Stitch formation

Sewing machines can make a great variety of plain or patterned stitches. Ignoring strictly decorative aspects, over three dozen distinct stitch formations are formally recognized by the ISO 4915:1991 standard (for a summary see [3], [4], or [5]), involving one to seven separate threads to form the stitch. Plain stitches fall into four general categories: lockstitch, chainstitch, overlock, and coverstitch (each described below).

Lock stitch is the familiar stitch performed by most household sewing machines and most industrial "single needle" sewing machines from two threads, one passed through a needle and one coming from a bobbin or shuttle. Each thread stays on the same side of the material being sewn, interlacing with the other thread at each needle hole. Industrial lockstitch machines with two needles, each forming an independent lockstitch with their own bobbin, are also very common. There are different types of lockstitch industrial machines. The most commonly used are the drop feed for light and medium duty, and walking foot for medium and heavy duty like the Class 7 with an impressive 3/4" foot lift. This makes the Class 7 able to stitch through heavy materials up to 3/4" with threads as strong as 57 lbs. Originally made by Singer in the US and Europe for supplying the demand of heavy duty clothing for the troops, for many years after the war this class was not available as new because the market was filled. With the outsourcing of many sewing manufacturing jobs, nowadays many Chinese Class 7 machines are available and built by Federal Specifications giving them equal performance as the original ones (FSN:3530-3111-1556, FSN: 3530-3111-3675, FSN: 3530-311-1556, FSN: 3530-3111-3075).

Chain stitch is less widely used than lockstitch, but it is preferred over lockstitch for applications like sealing bags of grain, garment seams likely to be altered, and as a "safety stitch" on serging machines. A chain stitch may be formed with either one or two distinct threads, one passed through a needle and the other, if used, manipulated by a looper, a device which moves back and forth but does not pass through the fabric. The needle thread is formed on both sides of the material being sewn, and on the bottom of the material either crosses through loops of itself (single thread) or loops of the second thread to prevent it from pulling back to the top of the material. Most household chainstitch machines are either very old, or toys intended for children. Industrial chainstitch machines are still heavily used in their application areas.

Lockstitch and chainstitch can be formed any distance from the edge of the material being sewn.

Overlock can only be formed at the edge itself, where one or more threads pass over the edge. Varieties of overlock stitch can be formed with one to four threads, one or two needles, and one or two loopers. Overlock sewing machines are usually equipped with knives that trim or create the edge immediately in front of the stitch formation. Household and industrial overlock machines are commonly used for garment seams in knit or stretchy fabrics, for garment seams where a clean finish is not required, and for protecting edges against ravelling. Machines using two to four threads are most common, and frequently one machine can be configured for several varieties of overlock stitch. Overlock machines with five or more threads usually make both a chainstitch with one needle and one looper, and an overlock stitch with the remaining needles and loopers. This combination is known as a "safety stitch". Household overlock machines are widely used.

Coverstitch is formed by two or more needles and one or two loopers. Like lockstitch and chainstitch, coverstitch can be formed anywhere on the material being sewn. One looper manipulates a thread below the material being sewn, forming a bottom cover stitch against the needle threads. An additional looper above the material can form a top cover stitch simultaneously. The needle threads form parallel rows, while the looper threads cross back and forth all the needle rows. Coverstitch is so-called because the grid of crossing needle and looper threads covers raw seam edges, much as the overlock stitch does. It is widely used in garment construction, particularly for attaching trims and flat seaming where the raw edges can be finished in the same operation as forming the seam. Machines with three needles are most common, and can be configured to use any two or all three of the needles. Machines with six or more needles are often used for applications like fastening elastic waistbands to garments. Household coverstitch machines are fairly rare, but are becoming more readily available.