Press Tools
6. Scrap – Strip Trip Layout For
Blanking
6.3
EVOLUTION OF A PROGRESSIVE BLANKING DIE
Figure
6-8 gives the blanked dimensions of a linkage case cover of cold rolled steel,
stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts made at
one setup, with the possibility of three or four runs per
year.
Step
1, Part Specification
- The
production is of medium class; therefore a second-class die will be used.
- Tolerances required: Except for location of the slots, all
dimensions are in fractions. The slot locations, though specified in
decimals, are not very close. Thus a compound die is not necessary; a two or
three-station progressive die will be adequate.
- Type of press to be used: Available for this production are
presses of 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or 200
mm.
- Thickness of material: Specified as 32 mm standard cold rolled
steel.
Step
2, Scrap-Strip Development
From the
production requirements, a single-row strip will suffice. After several trials,
the scrap strip shown in Fig. 6-9 was decided upon. Owing to the closeness of
the holes it was decided to make a four-station die.
The
scrap strip would be fed into the first finger stop, and the center hole would
be pierced.
The strip would then be moved in to the second finger stop, and the two
holes would then be pierced. At the third stage and third finger stop, a pilot
would locate the strip and the four corner holes would then be pierced. At the
fourth and final stage, a piloted blanking punch would blank out the finished
part.
Step
3, Press Tonnage
It is now
in order to determine the amount of pressure needed. Only the actual blanking in
the fourth stage need be calculated, since the work in the first three stages
will be done by stepped punches.
From
Table, the shear strength S of cold rolled steel is 40 kgs/mm². The length L of
the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut (stock thickness
t) equals 3.2 mm.
From the equation P = S L t
P = 40 kgs./mm² x 240 mm x 3.2 mm = 30,720 kgs. Or 30.7
tons.
This
tonnage is greater than can be handled by the available presses. To lower the
pressure, shear is ground on the blanking punch to reduce the needed pressure by
on third.
This, ? x
30.7 =
30.7 -
10.2 = 20.5
tons. A punch press of 25-ton capacity would do, but there is reported available
only a 30-ton press with a 190 mm shut height and a 50 mm stroke. This press is
selected. The bolster plate is found to be 300 mm deep, 140 mm from centerline
of ram to back edge of bolster, and 600 mm wide. Shank diameter is 64 mm.
Step
4, Calculation of the Die
(a) The die. The perimeter of
the cut equals 240 mm and therefore the thickness of the die must be 25 nm. The
width of our scarp-strip opening is 60 mm with 32 mm extra material on each side
of the opening, it will be 60 mm + 64 mm = 124 mm or 130 mm width. The distance
from the left side of the opening in stage 4 to the edge of the opening in stage
1 equals 3 C + 30 + 6 = 192 + 30 + 6 = 228 mm and plus 62 mm = 290 mm or 296 mm
long.
Therefore the die should be 2.5 x 130 x 296 mm long.
(b) The die plate. As a means of
filling in between the die and the die shoe, a die plate of machinery steel is
used. To secure the die plate to the die shoe M12 cap screws and dowels are
used. A minimum of twice the size of the cap screw for the distance from the
edge of the die to the edge of the die plate is needed, which will equal 25 mm.
Twice this distance = 50 mm and 50 mm added to the size of the die will result
in a die plate of 25 x 180 x 346 mm. Figure 6-10 shows the die and die plate
fitted together, and with the holes, which show the sharpening portion and the
relief portion.
Step
5, Calculation of Punches
Good
practice requires 10 per cent of the metal thickness to be removed from the
basic dimension of the blanking punch. This same value is used on the die opening,
since holes are to be pierced in the blank. The clearance rule will be applied to the die
opening in Stages 1, 2, and 3, and to the punch in Stage 4 (see fig6-11).
For
Stage 4: Blank
to be 60 mm square,
Stock
thickness = 3.2 mm; 10% = 0.32 mm.
Punch
= 60 – 0.32 = 59.68 mm.
Therefore
the die opening will equal 60.01 to 60 mm and the punch will equal 59.68 to
59.67 mm.
For
Stage 2: Slot to be 8 mm wide by 34 mm long.
Die =
8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm.
Punch
will equal 3.99 to 8.00 mm. wide, and 33.99 to 34.00 mm. long.
The punch
and the die opening will have straight sides for at least 3-11 also shows a 3 mm
shear for the die at Stage 4 and a 3 mm shear for the punches of Stage 2, and
also the stepped arrangement of the punches for all
stages
Step
6, springs
A solid
stripper plate can be used for this job.
Step
7, Piloting
Figures
6-9 and 6-11 illustrate the arrangement for piloting. In this case it is
direct piloting.
However, if the part did not have a center hole, and the slots and other
holes were too small, indirect piloting would have to be provided.
Step
8, Automatic Stops
Finger
stops will act as stops when a new scrap strip is being inserted but, after
that, an automatic spring drop stop must be used to halt the scrap strip.
