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Cracks in the window and doors

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About using blunt drill bits -

That used to be the advice given to 'first-timers' when drilling acrylic particularly, the trick - which works incidentally - is to use an old drill bit of the right size and drill it into some concrete or a cement block for a few seconds before drilling the perspex. There's no reason why it wouldn't work for polycarbonate too.

 

But - if you want to do the job properly what you should be doing is modifying the cutting edge of a sharp drill bit. When drilling different materials you need different shaped cutting edges and the cutting edge on the drill bit is very similar to the cutting edge on a lathe tool, except the drill bit is designed to cut while spinning.

 

There are two angles to consider, one is called the rake (or helix) angle and the other is the lip relief angle. On a drill bit the rake angle is the angle between the flute and the vertical axis (a line passing down through the centre of the drill bit. The drill bits you commonly buy in a hardware store have a rake angle that is suitable for the most commonly used material i.e. steel, the rake angle is around 8-10 degrees. The relief angle is the angle on the end/bottom of the drill compared to the horizontal plane i.e. a line at right angles to the vertical axis. Steel cutting drill bits have a relief angle of about 5-10 degrees.

 

The rake angle on the front of the cutting edge determines how much the drill wants to 'bite' into the metal, and the relief angle behind the cutting edge controls how deep it can bite. Consider - a high rake angle and the drill will want to bite viciously but if the relief angle is zero, the drill cannot enter the material it is cutting into because the flat back/end of the drill doesn't allow the cutting edge to bite. Add a little relief angle and it will cut slowly and produce thin swarf, and the cut will be very controlled. As the relief angle increases the swarf gets thicker and the cut becomes less controlled.

 

Next consider a moderate relief angle, say 5 degrees, and zero rake angle. The cutting edge is scraping away, like dragging a spade vertically across the dirt, and in steel the edge would blunt very quickly. Increase the rake to 5 degrees and it will cut steel nicely but if you want to cut aluminium optimally you need a higher rake angle than that. Unfortunately it isn't easy to increase the rake angle because it is a function of the helix angle (or flute) so you need to buy another drill designed for aly. If you're clever you can actually grind more rake into a standard drill and add a little more relief too, but for the most part a drill designed for steel cuts aly well enough.

 

However, if you want to drill brass nicely you should grind a vertical flat on the front face of the cutting edge (lip) because brass cuts perfectly with a zero rake angle. Anyone ever had problems drilling stainless? Screaming drills, overheating, blunt in a few seconds? For stainless the rake and relief both need reducing, and cut slower, with more pressure and use a coolant and then it will cut beautifully.

 

And finally - for efficiently and safely drilling plastics, including sheet acrylic and sheet polycarbonate, without being a blunt-drill-using-neanderthal ... you need a negative rake angle of about 5 degrees and a small relief angle to control the rate of penetration. In a horribly crude way that's what drilling into concrete or cement blocks does to your drill bit.

 

Happy drilling folks ...

Thanks HITC, that's great info. I'm guessing that you can buy specialised drill bits for plastics, for those of us who don't entirely trust the "drill into concrete first" method?

 

 

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I am particularly intersted in this thread because have always wondered if it was the fuel alone or the stress that caused the cracks in the polycarbonate windows. The post #38 by HITC got me enthused to experiment with some strips of polycarb and various solvents to see if it true and also to see if I could stop it by destressing with a heat gun. I tried the experiment with methylated spirits (denatured alcohol), petrol, enamel thinners, and lacquer thinners. I tried pouring a small amount of these solvents on pieces of unstressed, stressed and destressd polycarb. To destress I bent a strip of poly over double then lightly heated it with a heat gun. All of these had had the same result to a degree, but the lacquer thinner was the most aggessive, so i repeated the experiment and made a little video to show you (just did it with my phone, pretty crappy but you will see the results). Next time i put in a window i "might" try destressing it a bit, what do you think?[MEDIA=dailymotion]x3aeizg[/MEDIA]

Hi Rick,

 

This is an Amazing experiment! In my wildest Dream I wouldn't have expected this dramatic result with thinner on the stressed lexan and particularily the difference on the heat treated piece of lexan. How dramatic was the experiment with petrol? So, it turns out that the key factor for the fuel induced cracks is a question of rate of heat loss and only in places where the lexan sheet is under stress. That fits! I can also imagine that when during flight there is a spill of fuel that hits the lexan the heat loss is considerable as the wind flow will certainly increase the rate of evaporation. I agree, I would love to destress any new installed lexan in Aircraft if I only knew how. For instance, what temperature and how long? Also, should I treat both sides of the sheet? Once again, many thanks for the video. This is a great forum!

