FIELD OF THE INVENTION The invention relates to the technical field of two-thread knot stitch sewing machines, with special reference to those with so-called needle tip feed, intended for sewing leather or heavy fabrics.

DESCRIPTION OF THE PRIOR ART Among the machines with needle tip feed, those with dual fed are discussed, wherein the feeding of the material to be sewn is given by the combined action of the needle and of a feed foot, in cooperation with a leather-locking foot.

A machine like those mentioned, manufactured by the same Applicant, is illustrated by way of an example in Fig. 1 , and is especially suitable for sewing box soles in shoes and for sewing leather goods and the like.

In said machine 100, as in all the similar ones, needle A and feet P1 , P2 are vertical and stretch out from the bottom of head 1 of machine 100 towards an underlying device with rotating hook 2, commonly known as crochet which, in the specific case, is carried by an oscillating arm 3 with orientation means 4 arranged thereinbetween, suitable for maintaining the angular position of crochet 2 fixed, irrespective of the position taken by the oscillating arm 3; needle A is actuated with an up and down movement and, in suitable phase relation, with an oscillation movement on a vertical plane parallel to the plane of rotation of crochet 2, for the above material feeding. Also the feed foot P1 is actuated with an up and down movement by a mechanism M thereof of known type, and is constrained to needle A so as to follow the oscillation thereof; the leather-locking foot P2, arranged downstream of

the feed foot P1, is only actuated vertically, with a relative up and down movement determined by the same mechanism M in combination with a pneumatic jack S.

Figs. 1A, 1 B, 1C illustrate, according to arrow Z of Fig. 1 , mechanism M seated in the front side of head 1 , for actuating the feed foot P1 and the leather-locking foot P2; the same figures do not show the gears for moving needle A as they do not pertain to the invention.

Mechanism M, also visible in perspective in Fig. 2, is moved by a drive shaft B, actuated in rotation, which by an eccentric pin C, a first connecting rod D and a lever E, controls a secondary shaft F in alternating oscillation, to which an arm G is made integral.

The oscillation of arm G is transferred, by a second connecting rod H, to a square rocker arm L pivoted in intermediate position to a block N; rocker arm L is intended for actuating the upstroke and downstroke of bar Q which carries the feed foot P1 , with a small articulation rod Q1 arranged thereinbetween.

Block N is made integral to bar R which carries the leather-locking foot P2; the same bar R is elastically stressed downwards by said pneumatic jack S. Fig. 1A illustrates the return step of needle A and of the feed foot P1 (arrow 12), both raised from the material to be sewn, with the latter held in abutment with the plate of crochet 2 by the leather-locking foot P2, which is elastically pushed downwards by jack S.

Fig. 1 B illustrates the step wherein needle A and the feed foot P1 , at the end of the return stroke, are lowered, needle A for piercing the material and enter into crochet 2 and foot P1 for abutting the material; in this step, also foot P2 remains lowered abutted on the material, always pushed by jack S.

The downstroke of foot P1 is imposed by the rotation (in clockwise direction in the figures) of rocker arm L on the axis of fulcrum G1 , according to which the same rocker arm L is articulated to block N.

To carry out the next step, illustrated in Rg. 1C, wherein needle A and foot P1 carry the material (arrow T1), while the sewing threads are knotted by crochet 2, the leather-locking foot P2 must be raised.

The rotation of the eccentric pin C continues to move the arm G in the same direction (in counter-clockwise direction in the figure) that had made foot P1 move downwards, which cannot move down further since it already is in abutment on the material, and therefore forces rocker arm L to rotate according to the axis of the hinge G2 that connects it to the small articulation rod Q1 , thus determining the raising of fulcrum G1 , block N and, consequently, of bar R and foot P2, overcoming the elastic downward thrust given by the jack S.

At the beginning and at the end of the work, in order to introduce and extract the material, both the needle and the feet must be in a raised position; the needle is automatically stopped in raised position by a suitable controlled stopping device associated to the machine motor, whereas feet P1 , P2 are both raised by the jack S that, raising the bar R also raises the foot P2 and the block N, and therefore the rocker arm L and also the foot P1.

