the
necessarily small follower will not hold the blank. To prevent this
slipping, the face of the follower is covered with emery cloth. Often,
however, on rough work, the spinner will not stop to face the follower,
but will make a large shallow dent at the center of the blank; the
extra pressure required to force the metal against the form will
usually overcome the slipping tendency.
[Illustration: Fig. 7. Specimens of Metal Spinning]
Hand Tools
Hand tools, in great variety, form the principal asset of the spinner’s
kit. Spinning tools are made of tool steel forged to the required
shapes, and are hardened and polished on the working end. The round
steel from which they are made varies from ½ inch to 1½ inch in
diameter, according to the class of work upon which they are to be
used. The length of a spinning tool is about 2 feet, and it is fitted
into a wooden handle 2 inches diameter and 18 inches long, making the
total length of the handled tool about 3 feet, as shown in Fig. 8. As
the spinner holds this handle under the right armpit, he secures a
great leverage upon the work and is better able to supply the physical
power required to bring the metal to the desired shape.
[Illustration: Figs. 8 and 9. Spinning Tool and Swivel Cutter]
The commonest and by far the most useful of the spinning tools is
the combination “point and ball” which together with a number of
other tools, is shown in Fig. 11. This tool is used in doing the bulk
of the spinning operations--for starting the work and bringing it
approximately to the shape of the form. Its range of usefulness is
large on account of the many different shapes that may be utilized by
merely turning the tool in a different direction. Next in importance
comes the flat or smoothing tool which, as the name implies, is for
smoothing the shell and finishing any rough surfaces left by the point
and ball tool. The fishtail tool, so named from its shape, is used
principally in flaring the end of a shell from the inside, “spinning on
air,” as it is sometimes termed. This tool is used to good advantage in
any place where it is necessary to stretch the metal to any extent, and
its thin rounding edge proves useful in setting the metal into corners
and narrow grooves. Other tools are the ball tool which is adapted to
finishing curves; the hook tool, used on inside work; and the beading
tool which is needed in rolling over a bead at the edge of a shell when
extra strength or a better finish is desired.
When much beading of one kind is being done, a large heavy pair of
round-nose pliers (Fig. 10) with the jaws bent around in a curve and
sprung apart enough to allow for the thickness of the metal proves to
be a handy tool. After the edge of the shell has been flared out to
start the bead, the pliers are opened enough to admit the metal and
then closed and the stock guided around to form the bead as far as
possible. In this way the larger part of a bead is rapidly formed, one
jaw of the pliers acting as a spinning tool and the other corresponding
to the back-stick. During this operation, the pliers are, of course,
supported by being held against the T-rest.
[Illustration: Fig. 10. Spinners’ Pliers]
Closely allied with these spinning tools are two other tools (also
shown in Fig. 11) known as the diamond point and the skimmer. The
diamond point is for trimming the edges of the shell during the
spinning operation and for cutting out centers or other parts of the
work. The skimmer is for cleaning up the surface of a shell, removing a
small amount of metal in doing so, the amount depending upon the skill
the spinner used in the spinning proper.
[Illustration: Fig. 11. Hand Tools of Various Forms used in Spinning]
When the bottoms are to be cut from a large number of shells and it
is necessary that they be cut exactly alike, a tool known as a swivel
cutter is used. This tool (see Fig. 9) is simply an iron bar with a
cutter on one end, which swivels near the center around a pin in the
T-rest; thus by a slight movement of the arm the cutter is brought up
to the work, cutting a piece from the shell of exactly the same size
each time.
The Spinning Operation
In order to make clear the successive steps in spinning, let us briefly
consider the making of a copper head-light reflector, and the way the
work is handled when a few hundred pieces are to be made.
