Metal Spinning 3

Metal Spinning 3

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_.

 

 

 

 

CHAPTER II

 

TOOLS AND METHODS USED IN METAL SPINNING[2]

 

 

The principal object of this chapter is to describe in detail the

various operations of spinning metal so that a tool-maker or machinist

who has not access to a metal spinner, will be able to make his

own tools, rig up an engine or speed lathe, and make the simple

forms or models that are required in experimental work. To do this

intelligently, it is necessary to follow in detail every step in metal

spinning from the circular blank to annealing, pickling, dipping,

burnishing, etc., and also to know how to make the simpler forms of

spinning tools, what lubricants to use on the different kinds of

metals, what material to make the spinning chuck of, and how far the

metal can be worked before annealing.

 

Spinning metal into complicated and elaborate shapes, is an art fully

as difficult as any craft, and the man is truly an artist that can

make artistic and graceful outlines in metal, especially when only a

few pieces are required and the cost will not allow of making special

chucks to do the work on and with no outline chucks to govern his

design, the forms being made by skill and manipulation of tools alone.

Such skill is far superior to that of the Russian metal worker, who,

instead of making a vase or ornament of one piece, cuts up several

sections and soft solders them together, after covering them with crude

“gingerbread” work to disguise his poor metal work.

 

The amateur can imitate the Russian work, but never the work of the

skilled spinner. There are several grades of spinners, most of them

never attaining the skill of the model-maker or the facility for

handling the different metals. A man that has had several years of

experience spinning brass or copper would not be able to spin britannia

or white metal without stretching it to a very uneven thickness.

As brass or copper is harder than the other metals mentioned, they

resist the tool more and require more pressure in forming, and if the

operator used the same pressure on the softer metals, he would stretch

or distort them, so that they would be perhaps one-quarter of the

original thickness at angles and corners where the strain in spinning

would be greatest, which would ruin the articles. The best test for

skill in ordinary spinning, is to take a long difficult shape, after

being finished, and saw it in two lengthwise, and if the variation in

thickness is less than 25 per cent of the original gage, it is good

practice. Some spinners can keep within 10 per cent of the gage on

ordinary work, but they are scarce.

 

The spinning trade in this country is mostly followed by foreigners,

Germans and Swedes being the best. The American that has intelligence

and skill enough to be a first-class spinner, will generally look

around for something easier about the time that he has the trade

acquired. It is an occupation that cannot be followed up in old age,

as it is too strenuous, the operator being on his feet constantly, and

having to use his head as well as his muscles.

 

 

General Remarks on Metal Spinning Chucks

 

For common plain shapes, a patternmaker’s faceplate, with a tapered

center screw, is sufficient for holding the wood chuck. The hole in the

wood should be the same taper as the screw, thus giving an even grip on

the thread. If a straight hole only is used, and it is not reamed out

before screwing to the plate, it will only have a bearing on one or two

threads, and if the chuck is taken off and replaced on the faceplate,

it will not run true. Care should also be taken to face off the end

of the chuck flat, or to slightly recess it, so that it will screw up

evenly against the faceplate, as a high center will cause it to rock

and run out of true.

 

In large chucks (over five inches) it is best to have three or four

wood screws, besides the center screw. The holes for these can be

spaced off accurately on a circle in the iron faceplate, and drilled

and countersunk. It is best to have twice as many holes as screws; that

is, if four screws are used there should be eight holes, so that if the

chuck has to be replaced at any time and the wood has shrunk, it can be

turned one-eighth of a revolution further than the original chucking.

 

Where a chuck has to be used several times, it is better practice to

cut a thread in the wood and screw the chuck directly to the spindle

of a lathe, not using the faceplate. This thread can be chased with

a regular chasing tool, where the operator has the skill, or if not,

the wood can be bored out and a special wood tap used. Such a tap has

no flutes and it is bored hollow, there being a wall about ³/₁₆ inch

thick. One tooth does all the cutting, that is the one at the end of

the thread. The chips go into the hollow part of the tap. The end of

the tap for about ¼ inch should have the same diameter as the hole

before threading to act as guide for the cutting tooth.