 

 

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Thanks HITC, that's great info. I'm guessing that you can buy specialised drill bits for plastics, for those of us who don't entirely trust the "drill into concrete first" method?

See post 42

 

 

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Sorry Dazza, missed that somehow.

No worries mate, there is a company in Victoria whos website is www.plasweld.com.au . I dont think they sell drill bits as they are a plastic welding company but they do have some good tips on polycarbonate and acrylic ect. Worth a look. Click on the tips of the trade link.

 

 

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Rick did you try lesser bends than right in half?

 

HITC you say polycarbonate is totally impervious to fuel, I am 95% sure my screens are polycarb and on my roof I have two slightly curved sheets for overhead viewing. On one of those I splashed a drop of Mogas which I wiped off instantly but it had already cracked, not right through like ricks experiment but similar in depth to Hasses pics. On my rear cargo door I also had a spill, it is basically flat, and it didn't instantly crack but was discoloured immediately and eventually I replaced it when cracks started (they were from the rivets but only where the fuel had spilt).

 

If polycarb is affected like that when stressed then I would have thought around rivets would be a perfect environment for similar results? Seeing as unless bushes were used with the rivets they would have to put a little squeeze pressure on the screen. I don't have any scraps to test on but would love to see someone rivet some polycarb and then apply fuel or thinners.

 

 

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HITC you say polycarbonate is totally impervious to fuel ....

Hi SD, I'm just one of those people who doesn't automatically take what people say as gospel, I like to try things out for myself, then I know what the real facts are - usually.

 

I found out about the polycarb windshield thing by unfortunate experience many years ago when I destroyed my curved Macro windshield with a small fuel spill. From earlier experience with stress cracking I recognised exactly what had happened, nonetheless it had already ruined my day's flying.

 

A couple of weeks later I was in the clubroom when I overheard someone telling another how petrol destroys polycarbonate. I started to say that it wasn't the petrol and he leapt on my comment and told me I didn't know what I was talking about because he'd seen it happen with his own eyes, and then went on to tell me how petrol shatters polycarbonate immediately on contact. As far as he was concerned he knew for certain that petrol eats polycarbonate because he'd watched it happen. Of course it was a perfectly reasonable thing for him to think based on what he'd witnessed.

 

Once he'd settled down he realised it was my windshield he'd watched explode, and then of course thought I must be a complete imbecile if I still thought petrol didn't instantly melt polycarbonate.

 

I went out to my plane trailer and came back with a tin of petrol and a piece of Lexan, an offcut from the new screen I'd made, and dropped it into the petrol ... you can guess the rest no doubt. We ended up leaving that piece in the petrol for the rest of the weekend and it didn't harm it. I also showed a couple of demonstrations of it shattering when curved as Rick did in his excellent experiment video.

 

Your experience may well be different, I have no idea what brand the sheeting on your plane is and there may be subtle chemistry differences between brands. I've only ever used Lexan and Makrolon and not seen them discoloured but I'm not saying it couldn't happen, most things are porous to some extent, and/or to some substances. Certainly the dyes used to colour fuel are very good at getting into most surfaces.

 

I do know that you are correct about evaporative cooling causing cracking around rivets even if the sheet is installed without curvature. I recall a door panel on a Lightwing, IIRC, getting cracks around the rivets after a wing tank developed a leak. That is why I always use elastomeric washers under the fastener heads. The last time I installed a screen with rivets I made a 2mm thick sheet of 60 durometer polyurethane from 2 pack pour polyurethane and laboriously punched the washers out with a wad-cutter and then cut the inner holes with a hand-punch. It was a pain but only took about an hour which isn't long in the scheme of things. I prefer to use urethane because it isn't affected by cleaning chemicals, fuel spills and not much by UV.

