METHOD FOR PRODUCING A MODIFIED LOCKSTITCH, METHOD AND SEAM PRODUCED WITH SAID MODIFIED LOCKSTITCH AND SYSTEM FOR PRODUCING SAID MODIFIED LOCKSTITCH

Technical field

The present invention relates to a method for producing a modified lockstitch, a system for implementing said method and a sewing machine for producing said modified lockstitch.

The present invention has particular application in the production of seams with features identical to those of seams made with the lockstitch as defined by standards DIN 61400 of 1991 and ISO 4915:1991 , class 300 (stitch types from 301 to and including 312). That standard groups the stitch types into six classes.

The method currently used to produce a lockstitch provides for a concatenation of two threads together: the thread inserted by the needle into the fabric coming from above and wound in the spool positioned on the spool pin, with the hook thread coming from below and wound on the bobbin inserted inside the sewing machine.

The lockstitch is the most common, as it allows a type of universal stitching to be produced. Indeed, it is a simple, highly resistant stitch making it possible to produce a seam that is difficult to undo and inelastic, which makes the parts it unites joined very firmly together. The machines for producing it are well known all over the world and it can also be made in reverse, which is an important feature for automatically producing fixing stitches or locking stitches or backtacking. The top side and bottom side of the seam have the same appearance, which facilitates the packaging of some types of products that have both sides with visible seams. Unfortunately, however, as a bobbin has to be used for the hook thread, the duration of sewing is dictated by the size of the bobbin contained in the sewing machine, which cannot have unlimited dimensions. Furthermore, the location of the bobbin, beneath the sewing area, precludes the operator from having a continuous view of the quantity of thread available. The loop of thread inserted by the needle (needle thread) into the workpiece is bound to a second thread (bobbin thread).

The form of the resulting stitching is symmetrical and reversible, i.e. it can be performed either on one side or the other of the product with the same result. The execution of stitches can also take place in reverse. This latter feature is necessary, for example, for the execution of fixing stitches (or locking or backtacking) at the start and end of sewing, to prevent the stitches from coming undone.

It is the most versatile stitch and ensures a quite good hold, as well as the simplicity of construction of the machines that produce it.

The stitching is difficult to unravel and is less elastic than the chain stitch.

The needle thread is brought from the upper part of the leather into the lower one until reaching the bottom dead centre (PMI). When the needle starts to rise up again, a loop is formed which, by means of the hook, is interlocked with a second thread in the lower part (bobbin thread). Repeating this stitch (301 ) will form the seam.

The hook, whether oscillating or rotary, simply passes the lower thread through the loop formed by the upper thread, crossing the two threads at the centre of the fabric to be sewn. When the two threads are then pulled, they will be tightened to produce a stitch around the fabric.

During the formation of a stitch, the interlocking between the threads takes place exactly at the centre of the two layers of the workpiece, resulting in a seam that is identical on both sides of the product.

State of the art

The current technology for lockstitch formation is conditioned by the reloading length of the bobbin thread, which is a function of the limited diameter of the bobbin itself. This obliges the operator to reload the thread many times a day and entails costly reworking, because the bobbin thread loaded is not visible to the operator and work can be interrupted at any point. Furthermore, this method makes it obligatory to define a number of products that can be produced with each load of bobbin thread (e.g. footwear) and mentally keep count of them.

Some of these drawbacks have been partially overcome by the method described in patent US 6314899. The needle carrying the upper thread passes through the article until reaching the PMI. There it stops and begins to rise, thereby forming a loop. At this point, the oscillating hook starts up and inserts itself into the loop, enlarges it and feeds forward a Velcro thread. The hook moves back and leaves the Velcro thread in the loop while the upper thread is tightened over it, becoming engaged between the hooks, and tightens it against the added Velcro layer or strip. The blade cuts the segment of thread, forming a lockstitch that does not come undone. The lower surface is then coupled with a layer suitable for being coupled with the Velcro, to prevent the Velcro hooks from disturbing the fabric. The top side is aesthetically identical to the one produced with the traditional method, whereas the bottom side has improvable aesthetics. This solution eliminates the rotary hook, bobbin and bobbin case, but adds an oscillating hook (which also acts as a mechanism for feeding the lower thread) as well as a blade. This mechanism requires a larger stroke and more moving members, resulting in a slower production speed. Even though the lower thread has been eliminated with this method, the addition of the Velcro layer onto the article entails an increase in the cost of producing a seam.

