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Tapping Plexiglass Rear Window - Really?

Tacco

Well Known Member
About to fit and install the back window. Plans say to tap both the window and the rollover structure for the attachment screws. As paranoid as I am to avoiding any sharp edges on plexigalss canopy/window, I am perplexed as to why I should tap both the plexiglass and the underlying structure. Seems to me that I will introduce a helix of sharp edges at each screw hole by doing so...and what purpose does it serve versus a smooth hole? How did this go for those that have done this?
 
Things have changed (at least from what the lexan rear window install procedure was, and I don't think the plans say to countersink either because truss head screws are used).
Years of experience has shown that one thing that is a big challenge for many builders is threading/tapping a hole straight.

More recent plans sections (RV-14 and now acrylic rear window plans for RV-12) have you drill a hole in the window and the metal structure (in this case rollbar) and then tap with the window in place. The window is soft and the tap will start easily and self straighten when it gets to the metal structure.
After all the holes are threaded, the window is removed and all the holes in the window get enlarged to appropriate size for the screws that are used.

You did read the entire section first before starting to get an understanding of will be happening didn't you? ;)
 
Hi Scott, I was just going to try and put the OPs mind at ease, I remember tapping all those holes in the plex rear window and it worked out fine, just as it says on page 25A-03, step 2. When I read your response about later enlarging the tapped holes, I didn't remember doing that step. I think you actually leave the front row of holes (in the rear window) tapped, they don't get touched again. The rest of the holes in the rear window get final drilled to #27. I could have missed it though....bottom line, the tapped holes in the plex are holding up fine on my install. Certainly no sign of cracks at 43.0 hours :)
 
Hi Scott, I was just going to try and put the OPs mind at ease, I remember tapping all those holes in the plex rear window and it worked out fine, just as it says on page 25A-03, step 2. When I read your response about later enlarging the tapped holes, I didn't remember doing that step. I think you actually leave the front row of holes (in the rear window) tapped, they don't get touched again. The rest of the holes in the rear window get final drilled to #27. I could have missed it though....bottom line, the tapped holes in the plex are holding up fine on my install. Certainly no sign of cracks at 43.0 hours :)

Paul, Step 6 has you final drill the remaining holes in the window to #27 (by remaining it basically mean all holes because the ones common to the aft skin were drilled #30 and the ones common to the roll bar were drilled 36 and tapped).
 
Ahh, OK. I definitely missed that. I did see that step 6 said to 'Final drill' the #30 holes, so just assumed that was all. It's kind of funny, the forward rear window holes get 'final' drilled twice....once in step 2 and again in step 6. :)
 
Having just got to this bit, I also wondered.....

Sometimes, instructions of some importance get hidden or put at the end of a piece and can be easily over read.

As plans get revised, it would be great to collect some of the OMG gotchas and box them on the side of the particular print or passage in the instructions. Wouldn't take long and would be very helpful.

For an example - the quoting of all the part numbers in full means that sometimes you begin to speed read (bit like Tolkien books) and some of these little tidbits get missed :D
 
BUILDER HINT!!!!!!!

Having just got to this bit, I also pondered.....
Sometimes, instructions of some importance get hidden or put at the end of a piece and can be easily over read.
As plans get revised, it would be great to collect some of the OMG gotchas and box them on the side of the particular print or passage in the instructions. Wouldn't take long and would be very helpful.
For an example - the quoting of all the part numbers in full means that sometimes you begin to speed read (bit like Tolkien books) and some of these little tidbits get missed :D

NEVER "SPEED READ" PLANS!
 
Thanks guys.

I think Paul had it right the first time though. The holes in the roll bar and window were already "final drilled" to #36 in step 2. By "Final drill #27 the remaining #30 holes" in Step 6, I'm pretty sure it means the ones in the aft turtle deck skin and window in that area. Since they are #30. The ones associated with the roll bar were never drilled to #30 anyway. At least that my interpretation of the english.

