Abstract
M.Ing. (Mechanical Engineering)
The surface finish of machined components is generally described by conventional
surface trace measurements with trace lengths in the order of a few millimetres.
Surface roughness descriptors (such as the arithmetic mean roughness Ra) are then
used to quantify the surface finish. In general these techniques cannot however fully
capture the finer topography features on a sub-micron and nano-scale level.
An investigation was therefore performed in order to quantitatively characterise the
sub-micron or nano-scale surface features of turned Grade 4 titanium alloy. Submicron
surface features are believed to play a significant role in the performance of
machined components in various applications in the aerospace and biomedical fields.
The effects of the machining parameters on the sub-micron surface features were
investigated. Grade 4 titanium alloy bars were machined with different cutting
parameters. The cutting speed, feed rate and depth of cut were adjusted in an attempt
to characterise and control their effects on the sub-micron surface features.
Sophisticated atomic force microscopy is then required to quantify this roughness
component by using a variable length scale roughness descriptor.
It was concluded that there exists a significant sub-micron surface roughness
component that is not fully captured by the conventional roughness measurement
techniques. At low length scales the surface roughness was higher than theoretically
expected due to the existence of significant sub-micron surface irregularities. The
measured conventional roughness was observed to be lower than theoretically
expected in all cases. This is due to the reduction of the expected major surface
feature height as a result of the tool geometry and related shearing process.
It was found that the variable length scale surface topography was affected by cutting
speed and feed rate. In general at low scale lengths the roughness increases with an
increase in cutting speed. However at the conventional roughness length scale a local
maximum was displayed at an intermediate cutting speed (± 150 m/min). The feed
rate was found to affect the intermediate length scale surface topography by altering
the sub-micron to conventional topography transition region. Depth of cut had no
appreciable effect over the length scale range measured.