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Sheet Metal Working: Series of opera+ons that involve cu3ng, bending and drawing sheet metal. Sheet metal (from 0.4mm or 1/64in to 6mm or 1/4in thickness); Plate (from 6mm upwards). Opera+ons are usually performed as cold working.
Thickness= 0.4 to 6 mm (1/64 to 1/4 in) Advantages of Sheet Metal: • Rela+vely low cost. • Good dimensional accuracy and good surface finish. • High strength
Basic Types of Sheet Metal Opera<ons
Cu3ng: It involves processes such as punching, shearing and blanking. Bending: Deform the sheet around a straight axis. Drawing: Deform the sheet into convex or concave shapes.
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Mechanical Proper<es of Sheet Material
When subjected to a tensile force there are three deforma+ons to be measured: the longitudinal strain, the strain in the width direc+on and the strain in the thickness direc+on. The material is anisotropic.
Shearing: Shearing of sheet metal between two cu3ng edges: (1) just before the punch contacts work (2) punch begins to push into work, causing plas+c
deforma+on; (3) punch compresses and penetrates into work
causing a smooth cut surface; (4) fracture is ini+ated at the opposing cu3ng
edges which separates the sheet.
Forces in a Shearing Opera<on
Force = 0.7×UTS × t × L
L = Total length Sheared t = thickness
Shearing: A cu3ng metal opera+on usually along a straight line, between two cu3ng edges.
Punching and Blanking
Punching and blanking are very similar. In punching the cut piece is scrap, while in blanking the cut piece is the desired product.
Conven+onally sheared surface showing the dis+nct regions of deforma+on and fracture and (boYom) magnified view of the sheared edge. (Courtesy of Feintool Equipment Corp., Cincinna7, OH.)
Percent Penetra<ons Material % Penetra<on Silicon Steel 30 Aluminum 60 .10 C Steel Annealed 50 .10 C Steel Cold Rolled 38 .20 C Steel Annealed 40 .20 C Steel Cold Rolled 28 .30 C Steel Annealed 33 .30 C Cold Rolled 22
E.V. crane, Plas+c Working in Presses, John Wiley and Sons, Inc., New York, 1948, p. 36
Penetra+on=Roll over + Burnish
Characteris<cs of a Die Cut edge Roll Over – Flow of material around the punch and die .The larger the clearance the greater the roll over Burnish – The rubbed or “cut” por+on of the edge. The sharper the punch the wider the burnish Fracture – The angled surface where the material separates from the parent material Burr – The very sharp projec+on caused by a dull cu3ng on the punch or die.
Fineblanked surface of the same component as shown. (Courtesy of Feintool Equipment Corp., Cincinnati, OH.)
Die size determines blank size Db; punch size determines hole size Dh.; c = clearance
Clearance: It is defined as the distance between the punch cu3ng edge and the die cu3ng edge. It depends on the hardness and thickness of the material. As the thickness increases, the clearance must increase. The clearance typical values ranges from 4% to 8% of the thickness of the material. The recommended clearance is calculated by: c=a.t where c=clearance, t=thickness and a=allowance
Forces in a Punching Opera<on
Force = 0.7×UTS × t × L
L = Total length Sheared t = thickness
Metal group a _ aluminum alloys (1100, 5052) 0.045 aluminum alloys (2024 and 6061); brass, 0.060 so? cold rolled steel, so? stainless steel cold rolled steel, stainless steel, (hard & half-‐hard) 0.075
Angular Clearance: Allows the blank or the slug to drop off easily. Typical values ranges from 0.25degrees to 1.5 degrees
For a round blank of diameter Db is determined as: Blank punch diameter = Db -‐ 2c Blank die diameter = Db
For a round hole (piercing) of diameter Dh is determined as: Hole punch diameter = Dh Hole die diameter = Db + 2c
Forming Proper<es of Sheet Metal Sheet metal, due to its manufacturing process is not an isotropic material. Anisotropy is caused by the thermal processing of the sheet. Two types, namely crystallographic anisotropy and mechanical fibering.
Typical Range of Average Normal Anisotropy (Ravg) for various Sheet Metals
Zinc 0.2
Hot rolled steel 0.8-‐1.0
Cold rolled rimmed steel 1.0-‐1.35
Cold rimmed aluminum–killed steel 1.35-‐1.8
Aluminum 0.6-‐0.8
Copper and Brass 0.8-‐1.0
Titanium 4-‐6
Example: Low carbon steel exhibits an upper and lower yield strength. As a result during deforma+on, it shows stretch strain bands (Lueder’s bands). These can be eliminated by a reduc+on of thickness of 0.5 to 1.5% by cold rolling.
