valleys” become too small to hold sufficient lubricant
and wear debris. (3). Good anti-galling properties
were found to be achieved when surface roughness
Ra was in the range of 0.7 - 1.5 um and with Rpc in
the range of 37 - 57 peaks/cm. Both lower
Roughness Ra and higher Rpc were shown to be a
disadvantage as the resulting “valleys” are too small
to retain sufficient lubricant or the wear debris formed
during sliding leading to wear and “galling”.
Conversely higher roughness Ra and lower Rpc were
found to cause high localized contact pressures due
to the reduced number of contacts between the sheet
and tool. Other investigations (4) found similar results
for the importance of the "open roughness" with the
valleys to act as oil reservoirs and evacuate the wear
particles, they also found that a rolled sheet surface
with negative Rsk should be used to avoid “galling”.
Further investigations considered that the area of the
valleys should be greater than the area of the plateau
on the rolled sheet. (5) This higher ratio of valleys to
plateaus resulted in a reduced bearing area and a
lower potential for “galling”.
The overall painting appearance is determined by
such factors as; gloss, DOI and orange peel, each of
which relates to a specific area of wavelength
components in the surface texture on the rolled sheet
as shown in Figure 1. Several studies of the effect of
surface texture on the paint appearance have
concluded that for stochastic textures, such as
Shotblasting (SB) and Electro-discharge Texturing
(EDT), there is a correlation between low Ra and
high Rpc and improved paint appearance. A low
roughness of approximately 0.75 um Ra in
combination with high peak count, in the range of 74
peaks/cm, provided the "maximum" image clarity for
a 50 um paint film. (6) While Ra and Rpc are
important for good paint appearance these
investigations also revealed that they alone don’t
always produce good paint appearance, but that the
Waviness (Wa) also has a strong correlation with the
paint appearance.
Figure 1: