Adjusting the Curved Needle Sewing Machine

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by J.D. Belcher

(submitted by Dave King)

Originally published in Work Magazine, the illustrated weekly journal for mechanics. January 4th 1902


{Click on thumbnails for larger images)

The curved needle lockstitch sewing machine is used extensively in the manufacture of shirts, collars, cuffs and white goods generally. It has very few parts and consequently runs very smoothly, being easy and silent. It consists of a curved arm A (Fig.1), carrying an upper needle, another arm B having a presser foot to hold the fabric firmly; a rotating hook C, recessed to receive a spool carrying the under thread; and a feeder to produce the required length of stitch. The whole is operated by an eccentric cam D situated under the cloth plate E.

Curvedneedle fig1.jpg

Fig. 2 represents a hook for a curved needle machine. The groove A is about 7/8in.long, begins about 5/8in. from the extreme point, is about 1/8in. wide and 1/16in. deep, gradually tapering and narrowing till it reaches the circumference; at a short distance (B) beyond this the hook is chamfered. C represents a recess which admits a metal spool holding the under thread. A bridge D from the throat to the heel of the hook acts as a guard and prevents the needle springing on to the hook point. E is a washer or back wall of the hook, and reference to it will be made later.


The stitch differs from the usual lock stitch, and a knowledge of its formation will greatly aid the mechanic in adjusting this class of machine. The needle carrying the upper thread descends to its lowest point of travel, then rises slightly, forming the thread in to a loop, into which the point of the rotating hook enters (see Fig. 3). As the hook revolves, it makes the loop larger by drawing more slack; continuing its movement the thread is drawn into the groove A (Fig. 2) until it reaches the chamfered part B. At this point one line of the thread slips off and falls over the outer side of the bobbin or spool (the other line of the thread lies at the rear). The loop thus passing round the bobbin interlocks the under thread and forms a lockstitch. Now it can easily be seen that when the hook has made rather more than half a revolution the loop would slip off the hook and become uncontrollable, hence it is necessary to hold it until the hook has completed its action. For this purpose a small brush A (Fig. 4) is fixed so as to press slightly against the hook and thus retain the thread until the hook point has entered the loop made for the succeeding stitch, at which time the thread is released owing to the bevelled edge of the hook passing the brush and making an exit for it. At this point (see Fig. 4) there remains a long loop to be drawn in to the fabric. In most machines a take-up lever is attached for this purpose, but here the loop is drawn in to the goods by the action of the hook. After the hook has entered the loop, formed by the rising of the needle, it enlarges it, and in so doing draws up the slack of the previous stitch and locks it (see Fig. 4). The dotted lines indicate the upper thread, the full lines the lower or bobbin thread. The illustration represents the locking of the stitches in the centre of the material, and the hook just entering the loop is about to draw the previous stitch tight in to the fabric. Evidently each stitch is formed partly from the former stitch, but with the addition of some thread drawn from the upper reel. On this upper reel there must be a certain amount of tension or the hook in its action will draw thread from the reel instead of drawing up the previous loop. Therefore, a tension pulley is operated by a spring, and great care should be exercised in adjusting the tension and pressure of the brush against the hook.

Curvedneedlef234.jpg

To set a needle, loosen the needle yoke by means of a wrench; place the needle shank through the small hole in the needle yoke, having the long groove to the left hand; then screw the yoke up firmly. See that the needle passes through the centre of the hole in the needle plate; if it does not, bend it until it does. Turn the wheel until the hook point reaches the needle, when the eye should be 1/6in. below the point of the hook. However, in some machines the eye of the needle should be set level with the washer or back part of hook when the point of the hook is just up to the needle (see Fig. 2). When turning a square corner, raise the needle till the eye is nearly out of the goods, draw the thread upwards from the eye o the needle until quite tight, raise the presser foot, and turn the material to the desired angle and proceed to sew.


To time the hook, the instruction book directs that the needle be set so that when the point of the hook just reaches it, the centre of the eye will be level with the top edge of the back part of the hook (see Fig. 2). Thread the machine with cotton and have the stitch about 1/20in. long, and begin to sew on two thicknesses of unbleached calico. Turn the machine with the hand and stop it the moment the loop is cast off the hook on to the bobbin.
Curvedfig5.jpg
Raise the cloth presser and see whether the eye of the needle is 1/16in. above the cloth. (This is how it should be.) If it is too high, lower it half the distance it is too high, and again, time the hook by the needle; if the needle is too low, raise it half the distance and re-time the hook.


The Short lever F (Fig. 1) determines the stroke of the needle arm, and its correct position is as follows:- When the arm is at its lowest point, the centre of the needle clamp should be 5/8in. from the cloth-plate, and at its highest point barely 2 3/16 in. from the cloth-plate. The needle clamp should move about 1 1/2in. from highest to lowest position. A handy gauge (Fig. 5) for testing these distances can be made from sheet steel, 3in. by 3in. by 1/16in.


To regulate the tension of the under thread, as no special device is fixed, the thread should draw from the spool quite freely, the spool, when unwinding, turning towards the operator. The spool is held in position by a ring slide G (Fig.1) which is adjustable. If the spool has too much play, the upper thread, instead of passing to the front side of the bobbin, will fall inside and break; in such a case, remove the ring slide and screw in the adjusting screw H a little. Care must be taken not to set the ring slide too tight against the spool, or it will impede its free movement, which is imperative to the perfect working of the machine.


To set the brush according to the instruction book, make the stitch 1/20in. long. Use two thicknesses of unbleached calico only. Set the segment which holds the brush about 1/8in. above the lowest point to which it can go, and so that the left edge of the brush will be even with the point of the hook. Set the brush so that the lower two thirds will touch the hook; but it must not be pushed so hard against the hook as to divide the bristles nor press against the chamfered part of the hook. After the brush straightens the loop across the bobbin, the needle arm should descend 1/32in. only. If it does not, lower the segment. The thread must be perfectly free from the brush when the point of the hook has entered the new loop 1/4in.