  Large area Micromegas detectors will be employed for the first time in
high-energy physics experiments. A total surface of about $\mathbf{150~m^2}$ of
the forward regions of the Muon Spectrometer of the ATLAS detector at LHC will
be equipped with 8-layer Micromegas modules. Each layer covers more than
$\mathbf{2~m^2}$ for a total active area of $\mathbf{1200~m^2}$. Together with
the small strip Thin Gap Chambers they will compose the two New Small Wheels,
which will replace the innermost stations of the ATLAS endcap muon tracking
system in the 2018/19 shutdown. In order to achieve a 15$\mathbf{\%}$
transverse momentum resolution for $\mathbf{1~TeV}$ muons, in addition to an
excellent intrinsic resolution, the mechanical precision of each plane of the
assembled module must be as good as $\mathbf{30~\mu m}$ along the precision
coordinate and $\mathbf{80~\mu m}$ perpendicular to the chamber. The design and
construction procedure of the Micromegas modules will be presented, as well as
the design for the assembly of modules onto the New Small Wheel. Emphasis will
be on the methods developed to achieve the challenging mechanical precision.
Measurements and simulations of deformations created on chamber prototypes as a
function of thermal gradients, internal stress (mesh tension and module
fixation on supports) and gas over-pressure were essential in the development
of the final design. During installation and operation all deformations and
relative misalignments will be monitored by an optical alignment system and
compensated in the tracking software.
