WO1988006196A1

WO1988006196A1 - Sewing machine

Info

Publication number: WO1988006196A1
Application number: PCT/GB1988/000092
Authority: WO
Inventors: Colin Mccallum
Original assignee: James Seddon (U.K.) Limited
Priority date: 1987-02-17
Filing date: 1988-02-15
Publication date: 1988-08-25

Abstract

An electric sewing machine has an electric motor, clutch and sewing mechanism (3) which comprises a thread carrying sewing needle (5), an apertured base plate (7) through which the needle reciprocates, further thread carrying means, the thread or threads of which cooperate with the needle thread to form stitches, a presser foot (9) for holding a workpiece during formation of a seam of stitches therein against the base plate and feed means for feeding the workpiece between the presser foot and base plate, in which sensing means (15, 17, 19), control means incorporating delay means and thread severing means are provided, the sensing means for sensing a leading and a trailing edge of a workpiece during formation of a sewn seam therein and for supplying a signal to the control means when each of said edges is sensed, whereby the control means when actuated by the sensing means upon sensing by the sensing means of one end of the sewn seam causes the switching to a constant speed of operation of the sewing machine motor and thereafter, following a delay determined by the delay means, actuation of the thread severing means at said one end of the sewn seam.

Description

Sewing Machine

This invention relates to sewing machines and more particularly to sewing machines of the kind comprising an electric driving motor, electrically operated clutch means and sewing mechanism driven by the motor by way of the clutch means, the sewing mechanism comprising a reciprocable sewing needle adapted to be supplied with a first thread, an apertured base plate below the needle through which the needle reciprocates, further thread carrying means which during operation of the sewing mechanism provide at least one thread which co-operates with the needle thread to form stitches, a presser foot for holding a workpiece during formation of a seam of stitches therein against the base plate and feed means located beneath the base plate and movable in relation thereto during sewing of the seam of stitches to feed the workpiece between the presser foot and the base plate.

Such machines are referred to hereinafter as sewing machines "of the kind set forth" and are used in the garment manufacturing, for example, the shirt manufacturing industry with thread trimmers located below the base plate for severing the needle and further threads at the end of a seam of stitches.

One form of sewing machine of the kind set forth is known as a "lock stitch" machine. In this type of machine the further thread carrying means comprise a bobbin located below the base plate for providing a thread which during operation co-operates with the needle thread to form the stitches of the sewn seam. The thread trimmers of this type of machine when used in the shirt meanufacturing Industry leave about 20mm of needle thread and lOmms of bobbin thread trailing from the needle and bobbin respectively. On restarting the machine with a further workpiece the ends of the needle and bobbin threads tend either to bunch up to form a knot at the back of the workpiece which has to be unpicked and trimmed or form loose thread ends which have to be trimmed. These unpicking and trimming operations are labour intensive and carried out during garment inspection and there is a need for them to be replaced by a statistical quality control procedure.

Another form of sewing machine of the kind set forth is known as an "overlooking" machine. In such a machine, typically, as the sewing needle reciprocates and forms loops of thread which are open at the top above the fabric panel edges being seamed together and closed below those panel edges, threads respectively carried in upper and lower looper arm assemblies are formed in known manner into loops, the loops of thread of the upper looper arm passing around the open ends of the sewing needle loops above the fabric panels and the loops of lower looper arm passing through the closed end of the respective sewing needle loops below the fabric panels and around open ends of the loops of the upper looper arm. Thus, the loops formed by upper and lower looper arms are mutually secured and further secured above and below the fabric panels respectively to the upper and lower ends of the sewing needle loops. Overlooking machines, as well as lock stitch machines, are used in the garment manufacturing, for example the shirt manufacturing industry with thread trimmers for severing the needle and looper arm threads at the ends of a seam of stitches. These leave, as in the case of the lock stitch machine, lengths of needle and co-operating threads which have to be removed during garment inspection.

It is accordingly a principal object of the invention to provide a sewing machine of the kind set forth which can sew a seam of stitches in a workpiece and control accurately and to a small value the lengths of free threads at one or each end of said seam.

