This dial controls the tension of the upper thread. The tension of the thread will affect the quality of the stitches.
If the upper tension is too tight, loops will appear on the surface of the fabric therefore the tension is reduced by turning the upper tension-control dial to a lower number.
If the upper tension is too loose loops will appear on the back of the fabric therefore the tension is increased by turning the upper tension-control dial to a higher number.
The thread from the spool is passed from thread guide and then through the thread take-up lever. This lever controls the flow and movement of the spool thread.
During the threading of the machine, if the take-up lever is lowered the upper thread cannot be passed through the take-up lever. Therefore, one should raise the needle by turning the handwheel clockwise i.e., towards you so that the mark on the wheel points up.
The thread guide is used while winding the thread onto the bobbin and then in threading the machine. There may be one or more thread guides depending on the model of the machine.
The bobbin winder winds the thread into the bobbin for use as the lower thread (that is load thread from a larger spool to small round spool).
Spool pin are for thread spool to rotate freely during the sewing process. Depending on the model it can be horizontal or vertical in place. The thread spool sits right on the spool pin.
In modern machines an extra spool pin can be placed in the hole on the top of the machine so that a second spool of thread can be placed for sewing with the twin needle.
