Another type of loom employed in the making of fancy goods is what is known as the overshot loom. It is used for the introduction of a silk weft figure effect, and is probably the most pronounced form of elaboration introduced. It differs from the old rise and fall method in the economy of operation. The overshot continues to weave the body of the goods right along while the auxiliary shuttle is putting the silk figure in at the same time. Not only is it economical in the respect of greater yardage, but the method employed in binding the figure limits the use of silk to the actual figure displayed, and does not carry the silk, which is the most expensive material in the fabric, to the extreme selvage at every pick, as is the case where the rise and fall method is employed.
In the overshot system a specially designed dobby, shown at Fig. 3, is used for operating the lightly weighted threads of the binder warps. Two pairs of knives are employed, one of each pair operating far enough to raise the threads used in the binder warp to the level of the top main shed, while the other one of each pair carries the threads which are used for figure purposes to a higher level, so that the overshot shuttle may pass under them. This occurs every alternate pick of the loom, the body shuttle making two picks while the upper or overshot shuttle makes only one.
In levelling the harness, setting or timing of the loom, and making the shed for overshot work, the plans followed are identically the same as in ordinary single shuttle work, as the upper shuttle and upper shed are distinctly auxiliary and subordinate to the main shed. The binder warp, being necessarily but lightly weighted in its relationship to the upper and lower cloths it is binding together, allows for the figure threads to be strained out of their normal position, so that the upper shuttle may pass under them. In order to conform to this strained position of the binder figure threads, the upper shuttle must be acutely pitched downward at the nose so as to get a good clearance, and thus avoid any binding in its passages through the shed. This peculiar downward pitch of the shuttle is very important and cannot be over emphasized. It is shown in Fig. 4.
The overshot dobby is so constructed that a different set of draw knives operate on each alternate pick of the loom, one on the binder lift and the other on the rubber lift. This not only allows for a silk figure made with the shuttle but affords facilities for the introduction of a warp figure also, a combination which can often be made very effective, as shown in Fig. 5.
Fig. 4.—Showing Auxiliary Shed and Pitched Shuttle as Used in Overshot Work
Importance of Dobby Harness
Too much importance cannot be attached to the rigging of the dobby harness. A 30/9 ply linen cord is desirable and a lingo of about 16 to the pound. After deciding on the character of the tie-up required, and when the harness has been threaded in the compart boards, the lingoes should be looped on the strings, and then left to stand and settle for a couple of days before leveling. It is better still to run the dobby machine for a few hours, lifting all the harness and then dropping them, so as to settle the strings and take out any kinks or loose places which are bound to exist in a highly cabled linen cord of this character.
The labor required in the tying up and leveling of a string harness suggests the advisability of great care in determining the tie-up to be used, so that changes of pattern can be made easily without involving changes in the tie-up. In order to prolong the life of the harness, in the adjustment of which so much time and care must necessarily be spent, it is advisable to apply a dressing of boiled linseed oil, which should be thoroughly worked into the strings by running the harness for several hours, using one and one change cards. This should be followed by a dusting of talc or soapstone, which will add much to the smoothness of the finish.