All that said, here is an indication that Scott may be right with respect to Van's intent - the drawing, Fig. 2 25A-04, seems to show enlarged smooth holes in both areas of the window.

Firing off a question for Van's Support.

Last one for you guys though, will a plexiglas bit do well in the aluminum roll bar? Did you swap to a regular bit when you got that deep?
 
Your plexi bit will have no problem drilling through the roll bar, and later the front canopy bow.
I think you may find that the screws will not fit the #36 holes you 'match drilled' prior to tapping. The screw holes need the extra clearance to allow for the different expansion rates between plexi and the screws.
Cheers, DaveH
120485
 
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Thanks guys.

I think Paul had it right the first time though. The holes in the roll bar and window were already "final drilled" to #36 in step 2. By "Final drill #27 the remaining #30 holes" in Step 6, I'm pretty sure it means the ones in the aft turtle deck skin and window in that area. Since they are #30. The ones associated with the roll bar were never drilled to #30 anyway. At least that my interpretation of the english.

All that said, here is an indication that Scott may be right with respect to Van's intent - the drawing, Fig. 2 25A-04, seems to show enlarged smooth holes in both areas of the window.

Sometimes describing processes with written words can be challenging.
Drilling 40 and then FINAL drilling was in the context of the tapping process (since the process was started with a smaller hole, 36 is the final drilled hole for TAPPING.
Once the tapping process is completed and the window is no longer being used for the tapping process, the holes are final drilled for attachment with screws.

In this particular case the term final drill during the initial drilling is more meant to describe a process (drilling holes in multiple steps) than it is to mean that is the final finished size for the hole.

A lot of instruction steps are written in the context of expecting the builder to read through the entire section (and gain some understanding of what is to be done) before beginning, as described in Section 5 of the manual. If they are looked at from that viewpoint, many steps make more sense than if taken sentence by sentence by them selves.
 
Thanks for the clarification Scott, that makes sense. I have not sealed my rear window yet, so the next time I have it off, I will final drill those tapped holes in the plexiglass. I think that I would add 5 words to pg 25A-03, Step 6 to make it a little more clear to future builders, but I am probably one of the few that didn't figure it out!

"Final-Drill #27 the remaining #30 holes and the previously tapped holes in the F-1277B Aft Window."
 
Reading through this entire section to understand the context, is precisely what I'm doing. Haven't even taken the window down off the shelf yet. When the plans say "Final-Drill #27 the remaining #30 holes holes in the F-1277B Aft Window.", to assume that this also includes the holes that were previously drilled to #36 and tapped, is exactly that - an assumption. Nothing in the plans leads to that assumption, step by step or in total context. ....with the possible exception of Figure 2 on page 25A-04.

The plans are pretty good, having been vetted by so many builders thus far. I've learned to trust them. Every time I think it would make sense to deviate, I've been proven wrong.

Yes, is doesn't seem logical that you would leave them tapped. But, I also don't think tapping them in the first place seems logical. I can tap a straight screw hole in the roll bar and see no reason to risk running a tap through the window and then screwing a screw into it!....but that's what the plans say so there has to be a reason(?).

Let's see what the experts/authors say on Monday. I'm not in any hurry. But I bet they agree with Paul - the plans are unclear on this point....whether taken step by step or in total context. And there is nothing challenging about Paul's suggested clarification.
 
Good discussion

This is the kind of discussion I enjoy reading, especially regarding the kit plans, processes, and the perspectives of how different folks see things.

For the window installation, just keep in mind the purpose of the different drill steps is to ensure that the hole through the window is aligned with the hole in the structure (perpendicularly), and then to ensure that there is sufficient clearance around the fasteners for the window to move as it expands, contracts (at different rates than the aluminum structure and fasteners) and reacts to airframe movement.

1. Initial drill, which is your pilot hole to ensure proper hole placement.
2. Final drill of the structure for tapping size, which is also through the window, used to ensure proper alignment of the tap.
3. Final drill of the window, which ensures fastener clearance.