The present invention consists in a sewing machine comprising an electric driving motor, electrically operated clutch means and sewing mechanism driven by the motor by way of the clutch means, the sewing mechanism comprising a reciprocable sewing needle adapted to be supplied with a first thread, an apertured base plate below the needle through which the needle reciprocates, further thread carrying means which, during operation of the sewing mechanism, provide at least one thread which co-operates with the needle thread to form stitches, a presser foot for holding a workpiece during formation of a seam of stitches therein against the base plate and workpiece feed means located beneath the base plate and movable in relation thereto during sewing of a seam of stitches in a workpiece to feed said workpiece between the presser foot and the base plate, characterised in that sensing means, control means responsive to the sensing means and incorporating delay means and thread severing means are provided, the sensing means for sensing a leading and a trailing edge of a workpiece during formation of a sewn seam therein and for supplying a signal to the control means when each of said edges is sensed, whereby the control means when actuated by the sensing means upon sensing by the sensing means of one end of the sewn seam causes the switching to a constant speed of operation of the sewing meachine motor and thereafter, following a delay determined by the delay means, actuation of the thread severing means at said one end of the sewn seam.

Advantageously, vacuum forming means disposed in the sewing path following the sewing needle are provided for holding the sewing threads taut at the commencement of sewing of a seam of stitches in a workpiece.

Preferably, first, second and third sensing means are provided of which the first sensing means senses the leading edge of the workpiece as the latter enters between the presser foot and the base plate and sends a signal to the control means which sets the control means in a condition to allow the machine operator to engage the clutch thereby to drive the sewing mechanism at normal sewing speeds and effect raising of the thread clamping and severing means and operation of the vacuum forming means, the second sensing means senses the leading edge of the workpiece as it arrives at the sewing needle following engagement of the clutch by the machine operator and sends a signal to the control means to cause operation of the motor at a predetermined constant speed whilst the third sensing means senses the leading edge of the workpiece after sensing thereof by the second sensing means and sends a signal to the control means to operate, after a delay determined by the delay means, the thread clamping and severing means to control the length of free thread at the commencement of the stitched seam, the operation of the clamping and severing means causing switching off of the vacuum, return movement of the thread clamping and severing means and a signal to be sent to the control means which effects reversion of the motor under the control of the operator to normal operating speeds so that sewing of the stitched seam progresses under the control of the machine operator whereafter the first sensing means serves to detect the trailing edge of the workpiece and thereby sends a signal to the control means to set the latter to enable subsequent operation of the thread clamping and severing means, the second sensing means senses the trailing edge of the workpiece as it arrives at the sewing needle and sends a signal to the control means to operate the motor at said predetermined constant speed and the third sensing means senses the trailing edge of the workpiece and sends a signal to the control means which after a delay determined by the delay means effects disengagement of the clutch lowering of the clamping means and cutting of the threads held by the clamping means by the severing means, the clamping means continuing to hold the threads after severing thereof by the severing means. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

FIGURE 1 Is a side elevation of a sewing machine, of the kind set forth, namely a lock stitch machine adapted for operation in accordance with the present invention; FIGURE 2 is a fragmentary view to a larger scale than that of Figure 1 showing a detail of the machine of that Figure; FIGURES 3. -^ and 5 ar diagrammatic side elevations of certain components of the machine of Figure 1; and FIGURE 6 is a side elevation of a sewing machine, namely, an overloeking machine, according to the invention. In the drawings, like parts have been accorded the same references.

Referring first to Figures 1 to 5. a sewing machine 1 of the kind set forth is a rotary hook lock stitch machine and comprises an electric driving motor (not shown) , an electrically operated clutch (not shown) and sewing mechanism, generally indicated at 3. driven by the motor by way of the electronic clutch, the sewing mechanism 3 comprising a vertically reciprocable sewing needle 5. an apertured base plate 7 below the needle through which the needle reciprocates, a bobbin (not shown) located beneath the base plate for providing a thread which in known manner co-operates with the needle thread to form the stitches, a reciprocable presser foot 9 carried on a vertical post 11 serving, in operation, to hold a workpiece during formation of a seam of stitches therein against the base plate 7 and feed means (not shown) located beneath the base plate and movable in relation thereto during sewing of the seam of stitches to feed the workpiece between the presser foot and the base plate. As is usual, the presser foot is spring biassed towards its lowered position and can be raised by the machine operator, for example, by displacing by sideward knee movement a foot lift control. Also for engaging the clutch with the motor and sewing mechanism, the operator depresses a further control, e.g. a treadle.