 

Bolts or screws are a good option for fitting screens too, because you can better control the pressure on the material, but it's hard to make them so neat. I'll probably use rivets and urethane washers again for the DooMaw screen but just a few of them in the corners, the rest of the screen will be bonded in place using a 3M double sided adhesive urethane tape. This evaporative cooling issue is one reason that DooMaw has a flat panel windshield like the Tailwind and BD4, the main difference with mine is that I will fold the windshield at the front corner pillars instead of making it in three pieces with sharp corners. Polycarb sheet folds quite readily with a white stress discolouration along the fold line, but once it has turned white and stays folded rather than wanting to spring back flat again it has de-stressed itself and so doesn't crack with rapid cooling.

 

 

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my understanding of the poly carbonate market is the differences in brands is due to adding of UV protection and scratch resistant coatings.

 

 

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adding of UV protection and scratch resistant coatings.

Both those things are options within the same brand from my experience. Just add more $$$.

 

 

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How dramatic was the experiment with petrol?

Not so dramatic with petrol but was immediate cracking and failure of the part, not so much crazing.

 

Rick did you try lesser bends than right in half?

Yes, tried down to about 10-15 deg, same result with failure at the point of most stress.

 

would love to see someone rivet some polycarb and then apply fuel or thinners.

This morning I cut some bits of polycarb and .016" ali and riveted them together in various configurations and poured a small amount of thinners on each.

 

Firstly I made a multi rivet test, with two rivets through 1/8" holes- no extra clearance,67141280_IMG_20151022_095314_9321.jpg.ea3781ab6f44c8be34c52706936ec16e.jpg

 

then two rivets with 1/8" hole in the ali and 5/32" clearance hole in the polycarb,1054118420_IMG_20151022_095323_5541.jpg.4f2815d377953478ef09d1645fd56aa0.jpg

 

then two rivets with the clearance plus a nylon washer under the rivet head,1309743765_IMG_20151022_095439_0721.jpg.8ae296c05e124a3775d4fd3291d2935a.jpg

 

and one unriveted hole for control, no photo of that one but it had no cracking. So you can see all of the the rivetted holes had some cracking but the clearance holes were the smallest and least cracked, so I did some more tests with single rivet pieces with a clearance hole and destressing them with the heat gun for vaious times. Heat gun was on high setting, 150-200mm from piece, waving across piece not concentrated blast, and I allowed the part to cool to room temp befor applying the thinners.

 

first, heating for 1 minute1408301729_IMG_20151022_101455_0001.jpg.6fc748799b6d8c4981763e11846ab17c.jpg and as you see not one crack. Encouraged by the result I tried several more individual tests with varying heating times and found I could destress the rivet penetration with as little as 15 sec waving the heat gun over the part. Lowest time I did was 7 sec and the thinners produced 2 small 1 to 2mm cracks.

 

So from these test I reckon next time i rivet in a polycarb window i will drill clearance holes and spend a small time destressing with the heat gun.

 

Cheers

 

Rick

 

 

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for riveting polycabonate onto the tornado, the factory spec is to use a Large Flange rivet or a rivet washer on the back side if in contact with the PC

 

 

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cordless screwdriver is essential for drilling slowly, bosch also make a handy cutter for theirs1-PT-19921.jpg?file=1-PT-19921.jpg&id=9117,7

Not sure if I like the corkscrew or the pepper mill attachment best!

 

 

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Not so dramatic with petrol but was immediate cracking and failure of the part, not so much crazing.

 

Yes, tried down to about 10-15 deg, same result with failure at the point of most stress.

 

This morning I cut some bits of polycarb and .016" ali and riveted them together in various configurations and poured a small amount of thinners on each.

