I noticed a small crack on my lower cowl around a dimpled hole for a #8 screw where it attaches to the firewall. On further inspection, many of them have tiny cracks, both top and bottom cowls. Not the sort of thing I like to discover and a bit of a heart sinking moment. I had deburred all of the holes. These are the largest dimples I’ve done, and I hadn’t done any #8’s before starting the cowling.
I’ve discussed it with my mentor today, and spent some time on various forums trying to discover where I’ve gone wrong. So far I haven’t found the same issue on any smaller holes.
6114E949-5DA7-4651-A934-7062522DD534.png
It seems the cause can be not enough deburring, too much deburring, metal riding up the drill bit, hole too small, low quality dimple dies etc etc.
There appear to be a number of solutions including using a larger drill bit, smooth the hole very carefully, and using WD40 to lubricate (which apparently gets very good results).
My main concern is what to do now. Most of the cracks are tiny and difficult to see, but I’ve kind of resigned myself to having to remake the cowlings. I figure that the lower cowling in particular will have a cantilever force on it, as well as air loads, so there’s probably quite a load on the dimpled holes. I couldn’t find any cracks on the firewall dimples interestingly, and the loads on the firewall are being taken by the nutplates.
I did some test holes in .032 2024 T3 aluminium to see where I had gone wrong, yet none of them cracked. (even when I didn’t deburr the holes which doesn’t make sense to me)
A review of the Vans forum on the topic shows that it’s not an unusual occurrence. One engineer said he had never seen a crack propagate out of a dimple and it appears some people just accept it. What to do ?
Any advice is appreciated.
I’ve discussed it with my mentor today, and spent some time on various forums trying to discover where I’ve gone wrong. So far I haven’t found the same issue on any smaller holes.
6114E949-5DA7-4651-A934-7062522DD534.png
It seems the cause can be not enough deburring, too much deburring, metal riding up the drill bit, hole too small, low quality dimple dies etc etc.
There appear to be a number of solutions including using a larger drill bit, smooth the hole very carefully, and using WD40 to lubricate (which apparently gets very good results).
My main concern is what to do now. Most of the cracks are tiny and difficult to see, but I’ve kind of resigned myself to having to remake the cowlings. I figure that the lower cowling in particular will have a cantilever force on it, as well as air loads, so there’s probably quite a load on the dimpled holes. I couldn’t find any cracks on the firewall dimples interestingly, and the loads on the firewall are being taken by the nutplates.
I did some test holes in .032 2024 T3 aluminium to see where I had gone wrong, yet none of them cracked. (even when I didn’t deburr the holes which doesn’t make sense to me)
A review of the Vans forum on the topic shows that it’s not an unusual occurrence. One engineer said he had never seen a crack propagate out of a dimple and it appears some people just accept it. What to do ?
Any advice is appreciated.
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