A tip for installing plastic windows is that once the final holes are drilled, then go back and chamfer the window holes on both the inside and outside; I recommend a rounded end, fine grit stone on a dremel at slow speed removing just enough material to do the job - don’t overdo it. As Scott point out, truss head screws are used, so countersinking is not the goal, but rather rounding of the hole openings to stress relieve the material and minimize potential cracks.
 
Let's see what the experts/authors say on Monday.

Well, I avoid the title expert, but I am one of the authors, but go ahead and see what one of the tech support guys say.......

In the context of different wording..... The nemesis of technical writers is that no matter how you write it, there will be some people that don't understand, but would have if it was worded differently. That doesn't imply lack of anything on their part... just that everyone has there own way of understanding and sometimes it falls outside of the way something is written......
 
Well, I avoid the title expert, but I am one of the authors, but go ahead and see what one of the tech support guys say.......

In the context of different wording..... The nemesis of technical writers is that no matter how you write it, there will be some people that don't understand, but would have if it was worded differently. That doesn't imply lack of anything on their part... just that everyone has there own way of understanding and sometimes it falls outside of the way something is written......

Scott, while I truly appreciate your advice, I caution you that when you say things such as the above, the tag line you use “Any opinions expressed in this message are my own and not necessarily those of my employer.” will likely provide little relief for Van’s in a litigation process as you are no longer expressing an opinion as an independent forum participant, but rather using your position as an employee to emphasize the credibility of your advice. That’s for free...
 
Scott, while I truly appreciate your advice, I caution you that when you say things such as the above, the tag line you use ?Any opinions expressed in this message are my own and not necessarily those of my employer.? will likely provide little relief for Van?s in a litigation process as you are no longer expressing an opinion as an independent forum participant, but rather using your position as an employee to emphasize the credibility of your advice. That?s for free...

It is not for litigation protection, it is to emphasize that any info received through official channels that contradicts my info, should take precedence (and that is why I said to go ahead and check with tech support if he didn't feel comfortable with my answer).
 
Thanks guys. This is a good discussion. Gleaning the experience and expertise of others is exactly why I come here and what makes this forum valuable. Remember that why I asked this question initially was that I suspected leaving the holes tapped was not appropriate. It was not a question of my understanding of the plans, its an issue of what the plans say, and in this case, do not say. The plans simply do not say to drill out the forward window hols to #27. What is a matter of understanding is Scott's knowledge of the intent. I do not have his understanding of that intent (which Ron describes beautifully above).

Scott, now that you've self identified as one of the authors, I may have lots of questions!! ;)
 
I had to replace my Lexan window some time ago and upon doing the replacement I used a uni-bit to enlarge the holes. The plans called for enlarging the holes to #27 but I could not source a #27 drill bit for plastic. I was not unhappy as my experience was plastic drill bits can cause cracking even when you are extremely careful. With the Uni-bit the nearest size was 5/32, a bit larger than #27 but I figured that was a plus. The Uni-bit makes a very smooth hole in Lexan with NO cracking.
i-HbrvBqG-L.jpg

Note the black painted edge.
A caveat, this was for the Lexan and may not even be pertinent for the newer plexiglass rear window. I would be interested to know if others have used a Uni-bit in plexiglass.

Another RV-12 builder, Randy, did something similar making oversize holes and using a thin slice of rubber tubing as a kind of O-ring in the hole for the screw. I thought that was a neat idea.
http://www.rv12.pflanzer-aviation.com/finishkit09.htm

Another neat trick I learned from Randy's website was to paint a black edge on the part of the window where the sealant would be injected to make a smooth black line when viewed from the inside:
i-RnNSBff-L.jpg
 
Unibit worked great on my Lexan rear window as well. It has the added advantage of nicely chamfering the edge of the hole -- use a careful touch. It also produces a slightly larger hole which in my mind gives a bit more room for screw/Lexan expansion. Practice on scraps first to build your confidence.
 
