[Halld-tracking-hw] Carbn Fiber Parts

[Curtis A. Meyer]

Hi Everyone -

     yes, we are all planning to measure the position of the crimp pin 
after stringing to get
the best possible accuracy. I am not contesting that. However, making 
sure that that things
are as close as possible to where they should be should be our starting 
point.

     When we were setting all the specifications on the parts and hole, 
CMU wanted things
at the 1/2 mil level, but we had to back off due to cost. That said, we 
had somewhat
reasonable expectation that we would fall somewhere near the middle of 
the specs
with a distribution (like the Al parts) versus what we have with the 
Carbon fiber where
we have parts just out side the specifications on the low side.

     When I take a donut and a feedthru, put them through the endplate 
and feel the substantial
wiggle that is present. I a concerned to the point where I loose sleep 
(seriously) over the
thought of continuing with these parts.

      As per the gas tightness, I believe that our small number of 
controlled tests on a bench
appear to have sealed the donut to the feedthrough, but looking into the 
straw at the glue
joint, there is some glue oozed into the straw. This is indicative of 
what we saw when
we had gas leaks. We can also glue the feedthru to the endplate sample, 
it is difficult to
check gas tightness, but we can look. However, even with that done, I am 
worried about
guaranteeing all the seals holding in production.

      Our procedure, while controlled very well with the pneumatic 
gluing and the entry and
exhaust ports is very good, but if we have wiggle in the parts during 
installation, we have
trouble controlling things and I am very concerned about developing gas 
leaks during the
experiment. I am not willing to say that we can do it with what we have.

      Tim's comment on the straws fitting is a good one. I also recalled 
a tighter spacing
than the 9mil he recalls. That makes this much less of a problem.

      Curtis
On 7/16/10 2:49 PM, Tim Whitlatch wrote:
> Hi All,
>
> I will state what appears to be the 4 concerns and address each in 
> order and offer a solution.
>
> 1) Uncertainty in the Position of wire.
>
> 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
>
> *Possible solution*: 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.
>
> 2) Concern with gas seepage at joints.
>
> >From the average numbers supplied by CMU this morning;
> Clearance Fit between donut and straw is within .0027 with pin gage in 
> place inches (.0032 without)* (drawings allow .004 inches)
> Clearance Fit between donut and feedthru is .0028 inches (drawings 
> allow .004)
> Clearance Fit between feedthru and endplate is .0026 inches (drawings 
> allow .0025)
>
> *Looks like the donut gets squeezed by .5 mil without pin gage while 
> measuring
>
> >From above we note that the actual clearances are equal (within .0001 
> inches) or less than what was allowed by the drawings.
>
> *Possible solution:* 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.
>
> 3) Concern that the last straw will not be able to be installed due to 
> lack of space.
>
> 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.
>
> *Possible Solution: *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.
>
> 4) Crimp pin fit too tight
>
> *Solution:* Have vendor correct this.
>
> Cheers,
>
>        Tim
>
> Curtis A. Meyer wrote:
>> 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/


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