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  #1  
Old 06-24-2019, 11:07 AM
rcarsey rcarsey is offline
 
Join Date: Jul 2012
Location: North Brunswick, NJ
Posts: 133
Default aft window crack

It happened. I'd like to post a photo of it, but it is too graphic and horrendous to show in public, so I'll offer an illustration and narration of what happened.

After careful drilling of all the holes with official plastic drill bits, I declared success! No cracks, no spider webs.. perfect round holes that even a CNC machine would be proud if, if they had feelings.

Then came time to tap 6-32 the holes in the rollbar. All of the holes were cleco'd (#40 holes; then final-drilled #36). I removed one, top-center/left-side. Put my 6-32 tap in and started turning by hand. The plastic was no match for it.. nice threads in the plastic. Then the tap contacted the metal rollbar. I suppose there wasn't enough pressure on the tap, as it had a hard time getting its teeth into the metal. Meanwhile, as the tap turned, the gap between the plastic and rollbar grew bigger and bigger. SNAP!

Being a wishful thinker, gee, it sounded like a cleco had snapped into place.. but then after I looked at the window on a slight angle, I see it had cracked. Beginning at the hole next to the one I was working on, extending backwards about 3 inches.

Options?
  1. stop-drill it. Use tank sealer on the crack & hole to prevent leaks
  2. Same, but don't seal the window to the turtle skin and eventually replace the window
  3. Toss the window and start over now ($400).
  4. Have a beer and pretend like this didn't really happen.

Note: the crack is above the rollbar's brace to the baggage bulkhead. So its not visible from the interior.

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Rob Carsey, North Brunswick NJ
RV-12iS (N713)
Tail 95% done (VS fairing)
Wings 95% done (AOA)
Fuse 100% done
Finish 75% done (canopy, cowl & tail glass)
Avionics 0% done (IFR package ordered)
Dues paid 2019
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  #2  
Old 06-24-2019, 11:52 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
Posts: 1,548
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Suggestion to use a 6-32 Taper Tap‎. The gradual lead-in taper will allow the metal to begin cutting threads with minimal effort.
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Jim Stricker
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RV-12 E-LSA #120058 AWC July 2012 N633CM
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  #3  
Old 06-24-2019, 12:25 PM
Mich48041 Mich48041 is online now
 
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,922
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I assume that the aft window currently used is made of acrylic. Cement is available that is made specifically for acrylic. It will melt the acrylic and weld it together. A scrap piece of acrylic could be cemented to the crack to reinforce it.
It is a bad idea to tap the plastic unless it is firmly clamped to the aluminum. Any space at all between the two materials during tapping will result in the threads being out of alignment. That will prevent the two materials from being drawn together tightly when the screw is tightened.
An option is to not tap the acrylic. Instead, ream out the holes in the acrylic so that a screw slides though the hole without turning.
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  #4  
Old 06-24-2019, 01:20 PM
DHeal DHeal is offline
 
Join Date: Sep 2006
Location: Windsor, California
Posts: 834
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Stop drill it and put a decal over it.
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David Heal - Windsor, CA (near Santa Rosa)
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RV-12 E-LSA #120496 (SV w/ AP and ADS-B 2020) - N124DH flying since March 2014 - 840+ hours (as of Oct 2019)!
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  #5  
Old 06-24-2019, 02:25 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by Mich48041 View Post
It is a bad idea to tap the plastic unless it is firmly clamped to the aluminum. Any space at all between the two materials during tapping will result in the threads being out of alignment. That will prevent the two materials from being drawn together tightly when the screw is tightened.
An option is to not tap the acrylic. Instead, ream out the holes in the acrylic so that a screw slides though the hole without turning.
To comment requires an understanding of the recommended drilling tapping, and finishing processes.

The threads are not left in the window when completed. The window is only tapped during the tapping process to provide a means of aligning the tap so that the threads are not cut at an improper angle. The holes get final drilled later to provide for a slip fit for the screws

Also, there is a gap on some of the holes common between the roll bar and the rear window so the window will never lay flush against the roll bar at all hole locations.
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  #6  
Old 06-24-2019, 02:45 PM
Bob Y Bob Y is offline
 
Join Date: Jul 2017
Location: Piedmont, SC
Posts: 84
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Quote:
Originally Posted by Mich48041 View Post
...Cement is available that is made specifically for acrylic. It will melt the acrylic and weld it together. A scrap piece of acrylic could be cemented to the crack to reinforce it.
A fellow builder recommended this product to me when I cracked some of my holes:

ACRIFIX® 1R 0192 Bright

It worked very well. It’s UV curing. You can see some videos on line that show the adhesive is stronger than the base acrylic.
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  #7  
Old 06-24-2019, 10:00 PM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,538
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Something similar happened to me while building a WAR FW-190 in the 80s. Fortunately it was in an area where I could cut a hole with a circular saw and install a canopy vent. Not really helpful in your situation, but an example of necessity being the mother of invention.

Are you building ELSA? If so seems like any repair would need to be approved by the mothership.
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  #8  
Old 06-25-2019, 07:57 AM
Boyd Birchler Boyd Birchler is offline
 
Join Date: Sep 2014
Location: IN
Posts: 208
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Having stop drilled many cracks, my suggestion is to use no larger than a 1/16" drill. Drill with minimum pressure just past the end of the crack. The crack will eventually make it to the hole. Drilling at the perceived end of the crack is usually a mistake, by the time you drill the hole; the crack is likely to go beyond the hole.
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