Example Es+mate the force required for punching a 25mm diameter hole through a 3.2mm thick annealed +tanium alloy Ti-‐6Al-‐4V sheet at room temperature. Data: UTS = 1000MPa
F = 0.7×UTS × t × L = 0.7×1000MPa×0.0032m×π ×0.025=176kN
Cutlery manufacturing: Cutlery manufacture involves blanking the stainless steel or sterling silver to the proper shape. A series of rolling opera+ons then gives the piece the correct thickness. Aper heat treatment and trimming, the piece has a paYern embossed on it in a stamping opera+on. Finally, the piece is buffed and polished.
Blanking: The metal inside a closed contour is the desired part
Punching: The metal inside the contour is discarded.
Notching: Edges or corners of a material is punched.
Trimming: Cu3ng scrap or excess material for a fully or par+ally shaped part
Shaving: Finishing opera+on of a previously cut edge by removing a minimum amount of material.
Progressive Stamping Dies Common method to handle complex parts
Deep Drawing Blanking
Deep Drawing
Re-‐drawing
Ironing
Doming
Necking
Seaming
Complex 3D shapes can be made out of sheet metal.
Usually a cold working process. A punch forces a flat sheet metal into a deep die cavity. The die cavity is usually circular or rectangular. When the depth of the product is greater than its diameter, it is known as Deep Drawing and when the depth of the product is less than is diameter, it is known as Shallow Drawing. The sheet metal is supported on both sides by the blankholder, to avoid wrinkling If the Hold-‐down pressure (blankholder force) is too high the sheet will tear and if it is too low it will wrink.
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Draw beads may be used to control metal flow.
Es<ma<on of the Blank Diameter
π4Do2 =
π4d12 +πd1h⇒ Do = d1
2 + 4d1h
Formability Test: Deforma+on in sheet materials are carried out by either stretching and/or drawing. The ability of the sheet to withstand large degrees of streching or drawing deforma+on (shape change) without failure is known as formability.
Erichsen Test – Cupping Test
A round punch is forced into a clamped sheet un+l a crack (sudden drop in force) appears.
Forming Limit Diagram (FLD)
A FLD shows what combina+ons of the major and minor strains produce failure in a sheet metal. To develop the FLD, the major and minor engineering strains are obtained. The curves represent the limits of drawing between failure and safe regions,
By measuring the ellipses on the deformed paYern, the largest and shortest direc+ons of the ellipses are the major strains and minor strain respec+vely. Please note that the axes for these strains are 900 apart. This can be carried out at many different loca<ons in the work-‐piece. If both major and minor strains are posi<ve, the deforma+on are stretching, and the sheet metal will decrease in thickness. If the minor strain is nega<ve, this contrac<on may par+ally or whole compensate any posi<ve stretching in the major direc<on. The combina<on of tension and compression is known as drawing, and the thickness may decrease, increase, or stay the same, depending on rela+ve magnitude of the two strains.
Example:
A grid of 2.5mm circles is electroetched on a blank of AK sheet steel. Aper forming into a complex shape the circle in the region of cri+cal strain is distorted into an ellipse with major diameter of 4.5mm and minor diameter of 2.0mm. Is the component close to failure??
Major strain Minor strain
ε1 =4.5− 2.52.5
⋅100 = 80%
ε2 =2.0− 2.52.5
⋅100 = −20%
The coordinates indicate that the part is in imminent danger.
Limit Drawing Ra<o
It is defined as the ra+o between the largest diameter of the blank that can be drawn into a specific punch diameter without failure: The recommended drawing ra+os are the following: • for the first drawing: ~2 • for the second drawing: 1.2 – 1.25 • for the third drawing: 1.15 -‐1.18 • for further drawings: 1.1 -‐1.12
LDR = Maximum−Dblank
Dpunch
DR = Dblank
Dpunch
Reduc<on:
The value of the reduc+on (r) should be less than 0.5 for a cylinder.
reduction = r =Dblank −Dpunch
Dblank
Thickness to Diameter ra<o thickness− diameter − ratio = t blank
D blankThe thickness of the star+ng blank divided by the blank diameter. The recommended values for this ra+o are greater than 1%. As the ra+o decreases, the tendency for wrinkling increases.
The star+ng diameter of the blank must be of the right size for the final dimensions of the cup to be correct. Assume constant volume and neglect any thinning during the process.
Example: Determine if the following is feasible for manufacturing: A cylindrical cup with an inside diameter of 3.0in and height of 2.0in. Its star+ng blank size id 5.5in and its thickness 3/32in.
DR = Dblank
Dpunch
=5.53.0
=1.833< 2
r =Dblank −Dpunch
Dblank
=5.5−3.05.5
= 0.4545< 0.5
t blankD blank
=332( )5.5
= 0.017 > 0.01The drawing opera+on is feasible.