By trial spinning, the size of the blank required for one of the
reflectors is determined, and with the square shears the copper sheets
are cut into pieces an eighth of an inch larger each way. These squares
are then taken to the circular shears and cut to round shapes ready
for the spinning lathe. The spinning form, of kiln-dried maple, is
screwed to the spindle and the belt thrown to that step of the cone
pulley which will bring the speed nearest to 1,200 revolutions. From
the stock-room a follower is selected whose face will nearly cover the
bottom of the form. It is now “up to” the spinner. Holding a blank and
also the follower against the end of the form, he runs the tail-center
up to the center in the follower just hard enough to hold the blank
in place. Then, starting the lathe, he centers the blank by lightly
pressing against its edge a hard wood stick. As soon as it “lines up”
he runs the center up a little harder and clamps it in place. Some
spinners will “hop in” a blank with the lathe running, but this is
dangerous practice and sometimes the blank will go sailing across the
room. Often this happens in truing up the blank and for this reason it
is considered advisable to have a wire grating at the further side of
the lathe to prevent serious accidents; for a sheet metal blank is a
dangerous missile traveling at the high rate of speed which is imparted
to it by the lathe.
With a piece of beeswax (soap is sometimes used for economical reasons)
the spinner lightly rubs the rapidly revolving blank and then adjusts
the pin in the T-rest to a point near enough to the blank to obtain a
good leverage with the spinning tool. Holding the handle of his point
and ball tool under his right armpit and using the tool as a lever and
the pin on the rest as a fulcrum, he slowly forces the metal disk back
in the direction of the body of the form, never allowing the tool to
rest in one spot, but constantly working it in and out, applying the
pressure on the way out to the edge of the disk and letting up as he
comes back for a new stroke. In the meantime his left hand is busy
holding a short piece of hard wood (called the back-stick), firmly
against the reverse side of the metal at a constantly changing point
opposite the tool. The object of the back-stick is to keep the stock
from wrinkling as it is stretched toward the edge of the disk. Wrinkles
cause the metal to crack at the edges and for this reason they must be
kept from the stock as much as possible.
After a few strokes of the spinning tool have been taken, the shell
will appear about as shown at _B_, Fig. 12, and at this point it
is necessary to trim the shell at the edges with the diamond-point
tool. Trimming is required because spinning stretches the stock and
the resulting uneven edge will cause splits in the metal if it is
not trimmed occasionally. As a carpenter is known by his chips, so
a spinner is known by the way his work stretches. While the even
pressure of a good spinner will stretch the stock very little, the
uneven pressure of the inexperienced man will lead him into all sorts
of trouble on account of the way the stock will “go.” In either case
the metal always stretches least in the direction in which the sheet
stock was originally rolled, consequently giving the edge a slight
oval shape. In trimming zinc, the spinner holds a “swab” of cloth just
above the diamond point, to prevent the chips from flying into his
face and eyes--or those of his neighbors. With other metals the swab is
unnecessary.
The reflector is now taking shape. With each successive stroke the
spinner sets a little more of the metal against the form. Not only does
spinning stretch the metal, but it hardens it as well; therefore, at
the stage _C_ it becomes necessary to anneal the partially completed
reflector, which is done by heating it to a low red in a gas furnace.
In running through a lot of shells, the common practice is to spin them
all as far as possible without annealing, and after annealing the whole
lot, to complete the spinning.
[Illustration: Fig. 12. Successive Steps in Spinning a Reflector]
After replacing the shell upon the form, it is trimmed and worked
further along the form, gradually assuming the appearance shown at _D_.
At this time, the spinner goes back to the small radius at the front
end of the shell and with a ball tool he closes the annealed metal hard
down against the form, for the spinning has tended to pull the stock
slightly from the form at this point. The body of the reflector is now
practically completed and the spinner directs his attention to rolling
the bead at the outside edge. Slowly he begins to roll the edge of the
shell back, using his hook tool to complete the bead as far as possible
and exercising care to keep the back-stick firmly against the metal
so as to keep the wrinkles out. Now, with the diamond point, he gives
the edges a final trim, and with the beading tool closes down the bead
snugly against the rest of the shell, as shown at _E_. Lastly, the
swivel cutter is placed in the proper hole of the T-rest and a turn of
the tool cuts out the center to the exact size, and the reflector is
completed. If any burrs or rough places remain they are easily removed
at this time with the skimmer or diamond point, and a little emery
cloth gives the shell a finished appearance.
