Lecture 17 Drawing practice and forceLecture 17 Drawing practice and force

Course Hour: 2Course Hour: 2

Basic requirements: Acquainting with Basic requirements: Acquainting with

different type of drawing processes, different type of drawing processes,

advantages and disadvantages; grasping the advantages and disadvantages; grasping the

die pressure and drawing forcedie pressure and drawing force

Emphasis: drawing practiceEmphasis: drawing practice

Difficulties: drawing force and die pressureDifficulties: drawing force and die pressure

Drawing ProcessDrawing Process

Like extrusion but material Like extrusion but material pulled pulled out the orifice of thout the orifice of the required shape instead of being pushed in.e required shape instead of being pushed in.

Resulting shape variety is more limited than extrusionResulting shape variety is more limited than extrusion – – Rod (larger cross sections)Rod (larger cross sections) – – Wire (smaller cross sections)Wire (smaller cross sections)

AdvantagesAdvantages – – Very longer product than extrusion - wire, rod (tVery longer product than extrusion - wire, rod (t

otal length can exceed miles)otal length can exceed miles) – – Non circular cross sections can be producedNon circular cross sections can be produced DisadvantagesDisadvantages – – Higher forces requiredHigher forces required – – If exceed tensile strength further deformation cIf exceed tensile strength further deformation c

an an be non-uniformbe non-uniform – – Limited size reduction per pass (Maximum 63%)Limited size reduction per pass (Maximum 63%) – – Need multiple diesNeed multiple dies

Drawing ProcessDrawing Process

Drawing of round sectionsDrawing of round sections Drawing forceDrawing force – – The drawing stress The drawing stress σσd can be obtained by the samd can be obtained by the sam

e approach as that in extrusion,e approach as that in extrusion,

–– For strain-hardening materials Y is replaced by an For strain-hardening materials Y is replaced by an average flow stress. For a material that exhibits the average flow stress. For a material that exhibits the true stress-true strain behavior of The avetrue stress-true strain behavior of The average flow stress is given by rage flow stress is given by

the Drawing force is thenthe Drawing force is then F= AF= Affln(Aln(A00/A/Aff))

Die pressureDie pressure The die pressure at any diameter along the The die pressure at any diameter along the

contactcontact

The tension stress and die pressure distributions The tension stress and die pressure distributions are shown beloware shown below

Drawing at elevated temperatureDrawing at elevated temperature At elevated temperatures, the flow stress of At elevated temperatures, the flow stress of

metals is a function of the strain ratemetals is a function of the strain rate The average true strain rate in the deformation The average true strain rate in the deformation

zone in drawing can be given by zone in drawing can be given by

Optimum die angleOptimum die angle There is an optimum die angle in drawingThere is an optimum die angle in drawing The optimum die angle for the minimum force The optimum die angle for the minimum force

increases with reductionincreases with reduction Optimum angles in drawing are rather smallOptimum angles in drawing are rather small

Maximum reduction per passMaximum reduction per pass

If the drawing stress reaches the yield stress of the material, the product will If the drawing stress reaches the yield stress of the material, the product will

simply continue to yield further as it leaves the diesimply continue to yield further as it leaves the die

Thus, the maximum possible drawing stress can only be equal to the yield Thus, the maximum possible drawing stress can only be equal to the yield

stress of the exiting material. The maximum reduction per pass=(Astress of the exiting material. The maximum reduction per pass=(A00-A-Aff)/A)/A00=1-=1-

1/e=63%1/e=63%

Effect friction and die angle on maximum possible reductionEffect friction and die angle on maximum possible reduction

Examples of Tube-Drawing Examples of Tube-Drawing OperationsOperations

Examples of tube drawing operations, with and without an internal mandrel. Examples of tube drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thicknesses can be produced from Note that a variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes)the same initial tube stock (which has been made by other processes)

