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Unit 6: Lathe ThreadingAfter completing this unit, you should be able to:
• Determine the infeed depth.
• Describe how to cut a correct thread.
• Explain how to calculate the pitch, depth, and minor diameter, width of flat.
• Describe how to set the correct rpm.
• Describe how to set the correct quick change gearbox.
• Describe how to set the correct compound rest.
• Describe how to set the correct tool bit.
• Describe how to set both compound and crossfeed on both dials to zero.
• Describe the threading operation.
• Describe the reaming.
• Describe how to grind a tool bit.
Thread cutting on the lathe is a process that produces a helical ridge of uniform section on the workpiece. This is performed by taking successive cuts with a threading toolbit the same shape as the thread form required.
Practice Exercise:
1. For this practice exercise for threading, you will need a piece of round material, turned to an outside tread Diameter.
2. Using either a parting tool or a specially ground tool, make an undercut for the tread equal to its single depth plus .005 inch.
3. The formula below will give you the single depth for undertaking unified threads:
d = P x 0.750
Where d = Single Depth
P = Pitch
n = Number of threads per inch (TPI)
Infeed Depth = .75 / n
To cut a correct thread on the lathe, it is necessary first to make calculations so that the thread will have proper dimensions. The following diagrams and formulas will be helpful when calculating thread dimensions.
Example: Calculate the pitch, depth, minor diameter, and width of flat for a ¾-10 NC thread.
P = 1 / n = 1 / 10 = 0.100 in.
Depth = .7500 x Pitch = .7500 x .100 = .0750 in.
Minor Diameter = Major Diameter – (D + D) = .750 – (.075 + .075) = 0.600 in.
Width of Flat = P / 8 = (1 / 8) x (1/10) = .0125 in.
Procedure for threading:
1. Set the speed to about one quarter of the speed used for turning.
2. Set the quick change gearbox for the required pitch in threads. (Threads per inch)
Figure 1. Thread and Feed Chart
Figure 2. Setting Gearbox
3. Set the compound rest at 29 degrees to the right for right hand threads.
Figure 3. 29 Degrees
4. Install a 60 degree threading tool bit and set the height to the lathe center point.
Figure 4. 60 Degree Threading Tool
5. Set the tool bit and a right angles to the work, using a thread gage.
Figure 5. Using the Center gage to position the tool for machining Threads
6. Using a layout solution, coat the area to be threaded.
Figure 6. Layout
7. Move the threading tool up to the part using both the compound and the cross feed. Set the micrometer to zero on both dials.
Figure 7. Compound Figure 8. Cross Feed
8. Move cross feed to the back tool off the work, move carriage to the end of the part and reset the cross feed to zero.
Figure 9. End of the part and Cross feed to Zero
9. Using only the compound micrometer, feed in .001 to .002 inch.
Figure 10: Compound feed in .002 inch
10. Turn on the lathe and engage the half nut.
Figure 11: On/Off Lever and Half Nut
11. Take a scratch cut on the part without cutting fluid. Disengage the half nut at the end of the cut, stop the lathe and back out the tool using the cross feed. Return the carriage to the starting position.
Figure 12. Starting Position
12. Using a screw pitch gage or a rule check the thread pitch. (Threads per inch)
Figure 13. Screw Pitch Gage Figure 14. Screw Pitch Gage(10)
13. Feed the compound in .005 to .020 inch for the first pass using cutting oil. As you get near the final size, reduce the depth of cut to .001 to .002 inch.
14. Continue this process until the tool is within .010 inch of the finish depth.
Figure 15. Threading operation
15. Check the size using a screw thread micrometer, thread gage, or using the three wire system.
Figure 16. Three wire measurement
16. Chamfer the end of the thread to protect it from damage.
Reamers are used to finish drilled holes or bores quickly and accurately to a specified sized hole and to produce a good surface finish. Reaming may be performed after a hole has been drilled or bored to within 0.005 to 0.015 inch of the finished size since the reamer is not designed to remove much material.
The workpiece is mounted in a chuck at the headstock spindle and the reamer is supported by the tailstock.
The lathe speed for machine reaming should be approximately 1/2 that used for drilling.
Reaming with a Hand Reamer
The hole to be reamed by hand must be within 0.005 inch of the required finished size.