Mounted on the body of the machine by a strap 13 are first, second and third sensors 15.17 and 19 which at certain times during operation as hereinafter described transmit signals to electronic controls of the motor and/or other components of the machine. The sensing means 15 is directed at a point ahead of the sewing needle, the sensing means 17 at the point of contact between the foot 9 and plate 7 at which the sewing needle reciprocates and the sensing means 19 to the rear of the point of engagement of the foot 9 and plate 7•

Secured to the post 11 of the presser foot by a strap 21 is a thread clamping and severing means having a severing or chopper blade 23 and a clamp 25, the latter being carried so as to move in parallel with the blade. Further the blade 23 and clamp 2 by means of a spring 27 are mutually reciprocable so that when the blade and clamp are lowered, the clamp first engages the base plate 7 and the blade continues moving until it also engages the base plate and in so moving the blade stresses the spring 27 which holds the clamp firmly against the base plate. On return movement, the blade lifts first independently of the clamp until the stress in the spring 27 is relieved then the clamp and blade are further lifted in unison. For imparting reciprocating movement to the blade and clamp those components are mounted for displacement by a double acting piston and cylinder 29, shown diagrammatically in Figure 3. of which the cylinder 33 is held stationary relatively to the post 11 whilst the clamp and blade are fixedly secured to the piston. A spool valve 35 controls the supply of compressed air to the sides of the piston 31 from a source 37- The spool is solenoid actuated and when de-energised the spool occupies the position shown in Figure 3 ^ιn which the source 37 of compressed air is connected to supply air to the righthand side of the piston 31 whilst the region of the cylinder 33 to the left of the piston is vented to atmosphere. When the solenoid receives an energising pulse the spool moves to the left as seen in Figure 3 and the supply from the source 37 then connects with the left-hand side of the piston whilst the righthand side thereof is vented to atmosphere. Movement of the piston thus occurs so that the blade and clamp are lowered. The clamp first engages the plate 7 to trap the needle and bobbin threads and the blade continues moving until it also contacts the plate so severing the threads. The clamp remains engaged with the plate until a further pulse restores the spool to its initial position.

Formed in the base plate 7 is an aperture 39 which connects with a pipe 4l. As seen in Figure 4 compressed air supply source 43 connects with pipe 4l at a location spaced from the aperture 39. the supply of air to the pipe being by way of spool valve 45. The air supply to the pipe passes down the pipe in a direction away from the aperture 39 so that vacuum is applied to the aperture 39- The spool valve is solenoid actuated and in the position shown the spool obstructs the supply of air from the source 3. When the solenoid is energised the source, as shown, supplies air to pipe 4l and vacuum is applied to the hole 39-

The first, second and third sensing means, as shown in Figure 5. each comprise a light transmitting diode 47 and a photosensitive diode 49 which receives light transmitted from the- corresponding diode 47 and reflected back from the upper surface of the base plate 1. It will therefore be appreciated that as the base plate is covered by a workpiece in which a stitched seam is being formed the light paths from each diode 47 are successively first interrupted by the workpiece and then successively restored. At each change in the condition of each light path the corresponding diodes transmit a signal to the electronic control means of the machine and effect changes as described below.

In operation of the machine described, the operator switches electrical power to the motor which commences to run with the electically operated clutch disengaged. The clamp 25 is holding down the needle and bobbin threads and the vacuum to the hole 39 is switched off.

The machine operator now supplies a workpiece in which a seam of stitches is to be formed to the machine between the presser foot 9 which is in its lowered position and the base plate 7> Threads from the needle and the bobbin extend rearwards through aperture 39 into the pipe 4l.

As the workpiece is introduced between the presser foot and the base plate the leading edge thereof is sensed by sensing means 15 which thus transmits a signal to the electronic control means of the machine which sets those controls so that the electrically operated clutch is placed in a condition to be engaged when the operator activates the sewing mechanism 3- This the operator accomplishes by depressing the machine treadle whereupon a signal to the electronic control means causes the clamp 25 and blade 23 to be raised and the solenoid of spool valve 45 to be energised to apply vacuum to the upper part of pipe 4l to hold the needle and bobbin threads taut therein. Sewing of the seam commences with the motor speed under the control of the operator.

As the workpiece advances, the leading edge thereof is next sensed at the location thereof engaged by the needle by the sensing means 17 which sends a signal to the electronic control means of the machine which in response thereto switch the motor speed to a predetermined fixed speed, suitably 2000 r.p.m. This ensures that cutting of the threads always occurs at the same motor speed and, therefore, the same speed of the workpiece.