Mechanism M, thus configured, causes several problems resulting from the fact that the movements of the two feet P1 , P2 are interlocked. In the first place, the feed foot P1 must press on the material, pushing it on the plate of the crochet 2, for obtaining the resulting movement that allows raising the leather-locking foot P2, overcoming the elastic thrust given by the jack S; due to the pressure of the feed foot P1 , the material does not run smoothly on the plate of the crochet 2, making the feeding difficult. Moreover, an excessive pressure of the foot P1 could cause permanent marks on the leather or fabric that of course are unacceptable.

The disadvantages mentioned above are further increased when while sewing, there are considerable variations of the material thickness given by overlapped hems of flaps, tongues and the like, since in the mechanism M of the known type

described above it is not possible to change the initial setting, while working, according to the minimum and maximum thickness met.

A further disadvantage caused by the interlocking of the movements of the feet relates to the fact that the phasing between the downstroke of the feed foot P1 and the upstroke of the leather-locking foot P2 is a consequence of an interplay of thrusts and counter-thrusts, variable according to the material thickness and/or pliability and therefore quite uncertain, in clear disagreement with the mechanical needs for optimum machine operation. The above therefore makes it difficult to find a setting of the mechanism M which should be an acceptable compromise between the different needs; in fact, for such setting it is necessary to define the lever ratio between the first connecting rod D and the lever E, so that the movement of the arm G has a sufficient extent both for the first rotation of the rocker arm L, for bringing the feed foot P1 to impact on the material, and for the second one, for raising the block N and therefore the leather- locking foot P2; at the same time, it is necessary to adjust the angular position of the arm G on the secondary shaft F for determining the lowest height of the feed foot P1 , according to the material thickness, trying not to press thereon too much while obtaining a sufficient raise of the leather-locking foot P2, with the appropriate phase relative to the downstroke of the feed foot P1.

SUMMARY OF THE INVENTION

The object of the present invention therefore is to propose a foot control system which should be configured so as to make the movements of both independent, thus avoiding the problems described above and making the initial settings easier. Another object of the invention is to propose a control system which should be capable of allowing two settings of the feed foot, respectively for a minimum and for a maximum thickness of the material, with such two settings switchable to one

another, and vice versa, while working.

A further object of the invention is to propose a system which for its application should not require any modifications to the gears intended for actuating it, or to other nearby gears for space reasons.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will appear more clearly from the following description of a preferred embodiment, in accordance with the claims and with reference to the annexed drawings, wherein: - Fig. 1 shows a schematic side view of a sewing machine where it is possible to apply the subject control system;

- Figs. 1A, 1 B, 1C show, according to arrow Z of Fig. 1 , the known mechanism described in the introduction, in different operating moments;

- Fig. 2 shows a perspective view of the mechanism of Figs. 1 A, 1 B, 1 C; - Fig. 3 shows, according to arrow Z of Fig. 1 , the subject control system in non-operating position;

- Fig. 4 shows a perspective view of a part of the control system of Fig. 3;

- Figs. 5, 6, 7, 8 show, according to the same view of Fig. 3, the control system in as many operating moments.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to Fig. 1 reference numeral 100 denotes a sewing machine with dual needle tip feed of substantially known type, wherein main operating gears as well as the operation thereof have already been described in the introduction, with reference to Figs. 1 A, 1 B, 1C and 2.

The control system of the invention is intended for being applied on the same machine 100 instead of the known mechanisms, and is illustrated in Fig. 3 and in

the following ones.

Said system comprises first and second means M1 , M2, independent of each other, respectively provided for controlling said feed foot P1 and said leather- locking foot P2, in suitable phase relation with each other as well as with the up and down and oscillation movements of a needle A.

