3 Fundamentals of press design

Blanking processes
Using these equations, it is possible to determine the position of the
center of force S R (x S; y S) in any selected system of coordinates. The
equations can only be used for dies with simultaneously acting flat
ground punches and flat female dies. It makes sense to lay the coordinate
axes through as many centers of gravity as possible, so reducing
the distances to zero and simplifying the calculations. Steps should be
taken to avoid laying the axes between centers of gravity as otherwise
attention must be paid to +/– signs (generally applicable for x: to the
right positive values, to the left negative values; for y: upwards positive
values, downwards negative values). The formulas required to calculate
the centers of gravity of the individual perimeters or line elements can
be found where applicable in books of tables. More complex part
shapes can often be composed of basic figures (quadrants, rectangles,
etc.). Nowadays, computer programs are frequently used to determine
the surface center of gravity.
Where the two force action lines in the x and y direction interact, we
obtain the center of blanking forces S R (x S; y S), which should be ideally
in the center of the slide, or in the case of smaller tools, at the optimum
position of the centering pin.
Example:
A workpiece with outside dimensions 80 � 90 mm with a semicircular punched
hole (radius 30 mm) and two round holes with a diameter of 20 mm is to be produced
in two steps (Fig. 4.5.11a). Two blanking operations take place in the
press: piercing and blanking, as indicated in Fig.4.5.11band c. The center of
force S R is determined initially on the basis of the centers of gravity of the
punch perimeters, and then using the line centers of gravity for the purpose of
comparison.
U = U = π ⋅ 20 mm = 62. 83 mm
1 2
1
U3= ⋅π⋅ 60 mm+ 60 mm
2
= 94. 25 mm + 60 mm = 154. 25 mm
U = 280 ⋅ mm+ 290 ⋅ mm = 340 mm
4
l = l = π ⋅ 20 mm = 62. 83 mm
5 6
1
l7= ⋅π⋅ 60 mm = 94. 25 mm
2
l = 60 mm, l = l = 90 mm
8 9 11
l = l = 80 mm
10 12
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
➔
279