Following up?

Van?s Support confirms that ?all of the holes in the aft window will need to be match/final drilled to #27?

So Step 6 on page 25A-03 is incomplete and I interpret it to mean ?Final drill #27 the remaining #30 holes and the tapped #36, 6-32 holes in the F-1277B Aft Window?, instead of ?Final drill the remaining #30 holes in the F-1227B Aft Window.? No subsequent or preceding step will tell you to go back and #27 drill the tapped holes - as logical as that seems.

My window is complete and here are some notes. I apologize if I?m covering old ground for anyone.

1. I had a scrap piece of Plexigalss to abuse beforehand to get used to working with the stuff. It REALLY makes a difference to warm up the shop to 80+ degrees!

2. Use a block of wood to backup drilling through the Plexiglas to lessen the risk of tear through and cracks. This is particularly important when enlarging holes. Van?s support reiterated this to me and indeed, on my scrap piece - with a little pressure, speed, and no backup?.cracks. Section 5 recommends you not enlarge holes at all with a regular twist of plexi-bit. Unibit or reamer is recommended. Go slow!

3. I tried to be careful to maintain the 5/16 edge distance when placing holes. Pay attention to this in three places ? The two forward most holes in the Aft Turtle Deck, the two holes forward of the area where you removed material for clearance of the Brace Bracket in step 10, page 25A-02?and on the F-1231 Roll Bar. If you drill the holes in line with the Roll Bar rivets, you might find you?ll be too close when the window is final trimmed. I offset my holes slightly aft, being careful my tapped screws would still engaged not only the Roll Bar but also the F-1231B Straps underneath.

4. When drilling the Roll Bar, it is important to drill and tap perpendicular holes. Otherwise the screws might contact the window and the screw head will not sit flat on the plexiglass. Not good. I have one I have to deal with.

5. When final drilling the tapped holes, go back and ensure you have drilled out all the treads. #27 and the 6-32 tap diameters are pretty close. If you get slightly off-center, and somehow I did on one or two, you?ll leave some threads behind. Easily cleaned up with a rat tail file.

6. Lastly I found that I only needed to final trim about 1/8-1/2? of material from the forward edge of the window. Provided you secure the window somehow, a hand-held belt sander is great for achieving a straight clean trim line.

On to the canopy!
 
Thanks for the update, I will be removing the rear window soon to have the plane painted, so I will final drill those tapped holes before putting on the sealant and re-installing.
 
I re-read the plans again after reading this thread. I wouldn't have concluded in a million years to finish up the rear window by final drilling the tapped holes in the plexiglass. It struck me as quite odd that plexiglass would be secured with a rigid fastener at that point but the ELSA creed is follow the plans to the letter.
I made a quick trip to the shop yesterday to FINAL-final drill some holes.
Always amazed by the value of this website.
 
It?s interesting how carefully we do the tapping and protect the threads - only to face the dilemma of how to match drill a replacement window in a few years without damaging the threads in the crash arch. I managed it, but it wasn?t easy!
 
It?s interesting how carefully we do the tapping and protect the threads - only to face the dilemma of how to match drill a replacement window in a few years without damaging the threads in the crash arch. I managed it, but it wasn?t easy!

Rich
Any tips on how you did this whilst protecting the threads?
John
 
Rich
Any tips on how you did this whilst protecting the threads?
John

Could you just use the old rear window as a template for drilling the holes in the new one? That way, you could keep the drill away from the rollbar. It might be a little challenging on the curved portion, but if you could flatten them out a bit to mark the holes, it might work.
 
I went from the old flat lexan sheet to the new molded window, so I couldn?t use the old one as a template. I used Clecos to attach the holes except the roll bar. Then I center drilled each hole to the roll bar with a 1/16?? bit. I then removed the widow and final drilled each 1/16? hole. I had to enlarge a few holes in the window to get a good fit on some screws to the roll ran.
 
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