Press Tools
6. Scrap – Strip Trip Layout For
Blanking
6.2 EVOLUTION OF A BLANKING DIE
In the
planning of a die, the examination of the part print immediately determines the
shape and size of both punch and die as well as the working area of the die
set.
6.2.1 Die
Set Selection
A
commercially available standardized two-post die set with 150 mm overall
dimensions side-to-side and front-to-back allows the available 76 mm. wide stock
to be fed through it.
It is large enough for mounting the blanking punch on the upper shoe
(with the die mounted on the lower shoe) for producing the blank shown in Fig.
6-6, since the guideposts can be supplied in lengths of from 100 to 225
mm.
Since
the stock, in this case was available only in a width of 76 mm the length of the
blanked portions extended across the stock left a distance between the edges of
the stock and the ends of the blank of 6 mm or twice the stock thickness; this
allowance is satisfactory for the 3.2 mm stock.
6.2.2
Die
Block Design
By the
usual ‘rule-of thumb’ method previously described, die block thickness (of tool
steel) should be a minimum of 20 mm for a blanking perimeter up to 75 mm and 25
mm for a perimeter between 75 and 100 mm. For longer perimeters, die block
thickness should be 32 mm. Die blocks are seldom thinner than 22 mm finished
thickness to allow for grinding and for blind screw holes. Since the perimeter
of the blank is approximately 178 mm a die block thickness of 38 mm was
specified, including a 6 mm grinding allowance.
There
should be a margin of 32 mm around the opening in the die block; its specified
size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap screws and
dia. 10 mm dowels are located at the corners 20 mm from the edges of the block.
The wall
of the die opening is straight for a distance of 3.2 mm (stock thickness); below
this portion or the straight, an angular clearance of 1½° allows the blank to
drop through the die block without jamming. The dimensions of the die opening
are the same as that of the blank; those of the punch are smaller by the
clearance (6 per cent of stock thickness, or 2 mm), which result in the
production of blanks to print (and die) size.
The top
of the die was ground off a distance equal to stock thickness (Fig. 6-7) with
the result that shearing of the stock starts at the ends of the die and
progresses towards the center of the die, and less blanking pressure is required
than if the top of the die where flat.
6.2.3
Punch Design
The
shouldered punch (57 mm) long is held against a 6 mm thick hardened steel backup
plate by a punch plate 20 mm thick) which is screwed and doweled to the upper
shoe. The shut
height of the die can be accommodated by a 32-ton (JIC Standard) open-back
inclinable press, leaving a shut height of 240 mm. For the conditions of this
case study, shear strength S = 42 kg/mm², blanked perimeter length L = 178 mm
approx. and thickness T = 3.2 mm.
From
the equation P = SLT
The
pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons.
This
value is well below the 32-ton capacity of the selected press.
The shut
height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the punch into the die
cavity.
6.2.4
Stripper Design
The
stripper that was designed is of the fixed type with a channel or slot having a
height equal to 1.5 times stock thickness and a width of 80 mm to allow for
variations in the stock width of 75 mm. The same screws that hold the die block to
the lower shoe fasten the stripper to the top of the die block.
If,
instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a
spring-loaded stripper would firmly hold the stock down on top of the die block
and could, to some extent, flatten out wrinkles and waves in it.
A
spring-loaded stripper should clamp the stock until the punch is withdrawn from
the stock. The pressure that strips the stock from the punch on the upstroke is
difficult to evaluate exactly. A formula frequently used is
Ps = 2.5 x L x t kgs.
Where
Ps = stripping pressure, in kgs.
L
=
perimeter of cut, in mm.
t
= stock
thickness, in
mm.
Spring design is beyond the scope of this book; die spring data are
available in the catalogues of spring manufacturers.
6.2.5 Stock
Stops
The pin stop pressed in the die block is the simplest method for
stopping the hand-fed strip. The right-hand edge of the blanked opening is
pushed against the pin before descent of the ram and the blanking of the next
blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked
opening to ride over the pin and to engage the right-hand edge of every
successive opening.