The first means M1, in a preferred embodiment, are moved by the same drive shaft B of the machine 100, by the eccentric pin C that actuates the mechanism M of known type described in the introduction. Similarly to said mechanism M, the first means M1 are provided with a first connecting rod 10, coupled at an end thereof to said eccentric pin C and connected, with the remaining end thereof, to a slotted lever 11 , keyed on a secondary shaft 12, which is thus actuated in alternating oscillation. The presence of the slot on the lever 11 allows the position of the hinge point between the same and the connecting rod 10 to be changed, if required, for increasing or decreasing the angular width of the alternating oscillation of the secondary shaft 12.

A radial arm 13 is also keyed on the secondary shaft 12, whose end is articulated to a second connecting rod 14, in turn articulated to a wing 15a of a square rocker arm 15, pivoted in intermediate position to a block 16. The remaining wing 15b of the square rocker arm 15 is connected to a bar Q which carries the feed foot P1 , with a small articulation rod 17 arranged thereinbetween.

The linkage described above in intended for determining the upstroke and downstroke of the feed foot P1 , in mechanical phase with the movements of the needle A.

A block 16 is slidingly carried by a pair of vertical stems 161 , 162 that erect from a bracket 160 attached to head 1 of the machine 100.

At the bottom thereof, a bracket 160 supports a pair of pneumatic jacks 18, 19,

respectively first and second, arranged with vertical axis and with the stems facing upwards, passing through respective holes obtained in the same bracket 160.

The first pneumatic jack 18 is dual effect and the relevant stem 180 is constrained to said block 16, whereas the stem 190 of the second jack 19 (which is simple effect) is intended for pushing, according to modes better described hereinafter, an abutment 191 , with adjustable height, associated to the block 16. The abutment 191 is defined, for example, by a threaded bar 192, engaged in a corresponding threaded hole obtained in an appendix 16a, the latter made integral to the block 16.

The threaded bar 192 is provided at the top thereof with an operating knob 193 and the height position thereof, relative to the hole it is engaged with, is lockable by a lock nut 194. The above components, associated to the block 16, are intended for defining, for said feed foot P1, two working heights W1, W2 respectively provided for a minimum and maximum thickness of the material to be sewn. The second means M2 consist of a third pneumatic dual effect jack 20, supported by the head 1 of the machine 100 with the respective stem 200 facing downwards and made coaxially integral to the bar R that carries the leather-locking foot P2. The phase actuation of the third pneumatic jack 20, in one direction or in the other, is controlled by means not shown since known to the man skilled in the art; such means consist, for example, of cams keyed on said drive shaft B that directly actuate microswitches, connected to the corresponding solenoid valves provided in the pneumatic control system of the same jack 20. Optionally, instead of the microswitches there can be provided as many proximity sensors, which detect the passage of suitable cam profiles and provide the control signal to said solenoid valves. The preliminary operations to be carried out on first means M1 for adjusting the

lowest working height W1 of the feed foot P1 according to the minimum thickness of the material to be sewn shall now be described.

In the first place, feet P1 , P2 must be brought in the respective position of maximum raising, so as to not interfere with the insertion of the material to be sewn laying on the plate of the crochet 2; to this end, a pedal or knee control is used, not shown, which determines the concurrent actuation of both the first and the third pneumatic jack 18, 20, respectively for raising the block 16 relative to the bracket 160, then the feed foot P1 , and the bar R that carries the leather-locking foot P2 (Rg. 3). By manually operating on the machine 100, the feed foot P1 is moved to the phase bottom dead centre thereof, then the clamp 130 locking the radial arm 13 to the secondary shaft 12 is released.

By turning knob 193, the threaded bar 192 is raised fully and then, the above mentioned control is released; with such operation, the first jack 18 is fed in an opposite direction, so as to lower the block 16 to the end of stroke, the second jack 19 is arranged with the respective stem 190 fully extracted but which, in any case, does not hit the abutment 191 , whereas the third jack 20, by the relevant phase actuation means, remains fed so that the leather-locking foot P2 remains in the raised position thereof. By manually moving the mechanical means that from the radial arm 13 reach the bar Q of the feed foot P1 , the height of the latter is suitably adjusted so that it slightly touches the surface of the material (Fig. 5). The clamp 130 is then tightened again for locking the arm 13 to the shaft 12. At this point, the highest working height W2 of the feed foot P1 is adjusted according to the maximum thickness of the material to be sewn.