Figures 6-12 illustrate details of the completed drawing of the die.
6. Scrap – Strip Trip Layout For
Blanking
Figure 6-8 gives the blanked dimensions of a linkage case cover of cold rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts made at one setup, with the possibility of three or four runs per year.
Step
1, Part Specification
- The
production is of medium class; therefore a second-class die will be used.
- Tolerances required: Except for location of the slots, all
dimensions are in fractions. The slot locations, though specified in
decimals, are not very close. Thus a compound die is not necessary; a two or
three-station progressive die will be adequate.
- Type of press to be used: Available for this production are
presses of 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or 200
mm.
- Thickness of material: Specified as 32 mm standard cold rolled
steel.
Step
2, Scrap-Strip Development
From the production requirements, a single-row strip will suffice. After several trials, the scrap strip shown in Fig. 6-9 was decided upon. Owing to the closeness of the holes it was decided to make a four-station die.
The
scrap strip would be fed into the first finger stop, and the center hole would
be pierced.
The strip would then be moved in to the second finger stop, and the two
holes would then be pierced. At the third stage and third finger stop, a pilot
would locate the strip and the four corner holes would then be pierced. At the
fourth and final stage, a piloted blanking punch would blank out the finished
part.
Step
3, Press Tonnage
It is now
in order to determine the amount of pressure needed. Only the actual blanking in
the fourth stage need be calculated, since the work in the first three stages
will be done by stepped punches.
From
Table, the shear strength S of cold rolled steel is 40 kgs/mm². The length L of
the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut (stock thickness
t) equals 3.2 mm.
From the equation P = S L t
P = 40 kgs./mm² x 240 mm x 3.2 mm = 30,720 kgs. Or 30.7
tons.
This
tonnage is greater than can be handled by the available presses. To lower the
pressure, shear is ground on the blanking punch to reduce the needed pressure by
on third.
This, ? x
30.7 =
30.7 -
10.2 = 20.5
tons. A punch press of 25-ton capacity would do, but there is reported available
only a 30-ton press with a 190 mm shut height and a 50 mm stroke. This press is
selected. The bolster plate is found to be 300 mm deep, 140 mm from centerline
of ram to back edge of bolster, and 600 mm wide. Shank diameter is 64 mm.
Step
4, Calculation of the Die
(a) The die. The perimeter of
the cut equals 240 mm and therefore the thickness of the die must be 25 nm. The
width of our scarp-strip opening is 60 mm with 32 mm extra material on each side
of the opening, it will be 60 mm + 64 mm = 124 mm or 130 mm width. The distance
from the left side of the opening in stage 4 to the edge of the opening in stage
1 equals 3 C + 30 + 6 = 192 + 30 + 6 = 228 mm and plus 62 mm = 290 mm or 296 mm
long.
Therefore the die should be 2.5 x 130 x 296 mm long.
(b) The die plate. As a means of filling in between the die and the die shoe, a die plate of machinery steel is used. To secure the die plate to the die shoe M12 cap screws and dowels are used. A minimum of twice the size of the cap screw for the distance from the edge of the die to the edge of the die plate is needed, which will equal 25 mm. Twice this distance = 50 mm and 50 mm added to the size of the die will result in a die plate of 25 x 180 x 346 mm. Figure 6-10 shows the die and die plate fitted together, and with the holes, which show the sharpening portion and the relief portion.
Step
5, Calculation of Punches
Good
practice requires 10 per cent of the metal thickness to be removed from the
basic dimension of the blanking punch. This same value is used on the die opening,
since holes are to be pierced in the blank. The clearance rule will be applied to the die
opening in Stages 1, 2, and 3, and to the punch in Stage 4 (see fig6-11).
For
Stage 4: Blank
to be 60 mm square,
Stock
thickness = 3.2 mm; 10% = 0.32 mm.
Punch
= 60 – 0.32 = 59.68 mm.
Therefore
the die opening will equal 60.01 to 60 mm and the punch will equal 59.68 to
59.67 mm.
For
Stage 2: Slot to be 8 mm wide by 34 mm long.
Die =
8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm.
Punch
will equal 3.99 to 8.00 mm. wide, and 33.99 to 34.00 mm. long.
The punch and the die opening will have straight sides for at least 3-11 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for the punches of Stage 2, and also the stepped arrangement of the punches for all stages
Step
6, springs
A solid
stripper plate can be used for this job.
Step
7, Piloting
Figures
6-9 and 6-11 illustrate the arrangement for piloting. In this case it is
direct piloting.
However, if the part did not have a center hole, and the slots and other
holes were too small, indirect piloting would have to be provided.
Step
8, Automatic Stops
Finger
stops will act as stops when a new scrap strip is being inserted but, after
that, an automatic spring drop stop must be used to halt the scrap strip.
Figures 6-12 illustrate details of the completed drawing of the die.