Further advance of the workpiece next leads to sensing of the leading edge of the workpiece by the sensing means 19 which sends a signal to the electronic controls which respond by first sending an energising pulse to the solenoid of valve 35 to lower the clamp 25 and blade 23. The threads are thus trapped and cut whilst the vacuum is being applied. The electronic controls after the energising pulse to the solenoid of valve 35 is terminated effect de-energisation thereof so that the blade and clamp are raised whilst the solenoid of valve 5 is also de-energised to switch off the vacuum. At this juncture, the motor speed is switched from the constant speed control to the control of the operator who then at desired speed operates the machine to form the stitched seam.

As the end of the seam approaches, the sensing means 1 senses the trailing edge of the workpiece and sends a signal to the electronic control means which set the clamp 2 and blade 23 in a stand-by mode enabling them to be lowered at a later stage. The continuing advance of the workpiece now causes sensing means 17 to sense the trailing edge of the workpiece and as the sensing means 17 is directed at the point of engagement of the foot 9 with the plate 7. the signal it now sends to the electronic controls of the machine is an indication of the end of the seam and causes the controls to switch the motor back to its constant speed of operation until, as the workpiece further advances, the point is reached where the trailing edge thereof is sensed by the sensing means 19- When this happens sensing means 19 sends a signal to the electronic controls of the machine which effect disengagement of the clutch and lowering of the clamp and blade so that the threads are cut and held down thereafter by the clamp.

Thus the initial conditions are restored and a further workpiece can be supplied between the foot 9 and base plate .

The electronic controls of the machine include variable delay means for controlling the interval between the sensing of the leading edge of the workpiece by sensing means 19 and lowering of the clamp and blade. The delay means enable the length of free threads at the commencement of the seam to be controlled down to about 1.5mm. Likewise further variable delay means control the interval between sensing of the end of the sewn seam by the sensing means 19 and the lowering of the clamp and blade. These delay means control the length of free threads at the end of the sewn seam. It will be apparent that the shorter the delay at the commencement of the seam the longer the free ends of threads there produced whilst at the end of the seam the longer the delay the longer the free ends of threads there produced since at the beginning of the sewn seam the workpiece Is moving towards the blade location whilst at the end of the seam the workpiece is moving away from the blade location.

The embodiment of Figures 1 and 5 is concerned with the application of the invention to a lock stitch machine. The invention is also employed with an overlocking machine used, for example in shirt manufacturing, for sewing seams for securing together at their edges panels of a garment. Such seams are formed to secure together the shoulder edges of front and rear panels of a garment and the armhole edges thereof to armhole edges of the sleeve panel as well as to seam together side edges of said panels from the armhole and edges of the sleeve panel to close the sleeve. The application of the invention to an overlocking machine is now described with reference to Figure 6.

As with the lock stitch machine described in connection with Figures 1 to 5 of said co-pending application, the overlocking machine is provided with a sewing needle 5 which is reciprocably driven and passes at a lower part of its travel through an apertured base plate 7- A reciprocable presser foot 9 carried on a post 11 serves, in operation to hold the panels of the seam being sewn against the base plate. Feed means 12 located beneath the baseplate and movable in relation thereto during sewing of a seam of stitches feed the panels between the presser foot and the base plate as the seam is formed. Suitably the presser foot is spring biased towards its lowered position and can be raised by the machine operator displacing by e.g. sideward knee movement a foot lift control.

The sewing mechanism of the machine is driven by an electric motor by way of an electronic clutch which is engaged by the machine operator rocking a treadle. Mounted on the body of the machine is a strap 13 on which is mounted a sensor bar 14 carrying the sensors 15, 17. 19 each comprising a light emitting diode 47 for transmitting a beam of light on to the base plate and a photo-sensitive diode 49 for receiving such transmitted light by reflection from the base. The photo sensitive diode of each sensor thus only receives a reflected light beam when the point on the base plate at which the light is reflected is uncovered by fabric panels being seamed together. The sensing means 15, 17. 19 serves at certain times during operation to transmit signals to electronic controls of the motor and/or other components of the machine. The sensing means 1 is directed at a point ahead of the sewing needle, the sensing means 17 at the point of contact between the presser foot 9 and plate 7 and the sensing means 19 just to the rear of the foot 9-

