SMN15 Lederverarbeitung 02 2007 - Universal Sewing

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i n foT E C H N I C A L I N F O R M AT I O NSEWING 15NEEDLES FOR THE PROCESSING OF LEATHERPERFECT SEAMS FOR ALL APPLICATIONSThe leather worker’s trade is one of theare given priority during the constructiveoldest in the world.and sewing development of the needle:These days in most of the cases thefashion industry decides about the seamappearance, besides good durability. Agood seam construction should also offera decorative seam appearance. In orderto guarantee economic production, suchseams are produced on modern machinesand with high machine speeds.Customer’s requirements are in generalpermanent production security with minimum machine downtime and constantproduct quality. The following demands Minimum needle breakage Minimum skip stitches Minimum thread breakage Quality of the cutting point(sharpness and position) Maximum lifetimeThe results of this research and development work are realized in today’sGroz-Beckert cutting point needles.

i n fo SEWING 15THE SELECTION OF THE CORRECT NEEDLE POINTBased on the sort of leather and its compositionSoft leatherLeather with medium hardnessHard and thick n:R, SD (LL, LR)Depending on the requested seam ap-LR, VR, D, DH, DIpearance, all cutting points can be used.Based on the demands on seam appearanceStraightSlantedstitchesstitchesFilled stitch holesOpen stitch holesElevatedDeep-setstitchesstitchesThe pictures illustrating seams shown here and on the following pages, are based on the common threadingdirection from left to right.PA G E 2Seams were produced on leather of medium hardness.

i n fo SEWING 15THE RIGHT NEEDLE POINT FOR THE REQUIRED STITCH APPEARANCELR - pointApplication field:Cuts the leather to the right at a 45 angleFor the production of decorative seams onin the direction of sewing. Slight to mediumsoft to medium/ hard leather. Can be usedslanted stitch formation, depending onon almost every type of leather.the leather. The sewn thread is slightlyelevated. Stitch holes are easily visible.Suitable for short to medium stitches.Example:Shoes, bags andleather garments,trekking shoes,suitcases, VR - pointApplication field:Technically obsolete point style. CuttingFormerly recom-effect, seam appearance and seammended for hardcharacteristics identical to LR-point.leather. High qualityLR-points areunconditionallyapplicable for theseleather types.LL - pointApplication field:Example:Cuts the leather to the left at a 45 angleFor the productionBags, shoesin the direction of sewing. Straight stitchof straight seamsand car seats formation. The sewn thread is slightlywith a continuouslyelevated. Stitch holes are filled. Suitableclosed seam ap-for medium to short stitches.pearance. Can beused on almostevery leather type.D - pointApplication field:Example:Cuts the leather with a prominent triangularSuitable forBelts, suitcases,shape. Straight stitch appearance. The sewnstraight sewing.heavy shoes andthread is slightly elevated. Stitch holes areEffective on hardplastics relatively large. Medium to short stitchesand thick leatherpossible.as well as cartons.PA G E 3

i n fo SEWING 15THE RIGHT NEEDLE POINT FOR THE REQUIRED STITCH APPEARANCEDH - pointApplication field:Example:Leather is cut with a medium triangularSuited to producingUpholstery, bags,shape. Straight stitch appearance. Thestraight seams.shoes, tarpaulins,sewn thread is slightly elevated. StitchGood performanceawnings andholes are relatively large. Suited to mediumon medium-hardtents to long stitches.and hard leathers.SD - pointApplication field:Example:Round set point with a small triangular tipProducing stitchesShoes, leather gar-cutting edge. Cuts leather with a slight tri-that are almostments, fine leatherangular shape. Straight stitch appearance.straight and in-line.goods, leatherThe sewn thread is slightly elevated.Used on soft leatherembroidery (alsoSuitable for short to medium stitches.and multidirectionalon foils and lami-applications.nated materials) P - pointApplication field:Example:Cuts the leather perpendicular to theProduces decorativeShoes, belts andsewing direction. On thick and hard leatherstitches. Well suitedupholstery it creates a prominent decorative stitchfor heel and closeddue to the heavily slanted stitch formation.seams.The sewn thread is raised and exposed.The sewn thread fills the stitch holes.Short stitches possible.PCR - pointApplication field:Example:Stitch appearance same as the P-point.Producing decora-Shoes, upholsterytive stitches, partic-and bags ularly for the leftside of twin lockstitch machines.PA G E 4

i n fo SEWING 15THE RIGHT NEEDLE POINT FOR THE REQUIRED STITCH APPEARANCEPCL - pointApplication field:Example:Seam appearance like the P-point.For the productionShoes, bags andof very decorativeupholstery .seams, especiallyfor the right seamon twin lockstitchmachines.S - pointApplication field:Example:Cuts the leather in direction of sewing.For the productionShoes, bags, belts,Straight in-line stitch appearance. In certainof seams withleather clothesleathers and with particular stitch lengths,deep-set, straightand upholstery deep-set stitches are formed. Long andstitches. Suitablevisible stitch holes. Suited for medium tofor most leathers.long stitch lengths.DI - pointApplication field:Example:Stitch appearance similar to the S-point.For the productionSuitcases, bags,Better cutting effect due to the 4 cuttingof seams withheavy shoes edges. Precise stitches.deep-set, straight(also for awnings)stitches.Suitable for mediumand hard leathers.R - pointApplication field:Example:Standard round point.Used on soft leather,Sport shoes,Cloth point – without cutting effect.embroidery andgarments, carAlmost straight yet irregular and variablemultidirectionalseats. (also onstitch appearance. Sewing thread is slightlyapplications.foils and lamina-elevated. Irregular and inconsistent stitchted materials)holes.Suited to medium to long stitches.PA G E 5

i n fo SEWING 15SEAM STRENGTHThe threadThread related seam strengthmore thread is naturally in the seam. Therefore, withan increasing number of stitches the thread relatedseam strength is also increased.Thread related seam strengthThe higher the stitch rating (stitches per cm), theIncreasing number of stitchesThe leather and the needle pointsLeather tensile strengthThe higher the stitch rating and the larger the needle,Leather tensile strengththe more the leather seam is cut.Stitch-hole density andLeather tensile seam strength also depends on whichleather tensile strengthpoint style is chosen. The highest leather tensilecurvestrength is achieved with a P-point, which cuts theleather perpendicular to the seam direction. Thelowest leather tensile strength occurs with an S-point,which cuts the leather in direction of the seam (onconventional lockstitch machines).SPIncreasing number of stitchesThe curves of all other cutting points are between Pand S.The seamSeam tensile strengthIn practise, when tearing the seam (loading cross-Optimum stitch lengthwise) maximum seam strength is achieved when the“P-point”point of intersection of the thread related seamstrength and the leather tensile strength dependingon which cutting point is used.When changing the parameters cutting point andthread, the stitch length has to be adjusted to achievethe maximum seam strength again.Seam tensile strengthleather and thread break simultaneously. This is theMax seam strength“P-point”Max seam strength“S-point”Optimumstitch length“S-point”PA G E 6SPIncreasing number of stitches

i n fo SEWING 15SEWING OF LEATHER ON TWIN LOCKSTITCH MACHINESTwin lockstitch machines produce two parallel rows of stitches simultaneously. Ideally,the left and the right should have the same appearance. Due to different mechanicalsewing principles between the left and the right needle, a seam appearance that isexactly the same is not normally possible.Twin lockstitch machineDifferent loop pickupTwist shiftingR - point For the machinist the left needle is During the down-stroke the sewingWhen using a standard round point-R,threaded from right side to left. Thethread of the left needle is pulled overthe left-hand side stitches are usuallyneedle thread loop is penetrated by thethe right edge of the eye and theless slanted than the right. The left rowhook point and then pulled out againstsewing thread of the right needle isof stitches demonstrates stronger ply-the direction of the material feed.pulled over the left edge of the eye.twist shifting (untwisting). The right needle is threaded from the When using normal “Z” ply-twistThe degree of stitch appearance differ-left side to right. The needle thread loopthreads, the left needle produces anences depends on the properties of theis pulled out in the direction of theopposite rotational force than the right,thread, the leather and the selection ofmaterial feed.creating a different ply-twist shift in thethe point style of needle.thread. Generally the heavier the threadthe more pronounced this effect becomes. By using a CR-point designation on theleft hand needle, the ply-twist shiftingcan be reduced slightly.Needles with cutting points produce greater consistency of stitch appearance.This is most visible with the cutting points:LR - cutting pointLL - cutting point(on both sides)(on both sides)Stitch to stitch, left and rightstitches are regularly slantedtowards the left.This results in an attractiveseam of two duplicated rowsof stitches. Stitch holes areeasily visible.An LL cut in leather and theresultant stitch knot form produce thread twisted into thiscut and towards the directionof the seam. This results in astitch appearance that isalmost straight and with filledstitch holes.PA G E 7

Upholstery, bags, shoes, tarpaulins, awnings and tents Example: Shoes, leather gar-ments, fine leather goods, leather embroidery (also on foils and lami-nated materials) Example: Shoes, belts and upholstery Example: Shoes, upholstery and bags Application field: Suited to producing straight seams. Good performance on medium-hard and .