basics of sheetmetal operations

g India Business Center

Design For Manufacturability

Sheet Metal Part DesignBy

Suresh Sunnam

g India Business CenterTable of contents

Process details :•Types of sheet metal operations

•Press working principles

Tool details :•Types of press tools

•Types of Press Machines

•Guidelines :•Part design considerations.

•Heat treatment requirements.

Tolerancing :

Case studies :

g India Business CenterProcess details

Types of sheet metal operations

Sheet metal operations can be classified in to two types.

• Shearing operations

• Non Shearing operations.

•Shearing operations

Blanking : Shearing out a closed contour of a plane stock-strip to make a component is called blanking.The cut-out piece is the blank or component.

BlankScrap

Piercing : Cutting out holes in a blank strip or a semi-finished component using a press tool is called piercing.The cut out piece is scrap or slug.

ScrapPart


Notching : In notching,the punch does not cut on all the sides.It may cut on two or three sides making a change in the contour of the blank.The cut-out material is scrap.

Cropping : When the shape of the component is such that a single line shearing produces a component from a strip already sized to the component width,the operation is called cropping.Cropping is a scrap-less operation.


Parting : When contour is complicated and notching is employed in obtaining the contour,the final separation (cutting-off) of the component is achieved by parting.It can also be employed on simple works where width of strip and component are same.


Lancing : It is the combination of shearing and bending.A tongue is created by shearing three sides and the same is bent on the fourth side downwards using the same punch.

Louvering : Louvering is a combination of shearing and forming operation carried out in a single stroke of the punch.


Trimming : Trimming is an operation carried out to cut off the excess material such as flashes,fins etc.In forged components.Truing the edges of deep-drawn parts also may be called trimming.


•Non-Shearing operations

Bending : Metals take a permanent deformation if they are stressed beyond their elastic limits.In bending,deformation is made to sheet metal in a straight line along,across or in an angle to its sides to obtain a new configuration.Bending done on the edges contributes strength and rigidity to the component.

Flanging : When the width of bent down portion is small compared to the width of the component,it is called flanging.It may be in a straight line or curved.As the name indicates,it is done to create a flange on the part.


Coining : As the name indicates,it is an operation carried out to make coins and similar components by cold-forming a slug of equal volume.The opposite side of the component may have different markings as per the engravings made on the top and bottom punches.

Semi-Piercing : It is done to create a projection of a lug on the face of a component to facilitate location,Spot-welding etc.The amount of displacement of the material is controlled to avoid shearing of the portion.


Embossing : It is done to produce shallow depressions or projections in sheet metals in various shapes & Designs.The thickness of the material should remain the same as the stock in the embossed area also.

Curling : Curling is forming an edge of circular cross-section along the edge of a sheet metal part.Normally done to strengthen the edge often by keeping a wire inside for higher rigidity.


Crimping : It is normally done to close a seam or for setting down.The components are pre-bent.Sometimes crimping is done to fix eye-lets to cable-ends.

Deep-Drawing : Deep hollow components such as cups,containers,shells etc are produced from sheet-metals by stretching the sheet-metal into a die with a punch when the sheet is held between a pressure-pad and die-face,


Collar-Drawing : In order to provide adequate length for threads,the component is first pierced and subsequently drawn,maintaining hole size equal to core-diameter of the thread.This operation is called collar-Drawing.

Forming : Forming in general terms refers to all operations that impart a change of shape to a blank or component without causing change in its thickness.In forming,unlike in bending,the deformation may be in two or more directions including closed contours.


Press Working Principles

First Stage – Plastic deformation.

The stock material has been placed on the die,the press has been tripped, and the punch is being driven toward the die. The punch contacts the stock material and exerts pressure upon it.When the elastic limit of the stock material is exceeded,plastic deformation takes place.

Shearing operation :

Second Stage – Penetration.

As the driving force of the ram continues,the punch is forced to penetrates the stock material, and the blank or slug is displaced into the die opening a corresponding amount.This is the true shearing portion of the cutting cycle,from which the term “Shearing action” is derived.


