Most of the common packages on which the yarns are wound can be divided in to two groups.
(1)Parellal wound packages
(2)Cross wound packages.
(1)Parellal wound packages:-
These are double flanged bobbins,also known as warper’s bobbins on which yarn is wound in such a way that the coils of yarn are laid parellel sided or barrel shaped,Flanges are needed on either sides to support parellely laid coils.If flanges are not provided then coils at the two ends will COLLAPSE. To withdraw the yarn from these packages,package has to be rotated by pull of yarn.Hence high unwinding speed will lead to excessive unwinding tension & yarn will break.Also as the unwinding is stopped the package continues to rotate due to its inertia,hence yarn may continue to come out from package.So this package is not suitable for the process taking place at high speed.
(2)Cross wound packages:-
In this case the yarn is wound on cylindrical tubes or conical tubes.The yarn is laid on this in form of helices at the two extremes. In this type of winding the yarn wraps cross one another hence these packages are called cross wound packages.Because of laying in cross fashion there is no possibility of yarn coils collapsing at the two extremes.Hence these packages do not need flanges. The cylindrical cross wound package is known as CHEESE & the conical one as CONE. The yarn can be withdrawn from cone & cheese overend (& side ways unrolling also).The over end withdrawal allows unwinding at high speed without extreme increase in tension.Rotation of package for unwinding is not essential hence the unwinding from package stops almost at the same instant when withdrawal is stopped. For some special cases yarn is required to be withdrawn side ways also.
Bobbins may be made of card or plastic, the latter being perforated if the yarn is to be package dyed. Parallel-sided cheeses have tubular bobbins. For cones, the bobbin is of a conical form, i.e., a truncated cone; the angle of taper — the semivertical angle — depends on the end use for the resulting package. Table 1 lists four common tapers. The wound cone package may have a fixed taper, which gives it flat ends, in which case the package is referred to as straight-ended. Cones may also have an accelerated taper, where the taper of the package is greater than the bobbin, resulting in a concave end at the top (the nose) and a convex end at the bottom (the base) of the package. These are called dished ends.
Sewing is a creative and interesting skill. The knowledge of sewing give a confident feeling when it is applied to the construction of garments. The earlier method of sewing by hand is not applicable for all stages of garment making. Therefore, considerable emphasis is given to machine sewing. There are several machines in the market today, each with its own desirable features and advantages. Sewing machines range from most basic having only simple lock stitch to the electronic machines that use advanced computer technology having various functions for example piping, binding, ruffling, pleating, darning, hemming and even making buttonholes and attaching fasteners. A good sewing machine is required to obtain quality products. One has to be familiar with the characteristics of different types of machines for selecting appropriate machine, depending upon the ability and requirements of the person.
Sewing machines are now available in various models such as domestic model, tailor model, industrial model, portable and cabinet models. They may be operated by hand, treadle or electric motor.
Hand – Operated Sewing Machine:
This is the simplest form of sewing machine which is operated by hand. A detachable handle provided to the flywheel is used to operate the machine. This machine is generally suitable for domestic purpose because it does not help in speeding up the work.
Treadle Sewing Machine:
This machine is exactly like the hand sewing machine but it is operated by foot using an additional stand. In this type the balance wheel is operated by a belt with the help of lower stand, which is driven by feet. This machine operates faster than that of the hand-operated machine. This machine is suitable where there is no power supply. When handling
this machine both the hands are free to handle the fabric, speeding up the work. Even some of the heavy-duty machines are operated by this method.
Electric Sewing Machine:
This is the fastest sewing machine. One needs practice to handle it. In an electric machine the balance wheel comes to motion by a belt, which is attached to an electric motor.
PARTS OF A SEWING MACHINE AND THEIR FUNCTIONS:
The basic structure of sewing machine is the same whether it is hand-operated sewing, treadle sewing machine or electric sewing machine. The basic parts of a sewing are listed below and seen in Fig.1
1. Spool pin: It is fitted on top of the arm to hold the reel.
2. Thread guide: It holds the thread in position from the spool to the needle.
3. Tension disc: The two concave discs put together with the convex sides facing each other. The thread passes between the two. The tension of the thread is adjusted by a spring and nut which increases or decreases pressure
4. Take up lever: It is a lever fitted to the body of the arm. Its up and down motion feeds the thread to the needle and tightens the loop formed by the shuttle.
5. Needle bar: This is a steel rod to hold the needle at one end with the help of a clamp. Its main function is to give motion to the needle.
6. Bobbin case: This moves into position to catch the top thread and form the stitch as the needle is lowered into the bobbin chamber.
7. Presser foot: It is fixed to the presser bar to hold the cloth firmly in position when lowered.
8. Presser foot lifter: A lever attached to the presser bar for raising and lowering the presser foot.
9. Stitch regulator: This controls the length of the stitch.
10. Bobbin winder: A simple mechanism used for winding thread on the bobbin.
11. Fly Wheel: When this is made to revolve, it works the mechanism of the motion
12. Clutch or Thumb Screw: This is in the center of the fly wheel and it engages and disengages the stitching mechanism.
13. Slide Plate: A rectangular plate, which facilitates the removal of the bobbin case without lifting the machine.
14. Needle Plate or Throat Plate: A semi-circular disc with a hole to allow the needle to pass through it.
15. Feed dog: This consists of a set of teeth fitted below the needle plate. It helps to move the cloth forward while sewing.
