FUNDEMENTALS FOR DESIGNING PRESS TOOLS:
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CLEARANCE:
Clearance is the intentional space(gap)
between the punch and die cutting edges.
Proper clearances between the punch and die cutting
edges enables the fracture to meet, and the fracture portion
of the sheared edge has a clean appearance.
For improper clearances, cracks donot meet and
ragged edge results due to the material being dragged and
torn through the die.
Clearances are calculated by depending upon the
materials thickness and their cutting allowances. The usual
clearances preside of the die for various materials are
given below, in terms of the stock thickness “t”.

For copper, aluminium, brass and soft steel = 3 to 5% of t
For medium steel = 6% of t
For hard steel = 7% of t

Excessive cutting clearance provides larger burr
on the components and gives long tool life. Insufficient
cutting clearance prevents a clear break. It also increase
pressure on punch and die, thereby reduces the tool life.

Correct cutting clearance will allow the fractures
to meet evently resulting in a clear break and the sheared
edge as a clear appearance and minimum burr.

HOW TO APPLY CUTTING CLEARANCE
PIERCING OPERATION:
In piercing operation, clearance is given to the die.
The component size is equal to the punch .
Here slug is a scrap.
Die opening size = Hole to be pierced +2C.
BLANKING OPERATION:
In blanking operation, clearance is given to the punch.
The component size is equal to the die.
Here slug is desired part.
Blank punch size = Hole to be blanked – 2C

PUNCH AND DIE CLEARANCE AFTER CONSIDERING
THE ELASTIC RECOVERY OF THE MATERIAL:

In blanking operation, after the release of blanking
pressure, the blank expands slightly. The blanked part is
actually larger than the die opening that has produced it.
Similarly in piercing operation, after the strip is
stripped off the punch, the material recovers and the hole
contracts. Thus, the hole is actually small then the size of
the punch which produced it.
Thus to produce correct hole and blank sizes,
the punch size should be increase and the die opening
size should be decreased by an amount fo elastic recovery.
The elastic recovery will depend upon blank size,
stock thickness and material. It may be taken as between
0.0125mm to0.075mm.
For stock thickness upto 0.25mm,
this difference may be taken as zero.
For stock thickness 0.25mm to 0.75mm
—It may be equal to 0.025mm.
For stock thickness more than 0.75mm
—It may be taken as 0.05mm.


EXAMPLE: 1 FOR APPLICATION OF CLEARANCE:

A washer component has outer diameter is 30mm and
inner diameter is 10mm and stock thickness is 1.0mm .
material grade is mild steel.(soft steel)
Solution:
Cutting clearance = material thickness X cutting allowance
= 1.0 X 5%
= 1.0 X 5 / 100
=0.05mm per side.
Here elastic recovery taken as 0.05mm.

In blanking:
blank die opening size = part size – elastic recovery
= 30 – 0.05
= 29.95mm.
blank punch size = die opening size – 2C
= 29.95 – 2 X 0.05
= 29.85mm.

In piercing:
piercing punch size = part hole size + elastic recovery
= 10 +0.05
= 10.05mm.
piercing die opening size = punch size + 2C
= 10.05 + 2 X 0.05
= 10.15mm
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EXAMPLE 2: APPLICATION OF CLEARANCE:

Here a rectangular part has 50mm length, 30mm width
and material thickness is 1.5mm. It has dia 5mm holes
at 4 corners and a center hole is dia 15mm.

Solution:
Cutting clearance = material thickness X cutting allowance
=1.5 X 6%
=0.09mm preside.
Here, elastic recovery taken as 0.05mm.

Piercing-1: 4 Holes at corners
Punch size = 5.0 +0 .05 = 5.05mm
Piercing die size =5.05 + 2 X 0.09 = 5.23mm.

Piercing –2: center hole
Punch size = 15 + 0.05 = 15.05mm
Piercing die size = 15.05 + 2 X 0.09 =15.23mm.
Blanking
Die opening size = lenth is 50 – 0.05 = 49.95mm
=width is 30 – 0.05 = 29.95mm.

Blank punch size = 49.95 – 2 X 0.09 = 49.77mm.
= 29.95 – 2 X 0.09 =29.77mm.