Overlock
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An Overlock-Sewing Machine also known as a "Serger" sews and hems seams and cuts off the leftover cloth. They have no bobbins, using instead 3 or 4 thread cones. Some overlock machines can be set up to do a rolled or blind hem, but all can bind off the edge with an overcast stitch.
Technically, a "Serger" is an Overlock machine that uses 5 thread cones. In practice, a person referring to a serger generally means any overlocking sewing machine
Types of Overlock
A Coverlock refers to a machine that can only perform a coverlock stitch. Coverlocks have no knives, and thus perform a stretchable hem similar to a three or four thread overlock.
The three thread, single needle overlock consists of a chainstitch at the seam allowance and binds the edge at the same time, preventing the fabric from fraying and further reinforcing the seam. This stitch has a tremendous amount of give, making it useful for sewing elastic and other stretchy fabrics.
A four thread overlock consists of a chainstitch at the seam allowance and overlocks the seam, but adds an additional chainstitch at the edge of the seam to bind the overlock again. This adds strength but removes some of the stretch. A four thread is sometimes referred to as a "mock safety" overlock, for the similarity to a five thread overlock stitch. Four thread is used for the construction of most worn clothing, such as t-shirts, pants, and some undergarments. The four thread overlock uses two needles, as does the five thread.
The five thread overlock runs a safety stitch down the lips of the seam, further reinforcing the stitch. A five thread overlock has almost no give to it, but remains very strong. It is used for rugged clothing and stronger fabrics, such as denim and heavy nylon. It is not suited for general clothing construction due to this rigidity.
Home versus Industrial
Home overlock machines usually have a variety of stitch options and accessories, ranging from simple single or double chainstitching, to rolled hems and coverlocking. They are built with a more casual user in mind, sacrificing performance options for ease of use and general versatility.
Industrial overlock machines are built with raw output and quality as the primary features. Industrials only have one stitch - The knives cannot be disengaged, hems cannot be rolled, and the user cannot properly convert a five or four thread overlock to a three or four thread overlock. The machines are built with a production atmosphere in mind, and will withstand daily sewing for eight or more hours without breaking down constantly or requiring a large time investment for upkeep. They come with an automatic oil lubrication system, and sit on a large oil reservoir that needs to be filled once every four or six months, compared to the manual oiling of home machines that must be performed frequently. The stitch speed and quality are also increased. Home machines stitch at about 800 stitches per minute, while industrial machines sew at 6500 to 9000 stitches per minute. The stitch quality is preserved even at that speed, and will sew fragile fabrics without damage. The faster machines will incorporate a compressed air line, and an oil sprayer to keep the thread and needles cool so as not to burn the fabric. A differential feed is included on all industrial machines, as it becomes impossible to manually provide the proper stretch or bunching at speed. Higher end machines will include a walking foot to assist feeding. The power to the machine is from a separate, under-table mounted 1/3 or 1/2 horsepower electric motor that runs continuously, and a clutch is used to control the machine speed.
In addition to performance features, ergonomics becomes an issue on industrial machines. A person can only sew for so long on a poorly set up machine before fatigue sets in and affects product quality. With that in mind, all industrial machines come in three parts - the sewing head, the table, and the motor. The table is fully adjustable to provide a very comfortable fit to the user, and the machine is recessed into the table to provide a flat work surface. The two pedals that come with all industrial overlockers are completely adjustable in both position and ordering. One controls the machine speed clutch, and the other provides hands-free presser foot operation to allow easy positioning of difficult fabrics.
For other sewing-machine stitches see: Lockstitch.