16. Face plate: A cover which on removal gives access to the oiling points on the needle bar, presser bar and take-up lever.
17. Spool pin for bobbin winding: Spool of thread is placed on this at the time of bobbin winding.
Before starting actual machining, you should check that the needle of the machine is of correct size, is sharp and correctly set. The bobbin should be evenly set. Briefly, the various steps of prepreparation are:
• Winding the bobbin
• Upper Threading
• Drawing the bobbin thread
• Tension adjustments
• Pressure and feed adjustments
• Selection of thread and needle
A perfect stitch can be obtained only when the thread selected is suitable to the material to be stitched and the needle is of the correct size. For stitching on delicate thin fabrics, use fine thread and fine needle. For heavy fabrics, needles and thread size should be larger. The following Table 1 will be a guide to help selection of appropriate needle and thread sizes.
The natural fibre threads available in the market are cotton and silk. Synthetic threads are usually made from polyester and terylene. Threads whether natural or synthetic are produced in various thickness: higher the number, finer is the thread and smaller the number, coarser is the thread. It is important to remember that the same thread should be used for the bobbin and top spool.
Selection of needles
Machine needles are selected according to the weight and other characteristics of the fabric, as well as the thread type being used for construction. Generally, a needle should be fine enough to penetrate the fabric without damaging it and yet have an eye, which is big enough so that the thread does not fray or break. Needles come in various sizes, from very fine (size 9) for light weight fabrics to thick (size 18) for very heavy weight and dense fabrics.
COMMON MACHINE TROUBLES
The sewing like any other machine, gives troubles of stitching like thread breaking, uneven stitching, puckering, bending and breaking of needle, looping of threads, skipping of stitches, etc. Little problems with the sewing machine can be very irritating and time consuming. They can happen to even the most experienced seamstress. A person operating the machine should be able to rectify these and solve the problems. Some of the common machine problems are listed below:
• breaking needles
• looping of stitches
• skipping stitches
• variation in stitch length
• puckered seams
• upper thread breaking
• lower thread breaking
• machine not feeding properly
• machine working heavily
• layers feed unevenly
• fabric does not feed in straight line
• cause damage to fabric
• Puckering on both layers of fabric
• Puckering on under layer only
• Shows feed marks on the under side
• Fabric is damaged or holes around the stitches
CARE AND USE OF SEWING MACHINES
A sewing machine needs care for its smooth running. It should be cleaned and oiled regularly to ensure satisfactory sewing and long life. When not in use, your machine should be covered to prevent dust accumulation on it. Use a small dry brush or old toothbrush and soft cloth to remove dust and lint. You should always remove lint deposits, dust and thread bits before oiling any part of the machine. Use a pointed instrument like a needle to pick out the bits of thread and lint that cannot be brushed out.
It is necessary to oil and lubricate the machine periodically. If the machine is used everyday, oil it once a week. After oiling, wipe off the surplus oil and place a piece of folded fabric under the presser foot to absorb any excess oil. To oil thoroughly, remove the upper thread, needle plate, slide plate, face plate, bobbin case, needle and presser foot. Oil the holes on the underside first, after cleaning and then proceed to the upper side. Use only few drops of oil in each hole. Never use coconut oil. Machine oil of different brands may be used for different models of sewing machine, but should be used as recommended in the instruction book.
If the machine becomes gummed with oil, put a drop of kerosene or petrol in each oil hole and joints and run it rapidly for several minutes. Wipe off and re-oil it with machine oil. The motor of electric sewing machine should be greased periodically.
The knowledge of sewing give a confident feeling when it is applied to the construction of garments. The various parts of a sewing machine and their functions helps one to understand the working of a sewing machine. A brief up on the common machine problems help us to understand and rectify the problems. Above all the care of sewing
machine is rather important for a long service of the machine.
Roving machine is complicated, liable to faults, causes defects, adds to production costs and delivers a product that is sensitive in both winding and unwinding. This machine is forced to use by the spinner for the following two reasons.
Sliver is thick, untwisted strand that tends to be hairy and to create fly. The draft needed to convert this is around 300 to 500. Drafting arrangements of ring frames are not capable of processing
this strand in a single drafting operation to create a yarn that meets all the normal demands on such yarns.
Drawframe cans represent the worst conceivable mode of transport and presentation of feed material to the ring spinning frame.
· Fibre to fibre cohesion is less for combed slivers. Rollers in the creel can easily create false drafts.Care must be taken to ensure that the slivers are passed to the drafting arrangement without disturbance.
Therefore, a perfect drive to the creel rollers is very important.