Third Stage – Fracture.

Further continuation of the punching pressure then causes fracture to start at the cutting edges of the punch and the die.These are the points of greatest stress concentration.Under proper cutting conditions,the fracture extend toward each other and meet.When this occurs,the fracture is complete and the blank or slug is separated from the original stock material.

Closer Look at the shear portion


Importance of cutting clearance on the punch and die :Proper cutting clearance is necessary to the life of the die and the quality of the piece part.Excessive cutting clearance results in objectionable piece-part.Insufficient cutting clearance causes undue stress and wear on the cutting members of the tool because of the greater punching effort required.

Effect of less cutting clearance


Effect of larger cutting clearance


Bending operation :

In bending operations,the material is formed around a straight axis which extends completely across the material at the bend lines.A bending operation produces a plane surface which is at an angle to the original plane of the work piece or stock strip.

Some silent observations in bending :

1.The bend radius is tangent to the inner plane surfaces of the piece part.

2.The bend lines occur at the tangency of the bend radius with the inner plane surfaces.

3.At the bend lines,the bend radius is perpendicular to the inner plane surfaces.

4.The bend axis is located at the center of the bend radius.


Bend elements:


Calculation guide lines for Flat-blank length:

As a general principle the length of the neutral plane of a sheet is equal to the length of a blank.Estimating the required flat work piece length is a matter of determining the theoretical length of the neutral plane.To determine the theoretical length,it is necessary to calculate the bend allowance “A” for each bend.The estimated blank length will then be the sum of the lengths “L” of the bend legs and the allowances.Then,where “B” is the length of the flat blank.

Assumptions :

T= Material thickness.

R=Bend radius.

N=Bend angle (Degrees)

L=Length of flat plane (Legs)

A=Bend allowance.

C=Distance of inner surface to neutral plane.

B = L1 + A + L2.

To find “A” :

A= (R+C)*(2*PI*N)/360 ( OR ) A= (R+C) * 0.01745 N.


Distance “C” from the inner surface to the neutral plane is a variable factor depending largely upon the ratio of the stock thickness “T” to the bend radius “R”. Optimum value for “C” may be considered to be.

Where

R < 2 T C= 0.33T

R = 2T to 4T C=0.4T

R > 4T C=0.5T

Thus,for the bend of any angle

Where

R < 2T A = (R+ 0.33T) 0.01745 * N

R=2T to 4T A= (R+0.4T) 0.01745 * N

R > 4T A= (R+0.5T) 0.01745 * N


Types of Press Tools :

Blanking Tool :


Piercing Tool : Perforating Tool :


Progressive Tool : Compound Die :


Bending Tool : Forming Tool :


Types of Press Machines :

Press Break :

g India Business CenterPart Design Guidelines

Sheet Metal Part Design Guidelines :

Blank Design :A. Minimum Practical Section should never be less than material thickness or .060". A minimum section must be one and one half times material thickness for high shear strength material for the most practical stamping.

B. Radii on Blank Corners - Corners can be sharp if material thickness is 1/16" or less - over 1/16 " allow corner radii (R) equal to 1/2 material thickness. See illustration.

C. Practical Design For Economy Manufacture.

•W = .060 minimum for materials thinner than .060" wider if possible.

•W1 = Never less than material thickness, wider if possible.

•L = 5 x W is maximum depth, should be less if possible.

•L1 = 5 x W is maximum length, should be less if possible.

The above rules (a and b) apply for maximum economy. If followed, all blank periphery can be included in the blanking die, eliminating secondary tooling and operations.


Piercing Round Hole Design :To pierce holes with economical tools and operations, the hole diameter must not be less than the

sock thickness. If the hole diameter is less than the material thickness (or less than .060") it usually must be drilled and deburred and each of these operations is slower than punching.