The design of various types of stops adapted for manual and automatic
feeding is covered in a preceding discussion.
6. Scrap – Strip Trip Layout For
Blanking
6.2 EVOLUTION OF A BLANKING DIE
In the planning of a die, the examination of the part print immediately determines the shape and size of both punch and die as well as the working area of the die set.
6.2.1 Die
Set Selection
A commercially available standardized two-post die set with 150 mm overall dimensions side-to-side and front-to-back allows the available 76 mm. wide stock to be fed through it. It is large enough for mounting the blanking punch on the upper shoe (with the die mounted on the lower shoe) for producing the blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of from 100 to 225 mm.
Since
the stock, in this case was available only in a width of 76 mm the length of the
blanked portions extended across the stock left a distance between the edges of
the stock and the ends of the blank of 6 mm or twice the stock thickness; this
allowance is satisfactory for the 3.2 mm stock.
6.2.2
Die
Block Design
By the
usual ‘rule-of thumb’ method previously described, die block thickness (of tool
steel) should be a minimum of 20 mm for a blanking perimeter up to 75 mm and 25
mm for a perimeter between 75 and 100 mm. For longer perimeters, die block
thickness should be 32 mm. Die blocks are seldom thinner than 22 mm finished
thickness to allow for grinding and for blind screw holes. Since the perimeter
of the blank is approximately 178 mm a die block thickness of 38 mm was
specified, including a 6 mm grinding allowance.
There
should be a margin of 32 mm around the opening in the die block; its specified
size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap screws and
dia. 10 mm dowels are located at the corners 20 mm from the edges of the block.
The wall
of the die opening is straight for a distance of 3.2 mm (stock thickness); below
this portion or the straight, an angular clearance of 1½° allows the blank to
drop through the die block without jamming. The dimensions of the die opening
are the same as that of the blank; those of the punch are smaller by the
clearance (6 per cent of stock thickness, or 2 mm), which result in the
production of blanks to print (and die) size.
The top
of the die was ground off a distance equal to stock thickness (Fig. 6-7) with
the result that shearing of the stock starts at the ends of the die and
progresses towards the center of the die, and less blanking pressure is required
than if the top of the die where flat.
6.2.3
Punch Design
The
shouldered punch (57 mm) long is held against a 6 mm thick hardened steel backup
plate by a punch plate 20 mm thick) which is screwed and doweled to the upper
shoe. The shut
height of the die can be accommodated by a 32-ton (JIC Standard) open-back
inclinable press, leaving a shut height of 240 mm. For the conditions of this
case study, shear strength S = 42 kg/mm², blanked perimeter length L = 178 mm
approx. and thickness T = 3.2 mm.
From
the equation P = SLT
The
pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons.
This
value is well below the 32-ton capacity of the selected press.
The shut
height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the punch into the die
cavity.
6.2.4
Stripper Design
The
stripper that was designed is of the fixed type with a channel or slot having a
height equal to 1.5 times stock thickness and a width of 80 mm to allow for
variations in the stock width of 75 mm. The same screws that hold the die block to
the lower shoe fasten the stripper to the top of the die block.
If, instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a spring-loaded stripper would firmly hold the stock down on top of the die block and could, to some extent, flatten out wrinkles and waves in it.
A
spring-loaded stripper should clamp the stock until the punch is withdrawn from
the stock. The pressure that strips the stock from the punch on the upstroke is
difficult to evaluate exactly. A formula frequently used is
Ps = 2.5 x L x t kgs.
Where
Ps = stripping pressure, in kgs.
L
=
perimeter of cut, in mm.
t
= stock
thickness, in
mm.
Spring design is beyond the scope of this book; die spring data are
available in the catalogues of spring manufacturers.
6.2.5 Stock
Stops
The pin stop pressed in the die block is the simplest method for
stopping the hand-fed strip. The right-hand edge of the blanked opening is
pushed against the pin before descent of the ram and the blanking of the next
blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked
opening to ride over the pin and to engage the right-hand edge of every
successive opening.
The design of various types of stops adapted for manual and automatic
feeding is covered in a preceding discussion.