| Match: | #5 |
| Message: | #110167 |
| From: | "Rob W M Shipley" <rob(at)robsglass.com> |
| Subject: | Finish Drill / Deburr Pre-punched holes? |
| Date: | Apr 01, 2004 |
Too many experts and not enough pragmatism. Tedd McHenry kindly posted the links for the mil rivet specs. There's a problem here not addressed by those who feel Van's recommendations are akin to a quick spritz with holy water. Follow directions and you will NOT meet these specs. Charlie England's post echoes my own experiences. Match drill, deburr and dimple and your hole will be oversize on the thinner skins by as much as twice the allowed tolerance. You can't buy #41 dimple dies and everyone has done it this way for years so it works. If you match drill with a #41 instead of a #40 it works better, i.e. the hole is tighter, you need less swell from the rivet and the recommended rivet size is more likely to work. I'm sure the theoreticians out there won't like this suggestions much, however no one at EAA, AOPA and the guys at Vans considered it a bad idea, when I discussed it with them. Paul Parashak is concerned about "one of the surest ways to accelerate metal failures is to introduce a flaw. These miniscule cracks and stress areas that are left in are prime areas for crack propagation." I cannot believe that he has ever examined the holes which the average builder drills and deburrs. Look at one with a magnifying glass. They are ugly to varying degrees but definitely ugly. Cy Galley has a good grasp of the reality of drilling and deburring. "If you are getting so anal about drilling, I would propose that you use a chucking reamer instead of a drill. At least that way you will get a round smooth hole. You don't get smooth or round with a drill bit." If irregularities around the hole are significant, many riveted aluminium monocoque planes would not have remained flying for as long, (or longer), than many of us have been alive . He also makes the very pertinent observation that presumably all the lightening holes are punched by Van's. I haven't heard any suggestions from Van's or our list experts regarding the necessity of enlarging these to remove the work hardened edge and the attendant stress risers. Vans has not totally dismissed the idea of dimpling the punched hole without match drilling. Charlie England's discussion with Van "I asked Van, his own self, face to face, in person, at OSH last summer when they were going to go ahead &punch the holes 'full size' &save us all some build time. His answer (his *only* answer) was that there was enough variation in drilling patterns that they preferred to continue the match drilling thing for now." I also had asked Van's about the rumour I'd heard that they had experimented with this by building a wing or wings without drilling and was told it was true but too early to make any comments. I didn't think to ask them what plane they were for or if they would fly them. I'll ask Ken K. next time I see him. Van is most certainly not reluctant to tackle issues he considers dangerous for example his stand on not using engines larger than the O 320 in a 9. He is also notoriously conservative and yet he has not made any attempt to take on this topic in a similar forthright fashion. This is surprising if, as our theorists believe, this is a serious problem in practice. I wonder why. There are many discrepancies between our practice and theory - hole condition, size etc. and it would seem that little is known about dimpling and riveting punched holes in practice. I have read nothing about actual stress cracks seen in aircraft riveted together without match drilling. There is a lot of theory flying around and virtually no practical examples. We are very unlikely to get much information either, when we pillory the pioneers. The list punishes non conformists and unfortunately some listers become nasty about opinions that conflict with their own. This is NOT conducive to getting useful practical information. Let's be conservative or daring(reckless?) as we choose but let's learn something from this. It's just barely possible that the theorists are overestimating the dangers and that our pioneers can contribute something valuable. Make your own decision on what you can sift from the differing opinions and contradictory fact and build accordingly. It's just barely possible the pioneers are correct and that this is an issue the majority are being overly anal about. If so building our next project may get even easier. Rob Rob W M Shipley N919RV (res) Fuselage .....still! (Nomex most definitely on!)
| End Msg: | #5 |
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