[Halld-tracking-hw] feedthroughs and donuts
Beni Zihlmann
zihlmann at jlab.org
Fri Jul 16 10:50:31 EDT 2010
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
> Hi Everyone -
>
> we have carefully gone through the plastic donuts and feedthrus.
> We find that the
> ODs of both the donuts and the feedthrus are slightly under their
> specifications:
> donuts 0.6 to 0.7 mil too small and the feedthru about 0.3 mil too
> small (on average).
> We are extremely uncomfortable with the run-out that this will lead to
> when we assemble
> the chamber. We believe that this can adversely affect the the
> location of the wires at a
> level larger than 5 mils. We are also concerned that the run-out may
> exceed the spacing
> that we allowed for between tubes, making installation of the last
> element in a layer difficult.
> Finally, the run-out makes us prone to gas leaks at the input end of
> the tubes which worries
> us in terms of guaranteeing good gas flow in the chamber.
>
> I know that this introduces a delay, but we are very
> uncomfortable with the parts in hand.
> We feel that the following changes need to be made to reduce our
> perceived risk with
> the chamber assembly.
>
> 1) Increase the OD of the donut as it now stands by 1.0 to 1.5 mils.
>
> 2) Increase the OD of the feedthru as it now stands by 1.0 to 1.5 mils.
>
> 3) Increase the size of the pin-hole in the pin holder so the crimp
> pin fits
> into the hole.
>
> Curtis
>
> -----Note on measurements-----
> All measurements at CMU were performed with a brand new Mitutyo 293-344
> digital micrometer that reads to five decimal places and is guaranteed
> accurate
> to 0.00005 inches (1 micron). The has a ratchet-friction thimble so
> that you cannot
> squeeze the component too tightly.
>
> The ID of the straws which we received is difficult to accurately
> measure. We have made
> a part that slips in when we are cutting the straws that is 0.6106.
> From this, and the nature
> of the fit, we estimate that the ID of the tubes are about 0.611.
>
> We also note that because all the holes in the carbon fiber endplate
> were reamed using a
> 0.4375 tungston-carbide reamer, then checked with pin gauges, we
> believe that all holes
> are 0.4375, which is the lower end of the specification.
>
> Each of the parts that we received were measured many times in
> diffrent configurations.
> Those numbers are reported here, the largest first, follwed by the
> smallest. The largest
> number was measured with the micrometer on the small ridge.
>
> Donut OD: Design Specifications is 0.609 to 0.611
> -- the average of all of our measurements is 0.60784
>
> 0.60880 0.60730
> 0.60880 0.60710
> 0.60875 0.60720
> 0.60870 0.60700
> 0.60875 0.60715
> 0.60860 0.60690
> 0.60850 0.60680
> 0.60850 0.60695
> 0.60860 0.60680
> 0.60850 0.60700
>
>
> Donuts OD when mounted on the 0.4375" pin-go-gauge. The two measurements
> were taken 90 degrees apart near the glue trough.
>
> 0.60880, 0.60835
> 0.60870, 0.60815
> 0.60870, 0.60845
> 0.60875, 0.60810
> 0.60890, 0.60860
> 0.60880, 0.60830
> 0.60860, 0.60830
>
> Donut ID: Design Specification is 0.4375 to 0.4395
>
> All were checked with our 0.4375 go-gauge, 0.4396 no-go-gauge and
> passed.
> We noted that a 0.4380 pin-gauge would go through the holes.
>
> Feedthru OD Design Specifications are 0.4355 to 0.4375
> -- the average of all our measurements is 0.43491
>
> 0.43595 0.42400
> 0.43570 0.43410
> 0.43570 0.43400
> 0.43570 0.43435
> 0.43570 0.43410
> 0.43565 0.43395
> 0.43550 0.43450
> 0.43560 0.43400
> 0.43555 0.43430
> 0.43565 0.43410
>
> Feedthru OD when mounted on the 0.25000 pin-go-gauge. Measurements
> were taken
> 90 degrees apart near the glue trough
>
> 0.43575, 0.43525
> 0.43570, 0.43525
> 0.43560, 0.43515
> 0.43580, 0.43520
> 0.43565, 0.43525
> 0.43570, 0.43520
> 0.43565, 0.43525
>
>
> In all of the above cases, the typical smaller measurement will lead
> to the biggest slop
> in the fits. Also, if we take the measurements when mounted on the pin
> gauges to be
> the reference, then the donut OD is about 0.6-0.7mil under
> specification and the feed-thru
> is about 0.3 mil under specification. Essentially, with all the OD's
> undersized, and the IDs
> in the middle of the specifications, we have a build-up of loose
> tolerances.
>
> 1) Strawtube to donut is about 2.5 to 3.0 mil. This affects our
> ability to guarantee a gas-tight
> seal at the down stream end of the chamber as the gas enters the
> straws.
>
> 2) The feedthru to the donut is about 3.0mil. This also affects our
> ability to guarantee a
> gas-tight fit at the down stream end of the chamber.
>
> The two comments on gas above worry me. If we have leaks near the
> input to the straws,
> will we be able to guarantee good gas flow through the straws? A
> may be paranoid, but
> our performance is very sensitive to gas (as we have seen from
> our recent work with
> the preamplifiers).
>
> 3) The feedthru to the endplate is about 2.5mil. This most directly
> impacts the centering of the
> wire in the hole. This needs to be combined with the tolerances
> of the pin-holder.
>
> 4) The OD of the pinholder to the ID of the Feedthru is snug because
> of two small ridges
> near the top of the pin holder. The fit at the base of the
> pin-holder is very loose. I am
> not certain if this is an issue or not, but mention it for
> completeness.
>
> The above 1-3 also impact where the straw will be located relative to
> the center of the
> hole in the end plate. We have a potential build up of
> (2.5-3.0)+(3.0)+(2.5). The concern here
> is that this will be a systematic that pushes the straws apart, so it
> could consistently
> be above 5 mils. I do not believe that this will impact the
> electrostatics, but it could build
> up going around a layer of straws, and impact our ability both our
> ability to insert the last
> straw in a layer as well as place the "close-packed" layers in place.
> I believe that our design
> allowed for a 5 mil gap between straws.
>
>
--
***********************************************************
Benedikt Zihlmann
TJNAF
HallD 12C/ F350
Suite 8
12000 Jefferson Av.
Newport News VA, 23606
tel: 757 269 5310 fax: 757 269 6331 zihlmann at jlab.org
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