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University of Nevada Las Vegas |
MEG426/626 Manufacturing Processes |
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Department of Mechanical Engineering |
Fall Semester 2000 |
Sheet Metalworking
Most sheet-metal processing is performed at room temperature (cold working).
Tool: punch and die.
Shearing
:Four steps: 1). Before contact, 2). Plastic deformation, 3). Smooth cut surface by shearing, and 4). Fracture.
Surface characteristic: 1). Rollover, 2). Burnish, 3). Fractured zone (undesired), and 4). Burr (undesired).
Three principal operations:
Shearing: is a sheet-metal cutting operation along a straight line between two cutting edges.
Purpose: to cut large sheets into smaller sections for subsequent pressworking operations.
Machine: power shears or squaring shears.
Blanking: is to cut of the sheet metal along a closed outline in a single step to separate the piece from the surrounding stock.
Blank: the part being cut out, which is the desired product.
Punching: is similar to blanking, except that the piece that is cut out is scrap, called slug. The remaining stock is the desired part.
Clearance: the distance between the punch and die. Typically 4% - 8% of the sheet metal thickness.
c = a t
where c = clearance, in. (mm); a = allowance; and t = stock thickness, in. (mm).
Clearance is an important parameter for punch and die design.
In a blanking operation,
Db = diameter of a round blank,
Punch diameter = Db _ 2c
Die diameter = Db
In a punching operation,
Dh = hole diameter,
Punch diameter = Dh
Die diameter = Db + 2c
For both blanking and punching operations, the die opening must have an angular clearance of 0.250 to 1.50 on each side for the slug or blank to drop through.
Cutting forces
F = t L S
Where F = cutting force in sheet metalworking, t = stock thickness, in. (mm), L = length of the cutting edge, in. (mm), and S = shear strength of the sheet metal, lb/in.2 (MPa),
If shear strength is unknown, tensile strength can be used to estimate cutting force:
F = 0.7 t L TS
Where Ts = ultimate tensile strength, lb/in.2 (Mpa).
Bending
: involves straining of a metal sheet or plate around a straight axis.Tool: punch and die.
V-bending: the sheet metal is bent between a V-shaped punch and die.
Edge bending: involves cantilever loading of the sheet metal. A pressure pad is used to apply a holding force to hold the base of the part against the die.
Drawing
: is a sheet-metal forming operation used to make cup-shaped, box-shaped, or other more complex-curved, hollow-shaped parts.Clearance: the distance between the punch and die.
c = 1.1 t
Where c = clearance, in. (mm); t = stock thickness, in. (mm).
Drawing ratio:
where DR = drawing ratio; Db = blank diameter, in. (mm); and Dp = punch diameter, in. (mm).
The greater the ratio, the more severe is the operation.
DR � 2.0
The upper limit on the drawing ratio is a value of 2.0.
Reduction:
� 0.5
Force: the drawing force can be estimated roughly by:
where F = drawing force, lb. (N); t = original blank thickness, in. (mm); Ts = tensile strength, lb/in.2 (Mpa); and Db and Dp are the starting blank diameter and punch diameter, respectively, in. (mm).
where Fh = holding force in drawing, lb (N); Y = yield strength of the sheet metal, lb/in.2 (Mpa); t = starting stock thickness, in. (mm); Rd = die corner radius, in. (mm).
The holding force is usually about one-third the drawing force.
Exercises:
Textbook (page 539):
Problems 22.2, 22.5, and 22.20.
Created by Dr. Wang