At the rear end of the base plate 7 is provided a vacuum tube 18 the vacuum within which is controlled by a spool actuated pressurised air valve (not shown) similar to valve 4 of Figure 4. Instead of the tube 18, vacuum forming means can be provided as in the case of the embodiment of Figures 1 to 5. by a hole in the base plate and a pipe extending below the base plate from said hole, to which pipe compressed air can be supplied by way of a spool valve in a direction away from said hole. Also provided at the rear of the presser foot is a thread severing means 50 which comprises a pneumatic piston which is under the control of a pressurised air supplied spool valve and has a reciprocable rod 2 which terminates at the end thereof opposite the piston in a clevis 4 which pivotally connects with a crank 56 secured to a shaft 58 which extends towards the presser foot location above the base plate and is rotated by reciprocation of the piston. At its end adjacent the presser foot position, the shaft carries a chopper blade 60 which rotates with the shaft 58 relatively to a fixed blade (not shown) to the end of severing in scissors like manner the needle and looper threads.

For simplicity the looper arms and their threads have not been shown as they operate in well known manner.

In an overlocking sewing machine the formed stitches flow out of the rear of the machine from a finger which forms part of the base plate and the stitches are formed whether or not the fabric panels are present in which to form a seam. This is because the feeder mechanism of the machine moves relative to the base plate in an elliptical path so that stitches are formed in loops around the finger referred to of the base plate and feed off rearwardly therefrom. Accordingly, in an overlocking machine no clamping of threads is needed though it is desirable, as has been described, to employ vacuum forming means.

In operation when sewing a straight seam, as, for example, the seam which joins the side edges of each of the front body panels to the rear body panel from the bottom of the garment to the armhole and continues therefrom to join the edges of the sleeve so as to close the sleeve, the presser foot 9 is lowered and the garment panels disposed for sewing the seam are brought up to the presser foot and fed between the foot and the base plate. The sensor 15 senses the leading edges of the panels to be seamed and thereby a signal is sent to the electronic controls which are enabled and the electrically operated clutch is set to be engaged when the operator actuates the sewing mechanism by rocking the machine treadle. This treadle movement causes a signal to be sent to the electronic controls which cause energisation of the solenoid of the spool valve controlling the vacuum in the tube 18, so that vacuum is applied. Also, the treadle movement starts the motor and causes the clutch to be energised to the end that the motor is engaged. The feed of the fabric panels between the presser foot and base plate accordingly commences under the control of the operator until the sensor 17 senses, at the location of the sewing needle, the leading edge of the seam. When this happens a signal is sent to the controls which causes the motor to be switched to a constant speed mode, suitably, of, 5000 rpm. This ensures that cutting of the needle and looper threads•at the beginning of the seam always occurs at the same motor speed and, therefore, at the same fabric panel speed. Next, the sensor 19 senses the leading edge and a signal is sent to the electronic controls which cause the spool valve solenoid which controls the piston of the chopper mechanism to be energised after a delay provided by a variable delay means forming part of the electronic controls. The piston is thus moved and actuates the chopper, the delay imparted to the action of which controls the length of free threads at the commencement of the seam. The delay enables the length of these free threads to be controlled down to zero.

As soon as the chopper severs the threads the electronic controls receive a signal which causes de-energisation of the spool valve solenoid associated with the vacuum tube so that the vacuum is turned off and thereafter further causes the motor to revert to operator control so that the seam is sewn at the speed desired by the operator.