Illustration "B" indicates a hole diameter with a tolerance of plus or minus .002". We can pierce a hole within these limits on the punch side for approximately 25% to 30% of the material thickness as indicated in Illustration "C". The percent of thickness varies with the shear strength of the materials.

On holes where a machine finish is required, they can be punched undersized, redrilled and reamed to size as shown. (See Illustration "E".)

If the web (distance between the hole and edge of material) is a minimum of the stock thickness, the hole can be punched which is less expensive than drilling and deburring. (See Illustration D.)

A web that is less than the stock thickness will result in a bulge on the blank. Budge conditions would increase progressively as the web decreases, until there would be a complete break through. However, the bulge is hardly visible until the web is reduced to less than 1/2 the stock thickness. These examples would also apply to a web between holes. (See Illustration F.)

If a measurable bulge is not permitted, a drilling and deburring operation may be necessary.

As a suggestion, if the web is too narrow, the profile of the blank could be changed by adding an ear of sufficient dimensions and shape to eliminate the problem. (See Illustration G.)

Another alternate suggestion would be to change the contour of the blank to include the hole as a notch. (See Illustration H.) The notch could either be pierced or be wide enough so it could be included in the blank without a piercing or notching operation .(See Blank Design)

Caution: The addition of the word "thru" to any hole diameter, regardless of tolerances, indicates the requirement of the hole to be reamed. Reaming and the additional chamfer to remove burr add two extra operations to the cost of the part.


Minimum Ratios of Hole Diameter to Stock Thickness :Limitations are established in common practice for most economical production.

Recommended ratios are applicable to all common metals.

P = Punched Hole Diameter (0.062 min. dia.)T = Stock Thickness

Material Ultimate Tensile Strength (PSI) - Ratio P to T

32,000 - P = 1.0T50,000 - P = 1.5T95,000 - P = 2T


Piercing holes adjacent to Bends :Illustration "A" indicates that the minimum inside distance required from the edge of a hole to a bend

is 1-1/2 times the material thickness (T) plus the bend radius (R)

Otherwise, distortion will occur as indicated in Illustration "B" - or piercing after form must be considered.

Illustration "C" indicates a similar condition to "A", except for openings with an edge parallel to bend. In this case the following requirements apply for economical tooling and production:

•When "L" = up to 1" - 2T + R (minimum).

•When "L" = 1" to 2" - 2-1/2T + R (minimum).

•When "L" = 2" or more - 3T to 3-1/2T + R (minimum).


Limits of Extruded Holes:

R = Outer radiusH = Flange heightD = Inner diameterT = Material thickness

Specifications and Measurement of Formed Parts:

Preferred dimensioning and points to measure:

L = Linear dimensions; corner radiusR = Radii;R1 = Typical inside bend orR2 = Radius in flat blankT = Material thickness2 - Typical examples - additional views as needed.


Position of the Form :

"A" illustrates a design that is not desirable for quality or economy. When the form is inside the blank profile, as shown, the material must be torn through the stock thickness and the bend radius. If the part is under stress, this tear will likely cause fatigue failures. In addition, stock tooling cannot be adapted because the flat area adjacent to the form must be held in position during forming, which means extra tooling expense.

"B" illustrates a similar condition, but with the form just outside the blank profile. In this case, the tear extends to the center of the required bend radius.

"C" and "D" illustrates a possible solution by changing the blank profile to provide relief for the bend. Besides eliminating the chance of fatigue under stress, there is a possibility of using stock 90 degreevee punches and dies. The results are better quality and less expensive engineering charges.

If the relief notches in illustration "D" are wide enough compared to the material thickness and shear strength, or are designed like the relief in illustration "E", they can be included in the blanking operation for very little engineering cost and no extra operation.


Height of Form :"A" illustrates a 90 degree bend with

insufficient height (h) to form properly.

Consequently, stock must be added so the form is high enough (H), stock is then cutoff, which means additional tooling and an additional operation.

If "h" is not high enough, the cutoff tool may not have sufficient strength to stand up for a particular material or thickness. This may result in a higher cost secondary operation such as milling.