With the method described in that patent, moreover, production itself becomes more complex, as there is an extra layer to be cut into a template and shaped so as to form the lower thread (Velcro) and three layers are sewn together rather than two. What is more, the lower part is worked with a material, Velcro, which requires large reels in order to have a longer lasting supply of the lower thread, which has a larger diameter and thus needs larger storage spaces, precisely because the materials are bulkier. Another method known in the prior art to produce lockstitches is illustrated in document EP 0 580 260, which envisages joining an auxiliary lower thread in a fixed positional relation to the lower fabric to be sewn, or to the main upper thread or to the same auxiliary thread by means of spots of glue, and fixing by thermal fusion using a laser technique or chemical agents or micro-injections of glue.

Object of the invention

The object of the present invention is to propose a method for producing a modified lockstitch and a seam produced with said stitch which overcomes the drawbacks encountered both with the traditional method and with the method proposed in the cited prior art.

Another object of the present invention is to modify the current lockstitch and the mechanism that produces it automatically, on the respective sewing machine, in order to replace it with a new type of modified lockstitch and a system for producing said stitch, implementable on machines already on the market and/or newly constructed ones, which eliminates the disadvantages of the current stitch-machine system and preserves intact the essential positive features thereof.

It is likewise an object of the present invention to propose a method for producing a seam with a modified lockstitch that eliminates the need to have to reload the thread many times a day or do reworking, as the bobbin is not visible to the operator.

Finally, it is an object of the present invention to propose a method for producing a modified lockstitch and the associated seam which avoids the use of additional materials besides the two threads necessary to produce the seam itself.

Yet a further object of the present invention is to provide a system for producing a modified lockstitch which is easily installable on the sewing machines already present on the market as well as newly constructed ones. These and other objects are substantially achieved by a method for producing a modified lockstitch, a method for producing a seam with said modified lockstitch and an associated system implementable on already existing machines and/or newly constructed ones, according to what is described in one or more of the appended claims.

Additional features and advantages will emerge more clearly from the detailed description of a preferred and not exclusive embodiment of a method for producing a modified lockstitch, a method for producing a seam with said modified lockstitch and an associated system implementable on already existing machines and/or newly constructed ones, according to the present invention.

A first aspect of the present invention envisages a method for producing a modified lockstitch, comprising the steps of making a needle (1 ), carrying a main sewing thread (2), or more simply a main thread, descend from a top dead centre (PMS) to a bottom dead centre (PMI) along the sewing direction; piercing with the needle a workpiece to be sewn, passing completely through the thickness of such workpiece, from the visible upper surface to the non-visible lower surface, in such a way that the tip of the needle and the main thread exit and pass beyond the lower surface of the workpiece, reaching a bottom dead centre; making the needle and the main thread rise up towards the top dead centre along the sewing direction; the main thread thus forms a loop below the lower surface of the workpiece; feeding an auxiliary thread through the loop while the needle and the main thread are rising, before they reach the top dead centre; continuing to raise the needle and the main thread towards the top dead centre to pull said loop upwards and tighten it against the auxiliary thread, which in turn is pressed against the lower surface of the workpiece. The auxiliary thread is shaped and subsequently cut into individual discrete portions so that at least one discrete portion of auxiliary thread is present in the lower surface, inserted within a respective loop (4), and that said discrete portion of auxiliary thread has, at least partially, portions extending transversely to the main direction of extension of the discrete portion itself, so that the discrete portion cannot be pulled out of the loop. The method for producing a seam with a modified lockstitch implemented in accordance with the steps described above further comprises the step of advancing the workpiece to be sewn by one pitch and repeating the steps in sequence to produce a plurality of lockstitches until the seam is complete.

In said method, the auxiliary thread is fed in a direction transverse to the sewing direction.

