3 Fundamentals of press design

180 Sheet metal forming and blanking
the frictional shear stress divided by normal contact stress
μ τ / σ –
= [ ]
R N
At lower sliding velocities, dry or maximum friction conditions apply;
that is to say the dies/the workpiece are primarily separated by the reaction
or adsorption layers – so-called boundary layers – of a molecular
order of magnitude, whereby solid body friction is also possible in submicroscopic
ranges. Here, the workpiece slides on lubricating films of
molecular magnitude, i. e. on these boundary layers. In the absence of
sufficient lubrication, the boundary layers can break up locally and
weld joints can develop. Coefficients of friction in the range of µ = 0.10
to 0.15 are indication of dry friction conditions.
With an increase in sliding velocity, portions of the interface are supported
by hydro-dynamic fluid lubrication. The thickness of the lubricating
film increases, but the lubrication gap h [µm] still remains smaller
than the total of the surface roughness R t [µm] of both, the die and
the workpiece, i. e.:
h< Rtdie + Rtworkpiece
The frictional forces F R [N] drop sharply and come close asymptotically
to a limiting value, which is the transition point to pure hydro-dynamic
friction.
The influence of sliding velocity, normal stress and lubricant viscosity
is at its maximum in the area of boundary or mixed friction. Here,
the die/workpiece pair has the topography of the task of holding the
lubricant for as long as possible during the sliding action. As a rule, all
forming processes involving boundary friction operate with frictional
coefficients of µ = 0.05 to 0.10. With further increases in the sliding
velocity, complete hydro-dynamic friction can be achieved in which
the lubricant film h becomes greater than the sum of the surface roughness
R t of both workpiece and die. This condition, however, cannot be
achieved in practical metal forming operations:
h> Rtdie + Rtworkpiece
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998