Sewing machine thread chain cutter

A thread cutting device for industrial sewing machines having an elongated, unitary body, a compressed air passage, a simple vent hole and a cutting blade aligned parallel to the elongated body and pivotally mounted for cooperation with a fixed cutter blade.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The present invention relates to a thread-cutting device which has an 
improved structure and which is designed in particular for use on 
industrial sewing machines, for example sewing machines for producing the 
so-called overage stitch. 
DESCRIPTION OF THE PRIOR AND RELATED ART 
A device similar to that described and claimed in the present application 
can be found in Applicant's copending U.S. application Ser. No. 725,072. 
As is known, the thread-cutting devices which are commercially available 
nowadays come in various forms: some suck in the thread chin to be cut by 
means of a vacuum pump; others, however, obtain this suction effect by 
means of a Venturi tube, i.e. a shaped tube into which pressurized air is 
introduced and from which there extends, laterally to the direction of 
flow of the air, a suction mouth facing the fabric being sewn. 
Moreover, some thread-cutting devices are arranged along the working 
surface of the sewing machine, below the surface over which the fabric is 
fed, whereas others are arranged on one edge of the working surface and 
have a suction mouth which is raised relative to the said working surface. 
The thread-cutting devices which use a Venturi tube to suck in the thread 
chain to be cut and which, moreover, are arranged on one edge of the 
working surface, with the suction mouth in a raised position, are those 
which provide the best results in the opinion of the Applicants 
themselves. 
In fact, these devices are able to obtain a powerful suction effect using 
very simple means, and the compressed air can also be used, via a suitable 
external vent, to control directly the movements of the thread chain and 
blow back the latter on the fabric being sewn so as to fix it by means of 
the stitches themselves. 
Moreover, the fact that the device is positioned on the side of the working 
surface, instead of being embedded in the latter, prevents the fabric 
becoming caught up during sewing. 
However, even the thread-cutting devices of the said type, i.e. those which 
use a Venturi tube, possess drawbacks: in particular, the said devices are 
complex and costly to manufacture. In practice, to achieve a good suction 
effect in the case of a particularly wide suction mouth, the internal vent 
or nozzle from which the pressurized air is emitted must be given a 
complex shape. 
In the known embodiments, this internal vent consists of an appropriately 
shaped small cylinder which has inside it a passage for the pressurized 
air, which in turn emerges on a generatrix of the cylinder itself. 
The thread-cutting device must therefore be made from several elements 
which are joined together in succession. 
Until today, it appeared that this situation could not be remedied, since 
the size of the mouth is a decisive factor for ensuring insertion of the 
thread chain to be cut and cannot be reduced. 
Cutting is performed by means of a cutting element or blade which is able 
to swing angularly between a position where it is partially embedded in 
the device and a position outside the latter. It must also be pointed out 
that the said blade of the thread-cutting device in question needs to be 
frequently checked and serviced since it can easily accumulate fine dust 
particles produced as a result of cutting the thread chains as well as the 
fabric in general. These fine dust particles can hinder the movements of 
the blade and in particular can prevent it assuming its closed position 
inside the thread-cutting device. 
Finally, it must be pointed out that, in some cases, the projecting 
position of the thread-cutting device relative to the working surface can 
give rise to problems: consequently it appears suitable to position the 
thread chain cutter so that its suction mouth is flush with the working 
surface therefore appears to be preferable. 
SUMMARY OF THE INVENTION 
In view of the above, the general object of the present invention is to 
provide a new thread-cutting device which, on account of its improved 
structure, overcomes substantially the abovementioned drawbacks. 
Within the scope of this general object, an important object of the present 
invention is to design a very simple device which can be easily 
manufactured, at a low cost, by the industries in the sector. 
These and other objects which will become more apparent below are achieved 
by a thread-cutting device with an improved structure, intended in 
particular for industrial sewing machines, of the type comprising a rigid 
casing fixed on one edge of the working surface of the machine in a 
position adjacent to the fabric being sewn, a discharge tube extending 
from one end of the said casing to a suction mouth located on the latter 
and facing the said fabric, a blower apparatus supplied with compressed 
air and comprising at least one internal vent located in the said suction 
mouth and designed to direct the compressed air along the said discharge 
tube, and a cutting element which is able to swing to and fro and is 
arranged in the region of the said mouth, wherein the said blade element 
consists of a blade which is hinged with the said casing and mainly 
extends in a substantially straight line, and wherein the said blade, when 
in the closed position where it is fully inserted inside the said casing, 
is arranged so as to block a substantially large part of the said suction 
mouth.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the abovementioned figures, the thread-cutting device 
which is the subject of the present invention is indicated in its entirety 
by the reference number 1. It is applied to one edge of the working 
surface 3 of an industrial-type sewing machine 2. As shown in FIG. 2, the 
thread-cutting device 1 is immediately adjacent to the needle plate 4 
provided with a tongue 5 on which the thread chain 6 to be cut is 
initially formed. 
During sewing, the fabric 7 moves forward in the direction of the 
thread-cutting device 1. 
The thread-cutting device 1 consists, in outline, of a casing 8 provided 
with a discharge tube 9 which extends from a first end 8a of the casing 8 
to a suction mouth 10. Pipe 11, which is connected to a source of 
compressed air at one end, is provided with an internal vent 16 at its 
other end. The internal vent 16 points toward the end 8a in order to send 
a compressed air jet along the discharge tube 9 and away from the suction 
mouth 10. The compressed air tube 11 is formed in the thickness of the 
lower wall in the discharge tube 9, but its acutal location is not 
important for the purposes of the present application; what is important, 
on the contrary, is the fact that the casing 8 is of one piece 
construction. Therefore the compressed air tube 11 formed on the casing 8 
starts in the region of the rear end 8a, where it is operatively connected 
with any known compressed air generator, as hereinbefore specified, and 
debouches into the discharge tube 9 with the vent hole 16. 
