Whether the needle groove of the sewing machine is facing in or out

The short grooves are facing inward to the right, and the long grooves are facing outwards to the left.
The long groove is the groove from the bottom of the needle handle to the needle hole. It is in the direction of the needle threading. It is generally as wide as the needle hole (40% of the needle body diameter). The width and depth of the long groove are larger than the diameter of the suture, so that the suture can slide more freely after being inserted into the long groove.
The function of the long groove is to bury the suture in the groove so that the suture does not rub against the sewing material. After penetrating the sewing material, when the thread loop is hooked and expanded, the long groove protects the high friction of the needle thread against the sewing material.
The short groove is located on the side of the needle facing the hook, and its groove width and groove depth are smaller than the diameter of the suture.
Its main function is to insert the suture into the groove when the needle pierces the sewing material, and fix the position of the suture when it passes through the sewing material. When the needle is raised, because the suture is not fully embedded in the groove, there is a certain friction between the suture and the sewing material, which can promote the formation and stability of the needle loop.
Generally, the needle of the lockstitch sewing machine is a single-slot type; the double-thread chain stitch sewing machine uses a double-slot machine needle. The double-slot machine needle has a second long slot facing the hook side, generally starting from the bottom of the cone to the top of the needle hole. The second groove is slightly shallower than the first.