This invention relates to a sewing machine, and in particular to a mechanism for controlling the movement of the fabric feed dog.
More particularly, the invention relates to a motion conversion mechanism arranged to receive movement signals generated by a decoder unit situated in said machine and comprising a first element arranged to assume a plurality of working positions and a second element arranged to assume a first or a second working position, said mechanism being arranged to convert said movement signals into movements of a feed dog for the fabric, in such a manner as to cause the fabric to undergo a movement, the extent of which depends on which position has been assumed by said first element, and which is in either a forward or a backward direction depending upon whether the first or second working position has been assumed by said second element.
Motion conversion mechanisms of the aforesaid type which are known at the present time have certain drawbacks. Firstly, they are of low precision such that the transfer of movement from the decoder unit to the dog is excessively influenced by the various couplings between the intermediate mechanical members, with consequent inaccuracy in the positioning of the dog. A drawback of this type is particularly felt when the sewing machine is set for embroidery work, the result being that lack of uniformity is noted in the distribution of the stitches, caused by imperfect transfer of the movement commands for the dog. Such a drawback is most greatly felt when fabrics are to be sewn which do not respond in the same manner when subjected to forward and backward dragging movements by the dog. For example, if a button-hole is to be made, the number of stitches required for sewing one edge is not equal to the number of stitches in the opposite direction required for sewing the opposite edge, so that in the case of sewing machines which automatically sew the button-hole, the button-hole remains incomplete and the operator has to operate the machine manually in order to correctly finish the sewing operation.