Sheet Metal Operation MCQ Quiz - Objective Question with Answer for Sheet Metal Operation - Download Free PDF

Concept:

Blanking force is given by:

\( F = \tau \times \text{Perimeter} \times \text{Thickness} \)

For a circular blank, perimeter is \( \pi D \) and thickness is \( T \).

Therefore, \( F = \tau \times \pi D \times T \)

Given:

Initial blanking force = 10 kN

Initial diameter = \( D \), Initial thickness = \( T \)

New diameter = \( 1.7D \), New thickness = \( 0.5T \)

Calculation:

New force,

\( F_{\text{new}} = \tau \times \pi \times (1.7D) \times (0.5T) = \tau \times \pi D T \times (1.7 \times 0.5) \)

\( F_{\text{new}} = F_{\text{old}} \times 0.85 = 10 \times 0.85 = 8.5~\text{kN} \)

Explanation:

Combination Die in Manufacturing Industry

• In the manufacturing industry, a combination die is a specialized tool designed to perform multiple operations, such as cutting, bending, and forming, in a single stroke of the press.
• This tool is used to improve efficiency and precision in the production process by consolidating multiple steps into one operation.
• A combination die works by integrating different sections within the die to perform various operations simultaneously.
• When the press is activated, the die moves to perform tasks such as punching, blanking, bending, or forming a workpiece in one stroke.
• This multi-functional capability reduces the need for multiple dies and operations, thus saving time and increasing productivity.

Advantages:

• Increases production efficiency by combining multiple operations into a single stroke.
• Reduces the need for multiple dies and setups, thus saving time and costs.
• Improves precision and consistency in the production process.
• Minimizes material handling and reduces the risk of errors.

Disadvantages:

• Initial design and manufacturing of combination dies can be complex and costly.
• Requires skilled operators to ensure proper setup and maintenance.
• Limited flexibility in changing the design once the die is manufactured.

Applications:

• Combination dies are commonly used in the production of small metal parts where precision and efficiency are critical. Typical applications include the manufacturing of components for the automotive, electronics, and aerospace industries.

Explanation:

Diameter of blank = 20 mm

Sheet thickness, t = 2 mm

Clearance allowance, A_c = 0.04

∴ Clearance, C = A_ct = 0.04 × 2 = 0.08

Blank size = Die size = 20 mm

Punch size = Die size - 2 × C = 202 × 0.08 = 19.84 mm

Explanation:

Anisotropy plays an important role in the performance of deep drawing processes. The anisotropy is of two types. In normal anisotropy the properties differ in the thickness direction. In planar anisotropy, the properties vary with the orientation in the plane of the sheet. Whereas deep drawability of sheets increases with normal anisotropy, anisotropy leads to the formation of ears in cup drawing. Ears cause the wavy edge of a drawn cup.
Earing:
In deep drawing, the edges of cups may become wavy, called earing. Ears are objectionable on deep-drawn cups because they have to be trimmed off, as they serve no useful purpose, and interfere with further processing of the cup, resulting in scarp. Earing is caused by the planar anisotropy of the sheet metal, and the number of ears produced may be two, four or eight, depending on the processing history and microstructure of the material. If the sheet is stronger in the rolling direction than transverse to the rolling direction, and the strength varies uniformly with respect to orientation, then two ears will form. If the sheet has high strength at different orientations, then more ears will form.

The correct answer is Gets larger.

Key Points

• When a metal sheet with a hole is heated, both the metal and the hole expand due to thermal expansion.
• Thermal expansion causes the entire sheet to expand uniformly, including the dimensions of the hole.
• The expansion of the hole occurs because the material around the hole expands outward.
• This phenomenon can be understood by imagining the hole as a circle drawn on a rubber sheet; when the sheet is stretched, the circle gets larger.

Additional Information 

• Thermal expansion is a common property of materials where they change dimensions in response to temperature changes.
• In most materials, the expansion is relatively uniform in all directions.
• Metals, being excellent conductors of heat, show noticeable expansion when subjected to temperature changes.
• The coefficient of thermal expansion is a material property that indicates how much a material expands per degree of temperature increase.

Lancing - Creating a partial cut in the sheet, so that no material is removed. The material is left attached to be bent and form a shape, such as a tab, vent, or louver.

Parting - Separating a part from the remaining sheet, by punching away the material between parts.

Slitting - Cutting straight lines in the sheet. No scrap material is produced.

Notching - Punching the edge of a sheet, forming a notch in the shape of a portion of the punch.

Concept:

Shearing is a cutting operation used to remove a blank of required dimensions from a large sheet. The shearing operations which make use of a die, include punching, blanking, piercing, notching, trimming, and nibbling.

Punching/Blanking: Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal. If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking.

Piercing: It is a process by which a hole is cut (or torn) in metal. It is different from punching in that piercing does not generate a slug.

Perforating: It is an operation in which a number of uniformly spaced holes are punched in a sheet of metal. The holes may be of any size or shape. They usually cover the entire sheet of metal.

Notching: It is an operation in which a specified small amount of metal is cut from the edges.

Lancing: It is an operation of cutting a sheet metal through a small length and then bending this cut portion.