At the end of the seam, the sensor 1 senses the trailing ends of the fabric panels and this causes a signal to be sent to the electronic controls which are set in a stand-by mode to enable subsequent motor speed control and operation of the chopper to occur. It is to be noted that enabling of the controls to allow subsequent operation of the vacuum is not required. Next, the sensor 17 senses the trailing end of the fabric panels being seamed together and a signal is sent to the electronic controls which switches the motor to the constant speed mode. When next the sensor 19 senses the trailing end of the panels a signal is sent to the electronic controls which cause, after a delay provided by a variable delay means incorporated in the electronic controls, actuation of the chopper and thereafter stopping of the motor and disabling of the electronic controls so that the initial conditions are restored. The delay in the operation of the chopper at the end of the seam enables the length of free threads at the end of the seam to be controlled again down to zero. in the sewing of a curved seam such as secures the garment sleeve to the armhole, it is a requirement that no thread cutting occurs at the commencement of the seam in order that any risk is avoided of the chopper perforating the fabric. A special programmer for this sequence is therefore provided and operated by push-button or like means by the machine operator and is now described. As with a straight seam, the fabric panels to be seamed together are brought up to the lowered presser foot and fed between that foot and the base plate. The sensor 15 senses the leading edge of the panels to be seamed and a signal is thereby sent to the electronic controls which are enabled and the clutch is set to be engaged when the operator actuates the sewing mechanism by rocking the machine treadle. The treadle movement causes a signal to be sent to the electronic controls which causes the sensors 17 and 19 to be disabled and that in turn causes a further signal from the electronic controls to energise the clutch and start the motor so that sewing commences under the control of the operator. However the treadle movement in this instance does not actuate the vacuum forming means.

As the end of the seam approaches, the sensor 1 senses the location of the seam end and causes a signal to be sent to the electronic controls to set those controls for the subsequent operations. Next, the sensor 17 senses the trailing end of the seam and sends a signal to the electronic controls which cause the motor to switch to the constant speed mode, suitably, at 5000 rpm. The sensor 19 next senses the end of the seam and sends a signal to the electronic controls which causes, after a delay provided by the delay means which, as with a straight seam, is controllable, operation of the chopper solenoid valve to the end that the threads are severed whereupon a signal is sent to the electronic controls which cause disengagement of the clutch and, thereafter disabling of the electronic controls. This completes the armhole seam which is sewn before the straight, garment closure seam joining the side edges of the front and back panels and the edges of the longitudinal sleeve panel. The sewing of this latter seam causes cutting of the surplus thread at the beginning of the armhole seam because in an overlocking machine, during sewing of a seam, straight or curved, a knife operates continuously to remove surplus fabric on the side of the seam adjacent the free edges of the panels being sewn and this knife removes the surplus thread at the beginning of the curved armhole seam.

The embodiments of the invention described enable reliable and accurate control of the lengths of threads at one or each end of the sewn seam of the workpiece and obviate the necessity for the numbers of garment inspectors who are normally employed for trimming such threads. Accordingly, a sample (i.e. statistical quality control) procedure is employed to ensure that the product meets required standards and such procedures is not labour intensive.

Claims

1. A sewing machine comprising an electric driving motor, electrically operated clutch means and sewing mechanism driven by the motor by way of the clutch means, the sewing mechanism comprising a reciprocable sewing needle adapted to be supplied with a first thread, an apertured base plate below the needle through which the needle reciprocates, further thread carrying means which, during operation of the sewing mechanism, provide at least one thread which co-operates with the needle thread to form stitches, a presser foot for holding a workpiece during formation of a seam of stitches therein against the base plate and workpiece feed means located beneath the base plate and movable in relation thereto during sewing of a seam of stitches in a workpiece to feed said workpiece between the presser foot and the base plate, characterised in that sensing means, control means responsive to the sensing means and incorporating delay means and thread severing means are provided, the sensing means for sensing a leading and a trailing edge of a workpiece during formation of a sewn seam therein and for supplying a signal to the control means when each of said edges is sensed, whereby the control means when actuated by the sensing means upon sensing by the sensing means of one end of the sewn seam causes the switching to a constant speed of operation of the sewing meachine motor and thereafter, following a delay determined by the delay means, actuation of the thread severing means at said one end of the sewn seam.

2. A sewing machine as claimed in Claim 1, characterised in that vacuum forming means disposed in the sewing path following the sewing needle are provided for holding the sewing threads taut at the commencement of sewing of a seam of stitches in a workpiece.

3- A sewing machine as claimed in Claim 2, characterised in that the vacuum forming means comprise a hole formed in the base plate, a pipe extending beneath the base plate from said hole and means for supplying air under pressure down said pipe in a direction away from said base plate thereby to cause said sewing threads to be drawn towards said hole in the base plate.

4. A sewing machine as claimed in Claim 3. characterised in that the compressed air supply to the pipe is connected by way of a solenoid actuated spool valve which in respective positions of the sppol thereof connects and obstructs the air supply to the pipe.