Anisotropy Ra<o
There are two different types of anisotropy ra+o, namely, normal and planar anisotropy ra+o. Normal Anisotropy Ra<o (R) : Measured in a tensile specimen, it is the ra+o between the true strain in the width direc+on and the true strain in the thickness direc+on. The tensile specimen must conform specific technical standards. The longitudinal direc+on of the tensile specimen can be parallel or to a certain angle with respect to the rolling direc+on of the sheet.
R = εwidthεthickness
Raverage =R0 + 2R45 + R90
4
Planar Anisotropy Ra<o: It determines the varia+on of the true strain in the plane of the sheet (rolling plane). ΔR = R45 −
R0 + R902
The value of the normal anisotropy ra+o determines the limi+ng drawing ra+o and the value of the planar anisotropy ra+o correlates with the material propensity to earing. High values of normal anisotropy combined with low values of planar anisotropy provides op+mal drawability. The maximum value of the normal anisotropy also depends on the grain size of the material.
Note: If ΔR=0, no ears form. The height of ears increases as Δ R increases.
The rela+onship between average normal anisotropy and the limi+ng drawing ra+o for various sheet metals. Source: Aper M. Atkinson.
LDR =Maximum blank diameter
Punch diameter=Db
Dp
Example:
A special deep-‐drawing steel showed a 30% longitudinal elonga+on and 16% decrease in thickness when it is subjected to a tensile test. Es+mate the limi+ng drawing ra+o (LDR) for this steel.
l − lolo
= 0.3⇒ llo=1.3⇒ ln 1.3( ) = 0.26236
w−wo
wo
= −0.16⇒ wwo
= 0.84⇒ ln 0.84( ) = −0.1743
R =ln wo
w( )ln ho
h( )=ln wo
w( )ln wl
wolo( )=ln 10.84( )ln 0.84 ⋅1.3( )
=1.98
From the graph the LDR~2.7
Drawing Force
Fmax = πDpt UTS( ) Db
Dp
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%&&− 0.7
(
)**
+
,--
Wrinkling can be reduced if a blankholder is loaded by maximum punch force The force increases with increasing blank diameter, thickness, strength and the ra+o
Bending Some sheet are bend along certain lines to produce a desired shape. Bending introduces plas+c deforma+on to the part and it should remain in the desired shape (angle) aper the load is released. Spring-‐back is the part of deforma+on (the elas+c part) that recovers in the plas+cally deformed material once the load has been released.
Original shape Desired deformed shape Springback
When the load is released there is a decrease in the bending angle (Springback) due to the elas+c recovery of the material.
Bend allowance: The amount of deforma+on of the neutral axis of the sheet depends on the bend radius and bend angle. The final dimension of the neutral axis (in the bending area) is used to calculate the blank length for the bend part. R is the bend radius, α is the bend angle , t is the thickness and k is a constant. In an ideal case, the neutral axis remains at the center of the sec+on k=0.5. In prac+ce, k ranges from 0.33 (for R<2t) to 0.5 (for R>2t).
Lb =α R+ kt( ) =αo Ro + kt( ) =α f Rf + kt( )
As the part is bended, the longitudinal dimension of the flat length is increased. The bend allowance is the amount of material that need to be added to the flange dimensions (leg parts) in other to develop a flat paYern. Example: suppose that flanges lengths of 2” and 3” with an inside radius of 0.250” at 90degrees are required. Then the flat dimensions are (2”-‐(0.25+0.125)) and (3”-‐(0.25+0.125)), i.e 1.625 and 2.625 respec+vely. The length of the flat sheet (bend allowance) is 1.625 + 2.625 + 0.457 = 4.707” Bend deduc<on: It is the amount of material that has to be removed from the sum of the flanges to obtain a flat paYern.
Lb =α R+ kt( ) = π90180
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%& 0.250+ 0.33×0.125( ) = 0.457
Springback
Aper releasing the pressure of the forming tool, the deformed work-‐piece experience a dimensional change (strain) due to the elas+c recovery of the material.
Original shape Desired deformed shape Springback
Sprinback is found in all forming opera+ons, but is is more pronounced in bending. As the yield strength of the material increases or as the Modulus of elas+city decreases the springback deforma+on increases.
Overbending, i.e. bending to a smaller radius of curvature than the required can compensate for the springback of the material.
RoRf
= 4Roσ yield
Et!
"#
$
%&
3
−3Roσ yield
Et+1
For aluminum alloys and austeni+c steels the springback can be approximated by the equa+on where Ro is the radius of curvature before releasing the load and Rf is the radius of curvature aper releasing the load; t is the thickness, E is the Modulus of Elas+city and σyield is the yield stress.