The auxiliary thread is shaped and cut at the same time or is first shaped and subsequently cut or else is already shaped (on a reel or the like) and is only cut in the machine. The step of shaping the auxiliary thread envisages the step of modifying the geometry of the auxiliary thread, preferably modifying the geometry of the ends of each discrete portion or each segment of cut auxiliary thread. It is likewise possible to deform or modify the geometry of the auxiliary thread, and in particular of the cut discrete portion or segment of auxiliary thread, at any point of the extent thereof, thus not necessarily at the ends, with the aim of preventing its disengagement from the loop produced with the main thread.

The auxiliary thread is made of a plastically deformable material such as, for example, nylon, plastic, or metal.

The step of shaping the auxiliary thread envisages the step of plastically deforming the ends of each discrete portion of cut auxiliary thread.

Another aspect of the present invention envisages a system for producing a modified lockstitch, comprising a thread-holding mechanism adapted to feed and retain an auxiliary thread, a cutting head adapted to section said auxiliary thread into discrete portions and a shaping head adapted to shape the auxiliary thread.

The shaping head present in the system is adapted to act on said auxiliary thread by simultaneously shaping a terminal end of a discrete portion of auxiliary thread and an initial end of the subsequent discrete portion of auxiliary thread.

The shaping head is positioned at the cutting head (22), so that shaping and cutting occur at the same time.

The shaping head is arranged upstream or downstream with respect to the cutting head, considering the feed direction of the auxiliary thread.

The shaping head is adapted to modify the thread geometry or deform it by squashing in discrete portions.

In another aspect of the present invention, there is also comprised a sewing machine comprising a system according to what is described and claimed.

In another aspect, the present invention envisages a modified lockstitch comprising a main thread passing through a workpiece and adapted to create a loop below the workpiece to be sewn, into which a discrete portion of an auxiliary thread is inserted; said discrete portion is tightened by the main thread against a lower surface of the workpiece.

Brief description of the drawings

This description is provided with reference to the appended figures, which also have a purely illustrative and hence non-limiting purpose, in which:

- figure 1 is a schematic view of a workpiece sewn with a lockstitch seam produced with a traditional method, according to what is known in the art;

- figure 2 is a schematic view of a workpiece sewn with a modified lockstitch seam produced with the method in accordance with the present invention;

- figures 3a and 3b are two schematic views of a portion of auxiliary thread in accordance with the present invention;

- figure 3c is a side view of the portion of thread in figure 3b;

- figure 4 is a schematic view of a workpiece sewn with a modified lockstitch seam, produced with the method of the present invention; - figures 5-8 are some of the steps of the method for producing a seam in accordance with the present invention, with a first example of a system for producing the modified lockstitch in accordance with the present invention;

- figures 9-12 are some of the steps of the method for producing a seam in accordance with the present invention, with a second example of a system for producing the modified lockstitch in accordance with the present invention.

Detailed description of preferred embodiments of the invention

A method for producing a modified lockstitch, the associated method for producing a seam made up of a succession of modified lockstitches and a system for producing said modified lockstitch, implementable on sewing machines already on the market and/or newly constructed ones, is presented and described below.

With reference to the figures described above, 1 indicates a sewing needle carrying, within the eye thereof located at the tip, a main sewing thread 2, or more simply a main thread, for sewing a workpiece 10, made of fabric, leather, or another material. The method of the present invention also envisages the use of an auxiliary thread 3, which is engaged inside a loop 4 created by the primary thread 2, below a lower surface 10i of the workpiece 10.

The sequence of steps for producing a modified lockstitch envisages making the needle 1 , carrying the main sewing thread 2, descend from a top dead centre TDS (figures 5 and 9) to a bottom dead centre PMI (figures 6 and 10). Therefore, starting from the top dead centre, the needle, carrying the respective thread, begins descending until it pierces the workpiece 10 to be sewn (figures 6 and 10). The needle 1 pierces the workpiece 10 to be sewn, passing completely through the thickness thereof, from the visible upper surface to the non-visible lower surface 10i, in such a way that the tip of the needle 1 and the main thread 2 exit and pass beyond the lower surface 10i of the workpiece 10, reaching the bottom dead centre PMI. At the bottom dead centre, the tension of the thread on the needle, generally exerted by the sewing machine, is completely released.