Illustration "B" indicates how to determine the minimum inside height "H", which in this case equals 2-1/2 times the material thickness (T) plus the required bend radius (R).

The concept illustrated by "B" above is converted to a chart form below for your convenience. These recommended minimum formed height dimensions are general to cover most variables of design, size, material types, tempers and thicknesses but which will permit the most economical tooling and production. Proper design, small parts and easily formed material, such as Aluminum, Brass, Copper and Mild Steel may be formed with a slightly lower minimum inside formed height (roughly 20% less).


Specifications and Dimensions of Embossed Parts :

Preferred dimensioning and points to measure on embossed parts.


Recommended Limits of Embossed Parts :

Limits for depths of embossments to minimize fracturing.

FLAT V-BENDS - L (MAX) = 3T*

OFFSETS - L (MAX) = R1 + R2**

*Reduce to 2T for commercial grades of steel, one-quarter hard tempers, and alloys of aluminum.

**Reduce to .5(R1 + R2) for commercial grades of steel, one-quarter hard tempers, and alloys of aluminum.


Specification and Characteristics of Drawn Parts :

The specification should show the form of the part, state the material, specify dimensions and condition of symmetry.

D1 or D2 (not both) D3R1R2TL1L2R3


Recommended bending radii :

Stainless Steel :

MATERIAL SHEET THICKNESS

-- .012 .016 .020 .025 .032 .036 .040 .045 .050 .063 .080 .090 .112 .125 .160 .190

302 Annealed .06 .06 .06 .06 .06 .06 .09 .09 .09 .09 .12 .12 .16 .19 .22 .25

347-1A .06 .06 .06 .09 .09 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22 .25

1/4 Hard Cres .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22 .25 .31 .38

1/2 Hard Cres .06 .06 .06 .09 .09 .12 .12 .12 .16 .16 .25 .25 .31 .38 .50 .62

Full Hard Cres .06 .06 .09 .12 .12 .16 .16 .19 .22 .25 .31 .38 .44 .50 .62 .87


Recommended bending radii :

Aluminum :MATERIAL SHEET THICKNESS

-- .012 .016 .020 .025 .032 .040 .050 .063 .071 .080 .090 .100 .125 .160 .190

2024-0 & W .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .19 .22 .31 .36

2024-T3 .06 .06 .06 .09 .09 .12 .16 .22 .25 .31 .38 .44 .62 .75 1.00

2024-T36 .06 .09 .09 .09 .12 .16 .19 .25 .31 .38 .44 .50 .75 1.00 1.25

3003-0 .06 .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19

3003-H14 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .19 .22 .31 .38 .44

5052-0 .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22

6061-0 & W .06 .06 .06 .06 .06 .06 .06 .09 .09 .09 .12 .12 .16 .19 .22

6061-T4 & T6 .06 .06 .06 .06 .06 .06 .09 .09 .12 .12 .16 .19 .22 .31 .38

7075-0 & W .06 .06 .06 .06 .09 .09 .12 .16 .19 .22 .25 .31 .38 .50 .62

7075-T6 .06 .09 .12 .12 .16 .22 .25 .31 .41 .44 .50 .69 .87 1.00 1.25

7178-0 & W .06 .06 .06 .06 .09 .09 .12 .19 .22 .25 .31 .38 .50 .75 -

7178-T6 .06 .09 .16 .19 .22 .31 .38 .50 .56 .62 .62 .75 1.00 1.25 -


Specification and Characteristics of Drawn Parts :

g India Business CenterFeasible operations

• What are the feasible manufacturing operations that can be performed in this process ( bending upto 180 degrees / injection molding of components as thin as 0.02 inches in thickness)

• What are the general manufacturing operations

• What are the easier and common operations

• What are difficult operations

• What are not possible operations

g India Business CenterPart design Guidelines

• Provide the guidelines of each design operations

g India Business CenterCase studies

• some case studies of part designs…..

g India Business CenterDos and donts