The effect of the rapid expansion of the compressed air coming out of vent 
16 results in a venturi effect which in turn creates a vacuum, in the 
region behind suction mouth 10. This suction causes the thread chain to be 
drawn inside discharge tube 9 as soon as it is opposite suction mouth 10. 
Once the thread chain has entered suction mouth 10 it is subject to the 
blast of compressed air coming from vent 16 and blown toward the other end 
of discharge tube 9. 
A cutting element 12, which pivots on a pin 13 and acted on by a 
compression spring 14, is able to swing from a closed position where it is 
partially inserted inside the casing 8 into an open position indicated by 
the dot-dash line in FIG. 3. 
Said suction mouth 10 comprises a first slot or zone 10a which extends 
substantially horizontally and parallelly to the direction in which the 
sewn fabric 7 moves forward (arrow A). The upper edge 10c delimitating 
said slot is suitably sharpened so that it defines the cutting edge of the 
stationary blade which, together with the movable blade 12, constitutes 
the cutting mechanism of the thread chain cutter 1. When the thread chain 
6 is sucked into the casing 8 through the suction mouth 10, said thread 
chain enters the slot 10a because it is brought close thereto by the 
fabric 7 moving forward and it is therefore cut by said cutting mechanism 
due to the fact that the movable blade 12 comes into contact with the 
cutting edge 10c. But the suction mouth 10 also comprises a second wide 
opening or zone 10b which widens out, starting from the lower edge of the 
slot 10a, towards the front end 8d of the casing 8 and in an upward 
direction so that it extends higher than the cutting edge 10c and can form 
the additional always open opening above the upper edge 12a of the movable 
blade 12, which additional opening is the one that allows the thread chain 
cutter 1 to suck in also the fine dust particles present in the 
environment surrounding said cutter. Also, a small rod 15 can be 
positioned over the suction mouth 10 so as to prevent the fabric being 
accidentally sucked into the mouth itself. In an original manner, the 
blade 12 mainly extends in a substantially straight line and is hinged 
with the casing 8 so that, when it is in the said closed position where it 
is fully inserted inside the casing itself, a substantially large portion 
of the suction mouth 10 is blocked. 
FIG. 3 shows that, in the embodiment shown, the blade 12, when in the 
closed position, not only completely blocks the first zone 10a of the 
suction mouth 10, and hence cuts the thread chain in conjunction with the 
cutting edge 10c, but also blocks part of the second zone 10b of the 
suction mouth itself. 
Advantageously, in this arrangement, the said, internal vent consists of a 
hole 16 located at the rear end of the suction mouth 10 and moreover, 
advantageously, the casing 8 consists of a body made in a single piece. In 
practice, the casing 8 is shaped so as to define in itself the discharge 
tube 9 located at the exit end of casing 8, the compressed-air supply pipe 
11, the suction mouth 10 and the hole 16. FIG. 4 also shows that the 
casing 8 has a widened recess 17 designed to accommodate, with play, the 
blade 12. Moreover, the blade 12 has a beveled cross-section 18 
corresponding to the widened recess 17: together the widened recess 17 and 
beveled cross-section 18 define a through-opening in the direction of the 
suction mouth, which is designed to prevent fine dust accumulating in the 
region of the blade 12. 
Moreover, the latter is sharpened in the upper zone which makes contact 
with the casing 8. Operation of the thread-cutting device is as follows: 
In a manner known per se, the thread chain 6 is formed on the tongue 5 
shortly before the needle engages with the fabric 7. Then, feeding of the 
fabric 7 causes the thread chain 6 to unwind itself from the tongue 5; 
feeding of the thread chain into the suction mouth 10 is caused by the 
suction effect caused by the compressed air emerging from the hole 16. At 
this point, the thread chain enters the discharge tube 9 and is cut by the 
blade 12 which is actuated by means known per se partially shown in FIG. 
2. The actuation means partially shown in FIG. 2 consists of an arm 20 
operatively connected, for example, to the shaft 21 driving the trimming 
knife of the machine (not shown). The insertion or drawing of the thread 
chain 6 the suction mouth 10 and the discharge tube 9 is ensured by the 
same blade 12 which performs the cutting operation. The alternate opening 
and closing movements of the said blade obstruct, in fact, the suction 
mouth 10, thereby increasing the suction effect. The partial closure of 
the suction mouth 10 facilitates insertion of the thread chain 6 for two 
reasons: the first is that the closing movements are alternated with 
opening movements; the second is that the closing movement increases the 
sucking action on the thread chain and leaves the second zone 10b of the 
suction mouth 10 unobstructed, in particular the upper zone of the latter, 
which zone is necessary for fabrics of considerable thickness in order to 
prevent the mouth being completely closed by the latter. 
The main advantage of the increase in the suction effect, which is obtained 
in an original manner by the said arrangement of the blade 12, is that a 
hole 16 can be used as the compressed-air vent and, consequently, that the 
entire thread-cutting device can be formed as a single piece, with no 
complicated parts which have to be made separately. 
Moreover, the through-opening formed in the region of the blade 12 gives 
rise to a small continuous flow of air through the opening itself: fine 
dust is thus prevented from accumulating in the region of the blade 12. 
In the embodiment illustrated, the thread-cutting device is also arranged, 
advantageously, in a substantially lowered position, the bottom edge of 
the suction chamber being level with the working surface 3. 
This device does not create any problems as regards operation; in fact, the 
amount by which the device projects vertically relative to the working 
surface, is reduced. 
Although the present invention has been described in connection with a 
preferred embodiment, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the invention and the appended claims.