The workpiece is mounted to the headstock spindle in a chuck and the headstock spindle is locked after the workpiece is accurately setup. The hand reamer is mounted in an adjustable reamer wrench and supported with the tailstock center. As the wrench is revolved by hand, the hand reamer is fed into the hole simultaneously by turning the tailstock handwheel. Use plenty cutting fluid for reaming.
Reaming with a Machine Reamer
The hole to be reamed with a machine reamer must be drilled or bored to within 0.010 inch of the finished size so that the machine reamer will only have to remove the cutter bit marks. Use plenty cutting fluid for reaming.
Procedure:
1. Grip the tool bit firmly while supporting the hand on the grinder tool set.
2. Hold the tool bit at the proper angle to grind the cutting edge angle. At the same, tilt the bottom of the tool bit in towards the wheel and grind 10 degrees side relief or clearance angle on the cutting edge. The cutting edge should be about .5 inches long and should be over about ¼ the width of the tool bit.
3. While grinding tool bit, move the tool bit back and forth across the face of the grinding wheel. This accelerates grinding and prevents grooving the wheel.
4. The tool bit must be cooled frequently during the grinding operation by dip into the water. Never overheat a tool bit.
5. Grind the end cutting angle so that it form an angle a little less than 90 degrees with the side cutting edge. Hold the tool so that the end cutting edge angle and end end relief angle of 15 degrees are ground at the same time.
6. Check the amount of end relief when the tool bit is in the tool holder.
7. Hold the top of the tool bit at about 45 degrees to the axis of the wheel and grind the side rake about 14 degrees.
8. Grind a slight radius on the point of the cutting tool, being sure to maintain the same front and side clearance angle.
Grind front Grind side Grind radius
Lathe tool bits are generally made of four materials:
1. High speed steel
2. Cast alloys
3. Cemented Carbides
4. Ceramics
The properties that each of these materials possess are different and the application of each depends on the material being machined and the condition of the machine.
Lathe tool bits should possess the following properties.
1. They should be hard.
2. They should be wear resistant.
3. They should be capable of standing up to high temperatures developed during the cutting operation.
4. They should be able to withstand shock during the cutting operation.
Cutting tools used on a lathe are generally single pointed cutting tools and although the shape of the tool is changed for various applications. The same nomenclature applies to all cutting tools.
Procedure:
1. Base: the bottom surface of the tool shank.
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2. Cutting Edge: the leading edge of the tool bit that does the cutting.
3. Face: the surface against which the chip bears as it is separated from the work.
4. Flank: The surface of the tool which is adjacent to and below the cutting edge.
5. Nose: the tip of the cutting tool formed by the junction of the cutting edge and the front face.
6. Nose radius: The radius to which the nose is ground. The size of the radius will affect the finish. For rough cut, a 1/16 inch nose radius used. For finish cut, a 1/16 to ⅛ inch nose radius is used.
7. Point: The end of the tool that has been ground for cutting purposes.
8. Shank: the body of the tool bit or the part held in the tool holder.
9. Lathe Tool bit Angles and Clearances
Proper performance of a tool bit depends on the clearance and rake angles which must be ground on the tool bit. Although these angles vary for different materials, the nomenclature is the same for all tool bits.
• Side cutting edge angle: The angle which the cutting edge forms with the side of the tool shank. This angle may be from 10 to 20 degrees depending on the material being cut. If angle is over 30 degrees, the tool will tend to chatter.
• End cutting edge angle. The angle formed by the end cutting edge and a line at right angle to the centerline of the tool bit. This angle may be from 5 to 30 degrees depending on the type of cut and finish desired. For roughing cuts an angle of 5 to 15 degrees, angle between 15 and 30 degrees are used for general purpose turning tools. The larger angle permits the cutting tool to be swivelled to the left when taking light cuts close to the dog or chuck, or when turning to a shoulder.
• Side Relief (clearance) angle: The angle ground on the flank of the tool below the cutting edge. This angle may be from 6 to 10 degrees. The side clearance on a tool bit permit the cutting tool to advance lengthwise into the rotating work and prevent the flank from rubbing against the workpiece.
