4.2.1 Scrap-Strip Layout
It may be appreciated that for the economical production of blanks, the utilization of the strip should be of high order, say at least 75%. This has been explained earlier in Chapter 2. Figure 4.1 illustrates a simple example of a scarp-strip layout.
Introduction to the Design of Blanking, Piercing, Progressive
and Compound Dies
4
Design of Jigs, Fixtures and Press Tools, First Edition. K. Venkataraman.
© K. Venkataraman 2015. Published by Athena Academic Ltd and John Wiley & Sons Ltd.
Generally, for strips whose thickness exceeds 0.75 mm, the following formulae are used:
t = thickness of strip
B = clearance between successive blanks or clearance between the edge of the strip and blank
= 11
4t, for C < 63 mm
= 11
2t, for C ≥ 63 mm Here, C = L + B-lead or advance of the die where L = blank length
W = width of the strip H = blank width or height.
For strips with thickness equal to or less than 0.625 mm, the above formulae are not to be used. Instead, Table 4.1 is to be used.
Table 4.1 Strips widths and dimension Strip width, W (mm) Dimension B (mm)
0 –75 1.25
75 –150 2.30
150 – 300 3.00
≥ 300 3.75
So far, the parameters for single-row single-pass layout have been explained (please refer Fig. 4.3 later in this chapter for single-row single-pass layout).
In case the layout is decided to be double-row double-pass, as shown in Fig. 4.1, the clearance B will follow the following rule:
Fig. 4.1 Double-Row Double-Pass Layout
y Single-row double-pass, B = 11 2t
y Double-row double-pass with curved lines, B = 11 4t
y Double-row double-pass with straight and curved lines [as in Fig. 4.1 (c)], B = 11
4t
The variants, namely, single-row double-pass or double-row double-pass are basically strip layout designed to improve the utilisation factor. Utilisation factor is the ratio of the blank area to the total area of the strip utilised to create a single blank. The same can be written in the form of equation:
a
A = LH
It has already been explained that CW L = length of blank
H = height or width of blank C = advance or lead
W = width of strip
Generally, the utilisation factor is aimed at 70 – 75%.
4.2.2 Design of Die Blank
By the rule of thumb, for die blocks made up of tool steel for blanking materials ranging from low-carbon steels up to stainless steels, the thickness can be taken as given in Table 4.2.
Table 4.2 Perimeter of the blank and die thickness Sl. no. Perimeter of the Blank (mm) Die thickness (mm)
1 0–75 20
2 75–100 25
3 100–175 31
4 ≥ 175 38
As regards the top view of the die, i.e. its length and width, the same are decided as follows:
(i) Length of the die = Length of the blank (L)
+ twice the marginal clearance (2 × B) + twice the clearance between the strip and the screws + twice the diameter of the set-screw or dowel + twice the margin from the set-screw till the edge
of the die (2 × 10 = 20 mm) (ii) Width of the die = Width of the strip (W)
+ twice the marginal clearance (2 × B)
+ twice the clearance between the strip and the screws
+ twice the diameter of the sub-screw/dowel ( 2 × 12) + twice the margin from the set-screw till the edge
of the die (2 ×10 = 20 mm)
Thus, for a blank opening of 60 mm × 60 mm, the size of die shall be as follows:
Length of the die = {( 60 + 2.5) + 2 × 10 + 20 + 20} 122.5 mm (say 122 mm) Width of the die = {( 60 + 2.5) + 2 × 10 + 20 + 20} 122.5 mm (say 122 mm) (assuming that set-screws and dowels of 10 mm are chosen and that the thickness of the strip is 1 mm.) With reference to die opening, the size should be the same as that of the blank. It is ground for the blank size up to a depth of the thickness of the strip. Subsequently, a taper angle of 1.5 degrees is provided to allow for the blank to drop without jamming.
4.2.3 Punch Design
Although the die opening is exactly the same as that of the blank size, the punch sizes will be smaller than the die sizes by twice the clearance assumed per side. If a clearance of 3% of stock thickness is assumed per side, then the cross-sectional size of the punch will be = 0.94 (length of die) × 0.94 (width of die)
(Note: In case of rectangular punches).
In the case of the length of the piercing punch, the same can be calculated using the following formula:
L = Πd E d S ts
{ }
{ }
/ 1 2
8
L = length of punch
Ss = unit shear stress on the stock (in MPa) E = modulus of elasticity
t = thickness of the stock (in mm) d = diameter of the punched hole (in mm).
This formula is applicable if d/t ≥ 1.1.
In the case of rectangular blanks, the term d will be substituted by the perimeter of the blank, viz. L × H and diameter ‘d’ by ‘L’.
However, the length of the punch is generally assumed to be the blank length, or as the case may be for rigidity. Let us assume that the punch length is 60 mm and it is held against a hardened backup plate of 60 mm by a punch plate of 20 mm.
The whole assembly is screwed and dowelled to the upper shoe or the top bolster plate. By the rule of thumb, the upper and bottom bolster plates are assumed to be (1.25 × thickness of die plates) and (1.75 × thickness of die), respectively. Refer to Fig. 4.2.
Nomenclature Rule of Thumb for Design
t1 : Thickness of Die Plate t1 : Thickness of Die Plate (in cm) = 3 Fsh t2 : Thickness of Stripper Plate t2 : Thickness of stripper Plate n = 0.5t t3 : Thickness of Punch Holder Plate t3 : Thickness of Punch Holder Plate = 0.5t t4 : Thickness of Die Shoe t4 : Thickness of Die Shoe = t1
T1 : Thickness of Bottom Bolster Plate T1 : Thickness of Bottom Bolster Plate = 1.75 t1 T2 : Thickness of Top Bolster Plate T2 : Thickness of Top Bolster Plate = 1.25t
d : Diameter of Guide Pin d : Dimater of Guide Pin = t1 Fsh : Shearing Force (in tonnes)
Fig. 4.2 Method of Calculating the Die Thickness—Rule of Thumb
As per the rule of thumb, the stripper plate thickness will be half of the die plate thickness and for channel-type strippers, the channel clearance will be11
2t. . Thus, the total shut height for a 25 mm die plate will be:
Shut height = {die plate thickness + die shoe thickness + Stripper Plate thickness+
11
+ top bolster plate thickness (1.25 × die plate thickness)2 + bottom bolster plate thickness (1.75 × die plate thickness) + (punch holder plate thickness + backup pressure pad
thickness) + clearance of 10 mm}
= 25 + 25 + (12.5 + 1.5) + 31 + 45 + 18 + 10 (assuming 1 mm as the thickness of blank) 168 mm = 170 mm (say)
4.2.4 Stripper Design
It has been mentioned in the last paragraph that the stripper thickness can be assumed to be 1
2of the die thickness for channel-type strippers. In the case of spring-type strippers, the strippers enable wrinkle-free surface while blanking, piercing or any other operation like drawing, etc. The spring-loaded stripper further holds the stock until the punch is withdrawn from the operation.
The pressure which is required to strip the stock is difficult to evaluate. However, the following formula can be used:
Stripping pressure (in N), Ps = 5250 L × T where L = perimeter (in mm)
T = thickness (in mm).
Spring design can be done from any standard design data book.
In case of channel strippers, the top view of the stripper will be similar to that of a die fixed to the die shoe, as the same is also screwed and dowelled to the die plate. In the case of spring strippers, the top view will be to suit the punch holding plate as well as the diameter of the spring holding screws and the spring diameter.