Example
A 0.0359in thickness sheet (20-‐gage) is bent to a radius of 0.5in. Calculate the radius of the part aper it is bent and the required bend angle to achieve a 90o bend aper springback has occurred. Data: Yield Strength=40000psi; E=29x106psi
RoRf
= 4Roσ yield
Et!
"#
$
%&
3
−3Roσ yield
Et+1
0.5Rf
= 4 0.5 ⋅ 4000029 ⋅106 ⋅0.0359!
"#
$
%&3
−3 0.5 ⋅ 4000029 ⋅106 ⋅0.0359
+1= 0.942
Rf = 0.531in
Lb =αo Ro + kt( ) =α f Rf + kt( )
αo 0.5+0.03592
!
"#
$
%&= 90 ⋅ 0.531+
0.03592
!
"#
$
%&
⇒αo = 95.4o
Bending Force The bending load can be calculated from the following equa+on: Where UTS is the ul+mate tensile strength of the material (psi); L is the length of the bent part (in), t is the thickness (in); W is the width between the contact points (in) or 8t for V-‐bends K is 1.3 for die opening of 8t, 1.20 for die opening of 16t, 0.67 for U-‐bending, 0.33 for a wiping die
Fb =K ⋅L ⋅UTS ⋅ t2
W
Example:
Es+mate the force required for a 90 degrees bending of a St 50 steel of thickness of 2mm in a V die. The die opening can be taken as eight +mes the thickness. The length of the part is 1m.
Die Opening W=8*2=16mm UTS=500MPa
Fb =K ⋅L ⋅UTS ⋅ t2
W=1.33⋅1⋅500 ⋅ 0.002( )2
0.016=166.25kN
Minimum Bend Radius: On the inside of neutral plane, the metal is compressed, while on the outside of the neutral plane is stretched. The outer layers under tension should not be excessively stretched as there is the possibility of rupture. The amount of stretching depends on the sheet thickness and the bend radius. There is a minimum bend radius that depends on the material proper+es. Minimum bend radius 0.5t for sop materials, 3t for spring steels and 1t for the others.
Example: A sheet material is to be bended according to the dimensions given in the figure. Determine the following: (a) star+ng blank size and (b) the bending force necessary if a V-‐die will be used with a die opening dimension of W=1.0”. Data: E=30000ksi ; Yield Strength=40000psi and a tensile strength=65000psi.
Length L = 1.75’, and the length of the part is: 1.5 +1.00 + BA. R/t = 0.187/0.125 = 1.5 < 2.0, so Kba = 0.33 For an included angle A’ = 1200, then A = 600
Lb =α R+ kt( ) = π60180
!
"#
$
%& 0.187+ 0.33×0.125( ) = 0.239"
Length =1.5+1.0+ 0.239 = 2.739"
Force: F= (KbfTSLt2)/W = 1.33 (65,000)(1.75)(0.125)2/1.0 = 2,364 lb
Fb =K ⋅L ⋅UTS ⋅ t2
W=1.33⋅1.75 ⋅65000 ⋅ 0.125( )2
1= 2364lb
Bending and Forming Tubes
Stretch-‐Forming
The form die is pressed into the work with force Fdie, causing it to be stretched and bent over the form. F = stretching force. It is used extensively in the aircrap industry to produce parts of large radius of curvature. The materials used are very duc+le.
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Spinning Ideal for • Lower produc+on volumes • Large parts • Inexpensive tooling
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(a) Schema+c illustra+on of the shear-‐spinning process for making conical parts. The mandrel can be shaped so that curvilinear parts can be spun. (b) and (c) Schema+c illustra+ons of the tube-‐spinning process
Large metal sheets and plates are formed into curved sec+ons using rolls
Roll Bending
Con+nuous bending process in which opposing rolls produce long sec+ons of formed shapes from coil or strip stock
Roll Forming
High Energy Rate Forming Short +me – High Energy forming processes. It includes explosive forming, electrohydraulic forming and electromagne+c forming.
Use of explosive charge to form sheet (or plate) metal into a die cavity. Explosive charge causes a shock wave whose energy is transmiYed to force part into cavity. Applica+ons: large parts, typical of aerospace industry.
Explosive Forming
Electromagne<c Forming
Sheet metal is deformed by mechanical force of an electromagne+c field induced in the work-‐piece by an energized coil. Presently the most widely used HERF process Applica+ons: tubular parts
A pinched aluminum can, produced from a pulsed magne+c field created by rapidly discharging 2 kilojoules from a high voltage capacitor bank into a 3-‐turn coil of heavy gauge wire. Source: Bert Hickman, Stoneridge Engineering.
Hydroforming
Hydroforming uses water at high pressure to force the piece into a specific shape.
