[Halld-tracking-hw] Carbn Fiber Parts

[Curtis A. Meyer]

Hi Beni

    run-out refers to the build up off all the errors as the parts are 
put together.
Since they are all in the same direction, we get a build up rather than 
a randomization.
I am not referring to glue seeping out, that does not happen.  I'll try 
to restat what we
feel the critical issue for us are that concern us a great deal with the 
parts as they stand.

The wire position relative to the endplate is indeed controlled by three 
fits:

    1) Feedthru into the endplate that we know is a sloppy fit right 
now. It was comfortable
        with these parts back in May, but they were shrunk by about 1.5 
mils.
    2) The pin-holder into the feed-thru which is probably ok, although 
is ok because
         the two ridges on the side make it snug.
    3)  The pin into the pin holder, which currently does not fit, so 
this has to be changed.

     We think that this leads to 3-5 mil (75 to 125 micron) uncertainty 
in the position.

   The second issue we are concerned about is the gas seal at the 
downstream endplate.
This is where gas is fed into the straws, and if it simply leaks into 
the large volume around
the straws here, we will not be able to guarantee what the gas is in the 
straws themselves.
We know from our pretty serious looks at this with our prototypes that 
the fit of the donut
into the straw and the feedthru into the donut are the major problem 
areas and we had to
have things better than what we have with the current parts. Thus my 
suggestion for the
1 to 1.5 mil change in size.  We also have a potential leak point where 
the feed through
goes through the endplate. This is a loose fit now. This gluing is done 
by "slathering" glue
on the feedthru, pushing it through the endplate into the donut, 
cleaning up the glue around the
base at the endplate, then injecting glue into the port into the 
feedthru-donut seal. Thus, I am
concerned about the seal from the endplate into the gas volume around 
the straws as well.

    Finally, the layers with close pack are reasonably tight with the 
straw positions. We left
about 5 mil between straws. Where this hits us is when we go around a 
layer and then try
to fit the "last straw" in. If things have slid due to all the run-outs 
plus the errors in the hole
positions themselves, then it may be impossible to load the last straw 
in some layers without
damaging the chamber. I am not certain what our strategy to deal with 
this would be other than
to plug the holes, don't insert the straw, and try to use the close-pack 
layers to make the straw-to-straw
glue joints.

     We have been talking about alot since Tuesday and our conclusion 
here is that we feel with these
three factors the risk is bigger than we are willing to accept.

     Curtis

   Hi Curtis,
I am a little confused here. What do you mean by run-out? Is this the 
glue that
potentially can seep out? I also do not see where this 5 mil number for 
the wire
location is coming from. I am totally confused. First you glue the donut 
into the
straw and let  it cure.  So having the donuts  undersized by 0.6mil  in 
diamter will lead
to a variation of not more that 0.3 mil in the  location of the center 
of the donut
that defines the location of the wire at this point. Then you put the 
straw into the
frame and glue the feed through into it. At this point an 0.3mil toot 
small feedthru
will cause a shift of 0.15mil in the central positon which defines the 
wire location.
So now I am at max about 0.45 mil  off center for the wire location or 
the crimp
pin holder if you wish.  I do not see how you could get to 5 mil.
Secondly this run-out if I understand correctly that this is glue 
seeping out somewhere.
To try to mitigate such a problem by tighter tolerances is the wrong 
approach. If this
is really a problem then the viscosity of the glue has to be changed. In 
addition you do
not glue in only one straw at a time but many. One question here: When 
you glue in
the straws into the frame is the frame vertically or horizontally. If it 
is vertically this
isue of seeping glue to neighbouring straws is very unlikely. If 
horizontally then start
from the bottom and work up on both sides of the ring then any seeping 
will flow down
and only affect already installed straws.
Before doing any more modification I highly recommend to do some test 
glueing with
straws and the carbon fire spare plate you used to test the reaming. If 
all fits why try
to improve on something that works. We should try to optimize also on 
the cost/benefit
ratio.

cheers,
Beni


-- 
Prof. Curtis A. Meyer		Department of Physics
Phone:	(412) 268-2745		Carnegie Mellon University
Fax:	(412) 681-0648		Pittsburgh PA 15213-3890
cmeyer at ernest.phys.cmu.edu	http://www.curtismeyer.com/