Wednesday, May 25, 2011


Lockstitch sewing machine is very popular sewing machine and widely used in most of the conventional modern garments sewing system. The lockstitch machine is very easy to use but required more power to operate.

Principal feature and sewing elements of a flat-bed lockstitch sewing machine

The three-thread overlock sewing machine

Objectives of lockstitch sewing machine:
-Explain mechanization and automation relative to general- and special-purpose machines
-Examine the basic components of sewing machines and work aids
-Discuss the effect of equipment on product quality andperformance

Modern mass-production sewing requirements have resulted in many variations of the basic flat-bed lockstitch sewing machine. As we have seen, many of these developments are concerned with the form of the bed on which the material to be sewn rests. Cylinder-beds, post-beds, raised- and feed-off-the-arm beds have given rise to machines which differ greatly in appearance, although the actual stitch forming elements contained in these machines remain basically the same. These mechanisms may be grouped under one or other of the two main headings, Rotary hook or Oscillating Shuttle.

The principal features and sewing elements of a modern flat-bed lockstitch machine of the rotary hook type are as illustrated in the figure given below:

Principal feature and sewing elements of a flat-bed lockstitch machine

The primary components concerned with the formation of the stitch are:
(i) The needle,
(ii) The feed dogs,
(iii) Throatplate,
(iv) The presser foot,
(v) The rotary sewing hook, and
(vi) Bobbin and the bobbin case.

Each of these components is a precision product and for its proper functioning must be in perfect condition, with their positions accurately set and, in the case of moving parts, perfectly timed in relation to the movement of the other components.

Stitch forming action of the rotary-hook lock stitch machine:
Rotary-hook machine forms the plain stitch or simple two thread lock stitch by carrying the needle thread loop around a bobbin containing the under thread.

-Commencing with the needle at the lowest point of its stroke, the needle starts to raise the needle-thread which, being flexible, bulges out away from the needle to form a loop.

-The loop formed in the needle-thread is then entered by the point of the sewing hook.

-As the needle continues to rise and the hook progresses in its rotation, the needle-thread take-up arm provides sufficient slack thread to be drawn down through the fabric to increase the size of the loop.

-On its first rotation, the sewing hook carries the needle-thread loops around the bobbin case and bobbin, the inside of the loop sliding over the face of the bobbin-case whilst the outside passes around the back, to enclose the bobbin-thread.

-As the needle-thread take-up starts to rise, the loop is drawn up through the “cast-off” opening of the sewing-hook before the revolution is complete.

-During the second revolution of the sewing-hook the thread take-up completes its upward stroke, drawing the slack thread through the material and settling the stitch. Meanwhile, the feed dog has moved forward carrying the material with it and drawing the required length of under-thread from the bobbin.
-The presser-foot guards against the slippage by holding the fabric firmly against the teeth of the feed dog whilst the feed dog is carrying the fabric across the smooth face of the throat plat or needle plate.

Now, that it is clear to us that how a lock stitch is formed, there are many factors that can prevent the correct formation of the stitch. Namely they can be:
(i) Anything which inhibits the formation of the needle-thread loop,
(ii) Anything which prevents a correctly formed loop from being entered by the point of the sewing hook, or

(iii) Anything, which interferes with the free running of either, the needle-thread or the bobbin thread.

Apart from this there are other factors that influence the formation of a satisfactory seam, which are:
(i) The size and type of the needle and sewing thread,
(ii) The relative tensions in the threadlines, and
(iii) The type of fabric and the manner in which it is fed across the throatplate of the machine.
Characteristics of a lockstitch sewing machine:
(i) Uses the least amount of thread
(ii) Produces the flattest stitch
(iii) Gives the best hand
(iv) Allows stitches to blend into the fabric surface
(v) It is reversible and thereby it is used extensively for top stitching
(vi) Tightest and most secure stitch
(vii) Only stitch formation that can be back stitched
(viii) Has the least amount of elongation potential and thereby it is inappropriate to attach elastic or sew knit fabrics or bias seams that are expected to stretch.
This stitch is also known as overedge, edge-seaming or overseaming. It is used extensively in the making up of knitted goods and for serging woven fabrics in the tailoring industry.

In its basic form the stitch comprises three threads- a needle thread, and two looper threads. It is the needle thread which actually stitches the plies of fabrics together, whilst the looper-threads interlock with the needle thread above and below the fabric, and with each other on the trimmed edge of the fabric- for which purpose trimming cutters are provided. The interlocking of the looper-threads binds the trimmed edges of the fabric preventing fraying or unraveling.

The stitch has the advantage of very high extensibility. This property makes it eminently suitable for use in readily extensible fabrics, knitwear and the making up of tights.

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