PRESS TOOLS-Cutting Clearance

Cutting Clearance:Cutting clearance is the gap between a side of the punch and the corresponding side of the die opening when the punch is entered into the die opening.
Cutting clearnace should always be expressed as the amount of clearance per side.
Proper cutting clearance is necessary for the longer life of the tool.Quality of the piece part also depends on proper cutting clearance.A visual examination of the punched components will indicate the amount of clearnace and whether the punch and die have
- optimum cutting clearance or
- excessive clearance or
- misalignment
Optimum Cutting Clearance:
When optimum cutting clearance condition exists a small edge radius is formed.
The edge radius is the result of the plastic deformation (first stage of shearing).A highly burnished cut band is approximately one third the thickness of the stock material.The balance of the cut is the break resulting from fracture(third Stage).Excessive cutting clearance:the gap between the punch and the die is comparatively more in this case.The stock material reacts to the initial pressure on a manner approaching that of forming rather than cutting.Therefore the edge radius becomes larger.It does not blend smoothly with the cut band.The cut band becomes smaller.The break shows greater irregularities.
Heavy burrs are noticeable all along the cut contour.The burr results from the dragging of the material.Insufficient cutting clearance:When cutting clearance is slightly less,the width of the cut band will be more.If cutting clearance is too less two or more cut bands will be formed.Because of the steeper angle between the punch and the die cut edges the resistance of the stock material to fracture is increased.The resulting pressure will cause the initial fracture to originate at clearance rather than at the cut edges.Burr may be caused by compressive forces. Misalignment between punch and DieThe cutting characteristics also indicate whether the punch and die openings are in accurate alignment.Because of misalignment,clearance on one side increases and the other side decreases.The component will show the corresponding difference on cut band.
Burr side
The burr side is adjacent to the break.Burr should be practically nonexistent
- if the cutting clearance between the punch and die is optimum.
- if the cutting edges are sharp.
The burr side of a blank or slug is always towards the punch(die stars shearing)The burr side of a punched opening is always towards the die opening (punch starts shearing).The characterstics of the blank or slug and the punched opening are inversely idential.
Relationship of piece part size to punch and die size
when pierced or blanked piece parts are measured,the measurement is made at the cut band.
The actual cutting of the blank or slug is done by the cutting edge of the die opening .Therefore the die opening determines the size of the blank or slug.
The actual cutting of the opening in the stock material is done by punch.Therefore the size of a punched opening is determined by the punch.
Clearance calculation:
The ideal clearance can be caluculated by the following formula
Clearance=C*s*square root of (T max/10)
where C is a constant=0.005 for very accurate component
=0.01 for normal components
s=sheet thickness in mm
T max=shear strength of the stock material in N/mm square
T max for some materials:
St.steel=400N/mm square
MS=400N/mm square
Brass=200N/mm srqare
aluminium=100N/mm square
Copper=250N/mm square

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