Referring to the illustration Fig. 7, _A_, _B_ and _C_ represent the
three most important stages of spinning a shell like that shown at _C_.
Annealing is necessary between steps _A_ and _B_. _D_ is a shell spun
upon a form of the plug variety, and _E_ and _F_ are two views of a
shell spun after the method shown in Fig. 4, _F_ being the completed
shell. _G_ illustrates a very difficult shell to spin, on account of
the small follower that must be used; the length of the small diameter
also adds to the difficulty. _H_ shows a shell that must be spun upon a
sectional chuck, while _I_ is a plain easy job of ornamental spinning.
The ball shown at _J_ was spun from one piece of aluminum and it is
more of a curiosity than a specimen of practical spinning. It was first
spun over a form that would leave one-half of the ball complete and the
stock for the other half straight out like a short tube. Next a wooden
split chuck was made, hollowed out to receive the finished end of the
ball and the open end was gradually spun down and in until the ball was
complete with but a ¹/₁₆-inch hole at the end. This hole was plugged
and the hollow ball was done.
[Illustration: Fig. 13. An Interesting Example of Metal Spinning]
As another example of metal spinning, assume the shape shown in Fig.
13. The shell is to be 20 inches in diameter, 6 inches deep, and 0.060
inch thick. The metal to be used is zinc. This is an interesting metal
spinning job, and not a particularly difficult one. The shell can be
best spun with the aid of two spinning forms, such as are illustrated
in Figs. 14 and 15. These forms should be made of kiln-dried maple if
there are comparatively few shells to be spun. If there are many, the
forms should be made of cast iron. Fig. 14 shows the first form to be
used, which conforms to the outside of the shell as far as the centers
of the spherical ring. Beyond these points, the form is straight.
The blank to be spun is placed as indicated by the dotted lines, and
follower No. 1 is used to hold the work against the form. The chief
trouble will be met in properly starting the shell, because of the
small follower that must be employed. However, follower No. 2 may be
substituted after working the metal back against the form a few inches,
and as this gives a better grip on the shell, there will be no further
danger of slipping. After spinning the zinc shell to the shape of the
first form (Fig. 14) it will probably have to be annealed, but this can
only be determined by trial. In annealing zinc, the flame should not
be allowed to touch the metal. The half completed shell is then put on
form No. 2 shown in Fig. 15. It is an easy matter to spin the metal
round to complete the arc. The dotted line shows the position of the
shell before starting the last part of the spinning. Of course, it will
be understood that the shell must be trimmed several times during the
spinning, and if the trimming is frequently done, a well-shaped shell
should result. For spinning on form No. 2, follower No. 3 must be used.
Either beeswax or soap should be frequently rubbed over the work while
spinning. If it is necessary to cut out the center, it can be done
before removing the shell from the last form by simply removing the
follower and using a diamond point tool, or in large product work the
swivel cutter will work well. The shell will cling to the form without
the follower. The spinning speed should be from 800 to 1,000 R. P. M.
[Illustration: Fig. 14]
[Illustration: Fig. 15]
While the operation of spinning is a comparatively simple one to
describe, it is not easily learned, and to-day good all-around spinners
are hard to find. The limits of accuracy are not as closely defined
as in straight machine work, but there are times when good fits are
absolutely necessary, as in cases where two shells must slip snugly
together. In this chapter we have taken up only the plain every-day
kind of spinning, and were we to follow its work in the gold and
silversmith’s trade, we would see it evolve into a fine art. In order
to insure really good work coming from the spinning lathe, there is a
wide range of knowledge that the spinner must have. That knowledge may
be brought together and summed up by a single word--_judgment_.