The needle 1 and the main thread 2 must then be made to rise again towards the top dead centre PMS. While passing from the bottom dead centre PMI and rising up towards the top dead centre PMS, the main thread 2 forms a loop 4 below the lower surface 10i of the workpiece 10 (figures 6 and 10).

During the time and along the path in which the main thread 2 carried by the needle 1 forms the loop 4, a thread-holding mechanism 21 feeds the auxiliary thread 3 forward until bringing it inside the loop 4 (figures 7 and 11 ).

After the tip of the needle has pierced and passed through the whole thickness of the workpiece 10, the thread-holding mechanism 21 also begins to move in perfect synchrony with the needle 1 ; these two movements are synchronised so that the auxiliary thread 3 begins to pass through exactly when the loop 4 formed by the main thread 2 is at the maximum of its diameter.

The auxiliary thread 3 passes through the loop 4 while the needle and main thread are rising, before the latter reach the top dead centre PMS.

Then, the needle 1 and the main thread 2 continue to be raised towards the top dead centre PMS to pull the loop 4 upwards and tighten it against the auxiliary thread 3, which is in turn pressed against the lower surface 10i of the workpiece 10. Raising the needle 1 , in fact, restores the tension that the sewing machine exerts on the main thread 2 and the loop 4 begins to tighten around the auxiliary thread 3 (figures 8 and 12).

Sewing direction means the direction of movement of the needle 1 , which draws along with it the main thread 2, thus vertical or orthogonal to the surface on which the workpiece 10 rests or extends. The sewing direction can also be identified as the direction along which an imaginary line located between the top dead centre and the bottom dead centre of movement of the needle 1 extends.

Sewing line, by contrast, means the line on the plane of the workpiece or fabric 10 containing the sewn stitches.

The auxiliary thread 3 is fed in a direction transverse to the sewing direction, along a plane substantially parallel to the rest surface for the fabric or workpiece 10 to be sewn. The feed direction of the auxiliary thread 3 is determined by the plane in which the loop 4 lies, as the auxiliary thread 3 is fed forward in such a way as to pass through the central axis of symmetry of the loop 4 formed by the main thread 2. The feed direction of the auxiliary thread 3 can thus be parallel or transverse to the sewing line, provided that the auxiliary thread 3 is always passing through the centre of symmetry of the loop 4 formed by the main thread 2.

The auxiliary thread 3 is then cut into segments and shaped in such a way that in the lower surface 10i at least one discrete portion 3d of auxiliary thread 3 is present, inserted inside a respective loop 4 (figures 8 and 12). The discrete portion 3d of auxiliary thread 3 has, at least partially, a shape such that it cannot be pulled out of the loop 4. Each discrete portion 3d can be compared to a hook that locks the main thread 2 in the stitch produced. A non-limited example embodiment of the shape of said hook is shown in figure 3b.

In this configuration, therefore, there is an auxiliary thread 3 that extends rectilinearly and is cut into discrete portions 3d. At each cut, the thread is plastically deformed so as to take on a pattern or shape transverse to the axial extension of the thread itself: in this manner the ends, deformed or pre-shaped transversely to the main direction of extension of the thread, interfere with the loop 4 which, by tightening around the discrete portion 3d of auxiliary thread 3, prevents the discrete portion from being pulled out.

Various other shapes are further possible for the discrete portion 3d, which must necessarily have a shape that deviates from the purely rectilinear one, in order to create mechanical interference with the loop 4 and avoid disengagement therefrom. Other solutions can, for example, envisage a discrete portion 3d that has, along the extent thereof, an undulating or a zig-zag shape, so that the curvilinear or angled portions create an interfering play with the loop 4 of the main thread 2, thereby maintaining the connection and avoiding reciprocal disengagement. If the auxiliary thread 3 is pre-shaped with an undulating or a zig-zag pattern, or in any event a non-rectilinear pattern, the ends may also not have plastic deformations. What is fundamental is to have a discrete portion 3d that is not only rectilinear but also has portions (for example end or central portions) with a different extension from the main direction of extension of the discrete portion, such as to create the interference that is necessary to prevent its disengagement from the loop 4.

