
\subsection{The 'Frascati' combs}


The precision combs holding the tubeplugs at the HV and RO ends
during the glueing of a tubelayer were assembled at Frascati.
They consist of 4 aluminum pieces with glued steel precision rods
to position the plugs.
These pieces were produced using a master jig
as descibed in [ref.Frascati], and then glued together to
make one 72 tube BOL jig.
  The assembled combs were measured on a CNC machine at NIKHEF.
The analysis of the measurements showed :
\begin{itemize}
   \item the average z-pitch of the two complete combs is 35.0355mm  
     at a temperature~\footnote{The temperature was stable at the level
 of 0.1$^o$C during the
 measurements with the 3D coordinate measuring machine. However,
  we discovered inconsistencies on the absolute value of the
 temperature reading. Due to this problem we assign an uncertainty
 of  0.4$^o$C.}
 20.4 +- 0.4 $^o$C
     and deviations of $<$ 0.1 \mum  from this value  
     for the average z-pitch of the original jig pieces. 

    \item  The  rms of the individual  z  positions 
     and the rms of the  y-deviations from a straight line 
     are less than  2\mum.
     No significant systematic shifts in z or y were found
     at the glue joints of the 4 pieces.
\end{itemize}


\subsection{The Tube combs}


The combs supporting the tubes in between the precision combs
at 0.5m intervals ,have been machined at NIKHEF as solid aluminum pieces
with a width corresponding to 18 tubes.A total of 50 pieces were produced
en measured on the CNC.

 To assemble the 9 full width combs (72 tubes) 
the 36 best pieces were selected, taking into account
the precision of the wedges and the connecting surfaces.
The connections between the pieces were checked with the RASNIK malmonitor (see~\ref{tools})
device prior to their fixation.

The assembled combs were again measured on the CNC and the results were 
compared with the nominal height and Frascati pitch.
The  RMS deviations of the tube positions  are less than 15\mum in $Z$ and $Y$.

\subsection{The Sphere towers}

The sphere towers consist of a base block, one of 3 different
type of stacking blocks to adjust the spacer position  according to 
the type of layer to be glued,
and a top block with the sphere holder.  

Measurements with the CNC machine of all the separate parts allowed
a selection of optimal combinations of
base, stacking  and holder blocks resulting in a Y and Z accuracy
of   about 5~\um.


\subsection{Positionning of the mechanics on the table}

The accuracy of the $Y$ position of the mechanics is limited by 
the flatness of the granite table. Over short distances (one meter) 
this accuracy is about 10~\um. Over longer distances, 
temperature gradients start to play a role, 
which make the table bend and can lead to a sagitta of 25~\um over five meter.

The combs and towers are  initially positionned with low accuracy. 
The high precision is reached by adjustments and 
many remeasurements in an iterative way.

Both the precision and tube combs are positionned on the table
with a sliding fixation  such that 
they are allowed to expand freely in the direction away 
from the reference bar in case of temperature variations.
This was checked with laser interferometer measurements
and local heating of some combs with approximately +5 deg
temperature increase.



\subsection{ Frascati comb positioning}
 In order to properly combine the Frascati combs with tube combs
  a 20\um steel foil was used to shim the height
 of the Frascati combs on the assembly table. This was needed
 due to a small change in the tube and plug geometry after
 the combs were already machined. 

   The relative height  above the table surface  of the precision wedges
was checked several times before and during chamber construction
with heigth-meter measurements. Average deviations from the nominal value
were found to be less than 5\mum with an rms below 4\mum 
for different positions along the combs. 

The relative torque of the Frascati combs is measured by using tiltmeters.
 Despite the high intrinsic precision precison of these tiltmeters,
 a measurement better than than 10~\urad seems impossible. We measured the
torque to be zero whithin the scatter of the measurements.

The $X$ distance was based on a test-TUBE with known length
and rechecked by tape-measure. The X-distance between the tube supports is
 stable within 500~\um, dominated by the low precision of the X coordinate of these supports themselves.


To measure the angle (the parallelness) between the two Frascati combs, a laser beam in the $X$ direction is used, aligned by the silicon sensor on the first wedges of the Frascati combs. 
A penta prism, accurate to 10~\urad, reflects the laserbeam  over
 an angle of 90 degrees into the $Z$ direction.
The Frascati combs are aligned with respect to this beam, leading to a
 straigthness in $X$ within 100~\um. 
Thus, seen in the $X$-$Z$ plane, the Frascati combs describe a rectangular 
shape with angular deviations of about 100~\urad.


\subsection{ Tube comb positioning}
 The tube combs are positionned on the assembly table with one side fixed
to a reference bar. Each jig position is adjusted in $Z$ such that
the tube positions of the 9 combs deviate less than 50\mum from straight lines 
connecting the two precision combs at each end of the table.

 For this we used a laser and silicon-sensor combination (see ~\ref{tools})
 The laser beam defines a straight line between the two precison combs 
 roughly along path of  a tube. The relative beam position
 in $Z$ and $Y$ is measured by the silicon sensor.

 These measurements are repeated at 
 several positions along the precision combs.
  In Fig.~\ref{sisens} we show the distribution of the deviations for
 some of the measurements. The rms of the deviations is typically
 16 \mum in $Z$ and 11 \mum in $y$, thus amply fulfilling the requirement
 of 100 \mum rms for the centrisity of the wire with respect to
 the tube centre. These measurements will be  repeated regularly
 as part of the QA/QC procedure.
\begin{figure}[tphb]
\begin{center}
\setlength{\unitlength}{1cm}
\begin{picture}(14,10)
\put(4,0){\mbox{\epsfysize=8cm\epsffile{sisens.ps}}}
\end{picture}
\end{center}
\caption{\sl 
  Results of silicon sensor measurements of the relative Z and Y 
position of the Frascati and Tube combs. The deviations from a 
straight line fit through the data are shown.}
\label{sisens}
\end{figure}


  Heigth meter measurements directly comparing  the relative height of corresponding
wedges in the Frascati combs and in the tube combs next to them
showed deviations less than 20\mum from the nominal value. 
This ensures that the alignment platforms
which are glued onto the first tubelayer in this area can be positionned with 
with sufficient precision.

\subsection{ Sphere tower positioning}
The sphere towers will support the spacer during the gluing procedure and
so determine the relative wire positions between the tube layers.
The towers are initially positioned  with an accuracy 
in the $Z$ direction of about 100~\um using standard optical tools. 

The alignement in $Y$ of the three reference towers is checked with
 the silicon sensor.
The results indicate that the middle tower with respect
 to a line defined by the $Y$ of the outer towers deviates by 
 -25~$\pm$~10~\um. The non
 reference side is checked in a similar way, which reveals the same deviation.
The intrinsic accuracy of these towers, does not allow such deviation,
 leading to the conclusion that the surface of the granite table, 
at the position of the towers, has deviations of the same order.
// We tried
// to find more evidence, but to measure with this accuracy over 5~m is far from
trivial.
Measurements with standard optical tools also
 suggest a negative deviation in heigth of the central towers.

   On the reference side of the table the sphere  towers are fixed to 
the corresponding  Frascati combs.
In order to assure
that the two multilayers are glued to the spacer without a 
significant stereo angle between them, the distance between the wedges
and the spacer fixation in $Z$ must differ by less than 10\mum
for the two precision combs. This is because the HV and RO
ends of a chamber are interchanged with respect to the two
precision combs for glueing either of the two multilayers
(see~\ref{global}).
 For the QA/QC of this requirement we use an auxiliary device
called the BOL-monitor as described in~\ref{tools}. 

From these measurements we conclude that the sphere towers are positioned
within specification.