· The drafting arrangement drafts the material with a draft between 5 and 15.The delivered strand is too thin to hold itself together at the exit of the front bottom roller.
· Bobbin and flyer are driven separately, so that winding of the twisted strand is carried out by running the bobbin at a higher peripheral speed than the flyer.
· The bobbin rail is moving up and down continuously, so that the coils must be wound closely and parallel to one another to ensure that as much as material is wound on the bobbin.
· Since the diameter of the packages increases with each layer, the length of the roving per coil also will increase. Therefore the speed of movement of bobbin rail must be reduced by a small amount after
each completed layer
· Length delivered by the front roller is always constant. Owing to the increase in the diameter of the package for every up and down movement, the peripheral speed of package should keep on changing , to maintain the same difference in peripheral speeds between package and flyer.
· There are two types of drafting systems.
3/3 drafting system
4/4 drafting system
In general 3/3 drafting system is used, but for higher draft applications 4/4 drafting system is used.
· The draft often has limits not only at the upper limit (15 to 20), but also at lower limit. It is around 5 for cotton and 6 for synthetic fibers. If drafts below these lower limits are attempted, then the fibre masses to be moved are too large, the drafting resistance becomes too high and the drafting operation is difficult to control.
It is advisable to keep the break draft (predarft) as low as possible, because lower break draft always improves roving evenness.
· In general two condensers are used in the drafting arrangement. The purpose of these condensers is to bring the fibre strands together. It is difficult to control, Spread fibre masses in the drafting zone and they cause unevenness. In addion, a widely spread strand leaving the drafting arrangement leads to high fly levels and to high hairiness in the roving. The size of condensers should be selected according to the volume of the fibre sliver.
· Flyer inserts twist. Each flyer rotation creates one turn in the roving. Twist per unit length of roving depends upon the delivery rate.
Turns per metre = (flyer rpm)/(delivery speed (m/min))
Higher levels of roving twist, therefore, always represent production losses in Roving frame and possible draft problems in the ring spinning machine. But very low twist levels will cause false drafts and roving breaks in the roving frame.
· Centrifugal tension is created at the bobbin surface as the layers are being wound and is created by the rotation of the package. Each coil of roving can be considered as a high-speed rotating hool of roving on which centrifugal tension increases with increasing diameter of the package. centrifugal tension in the roving is proportional to the square of the winding surface velocity.In this context, centrifugal force acts in such a manner as to lift the top roving strand from the surface of the package so that the radial forces within the strand that hold the fibres together are reduced and the roving can be stressed to the point of rupture. Breaks of this type may occur at the winding-on Point of the presser or in strands that have just been wound on the top surface of the package. This phenomenon is known as “bobbin-bursting”. This phenomenon will be prominent if the twist per inch is less or the spindle speed is extremely high when the bobbin is big.
· Apart from inserting twist, the flyer has to lead the very sensitive strand from the flyer top to the package without introducing false drafts. Latest flyers have a very smooth guide tube set into one flyer leg
and the other flyer leg serves to balance the flyer. The strand is completely protected against air flows and the roving is no longer pressed with considerable force against the metal of the leg, as it is in
the previous designs. Frictional resistance is considerably reduced, so that the strand can be pulled through with much less force.
· False twisters are used on the flyers to add false twist when the roving is being twisted between the front roller and the flyer.Because of this additional twist, the roving is strongly twisted and this reduces the breakage rate. Spinning triangle is also reduced which will reduce the fibre fly and lap formation on
the front bottom roller.
· Because of the false twister, the roving becomes compact which helps to increase the length wound on the bobbin. This compactness helps to increase the flyer speed also.
· Roving strength is a major factor in determining winding limitations. It must be high enough for the fibres to hold together in a cohesive strand and low enough for satisfactory drafting at the spinning machine. The factors affecting roving strength are as follows:
the length, fineness, and parallelisation of fibres
the amount of twist and compactness of the roving
the uniformity of twist and linear density.
· BUILDER MOTION: This device has to perform the following tasks
to shift the belt according to the bobbin diameter increase
to reverse the bobbin rail direction at top and bottom
to shorten the lift after each layer to form tapered ends
· Shifting of the belt is under the control of the ratchet wheel. The ratchet wheel is permitted to rotate by a half tooth. The bobbin diameter increases more or less rapidly depending upon roving hank. The belt must be shifted through corresponding steps. The amount of shifting, which depends upon the thickness of the roving, is modified by replacement of the ratchet wheel or by other gears.If a ratchet wheel with fewer teeth is inserted, then the belt is shifted through larger steps, i.e. it moves more rapidly, and vice versa.
· To form a package, the layer must be laid next to its neighbours. For that the lay-on point must continually be moved. The shift of the winding point is effected by moving the bobbin rail. This raising and lowering is done by rails.Since the package diameter is steadily increasing, the lift speed must be reduced by a small amount after each completed layer.
· During winding of a package, the ratchet is rotated at every change-over.Reversal of the bobbin layer occurs little earlier for every reversal.This gives a continuous reduction in the lift of the rail . Thus bobbins are built with taper.