• End Relief (clearance) angle: the angle ground below the nose of the tool bit which permits the cutting tool to be fed into the work. This angle may be 10 to 15 degrees for general purpose cut. This angle must be measured when the tool bit is held in the tool holder. The end relief angle varies with the hardness and type of material and type of cut being taken. The end relief angle is smaller for harder materials, to provide support under the cutting edge.
• Side Rake Angle: The angle at which the face is ground away from the cutting edge. This angle may be 14 degrees for general purpose tool bits. Side rake centers a keener cutting edge and allows the chip to flow away quickly. For softer materials, the side rake angle is generally increased.
• Back (Top) Rake: The backward slope of the tool face away from the nose. This angle may be about 20 degrees and is provide for in the tool holder. Back rake permits the chips to flow away from the point of the cutting tool.
1. What is pitch for ¼-20 tap?
2. To what angle must the compound be turned for Unified Thread?
3. Explain why you swivel the compound in Question 2.
4. What is the depth of thread for UNF ½-20 screw?
5. How would you make a left-hand thread? This is not covered in the reading—think it out?
6. What Tool bit do we use for cutting thread?
7. Please describe Center Gage.
8. What do we use to check the thread pitch(Thread Per Inch)?
9. The first and final pass, how much do we feed the compound in?
10. Name four material that use to make Tool bits.
This chapter was derived from the following sources.
Introduction
Raw Materials
Spinning of Polyester and Cotton Fibres
Twisting
Thread Manufacturing
Thread Types
Wet Processing
Finishing
Thread Sizing
Apparel Thread Conversion Table
Sewing thread has a variety of descriptions. We generally refer to it as, "two or more yarns twisted together to form a single strand in a plied or corded construction".
However, these days there are other 'Continuous Filament' constructions manufactured using technologies, like intermingling, texturising and air jet texturising which can produce a thread of a single ply construction.
Coats produces many different threads and this will be explained within this technical bulletin.
Most sewing threads today are based on synthetic materials, such as polyester or nylon, which have, to a large extent replaced the use of natural fibres like cotton and linen that used to be the material of choice.
Handicraft threads are still predominantly made out of cotton, especially when the handicraft work is done by hand. However, if the handicraft effect is being created by a machine, (as in the case of embroidery of logos), then synthetic threads are rapidly replacing cotton and rayon threads.
A few important characteristics of the fibres and filaments which are used for sewing threads are:
The finish is ultimately determined by the sewing performance and the lubricants used, but seam strength and seam durability is directly related to the properties mentioned above.
Polyester raw materials are initially produced using polymer chips or granules, which are then melted and fed under pressure through a device known as a spinneret.
A spinneret is a metal disc which contains holes. The molten polymer is extruded through the spinneret to form continuous filaments.
For the vast majority of sewing threads the holes in the spinneret are circular, but for high lustre filaments used to manufacture embroidery threads the spinneret profile is triangular producing trilobal filaments.
These filaments are then cooled and collected together to form a continuous filament yarn. This is one of the main components for making polyester based sewing threads. This process is known as Melt spinning.
The diameter of the filaments is determined by the size of the holes in the spinneret and the pressure of the pump.
To obtain filaments suitable for the production of Continuous Filament and Corespun sewing threads, the yarns are individually drawn to orientate the molecular chains to produce particularly high tenacity filaments. The Filaments that are used for the production of sewing threads have a circular cross-section and a smooth surface, making them more lustrous than the natural fibres due to enhanced light reflectivity.
However, for special end uses such as machine embroidery - multi-lobal Polyester cross-sectional filaments are available which offer greater light reflectance values than circular cross-section filaments.
Coats produces a variety of different continuous filament threads with different physical characteristics which satisfy particular sectors of the sewing thread industry. The main continuous filament threads produced by Coats are made from Polyester or Polyamide.
This continuous filament polyester can also be used to produce polyester staple fibre. In order to do this we take a number of melt spun continuous filament yarns which are then brought together into a tow or rope-like structure. This rope like structure is then drawn or stretched to align the molecular chain thereby increasing the tenacity of the filament. It is then crimped, given a crinkle, to produce a better frictional surface.
The tow can then either be cut into a suitable regular staple length of around 38mm, in order to mimic the best quality cotton fibre to create Staple Spun yarns. Or it can be stretched broken to produce a longer, though more variable length fibre, and used to produce Tow Spun or Craq spun yarns.