The modified lockstitch obtained with this method thus comprises a main thread 2 passing through a workpiece 10 and defining a loop 4 below the workpiece. The stitch is characterised by the presence of a discrete portion 3d of the auxiliary thread 3, which is inserted into the loop 4 and tightened by the main thread 2 against a lower surface 10i of the workpiece 10.

The method for producing a seam using the modified lockstitch envisages the steps described previously. Furthermore, to produce a seam, after the first stitch has been sewn, the workpiece must be made to advance in order to thus be able to proceed to produce a new stitch. By repeating all the steps, including advancing the workpiece by one pitch, the desired stitching is obtained every time after a modified lockstitch has been produced.

The auxiliary thread 3 is fed in a direction transverse to the sewing direction. In particular, the auxiliary thread 3 is fed in such a way as to pass through the centre or through the axis of a hypothetical circle formed by the loop 4. Furthermore, the auxiliary thread is cut and shaped at the same time or is first shaped and subsequently cut.

The auxiliary thread 3, therefore, can be cut and shaped simultaneously with the cutting directly in the machine or else it can be shaped in the machine shortly before the cutting, after being unwound from the reel, and then cut in the machine during the sewing process.

Furthermore, it is likewise possible to use an auxiliary thread already shaped during the production thereof (found on the market) and cut in the machine during the sewing process.

The auxiliary thread 3 can be shaped by modifying the geometry of each discrete portion 3d of auxiliary thread 3 cut. It is possible to deform or modify the geometry of the ends of each discrete portion or it is possible to deform or modify the geometry of the auxiliary thread, and in particular of the segment defined by the discrete portion that is obtained by cutting the auxiliary thread, at any point of the extent thereof, with the aim of preventing its disengagement from the loop produced by the main thread. The auxiliary thread 3 can also be shaped by deforming the ends of each discrete portion 3d of the auxiliary thread 3.

Preferably, the auxiliary thread is made of a material such as to be able to be plastically deformed, such as, for example, plastic, nylon, metal, etc.

The method envisages that the cutting may occur midway along the already shaped central portion of the auxiliary thread, so as to simultaneously create both a terminal end 3t of a portion of auxiliary thread already inserted into the loop 4 of the stitch that is being produced, and an initial end 3i of the subsequent portion of auxiliary thread 3 which must be inserted into the loop of the subsequent stitch (figures 9-12). Alternatively, the cutting can occur at only one end, the terminal one 3t, as illustrated in figures 5-8.

During the same path as in the previous step, the needle will continue to tighten the main thread 2 around the auxiliary thread 3 and the discrete portion 3d of the auxiliary thread 3, at the moment it is formed and cut, by virtue of the shaped or deformed ends, will be tightened all the way against the lower surface 10i of the workpiece 10 to be sewn, thereby locking the main thread 2 in position, and preventing the stitch from being pulled out. The needle 1 then continues its upward stroke, outside the workpiece 10, until reaching the top dead centre PMS, while the article will advance by one pitch (pitch whose length is established in the initial phase of adjusting the sewing machine, before work begins). By repeating the process, one obtains the seam.

The present invention also relates to a system 20 for producing a modified lockstitch, applicable and installable on a sewing machine already on the market, in order to be able to produce a modified lockstitch in cooperation with the traditional elements already present on a sewing machine. This system replaces the current hook devices. The system 20 comprises a thread-holding mechanism 21 adapted to feed and retain the auxiliary thread 3, preferably fed from a reel, not illustrated in the appended drawings. The system 20 also comprises a cutting head 22 adapted to section the auxiliary thread 3 into discrete portions 3d and a shaping head 23 adapted to shape the auxiliary thread 3.

The shaping head 23 is adapted to act on an area of the auxiliary thread 3 by simultaneously shaping a terminal end 3t of a discrete portion 3d of the auxiliary thread 3 and an initial end 3i of the subsequent discrete portion 3d of auxiliary thread 3.

The system can envisage a shaping head 23 positioned at the station of the cutting head 22 so that the shaping and cutting occur at the same time in the same station.

Alternatively, the shaping head 23 is arranged upstream or downstream of the cutting head 22, considering the feed direction of the auxiliary thread 3.

The shaping head 23 is adapted to modify the thread geometry or deform it by squashing in discrete portions.

