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           Types of sewing machines 
    
    There are two types of sewing machine.  

    The older is the chain stitch machine, which 
uses a single thread to make a chain rather like 
tambour embroidery, but the loops are poked down, 
rather than being pulled up.  
    A chain-stitch machine uses a needle with an 
eye in the point to poke loops through the fabric.  
Each time the needle is poked through the fabric, 
it is also poked through a loop left by the 
previous stitch, and a finger catches the thread 
to hold the new loop.  As the needle is withdrawn, 
the old loop is tightened around the new one.  
    At one time, industrial chain-stitch machines 
were much used in the manufacture of cheap 
clothing, because the seamstresses never had to 
interrupt their work to change bobbins.  
    Chain stitch seams are thicker than lock-
stitch seams, they unravel easily, and customers 
learned to spot inferior products by looking for 
chain stitching, so plain chain-stitch machines 
went out of style.  Chain-stitch machines that use 
three or more threads to overcast an edge 
continued to be used for making T-shirts and the 
like, and home overlock machines have become 
available and are enjoying great popularity.  In 
some places, overlock machines were first used to 
prevent serge from ravelling, and so became known 
as "sergers".  Whether a machine is called a 
"serger" or an "overlocker" is purely a matter of 
dialect; both words refer to the same machine. 
    
    A lock stitch machine is more expensive to 
make than a one-thread chain stitch machine, and 
you have to stop every forty yards or so to change 
the bobbin, but the stitches are more secure, and 
seams are only two threads thick instead of three.  
For home use, lock-stitch machines drove chain-
stitch machines off the market almost instantly. 
    Like the chain-stitch machine, the lock-stitch 
machine pokes a loop of thread through the fabric.  
But instead of catching the loop and holding it 
for the next stitch, the machine passes a bobbin 
through it.  As the needle is withdrawn, both the 
needle thread and the bobbin thread are tightened, 
so that the threads end up twisted together in the 
middle of the fabric. 
    When the machine is properly adjusted, that 
is.  If the bobbin tension is tighter than the 
upper tension, the bobbin thread runs straight 
along the fabric, passing through loops of needle 
thread.  If the tension is unbalanced the other 
way, the needle thread will run along the surface 
of the fabric passing through loops of bobbin 
thread. 

    There are two kinds of sewing-machine bobbins.  
Both are little spools in the form of  two disks 
on the ends of a stick.  
    The stick of a *long bobbin* is long and thin, 
and the disks are small.  This sort of bobbin is 
placed in a shuttle, and the shuttle always passes 
through the loop of thread point first, so the 
machine cannot sew backward, and I doubt that it's 
possible for a long-bobbin machine to zig-zag.  
When these machines were still common, one could 
buy zig-zag attachments that moved fabric back and 
forth the way a buttonholer does. 
    The stick of a *round bobbin* is short and 
thick, and usually hollow.  The disks are large -- 
wider than the length of the stick, so we call 
them sides instead of ends.  A round bobbin is 
placed in a "bobbin case" and the bobbin case is 
placed in a "shuttle race" which carries the loop 
of thread around the bobbin.  (Some machines have 
removable bobbin cases, and some have cases that 
are part of the shuttle race.)  Since the loop can 
be carried around the bobbin in either direction, 
round-bobbin machines are usually capable of 
sewing backward, and the mechanism can be made to 
move back and forth to make zig-zag stitches. 
    Some treadle machines use round bobbins, but 
many early electrics are long-bobbin machines. 
    Whether the bobbin is long or round, it is 
very important to put it into the shuttle or 
bobbin case correct end first.  The thread coming 
off the bobbin should point in the same direction 
as the slot in the shuttle or bobbin case, so that 
it is pulled in the unwinding direction when you 
pull it into the slot, and it makes a sharp turn 
inside the tension mechanism.  (Unless it is a 
White machine, in which case you put it in the 
other way.) 
    The bobbin itself is usually symmetrical, and most can 
be put into the bobbin winder either way.  But once it is 
wound, it acquires a handedness -- when you look at one end 
or side, the thread is wound clockwise, and when you look at 
the other end, the thread is wound counterclockwise.  
    
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