Sheet Metal Forming

Snap-Thru and Damping Systems
Snap-thru forces, also called reverse ton-
nage, are evident while blanking thick or very 
strong materials such as advanced high-strength 
steels. During blanking, some portion of the 
material is sheared and the rest is fractured. 
The fracturing happens much faster than shear-
ing, and there is a significant reverse tonnage at 
the end of fracture on the press. The press com-
ponents reach their maximum deflection just 
before fracture. After fracture, they spring back 
to their original shape and beyond, unopposed 
at a very high velocity, and there is a sudden 
release of energy. The ram is accelerated to a 
high speed. The drivetrain of the press has 
clearances around every moving component. 
When the clearances have reversed, the ram 
stops suddenly, expending all the energy and 
sending a shockwave. The press deflects in the 
opposite direction, as when the forward ton-
nage was developed, and generates the snap-
thru forces. The reverse tonnage or snap-thru 
forces can be as high as 50% or more of the 
regular blanking tonnage in the case of very 
strong materials (Fig. 1.18).
There are different ways to resolve the prob-
lem of snap-thru forces. When possible, stag-
gering punches would minimize the reverse 
load. Punch stagger is explained in an earlier 
section. Nitrogen cylinder cushions can be built 

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Chapter 1: Blanking / 11
to absorb the shock energy during snap-thru. 
However, this would require additional forward 
tonnage equivalent to the cushion capacity. Hy-
draulic shock dampers are the most widely used 
solution for combating snap-thru shocks (Ref 
1.12). They do not require additional forward 
tonnage and do not lower the press capacity. 
They are large-capacity shock absorbers with a 
short stroke. Springs hold up cylinders in their 
raised position, and when the ram moves down-
ward, the cylinder is forced to close and hydrau-
lic oil is forced to escape through small orifices. 
During fracture, the energy released is used to 
force more oil to escape through the orifices, 
thus generating a lot of heat. These are hydrau-
lic dampers. Properly sized and adjusted shock 
dampers will eliminate or at least lower reverse 
tonnage to a large extent.