For sewing threads high tenacity fibre is usually used. Despite their differing characteristics all synthetic threads must be properly lubricated if it is to perform properly at the sewing machine.
Cotton is a natural fibre and is the fibre on which Coats originally built its worldwide business. Cotton is produced in many different grades.
For sewing threads, Coats selects from the world’s long and extra-long staple cotton crops. The cotton fibres are carefully selected based on their grade, strength, length, fineness and maturity.
Under the microscope, cotton fibres have a flattened ribbon-like appearance with frequent convolutions. In a cross section of cotton you will notice that they look like collapsed, hollow tubes. These fibres are very fine having a typical diameter of 20 microns and a fibre length averaging approximately 38mm.
Although less common now as an industrial sewing thread, cotton is still dominant in the handicraft thread market.
The process of manufacturing a spun polyester sewing thread includes the steps; Opening, Carding, 1st and 2nd stage Drawing, Roving, Spinning, Clearing/Splicing & Winding, Assembly Winding, and finally, Twisting.
The process of manufacturing a cotton sewing thread includes the steps; Opening, Cleaning, Carding, 1st Stage Drawing, Sliver Lap Forming, Ribbon Lap forming, Combing, 2nd Stage Drawing, Roving, Spinning, Clearing/Splicing & Winding, Assembly Winding and Twisting.
This process is to open the bales of staple fibre and blend them together.
To remove the dirt, leaf and seed fragments that are mixed with the fibres.
The fibres in lap form are then passed to the Cards, which separate the individual fibres from each other and releases further quantities of impurities. The fibres are then brought together in a sliver or a tow, which is like a rope of fibre.
Typically 6 or 8 slivers are combined and blended together, then drawn out using a drafting roller system to produce a single sliver similar in size to the single input sliver. This machine operates by having the pair of front rollers revolving faster than the back rollers.
A number of slivers are presented to the Sliver Lap machine which aligns them side by side, draws them out. The fibres, now in sheet form, are rolled up into what is called a lap.
These laps are then presented to the Ribbon/lap machine where a number of laps are fed together on top of each other to another drafting system similar to the Sliver/lap machine. Again, the output from this machine is a lap, which is the input package for the combing machine.
These processes are designed to blend together the staple fibres, align them parallel to each other and enhance regularity.
The next stage of production presents the laps to the Comber, which combs the fibres as they are pulled from the lap. The function of combing is to remove short fibres and to lay the remaining long fibres parallel to each other. The production from the comber returns the fibre to a sliver form.
After combing [Cotton ONLY], a number of slivers are again combined and passed through the Post-comb draw frame to help eliminate irregularities and to ensure that the fibres are properly placed parallel to each other. The sliver produced here is approximately the size of the input sliver but is still many times heavier than the yarns required for thread.
For polyester fibres the sliver lap, ribbon lap and post combed drawing processes are replaced by the two draw frame processes. This means that the process route is shorter in the preparation section.
The sliver from post-comb drawing of cotton or 2nd stage drawing of polyester are sent to the Speed frame where each individual sliver is presented to an individual set of drafting rollers and drawn down to a weight suitable for ring spinning. These machines, sometimes referred to as “slubbing” machines produce a finer version of a sliver which is known as a Roving. Because at this stage the number of fibres in the cross section is considerably reduced the roving is relatively weak. A slight twist is inserted into the roving to hold it together and facilitate the Ring Spinning process, as it is wound onto a bobbin.
The final stage of yarn production is done on the Ring spinning frame, where the Roving is drawn out to the required weight or thickness and the relatively high level of singling twist, required for both strength and balance in the completed thread is inserted. This twist is normally in the “S” direction, though some threads have their singles twist in the “Z” direction. The drawing or drafting down is done by a series of rollers running at different speeds which draw the roving finer. After leaving the final drafting roller the singles twist is imparted creating a singles yarn. The resultant fine yarn is wound onto a ring tube. If the staple fibre used was 100% cotton then the yarn is 100% staple spun cotton. If the staple fibre used was 100% polyester then the yarn is 100% staple spun polyester.
