Press Tools
6. Scrap – Strip Trip Layout For
Blanking
In designing parts to be blanked from strip material,
economical stock utilization is of high importance. The goal should be
at least 75 per cent utilization. A very simple scrap-strip layout is shown in
Fig. 6-1.
6.1 SCRAP
ALLOWANCE
A
scrap-strip layout having insufficient stock between the blank and the strip
edge, and between blanks, will result in a weakened strip, subject to breakage
and thereby causing misfeeds. Such troubles will cause unnecessary die
maintenance owing to partial cuts, which defect the punches, resulting in nicked
edges. The following formulas are used in calculating scrap-strip dimensions for
all strips over 0.8 mm. thick.
t = specified thickness
of the material
B = 1.25 t when C is less than 64 mm
B = 1.5 t when C is 64 mm or longer
C = L + B, or lead of the die
Example: A rectangular part, to be blanked from 1.5 mm
thick steel (Manufacturers Standard) is 10 X 27 mm. If the scrap strip
is developed as in Fig. 6-2, the solution is
t = 1.5 mm
B = 1.25 X 1.5 = 1.875 mm
C = 10 + 1.875 = 11.875 mm
W = 27 + 3.75 = 30.75 mm
Nearest
commercial stock is 32 mm. Therefore, the distance B will equal 2.3mm.
This is acceptable since it exceeds minimum
requirements.
6.1.1
Minimum Scrap-Strip Allowance: If the material to be blanked
is 0.6 mm thick or less, the formulas above should not be used. Instead, dimension
B is to be as follows:
Strip width W
Dimension B
0 - 75 mm
1.3 mm
76 – 150 mm
2.4 mm
150 – 300 mm
3.2 mm
Over 300 mm
4.0 mm
6.1.2
Other Scrap-Strip Allowance Applications:
Figure 6-3,
6-4 and 6-5 illustrates special allowances for one-pass
layouts:
View
D. For layouts
with sharp corners of blanks adjacent, B = 1.25 t.
Fig.
6-4 Allowances for
one-pass layouts.
6.1.3
Percentage of Stock Used:
If the area of the part is divided by the area of the scrap strip used,
the result will be the percentage of stock used.
If A =
total area of strip used to produce a single blanked part, then
A = CW
(Fig. 3-35), and a = area of the part = LH.
If C =
11.5 mm and W
= 32 mm then A = 11.5 X 32 = 368 mm²
If L X
9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6 mm²
Percentage of stock used:
a
256.5
=
70% approx.
A
368
6. Scrap – Strip Trip Layout For
Blanking
6.1 SCRAP
ALLOWANCE
A scrap-strip layout having insufficient stock between the blank and the strip edge, and between blanks, will result in a weakened strip, subject to breakage and thereby causing misfeeds. Such troubles will cause unnecessary die maintenance owing to partial cuts, which defect the punches, resulting in nicked edges. The following formulas are used in calculating scrap-strip dimensions for all strips over 0.8 mm. thick.
B = 1.25 t when C is less than 64 mm
B = 1.5 t when C is 64 mm or longer
C = L + B, or lead of the die
Example: A rectangular part, to be blanked from 1.5 mm
thick steel (Manufacturers Standard) is 10 X 27 mm. If the scrap strip
is developed as in Fig. 6-2, the solution is
t = 1.5 mm
B = 1.25 X 1.5 = 1.875 mm
C = 10 + 1.875 = 11.875 mm
W = 27 + 3.75 = 30.75 mm
Nearest commercial stock is 32 mm. Therefore, the distance B will equal 2.3mm. This is acceptable since it exceeds minimum requirements.
6.1.1
Minimum Scrap-Strip Allowance: If the material to be blanked
is 0.6 mm thick or less, the formulas above should not be used. Instead, dimension
B is to be as follows:
Strip width W
Dimension B
0 - 75 mm
1.3 mm
76 – 150 mm
2.4 mm
150 – 300 mm
3.2 mm
Over 300 mm
4.0 mm
6.1.2
Other Scrap-Strip Allowance Applications:
Figure 6-3, 6-4 and 6-5 illustrates special allowances for one-pass layouts:
View
D. For layouts
with sharp corners of blanks adjacent, B = 1.25 t.
Fig. 6-4 Allowances for one-pass layouts.
6.1.3
Percentage of Stock Used:
If the area of the part is divided by the area of the scrap strip used,
the result will be the percentage of stock used.
If A =
total area of strip used to produce a single blanked part, then
A = CW
(Fig. 3-35), and a = area of the part = LH.
If C =
11.5 mm and W
= 32 mm then A = 11.5 X 32 = 368 mm²
If L X
9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6 mm²
Percentage of stock used:
|
a |
|
256.5 |
|
|
|
|
|
|
= |
70% approx. |
|
A |
|
368 |
|
|