CUTTING FORCE

CUTTING FORCE:

       “Cutting force is the force applied on the stock material in order to cut out the blank or slug”.

This determines the capacity of the press to be used for particular tool. The area to be cut is found by multiplying the length of cut by stock thickness.

Formula for calculating the cutting force:

Cutting force  = L x S x T max 

L = Length of periphery to be cut in ‘mm’.

S = Sheet thickness in ‘mm’

T max = Shear strength in N/mm2  

Shear and tensile strengths for most materials are not the same.Generally for calculation purposes Shear strength  is considered  as 80% of tensile strength.

The fig. Represents the typical load curve of cutting force of blanking or piercing punch.

Formula to calculate the press force:

  Press force  = Cutting force  + stripping force

  (striping force  = 10% - 20% of cutting force)

       The following table gives the shear strength  (T max = 0.2 for tensile strength σ max ) of several materials.

MATERIAL	T max in N/mm2
Steel with 0.1% carbon	240 -300
Steel with 0.2% carbon content (deep draw steel)	320 - 400
Steel with 0.3% carbon	360 - 420
Steel with 0.4% carbon	450 - 560
Steel with 0.6% carbon	550 - 700
Steel with 0.9% carbon	700 - 900
Silicon  steel	450 - 550
Stainless steel	350 - 450

MATERIAL	T max in N/mm2
Copper	200 -  400
Brass	350 -  400
Bronze	360 -  450
German silver (2 - 20% Ni, 45 - 75% Cu)	300 - 320
Tin	30 -  40
Zinc	100 - 120
Lead	20 - 30
Alluminium 99% pure	20 - 120
Alluminium manganese alloy	150 - 320
Alluminium silicon alloy	120 - 250
Paper & card board	20 - 50
Hard board	70 - 90
Laminated paper or rosin impregnated paper	100 - 140
Laminated fabrics	90 - 120
Mica	50 -  20
Plywood	20 -  40
Leather	7
Soft rubber	7
Hard rubber	20 - 60
Celluloid	40 - 60

Example:

Calculate the press force required to produce the following component. Sheet thickness 2mm. Material is brass.

Cutting force = L x S x T max

= 126 x 2 x 400

= 100800 N

= 100.8 KN

Press force      =  Cutting force +Stripping force

= 100800 + 20% 100800

= 120960 N

= 120.960 KN.

METHODS OF REDUCING THE CUTTING FORCE:

It sometimes becomes necessary to reduce the cutting force to prevent press over loading 

Stepped punches to be used.

To grind the face of the punch or die to a small shear angle.

•  In some cases it will be necessary to reduce cutting force to prevent over loading press

• A method to reduce press force is to grind the face of the punch or die at a small shear angle with reference to the horizontal plane.

• This reduces the area of contact during shear at any one time.

• Providing shear angle also reduces shock to the press and smoothness out the cutting operation.

• To prevent distortion on the stock material for blanking operation the shear angle will be on the die member for piercing operation the shear angle will be on the punch member.

• Another method to reduce cutting force is to step punch lengths.

• Punches or groups of punches are made progressively shorter by about one sheet thickness.