principles of machine operation
The basic working principle of sewing machines is very simple: two threads, one needle, up and down, stitch formed. The sewing machines are made of many pieces which synchronously work together to form stitches. The mechanism of a sewing machine can be divided into the four areas:
1. Motive source
2. Stitch formation
3. Fabric movement
4. Thread control
1. MOTIVE SOURCE Whether hand driven or powered by an electric motor or treadle belt, the connection to the machine is the balance wheel. This serves to even out any ‘lumpiness’ in the machine to give a smooth action.
2. STITCH FORMATION
Needle movement: The rotating driveshaft in the arm of the machine connects to the needle bar through levers in the head to produce the up and down movement of the needle. The sewing machine needle is a highly sophisticated piece of design which we look at in detail on the following page.
Stitch making: The drive shaft has a vertical crank attached to it, going down under the base plate. Again, by a series of levers, this connects to a hook ring. The hook ring picks up the upper thread and guides it round the bobbin holding the lower thread. Using the up and down movement of the needle and the rotation of the hook ring the two threads are looped together to form the stitch.
The Needle: The needle used in most of the domestic machines we handle has the following features:
· A flat side at the top end to secure it to the needle bar. (Interlockers and some industrials use round needles).
· On the same side of the needle, a thinning of the needle just above the eye.
· On the opposite side of the needle, a groove runs from near the top, down to the eye.
When the needle pierces the fabric on the down stroke, the thread on one side will lie in the groove. When the needle starts to rise again, the thread in the groove rises with it, but on the other side, the thread is pinched between the cloth and the side of the needle this friction against the fabric causes the thread to be retarded and a loop is formed just above the eye of the needle. The upward stroke of the needle creates the crucial moment in the sewing cycle and, as you can see from the illustration, the action takes place beneath the cloth.
The Hook Ring: As the hook ring turns in its part circle, the hook picks up the loop made by the needle and drags it round the lower bobbin. The thread then slips off the hook and, as the needle bar rises, is pulled round the lower bobbin to make the stitch. Below, you can see that the bobbin thread has been encircled by the stitch loop formed by the top thread. This action in the stitch cycle is the most important combination of movements performed by the machine. And yet, at normal sewing speed, it only takes about a fifth of a second to complete, which is approximately the time required to wink your eye. Accurate synchronization (or timing) between the simultaneous tasks of stitch formation is essential for the machine to work. If you thread up a machine, and sew a few stitches slowly on the edge of a piece of cloth with the cover plate open, you can see this stitch formation for yourself.
|
Bobbin thread |
|
Upper thread |
|
Needle |
Figure 1. 5 hook ring. Photo credits; Cholla.mmto.org/sewing/operation
3. FABRIC MOVEMENT
At each revolution of the machine the teeth of the feed dog rises lightly above the base plate. The teeth grip the underside of the cloth and feed it forward a tiny measured amount deter-mined by the setting of the stitch length regulator. The movement of the feed dog is generated through yet more levers, from another vertical shaft working from the drive shaft.
Fabric control: The fabric is held tight during stitching by the presser foot squeezing the fabric against the teeth of the feed dog or the throat plate while the feed dog is below the plate. The amount of pressure can be regulated by the presser regulating screw on top of the head and released completely by raising the presser bar lifter.
4. THREAD CONTROL
Stitch tension: The tightness of the stitch is controlled by two different tension mechanisms:
(1) The bobbin has a small spring on its case, which delivers the thread under a slight tension.
(2) The main tension mechanism is on the upper thread, on the head of the machine. The thread passes between two saucer shaped discs. The pressure between the discs can be varied by the tension regulating thumb nut.
(3) The upper tension is released completely when the presser bar lifter is raised. The lifter presses on a pin which in turn pushes the discs apart. When the tension is released the fabric can be removed easily.
(4) To provide sufficient thread for this loop (not under tension) the thread take up lever on the head travels downwards, rising again to pull the stitch tight.
Thread take up spring: As the slack thread lever reaches the top of its travel, the feed dog is also moving forward to make the stitch. At this point the thread is under extra tension and to avoid snap-ping the thread at long stitch lengths - it is passed through the thread take up spring in the tension mechanism which cushions the strain.
Figure 1. 6 ; stitch formation. Photo credits; Cholla.mmto.org/sewing/operation