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Hello All,<br>
<br>
Now that we have got all the issues out, I propose we have a video
conference Monday 2pm to discuss our next moves. Does that work for all
interested parties?<br>
Enjoy the weekend!<br>
Thanks,<br>
<br>
Tim<br>
<br>
Curtis A. Meyer wrote:
<blockquote type="cite" cite="mid4C40B5E1.4050507@ernest.phys.cmu.edu">
Hi Everyone -<br>
<br>
yes, we are all planning to measure the position of the crimp pin
after stringing to get<br>
the best possible accuracy. I am not contesting that. However, making
sure that that things<br>
are as close as possible to where they should be should be our starting
point.<br>
<br>
When we were setting all the specifications on the parts and hole,
CMU wanted things<br>
at the 1/2 mil level, but we had to back off due to cost. That said, we
had somewhat<br>
reasonable expectation that we would fall somewhere near the middle of
the specs<br>
with a distribution (like the Al parts) versus what we have with the
Carbon fiber where<br>
we have parts just out side the specifications on the low side.<br>
<br>
When I take a donut and a feedthru, put them through the endplate
and feel the substantial<br>
wiggle that is present. I a concerned to the point where I loose sleep
(seriously) over the <br>
thought of continuing with these parts.<br>
<br>
As per the gas tightness, I believe that our small number of
controlled tests on a bench <br>
appear to have sealed the donut to the feedthrough, but looking into
the straw at the glue<br>
joint, there is some glue oozed into the straw. This is indicative of
what we saw when<br>
we had gas leaks. We can also glue the feedthru to the endplate sample,
it is difficult to <br>
check gas tightness, but we can look. However, even with that done, I
am worried about<br>
guaranteeing all the seals holding in production. <br>
<br>
Our procedure, while controlled very well with the pneumatic
gluing and the entry and<br>
exhaust ports is very good, but if we have wiggle in the parts during
installation, we have<br>
trouble controlling things and I am very concerned about developing gas
leaks during the<br>
experiment. I am not willing to say that we can do it with what we have.<br>
<br>
Tim's comment on the straws fitting is a good one. I also recalled
a tighter spacing <br>
than the 9mil he recalls. That makes this much less of a problem.<br>
<br>
Curtis<br>
On 7/16/10 2:49 PM, Tim Whitlatch wrote:
<blockquote type="cite" cite="mid:4C40A9BF.50503@jlab.org">
Hi All,<br>
<br>
I will state what appears to be the 4 concerns and address each in
order and offer a solution.<br>
<br>
1) Uncertainty in the Position of wire.<br>
<br>
If all allowable tolerances are added up including the location of end
plate holes, the uncertainty of wire position can be as much as
.002+.002+.0025+.001+.003+.0025 = .013 inches.The uncertainty in the
stereo wire positions can be as much as .014 inches. This includes all
maximum clearance between mating parts and end plate hole locations <br>
<br>
<b>Possible solution</b>: As discussed last summer, the plan is to
use
either photogrammetry or a portable coordinate measurement machine to
determine the as built locations of each crimp pin. This can probably
get us to within knowing the actual location within about.003 inches.<br>
<br>
2) Concern with gas seepage at joints.<br>
<br>
>From the average numbers supplied by CMU this morning;<br>
Clearance Fit between donut and straw is within .0027 with pin gage in
place inches (.0032 without)* (drawings allow .004 inches)<br>
Clearance Fit between donut and feedthru is .0028 inches (drawings
allow .004)<br>
Clearance Fit between feedthru and endplate is .0026 inches (drawings
allow .0025)<br>
<br>
*Looks like the donut gets squeezed by .5 mil without pin gage while
measuring<br>
<br>
>From above we note that the actual clearances are equal (within
.0001
inches) or less than what was allowed by the drawings.<br>
<br>
<b>Possible solution:</b> It is important that we test the
components
in hand using the practice carbon end plate supplied by Meyer tool.
Glue and leak check. This will tell us whether we have a problem or not.<br>
<br>
3) Concern that the last straw will not be able to be installed due to
lack of space.<br>
<br>
If allowable tolerances are added up for possible straw tube location
discrepancies, we get .002+.002+.0025 = .0065 inches. For stereo holes
this goes up to .0075. The minimum nominal clearance between any 2
straws is .009 inches (assuming maximum possible straw tube OD of .622
inches). Therefore, this has already been taken into consideration and
does not pose a problem.<br>
<br>
<b>Possible Solution: </b>The last 3 straws in any row can be fit
checked prior to gluing them in order to avoid any remote possibility
that the last does not fit.<br>
<br>
4) Crimp pin fit too tight<br>
<br>
<b>Solution:</b> Have vendor correct this.<br>
<br>
Cheers,<br>
<br>
Tim<br>
<br>
Curtis A. Meyer wrote:
<blockquote type="cite" cite="mid4C40773D.3050201@ernest.phys.cmu.edu">
<pre wrap="">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
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<br>
<pre cols="72" class="moz-signature">--
Prof. Curtis A. Meyer Department of Physics
Phone: (412) 268-2745 Carnegie Mellon University
Fax: (412) 681-0648 Pittsburgh PA 15213-3890
<a href="mailto:cmeyer@ernest.phys.cmu.edu">cmeyer@ernest.phys.cmu.edu</a> <a href="http://www.curtismeyer.com/">http://www.curtismeyer.com/</a>
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