If the thread is to be a Corespun thread, then a pre-stabilised yarn of Continuous Filament Polyester is fed in at the back of the front rollers on the ring spinning frame, so that no stretching of the filament occurs, and the staple fibres in the roving form an outer sheath around the filament core.
If the staple fibre used was 100% cotton and the filament core is 100% polyester then the yarn is Polyester / Cotton corespun.
If the staple fibre used was 100% polyester and the filament core is 100% polyester then the yarn is Polyester / Polyester corespun, sometimes referred to as Poly / Poly.
Spun synthetic yarns can also be produced directly from the tow of continuous filaments supplied by the filament producers by stretching it to breaking point. This is done on special machinery where the tow is passed between two sets of rollers running at different speeds, the second set being the faster. Each filament is broken before it reaches the second rollers but is carried through by its neighbours. This results in a more variable but longer length staple fibres, which are then drawn out and twisted into yarns through a series of drawing and combining operations for subsequent standard processing into finished sewing threads. Such yarns are known as Schappe, Craq, or Tow Spun yarns.
The package from the ring spinning frame weighs between 100 - 150 grams. The purpose of the Clearing, Splicing and Winding process after spinning, is to take the yarn from the ring tubes, pass them through electronic fault detectors (Clearing) to remove faults which may impede the sewing performance of the final thread. The ends of yarn are joined together (Splicing), either due to the removal of a fault or just to join the short lengths of yarn from the ring tubes to produce longer lengths. This process is called Single End Winding.
Once we have our "yarns" the next stage is to convert these into "grey" or "greige" thread.
The packages (or cheeses) of "cleared" yarn from Single End Winding are sent to the Assembly Winding process to prepare a suitable package for the twisting machines. This process simply assembles the required number of ends of single yarn for the eventual thread and lays them side-by-side under equal tension.
All conventional sewing threads begin their construction cycle as simple yarns. As you’ve seen, spinning together relatively short fibres or twisting fine continuous filaments produces these basic yarns. Because of their fineness, these fibres and filaments have a large area of intimate contact with each other when held together with their axis parallel. This produces the coherence and strength combined with flexibility, which is essential in any good sewing thread, and it is the twist that is inserted in the basic yarns, usually in the "S" direction, which produces the consolidating force. This is referred to as the "Singling Twist".
The twist in the singling yarn is balanced by applying a twist in the opposite, usually "Z", direction when typically two, three or four yarns are combined to form a sewing thread. This is referred to as the “Finishing Twist”.
Without the correct level of finishing twist, a conventional thread cannot be controlled during sewing. The individual plies would separate during their repeated passages through the needle and over the sewing machine control surfaces. Twist is therefore defined as the number of turns inserted per metre (or turns per inch) of the yarn or the thread produced.
If the twist is too low, then the yarns may untwist, fray and break, if it is too high then the resulting liveliness in the thread may cause snarling, looping, knots or spillage from the final package.
As you can see the term "S" or "Z" twist direction is derived from the diagonal of these letters following the direction of the twist. “S” twist is sometimes referred to as right twist and “Z” twist is sometimes referred to as left twist.
The continuous filament yarns shown at the beginning of this session can also be twisted to make continuous filament sewing threads. Continuous filament yarns can also be assembled in 2, 3, or 4 plies to create continuous filament polyester or nylon sewing threads. These include textured threads.
A whole range of sewing threads can be produced from the few raw materials we’ve discussed today.
Corespun is the best general-purpose apparel sewing thread available in the market today. Corespun combines the strength and elongation characteristics of the Continuous Filament Core with the sewing performance and surface characteristics of the spun fibre threads.
The filament component is merged with the staple fibres in the yarn spinning operation. The filament takes up its position in the centre of the yarn with a protective sheath of staple fibres wrapped around it. These composite yarns are then twisted to form a plied thread.
The two main Corespun constructions are Polyester Filament Yarns covered and protected by Polyester fibres or Polyester Filament yarns covered and protected by Cotton fibres.
Staple Spun Polyester threads are manufactured from high tenacity staple fibres. A typical high tenacity fibre used for sewing thread would be 1.2 denier, which is a measure of the linear density and 38mm long with a tenacity of at least 7.5 grams per decitex. Some thread producers use a fibre length of 45mm or even 55mm depending on the machinery they have available. SSP threads are produced in a wide range of constructions and sizes, tex and ticket numbers, to accommodate most general sewing applications.