Explanation:

Blanking: 

• In blanking, the piece being punched out becomes the work-piece and any major burrs or undesirable features should be left on the remaining strip. Blanking is a shearing operation.

Stretch forming: 

• It is a method of producing contours in sheet metal. Thinning and strain hardening is inherent in the process. Stretch forming is a sheet metal forming process in which the sheet metal is intentionally stretched and simultaneously bent to have the shape change. It is a type of cold drawing. Stress-induced is mainly tensile.

Coining:

• It is essentially a cold‐forging operation except for the fact that the flow of the metal occurs only at the top layers and not the entire volume. In the coining process, compressive forces are there.

Deep Drawing: 

• When cup height is more than half the diameter is termed deep drawing. During deep drawing, in the flange of blank, there is bi‐axial tension and compression stresses.

Concept:

In punching and blanking, the shearing action on the sheet takes place which will depend on the shear strength of the metal.

Punching force F = Lt τ

Where L is Length of the periphery, t = thickness, τ = Ultimate shear strength

For circular section L = πd

Explanation:

Punching and blanking:

• Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal.
• If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking.

Shear:

• The working faces of the punch or die are ground off so that it does not remain parallel to the horizontal plane but are inclined to it. This angle of inclination is called shear.
• The provision of shear distorts the material being cut.
• When shear is on the face of the punch, the blank cannot be flat and when shear is on the die, the piercing cannot be flat.
• So, for the blanking operation, shear is provided on die face and for punching operation shear is provided on the Punch face. 
• Shear reduces the punch load, acts gradually and it is always less than equal to the thickness of sheet metal.

Additional Information

Punch load:

The general formula to calculate the punch load is given by:

F = τ_u × perimeter × thickness

F = τ_u × πd × t     (For circle)

F = τ_u × 4a × t     (For square)

F = τ_u × 2(l + b) × t   (For rectangle)

Explanation:

• Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal.
• If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking.
• The provision of shear distorts the material being cut.
• When shear is on the face of the punch, the blank cannot be flat and when shear is on the die, the piercing cannot be flat.
• So, for the blanking operation, shear is provided on die face and for punching operation shear is provided on the Punch face. 

Three types of dies are discussed below:

Progressive d ies perform two or more operations simultaneously in a single stroke of a punch press so that a complete component is obtained for each stroke. The progressive die is long and has patterns made for a different operation like punching, blanking, coining, piercing etc.

In Compound dies, two or more operations may be performed at one station. Such dies are considered as cutting tools since only cutting operations are carried out. The figure shows a simple compound die in which a washer is made by one stroke of the press. The washer is produced by simultaneous Blanking and Punching operations. Compound dies are more accurate and economical in mass production as compared to single operation dies.

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A combination die is the sam e as that of a compound die with the main difference that here non‐cutting operations such as bending, drawing and forming are also included as part of the operation.

Concept:

Spring back:

• Because sheet metal generally is thin and is subjected to relatively small strains during forming, sheet metal parts are likely to experience considerable spring back due to elastic deformation.
• This effect is particularly significant in bending and other forming operations where the bend radius to thickness ratio is high.

        ​​

Blank holding force (BHF):

• When force is applied by a punch on sheet material, the natural tendency of the sheet edge is getting lift up, and therefore because of the lifting of edges the wrinkles or foldings are created at the inside corner of the deep-drawn component.
• That lifting of edges called spring back during elastic deformation.
• Therefore to eliminate the wrinkles, the lifting of edges should be restricted.
• It is done by applying a blank holding force(BHF) with a compressed spring.
• BHF aligned in such a way that it should only avoid the lifting of the edges, but simultaneously it should allow the material to slip in the cavity, and therefore it is generally applied one-third of the drawing load.

Concept:

In punching and blanking, the shearing action on the sheet takes place which will depend on the shear strength of the metal.

Punching force F = Lt τ

Where L is Length of the periphery, t = thickness, τ = Ultimate shear strength

For circular section L = πd

∴ F = πDtτ

\(\tau = \frac{F}{{\pi Dt}}\)

Calculation:

Given t = 8 mm, D = 20 mm, F = 110 kN;

\(\tau = \frac{{110\; \times\; {{10}^3}}}{{\pi\; \times \;20\; \times \;8}} = 218.83\;MPa\)

Nearest option is 219 MPa

Concept:

• Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal.
• If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking.

Blanking operation: 

• Die size = blank size
• As the Blank is fixed since the blank here is a useful part. The clearance will be provided on the punch in the blanking operation.
• Punch size = blank size – 2 x clearance.

Piercing/Punching operation:

• Punch size = hole size
• As the Hole size is here is a useful part. The clearance will be provided on the die in the punching operation.
• Die size = blank size + 2 x clearance.

Concept:

Bend allowance (L_b) is the length of nuteral axis of before or after bending.

(L_b) = α (R + Kt)

where α: - bend angle (in radians)

R: - bend radius

K: - stretch factor

for R > 2t, K = 0.5

for R < 2t, K = 0.33

t: - plate thickness

Calculation:

Given, t = 2 mm, R = 150 mm, α = 1.6 radian

For K, we need to check the 2t value, 

2 × 2 = 4 < 150, therefore k = 0.5

Now, bend allowance

(Lb) = 1.6 (150 + 0.5 × 2) = 241.6 mm