Lecture 18 Drawing die and equipmentLecture 18 Drawing die and equipment

Course Hour: 2Course Hour: 2Basic requirements: Acquainting with Basic requirements: Acquainting with drawing equipment, grasping the principles drawing equipment, grasping the principles of die designs for drawing processes; of die designs for drawing processes; acquainting with die wear and residual acquainting with die wear and residual stress in drawingstress in drawingEmphasis: Die design principlesEmphasis: Die design principlesDifficulties: Residual stressDifficulties: Residual stress

Die for Round DrawingDie for Round Drawing Die angles usually range from 6 to 15 degreesDie angles usually range from 6 to 15 degrees The purpose of the die land is to size and set the final The purpose of the die land is to size and set the final

diameter of the productdiameter of the product When the die is reground, the die land maintains the exit When the die is reground, the die land maintains the exit

dimension of the die openingdimension of the die opening A rod or wire is fed into the die by first pointing it by swagingA rod or wire is fed into the die by first pointing it by swaging

Die for Round DrawingDie for Round Drawing

Tungsten- carbide die insert in Tungsten- carbide die insert in a steel casing. Diamond dies, a steel casing. Diamond dies, used in drawing thin wire, are used in drawing thin wire, are encased in a similar mannerencased in a similar manner

Die material are usually tool steel, carbides, or diamondDie material are usually tool steel, carbides, or diamond In addition to rigid die, a set of idling rolls are also used in In addition to rigid die, a set of idling rolls are also used in

drawingdrawing

Die wearDie wear Die wear is the highest at entryDie wear is the highest at entry The factors, which influence the wear are pressure, variation in diameter of The factors, which influence the wear are pressure, variation in diameter of

the wire and vibration, and the presence of abrasive scale on the surfacethe wire and vibration, and the presence of abrasive scale on the surface

Roll StraighteningRoll Straightening

Cold DrawingCold Drawing

Cold drawing of an extruded channel on a draw bench, to Cold drawing of an extruded channel on a draw bench, to reduce its cross-section. Individual lengths of straight rod reduce its cross-section. Individual lengths of straight rod or of cross-sections are drawn by this method.or of cross-sections are drawn by this method.

Multistage Wire-DrawingMultistage Wire-Drawing

Two views of a multistage wire-drawing machine that is typicaTwo views of a multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiringlly used in the making of copper wire for electrical wiring

LubricationLubrication Dry drawingDry drawing

– – The surface of the rod or wire is coated with various The surface of the rod or wire is coated with various

lubricantlubricant

– – The most common lubricant is soapThe most common lubricant is soap

– – The rod is first treated by picklingThe rod is first treated by pickling

– – Soft metal may also coated high-strength material as Soft metal may also coated high-strength material as

lubricantlubricant

– – Conversion coating may consist of sulfate or oxalate Conversion coating may consist of sulfate or oxalate

coatings (soap, polymers)coatings (soap, polymers) Wet drawing, Dies and the rod are completely immersed in Wet drawing, Dies and the rod are completely immersed in

a lubricant. Typical lubricants are oil and emulsions a lubricant. Typical lubricants are oil and emulsions

containing fatty or chlorinated additives, and various containing fatty or chlorinated additives, and various

chemical compoundschemical compounds

Residual stressResidual stress Cold drawn rod, wire, or tube usually contains residual stress due to inhomogeneous Cold drawn rod, wire, or tube usually contains residual stress due to inhomogeneous

deformationdeformation

For very light reduction, the surface residual stresses are compressive, which For very light reduction, the surface residual stresses are compressive, which

improves fatigue life of the componentsimproves fatigue life of the components

Residual stress can also be significant in stress-corrosion cracking over a period of Residual stress can also be significant in stress-corrosion cracking over a period of

time, and in warping of component when a layer is subsequently removed, such as time, and in warping of component when a layer is subsequently removed, such as

by machiningby machining

HomeworkHomework Why a cold-drawn rod, wire, or tube c

ontains residual stress? In which situation the surface residual stresses are compressive?

Which stress environment is desired in metal working operations? Why?