20.11 Distinguish between redrawing and reverse drawing.
Answer. In redrawing, the shape change is significant enough (e.g., drawing ratio greater than 2.0)
that it must be carried out in two drawing steps, probably with an annealing operation between the
steps. In reverse drawing, two draws are accomplished on the part, one in one direction, the second
in the opposite direction.
20.12 What are some of the possible defects in drawn sheet-metal parts?
Answer. Drawing defects include (1) wrinkling, (2) tearing, (3) earing, and (4) surface scratches, as
described in Section 20.3.4.
20.13 What is an embossing operation?
Answer. Embossing is a sheet metalworking operation used to create indentations in the sheet, such
as raised lettering or strengthening ribs.
20.14 What is stretch forming?
Answer. Stretch forming of sheet metal consists of simultaneously stretching and bending the
sheet-metal workpart to achieve shape change.
20.15 Identify the principal components of a stamping die that performs blanking.
Answer. The principal components are the punch and die, which perform the cutting operation.
They are attached respectively to the punch holder (a.k.a. upper shoe) and die holder (a.k.a. lower
shoe). Alignment of the punch and die during the stamping operation is achieved by means of guide
pins and bushings in the punch holder and die holder.
20.16 What are the two basic categories of structural frames used in stamping presses?
Answer. Two basic categories of press frame are (1) gap frame, also called C-frame because its
profile is the shape of the letter “C”, and (2) straight-sided frame, which has full sides for greater
strength and stiffness of the frame.
20.17 What are the relative advantages and disadvantages of mechanical presses versus hydraulic presses
in sheet metalworking?
Answer. The main advantage of mechanical presses is faster cycle rates. Advantages of hydraulic
presses are longer ram strokes and uniform force throughout stroke.
20.18 What is the Guerin process?
Answer. The Guerin process is a sheet-metal forming process that uses a rubber die that flexes to
force the sheet metal to take the shape of a form block (punch).
20.19 Identify a major technical problem in tube bending?
Answer. A major technical problem in tube bending is collapse of the tube walls during the bending
process.
20.20 Distinguish between roll bending and roll forming.
Answer. Roll bending involves the forming of large sheet and plate metal sections into curved
forms. Roll forming involves feeding a lone strip or coil through rotating rolls so that the shape of
the rolls is imparted to the strip.