The thread-holding mechanism 21 (on its own and/or with the contribution of a fixed part of the sewing machine) will squash the auxiliary thread 3, transforming the cross section thereof into a figure similar to a rectangle, exactly over the terminal end 3t of a discrete portion 3d of the auxiliary thread 3 formed in the present cycle and the initial end 3i of the discrete portion 3d of the auxiliary thread 3 that will be formed in the subsequent cycle. The mechanism will then cut it in the midpoint of the rectangle itself. After the cutting and shaping, the thread-holding mechanism 21 (holding the auxiliary thread) will stop and wait for a new cycle. The discrete portion of thread will remain in this position, at 0.5-1 .0 mm from the lower surface 10i of the workpiece 10. Another dedicated mechanism will maintain the discrete portion 3d of the auxiliary thread 3 in position until it is tightened against the workpiece 10. The needle will instead continue its upward stroke, outside the workpiece, until reaching the top dead centre, thereby tightening the discrete portion 3d of the auxiliary thread 3 against the lower surface 10i of the workpiece 10 to be sewn. At this point, the workpiece will advance by one pitch.

The advancement of the auxiliary thread 3 is generated by suitably constructed gears located on the thread-holding mechanism 21 itself.

The auxiliary thread 3 must cross the loop until the main thread 2 is located at the midpoint of the total length of the discrete portion 3d.

The auxiliary thread 3 is preferably a deformable or at least partially deformable thread, or one that has portions such as to define areas with larger overall dimensions, in order to be able to create the initial and terminal ends of the discrete portion 3d and prevent the individual discrete portion 3d, or hook, from being pulled out of the loop 4 and disengaging from the main thread 2.

Purely by way of non-limiting example, a material that could be used as an auxiliary thread 3 is a plastic or nylon thread with a cylindrical shape, of any colour, and with a diameter ranging from 0.2 mm to 1 .2 mm. However, other materials may be used, provided that they enable the creation of portions with larger overall dimensions, obtained for example by shaping or deforming the thread itself.

The seam that is obtained with this method appears, on the upper surface 10s, aesthetically identical to the one obtained with the traditional method; on the lower surface 10i, by contrast, it appears with a plurality of discrete portions 3d arranged substantially parallel to one another, aligned along the sewing trajectory and each arranged transversely to said sewing trajectory.

The above-described system can also be implemented on already known sewing machines available on the market. In fact, it can be positioned below the sewing plane, where the hook is normally housed.

The movements of the sewing machine are generated by an electric motor. The mechanisms in the upper part (needle, etc.) are controlled by a camshaft and those in the lower part (thread trimmer, hook, presser foot, feed dog, etc.) through a shaft that draws rotational motion from the camshaft through a pulley.

One operative technical solution envisages the use of the same lower shaft to move all the mechanisms necessary for producing the modified lockstitch.

This will give rise to a working system that is continuous, with no interruptions, because the main thread carried by the needle will be fed from entire spools of various colours, whilst the auxiliary thread, for generating the hooks or discrete portions, will be placed on reels with several thousand metres of length and in any case always visible and under the operator’s control.

The method for producing a modified lockstitch, the method for producing a seam with said modified lockstitch and the system for producing the lockstitch of the present invention reach the proposed aims and achieve considerable advantages.

Compared to the current methods, the method described herein makes it possible to completely eliminate the lower thread, the bobbin and the hook, in addition to eliminating the bobbin rewinding mechanism. The elimination of the lower thread means having to change only one thread (and not two) every time the colour of the article to be produced changes.

For the formation of the hooks or discrete portions, a large reel of thread is used (as a supply unit that would last for months and be within the operator’s view), so a continuous thread is worked with. This would make it possible to eliminate the frequent sewing interruptions due to the limited length of the bobbin thread, it would eliminate the presence of the lower thread, thus also eliminating the problem of the colour tone between the two threads used, it would no longer be necessary to reload the bobbin, thus obtaining a reduction in production costs (elimination of bobbin replacement downtimes, reduction of sewing times, elimination of reworks, reduction of thread changes, etc.). A sewing machine featuring this system for producing the modified lockstitch is simplified, more efficient, less costly and easier to maintain.