Areal surface texture parameters on surface

Additive manufacturing (AM) techniques enable
the manufacture of components with free-form
geometries and complex internal and external
features. X-ray computed tomography (CT) is
increasingly being used to inspect internal features
of AM parts. An advantage of the CT process,
compared to optical and stylus instruments
with limited acquisition slope angles, is the ability
to reconstruct reentrant features (undercuts).
Processing reentrant features provides an advantage
in the computation of surface parameters.
If the surface includes many reentrant features,
their elimination can lead to a biased estimation
of parameters related to the height or the area
of the scale limited surface. A unified framework
capable of handling free-from surfaces, with
generic form surface, reentrant features and unevenly
spaced points, such as those from CT reconstruction,
will be proposed. Standard software
instruments employed for roughness parameter
require evaluation of height data on a rectangular
grid. This allows the computation of areal parameters
based on discrete methods with good
approximation, dependent upon the sample size.
The reconstruction from CT volume to mesh allows
performance of an adaptive meshing based
on the maximum allowable distance between the
implicit function (implicit surface defined by a constant
grey value) and the final triangular mesh [1].
With irregular meshes it is not possible to perform
the integral with the discrete approximation and
a bias on the parameters computation can arise.
In this paper an approach that approximates a
generic mesh based on locally refined (LR) Bspline
is proposed [2]. The approach can be applied
to a generic form surface because the local
stretching of the surface is taken into account.
Mesh parameterisation enables to handle undercuts,
each acquired point is described as a function
of two abstract parameters. In this paper the
proposed method will be compared with the discrete
(ISO 25178-2 compliant [3]) method implemented
in standard software packages [4]. Since
filtering techniques based on a general mesh are
not yet defined in the standard, the primary surfaces,
the surface after removing the form, will be
analysed. The areal parameters of a Rubert sample
(casting plate 334, nominal Ra of 25 µm) measured
with a focus variation (FV) instrument will
be evaluated. Two form surfaces will be taken into
account: plane and cylinder. Robustness of the
discrete method will be finally evaluated with the
mesh reconstructed from two CT measurements:
the Rubert sample and an AM part.