Staple Spun Cotton threads are generally from high-grade long staple fibres. These cotton threads are of three distinct types, Soft, Mercerised and Glace or Polished.
Soft threads receive no special treatment other than bleaching or dyeing and the application of a uniform, low friction lubricant.
Mercerised threads are treated under tension, in a solution of caustic soda which causes the fibres to swell and become rounder in the cross-section. This process enhances the lustre and increases the strength of the fibres. The dye affinity is also enhanced by this process.
Glace cotton threads are produced from soft cotton threads by giving them a polishing treatment. This process applies a coating of starch to the surface of the thread, brushes the fibre ends into the body of the thread and dries them to form a smooth surface on the thread. The polishing process increases the strength of the thread by about 10% but more importantly this process protects the thread from abrasion during heavy-duty sewing operations. Glace finishes are also applied to Polyester / Cotton corespun threads.
Continuous Filament Threads - Coats produces a variety of different continuous filament threads with different physical characteristics which satisfy particular sectors of the sewing thread industry. The Continuous filament threads produced by Coats are made from Polyamide [nylon], Polyester and Rayon.
Continuous Filament threads are significantly stronger than their equivalent size in Corespun, SSP (Staple Spun Polyester) or Cotton.
Some varieties of Continuous Filament Threads are:
Soft CF Thread which is the simplest form. In the production of a continuous filament yarn the filaments are gathered from the spinneret into a continuous strand, each strand comprising a specific number of filaments dependant on the desired characteristics.
These strands are then combined and twisted conventionally into plied constructions similar to the post spinning processes used for spun threads.
These threads are used in sewing applications where the seam strength is particularly important such as footwear and fine leather goods.
Some of these threads are given an additional process called Bonding.
In this case, the threads are coated with a soluble resin like Nylon or Polyurethane for Continuous Filament Nylon and Polyester or Polyurethane for Continuous Filament Polyester.
After application, the resin is cured and dried which has the effect of holding the plies together. The application of the bonding agents also reduce the abrasion on the thread during the sewing operation.
Trilobal Polyester is a specific type of Continuous Filament Polyester mainly used for embroidery thread.
Trilobal Polyester has been modified to maximise the lustre of the thread created by the individual filaments having a triangular cross section.
Continuous Filament threads are modified by further processing to produce what we now call Textured threads. These can be produced from Nylon (Textured nylon or TXN) or from Polyester (Textured Polyester or TXP).
These threads are designed to be bulkier and softer than the continuous filament twisted threads and can be produced in different ply constructions. The most common method of producing these threads is by false twisting. In this process, the filament yarn is subjected to heat, by contact or by convection, to soften the filaments. The yarn is then subjected to a rotational force which results in twist being inserted. As the thread exits the twisting zone of the machine, the twist is removed, but as the yarn is still in a thermoplastic condition the yarns memory retains some of the distortion imposed in the twisting zone. This process results in the individual filaments adopting a crimped shape giving a soft bulky thread with high stretch characteristics. They are most suitable as under threads in lightweight chainstitching, overlocking and coverseaming operations.
Locked Filament Polyester sewing threads are produced by a technique involving the heating and stretching of the continuous filaments. The filaments are entangled and heat treated to produce a consolidated thread. Locked Filament Polyester threads have many of the characteristics and the performance of threads produced by conventional spinning and twisting routes but the thread does have a different handle or feel to it.
All synthetic threads, Polyester / Cotton Corespun threads and some 100% Cotton threads are wet processed on what are called cheeses. These cheeses are produced on the twisting frame or at a separate process called cheesewinding for dyeing.
The packages for dyeing are wound so that they are of a constant density and permeability to allow the dye liquor to be pumped evenly through the packages.
A cheese dyeing kier is used to dye sewing thread. The cheeses of grey thread are stacked onto the perforated hollow spindles of the dye machine carriers and compressed under controlled conditions to create a sealed column of thread of uniform density.
The lid of the dye kier is clamped shut. The dye liquor is pumped in under pressure and raised to the dyeing temperature.
The dye liquor flows through each thread package, alternately in-to-out and out-to-in at pre-set intervals. These cycles, together with the temperature, timing and the addition of all the other chemicals called for by the dye recipe for a specific shade, are regulated automatically by computer controls in modern dye kiers. This ensures even, level dyeing to precise standards.
When dyeing 100% cotton threads it is sometimes necessary to add bleaching cycle to the dye process in order to remove the natural colouring matters and chemical impurities in the Cotton fibres before they are dyed.
Different classes of dyestuffs are required for different substrates of sewing threads. Polyester is dyed using disperse dyestuffs at around 130°C whereas Cotton can be dyed with direct, reactive or vat dyestuffs usually at temperatures at below 100°C.
Polyester / cotton corespun threads contain two different components, unlike the sewing threads which are made from just one substrate, each component requires a different class of dyestuff and therefore the dyeing process is more complex and takes more time. The dyestuffs are selected to produce the same shade on both substrates and the two separate dye cycles are carried out consecutively in the same overall dyeing operation.
The dyed packages are then uniformly dried using centrifuges to extract excess moisture and then the cheeses are finally dried using hot air drying systems or radio frequency dryers.
Cotton threads are sometimes Mercerised. In this process, the Cotton fibres are impregnated with a caustic soda solution, which causes them to swell and in so doing, untwists their natural convolutions, before being stretched and the caustic solution washed out.
This produces a stronger, lustrous thread with an increased affinity for dye liquors. Mercerising is usually done in hank form and the dyeing process is also done in hank dyeing machines.
Before undergoing the mercerising process the grey 100% cotton thread is sometimes gassed or singed that is, run through a carefully controlled gas flame to remove extraneous fibres from the thread surface. This further enhances the lustre of the final product.
During sewing, considerable heat is generated at the needle of the sewing machine and an average temperature of 330 deg Celsius is common. This heat is generated through friction as the needle of the sewing machine punches its way through the layers of fabric being sewn. Both Polyester and Nylon threads will melt at temperatures below 300 deg Celsius. Most thread breaks that occur during sewing are caused by the synthetic thread melting in the needle eye when the sewing operation stops momentarily to change direction or while starting a new seam. They are not always caused by excessive thread tension.
To prevent thread damage at the needle eye and to help the passage of the thread through the thread path of the sewing machine, most threads are lubricated after dyeing. Most lubricants are combinations of Paraffin Wax that helps reduce friction and Silicone that promotes needle cooling. The lubricants can be applied at the dyeing kier, at a separate lubricant application process or more commonly at final winding. The Lubricant Content and the Friction Index helps ensure good sewing. The amount of lubricant applied is critical to successful sewing and the actual level of lubricant is carefully controlled.
After the lubricant and any special finishes have been applied, the thread is passed to Final Winding where the thread is wound onto the support (cone, vicone, tube etc) to the length that the customer requires. The type of support used will depend on the substrate of the thread being wound and the end use of the thread.
The final operation is Finished Goods Packing, where the product is labelled, wrapped and boxed prior to being sent to the warehouse for immediate despatch to the customer or for warehouse stock.
The preferred method of sizing sewing thread for Coats Global Offer Products is the Tex System. Tex is the weight in grams of 1,000 metres of thread or yarn. The higher the number the coarser the thread.
An alternative sizing unit in common use for synthetic and synthetic blended threads is Metric Ticket. The metric ticket number is derived from taking the number 1,000 divided by the tex number multiplied by 3. The higher the number the finer the thread.
100% cotton threads still utilise the Ne [number] English Sizing System known as Cotton Count and Cotton Ticket number.
Here is a useful apparel thread size conversion table. This includes the US ticketing system as well as Singer and Metric needle sizing.
Apparel Thread Conversion Table Tex Size US Ticket Metric Ticket Cotton Count Singer Needle* Metric Needle* 18 120, 100/80 160 60/2 9 - 11 65 - 75 21 100 140 - 9 - 11 65 - 75 24 100, 100/60 120 - 10 - 11 70 - 75 27, 30 70, 70/40 100 60/3 12 - 16 80 - 100 35 70 80 - 12 - 16 80 - 100 40 50, 60/36 75 40/3*Needle size recommendations are nominal and ultimately depend on the sewing application.
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