 

It is essential that a chuck should run very true and be balanced

perfectly, as the high speed at which it runs will cause it to vibrate

and run out of true, causing the finished metal to show chatter marks.

The best wood for chucks is hard maple, and it should be selected for

its even grain and absence of checks and cracks. It is best to paint

the ends with paraffine or red lead, or to immerse the chucks in some

vegetable oil after turning. Cottonseed oil is very good for this

purpose, but care should be taken not to soak the chucks too long.

 

For a man not skilled in spinning, it is better to use metal chucks

than wood, for if there are many shells of a kind, the operator is

liable to bear too hard on the tool, thus compressing the chuck and

making the last shells smaller than the first. Corners and angles not

well supported might also be knocked off. The writer prefers cold

rolled steel for chucks up to 6 inches in diameter and cast iron for

the larger ones, but where good steel castings can be obtained, a

good chuck can be made by turning roughly to shape a wood pattern,

allowing enough for shrinkage and finishing, and hollowing out the back

to lighten it. When the chuck is finished all over in the lathe, it

should balance much better than a cast iron one, as there are not the

chances of having blow holes in the iron, thus throwing the chuck out

of balance.

 

 

Annealing

 

The distance that metal can be drawn without annealing, can only be

learned by experience. A flat blank rotated in the lathe, being soft,

will offer little resistance and it can be gradually drawn down by a

tool held under the chuck and against the blank. This tool is pushed

from the center outward and forward at the same time, and every time

it passes over the blank or disk the metal becomes harder by friction,

and the change of formation and the resistance at the point of the tool

greater. This can be felt as the tool is under the operator’s arm. When

the spring of the metal is such that the tool does not gain any, but

only hardens the metal, the shell should be taken off and annealed. If

the metal has been under a severe strain, it should be hammered on the

horn of an anvil or any metal piece that will support the inside. The

hammer should be a wood or rawhide mallet, but never metal, the object

being to put dents or flutes in the metal to relieve the strain when

heating for annealing; if this is not done the shell will crack.

 

After annealing the shell it should be pickled to clean the oxide or

scale from the surface; otherwise the metal will be pitted. When the

scale is crowded into the metal and when it will not finish smooth

after spinning to shape, the metal can be finished by skimming or

shaving the outer surface which cuts out all tool marks; it can then

be finished with medium emery cloth or the shell can be bright dipped,

and be run over with a burnishing tool before buffing. Burnishing can

be done on the spinning chuck, but the speed should be higher than for

spinning; this requires some skill for a good job, and it can be done

only on metal chucks.

 

Annealing is best accomplished in a wood or gas oven, where a forge

fire is used. The metal should never touch the coke or other fuel,

but it should be held in the flame above the fire. Where only part

annealing is required, the shell can be immersed in water, the part to

be annealed being exposed above the water, and a blowpipe used on it.

The remainder of the shell will then be hard. This way of annealing is

sometimes necessary on a special shapes.

 

Brass should be heated to a cherry red, and held at that point for a

few minutes, in a muffle furnace. If an open furnace is used, just

bring the metal to a cherry red and then dip it in water; this method

is better than when waiting for it to cool, the action being just the

opposite to that on steel. Brass such as the common yellow brass is not

suitable for spinning, there being but 55 per cent copper and 45 per

cent zinc. There are two grades of brass suitable for spinning. These

are known as “spinning and drawing,” having 60 per cent copper and 40

per cent zinc, and “extra spinning and drawing” having 67 per cent

copper and 33 per cent zinc. There is also a better grade known as “low

brass” having from 75 to 80 per cent copper; it has the color of bronze and is only used on very deep and difficult spinning.