 

Firstly I made a multi rivet test, with two rivets through 1/8" holes- no extra clearance,[ATTACH]38877[/ATTACH]

 

then two rivets with 1/8" hole in the ali and 5/32" clearance hole in the polycarb,[ATTACH]38878[/ATTACH]

 

then two rivets with the clearance plus a nylon washer under the rivet head,[ATTACH]38879[/ATTACH]

 

and one unriveted hole for control, no photo of that one but it had no cracking. So you can see all of the the rivetted holes had some cracking but the clearance holes were the smallest and least cracked, so I did some more tests with single rivet pieces with a clearance hole and destressing them with the heat gun for vaious times. Heat gun was on high setting, 150-200mm from piece, waving across piece not concentrated blast, and I allowed the part to cool to room temp befor applying the thinners.

 

first, heating for 1 minute[ATTACH]38880[/ATTACH] and as you see not one crack. Encouraged by the result I tried several more individual tests with varying heating times and found I could destress the rivet penetration with as little as 15 sec waving the heat gun over the part. Lowest time I did was 7 sec and the thinners produced 2 small 1 to 2mm cracks.

 

So from these test I reckon next time i rivet in a polycarb window i will drill clearance holes and spend a small time destressing with the heat gun.

 

Cheers

 

Rick

So can i destress rivetholes without clearance? say i heat my door now before i even hang it on the fuselage?

 

 

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So can i destress rivetholes without clearance? say i heat my door now before i even hang it on the fuselage?

This is experimental aviation, what is the worst that could happen? Oh yes you could scorch a big patch on your window. Take it easy.

 

 

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So can i destress rivetholes without clearance? say i heat my door now before i even hang it on the fuselage?

Hi Skee,

 

As Rank says above, you can do whatever you want. You could damage the polycarb or the paintwork if you cook it too hard.

 

I did these tests for my own interest and edumacation and share the results for any interested, you may also do some tests yourself before making a decision to cook your doors.

 

I did a few more tests on rivets without a clearance hole in the polycarb - I heated them for 15, 30, 45, 60 seconds and only the 60 second one did not crack when thinners was applied. So quite a bit of heat is needed to destress and maybe more if the doorframe sinks the heat away before you can heat the polycarb.

 

Cheers

 

Rick

 

 

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Thanks for all that work Rick, certainly interesting.

 

As for heat treating my screens to avoid it, I'm not too worried and don't mind fitting new screens occasionally. I've found that after my first 150 hours my front screen was needing replacing due to scratches from bugs and cleaning anyway. The side screens don't get scratched as bad and since I've replaced the fuel damaged ones I've been more careful:whistling: So I'm hoping they will last a fair while. Knowing my skill level I'd heat it till it was hazy and then have to replace it anyway so I think keep the fuel away from them and all should be good:thumb up:

 

 

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I did buy the Canopy cover, maybe should make it a rule to always have it on when refueling...

 

85472254_20151002_1820222.jpg.834f512f28a805593660a2c174f3c5d0.jpg

 

 

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This is a interesting thread and just as a coincidence last week I completed a course which licences me as a Electrofusion and extrusion welder. A company from Melbourne called Polysmart flew up a instructor to teach and test us at work. Anyway we only deal mainly with HDPE but the extrustion guns can take different types of medium which is compatible with the material you're welding. Interesting stuff.

 

The extrusion welding gun cost around $6000 so its not cheap.

 

 

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This is experimental aviation, what is the worst that could happen?

I think I'm going to put that on a little plaque right in front of the passenger's seat.

 

 

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This is a interesting thread and just as a coincidence last week I completed a course which licences me as a Electrofusion and extrusion welder. A company from Melbourne called Polysmart flew up a instructor to teach and test us at work. Anyway we only deal mainly with HDPE but the extrustion guns can take different types of medium which is compatible with the material you're welding. Interesting stuff.The extrusion welding gun cost around $6000 so its not cheap.

Just a quick edit- I was included in the 1800 people made redundant at Origin.I received DCM phone call this morning, it wasnt really a surprise as the industry is going through downsizing. Its not the first time I have been made redundant, it happened in the construction industry as well.

 

Origin was a great company to work for and I will miss the close freindships made over the last 3.5 years I worked for them.

 

But nobody is owed a living so I guesd I will go back to plumbing swimming pools.

 

 

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