5. A sewing machine as claimed in Claim 2, Claim 3 or Claim 4, characterised in that first, second and third sensing means are provided of which the first sensing means senses the leading edge of the workpiece as the latter enters between the presser foot and the base plate and sends a signal to the control means which sets the control means in a condition to allow the machine operator to engage the clutch thereby to drive the sewing mechanism at normal sewing speeds and effect raising of the thread clamping and severing means and operation of the vacuum forming means, the second sensing means senses the leading edge of the workpiece as it arrives at the sewing needle following engagement of the clutch by the machine operator and sends a signal to the control means to cause operation of the motor at a predetermined constant speed whilst the third sensing means senses the leading edge of the workpiece after sensing thereof by the second sensing means and sends a signal to the control means to operate, after a delay determined by the delay means, the thread clamping and severing means to control the length of free thread at the commencement of the stitched seam, the operation of the clamping and severing means causing switching off of the vacuum, return movement of the thread clamping and severing means and a signal to be sent to the control means which effects reversion of the motor under the control of the operator to normal operating speeds so that sewing of the stitched seam progresses under the control of the machine operator whereafter the first sensing means serves to detect the trailing edge of the workpiece and thereby sends a signal to the control means to set the latter to enable subsequent operation of the thread clamping and severing means, the second sensing means senses the trailing edge of the workpiece as it arrives at the sewing needle and sends a signal to the control means to operate the motor at said predetermined constant speed and the third sensing means senses the trailing edge of the workpiece and sends a signal to the control means which after a delay determined by the delay means effects disengagement of the clutch lowering of the clamping means and cutting of the threads held by the clamping means by the severing means, the clamping means continuing to hold the threads after severing thereof by the severing means.

6. A sewing machine as claimed in any preceding claim, characterised in that each sensing means comprises a light transmitting and a light receiving diode of which the transmitting diode is directed to reflect light from the base plate to the receiving diode.

7- A sewing machine as claimed in any preceding claim, and in which the machine is a lock stitch machine and has a bobbin located beneath the base plate for providing a thread which during operation of the sewing mechanism co-operates with the needle thread to form stitches, the thread severing means are mounted above the base plate for reciprocal movement relatively to the base plate and thread clamping means are secured alongside the thread severing means and resiliently relatively thereto so that upon reciprocation of the thread severing means the thread clamping means first trap thread against the base plate and the severing means then cut said thread.

8. A sewing machine as claimed in Claim 5 and in which the machine is an overlook machine, characterised in that means are provided for preventing the second and third sensing means and control means from effecting severance of threads at one of the ends of the sewn seam.

9• A sewing machine as claimed in Claim 8, characterised in that the means for disabling the second and third sensing means are effective upon sensing by the first sensing means of the end of the sewn seam at which sewing Is commenced.

10. The method of operating a sewing machine comprising an electric driving motor, clutch means and sewing mechanism driven by the motor by way of the clutch means, the sewing mechanism comprising a reciprocable sewing needle adapted to be supplied with a first thread, an apertured base plate below the needle through which the needle reciprocates, further thread carrying means which during operation of the sewing mechanism provide at least one thread which co-operates with the needle thread to form stitches, a presser foot for holding a workpiece during formation of a seam of stitches therein against the base plate and workpiece feed means located beneath the base plate and movable in relation thereto during sewing of the seam of stitches to feed the workpiece between the presser foot and the base plate, there being provided sensing means, control means responsive to the sensing means and incorporating delay means and thread severing means, characterised by supplying signals from the sensing means to the control means when the sensing means senses a leading and a trailing edge of a workpiece to cause the control means when one edge of the workpiece is sensed at the sewing needle to switch the electric motor -to drive the sewing mechanism at a constant speed of operation and after a delay determined by the delay means to actuate the thread severing means to sever free threads at said one edge of said workpiece.

11. The method claimed in Claim 10, characterised by the control means, when supplied with signals from the sensing means having sensed the leading and trailing ends of the sewn seam at the sewing needle, effecting, at each of the leading and trailing edges of the workpiece, switching of the electric motor to drive the sewing mechanism at a constant speed of operation and, following such switching, after respective delays at the leading and trailing edges of the workpiece determined by the delay means, actuating the thread severing means to sever free threads at each of said edges of the workpiece.

12. The method claimed in Claim 10, and in which the sewing machine is an overlocking machine, characterised by preventing the signal supplied by the sensing means to the control means at the beginning of the seam of stitches from operating the control means to effect severance of free threads at said beginning of the seam of stiches.