To this end, the knob 193 is rotated so that the abutment 191 goes in abutment on the stem 190 of the second jack 19 and continuing the rotation, the block 16 and therefore the feed foot P1 raise; to this end, the thrust action of the second

jack 19, on the relevant stem 190, must overcome the force with which the first jack 18 pulls the block 16 downwards.

In the non-limiting construction solution shown, wherein the first and the second jack 18, 19 are similar, this does not happen, even in the presence of equal pressure of the air, due to the difference of the useful surface of the piston relative to the actuating direction; in said condition, in fact, the first jack 18 is fed on the stem side, whereas the second jack on the opposite side, and therefore the useful surface of the first one is smaller; of course, a similar result can be obtained with jacks of different sizes and/or with suitably different feeding pressures.

Foot P1 is raised above the working height W2 to be defined; then, a zone of material with the maximum thickness is arranged below foot P1 and the knob 193 is rotated in the opposite direction for lowering the same foot P1 again and bring it to slightly touch the material surface (Fig. 6). The highest working height W2 is thus defined.

As regards the leather-locking foot P2, no height adjustment operations are needed, according to the material thickness, since the direct pneumatic control, given by the third jack 20, is in se elastic, and thus capable of self-adjusting to the different working heights. The system is thus adjusted and it is possible to start to sew the material in the zones with maximum thickness, which are usually prevailing, with the foot at the highest working height W2 (Fig. 7); when zones with minimum thickness are to be sewn, the operator operates a special pedal or knee control that interrupts the air feed to the second jack 19. As the upward thrust given by the latter is not provided anymore, the first jack 18 prevails and pulls the block 16 downwards, bringing it to the end of stroke, and as a consequence the feed foot P1 arranges at the lowest working height W1 determined before (Fig. 8).

If the material to be sewn has a regular thickness, it is possible to carry out only the first part of the procedure, for adjusting the lowest position W1 of the feed foot P1 , with abutment 191 raised to avoid intercepting stem 190, after which it is possible to start to sew without further operations; in this case, the second jack 19 is non-operating.

At the end of the work, the pedal or knee control is operated again to concurrently raise feet P1 , P2, to allow the material to be removed.

In all the working situations described above, every time the feed foot P1 reaches the phase bottom dead centre thereof, it slightly touches the material without pressing it, so that the feeding of the same is not hindered and the material is not damaged.

This fundamental advantage, as compared to the known mechanism described in the introduction, is due to the fact that in the proposed system the actuation of the feed and leather-locking feet is independent. The advantage obtained with the above possibility of adjusting the feed foot on two working levels, which can be switched from one to the other and vice versa, while sewing, without stopping the machine, is important as well. The same feed foot P1 is in any case capable of carrying out the other functions, not mentioned so far but well known to the man skilled in the art, in particular for guiding the needle during the material piercing step at the beginning of each stitch and for holding the same material during the need extraction step at the end of the same stitch.

A further advantage, obtained with the independent actuation of the feet, is the possibility of calibrating the reciprocal phasing between the same only according to the optimum working conditions, without having to find difficult compromises according to the material thickness and avoiding that any variations of said thickness, while working, change the value thereof. In the proposed system, the preliminary adjustment operations are very simple

and quick, with clear advantages in terms of time.

Furthermore, it should be noted that the above embodiment of the invention allows the proposed system to be applied to existing machines, since the mechanical movement is taken by the same eccentric pin, on the drive shaft, which actuates the known mechanism and since the relevant components have such overall dimensions as to not interfere with other nearby gears.

Against the advantages obtained, finally, it is noted that the costs for manufacturing the system are comparable to those of the known mechanism.

It is understood, however, that the above is described by way of a non-limiting example and therefore any detail changes made to the system, for construction or functional reasons, are to be understood as falling within the scope of protection as defined in the following claims.