[woxofswa]

In assembling the H/S, I was very careful to Mach C/S the rear spar flange to accept the skin dimple. When cleco'd, the skin was nice and flush, but when rivited, the skin lifted to a uniformly thin gap at the aft most edge of the flange. The gap isn't very deep as the sin is tight at the rivit line. Is this normal or is this bad form? Do I press on or get a bunch of drill out experience? Thanks in advance. LUV the forum.

 

[Rick S.]

Myron,

Most likely you did not get the machine countersink deep enough to accept the dimple. The strength of those two parts is in a critical area and depends on the two parts being riveted together without a gap. So, my suggestion is to drill them out, make yourself a little go/no go gauage using scrap aluminum out of several different thicknesses of material. Drill and dimple all the normal hole sizes, #30, #40, #19 etc in each thickness. When it comes time to set your countersink start small and work up to the correct size a few detents at a time until your guage fits in the countersink enough to allow the metal to sit flush but no so much to allow the dimple to wiggle in the countersink. Make sense?

Look straight into the two parts, if you can see light, you need to correct them. If it's only on the very edge, then I still think your countersinks are not deep enough or are not uniformly deep enough. One tip, make sure you keep the countersink cages clean after each cut, chips will allow the depth to shallow as they gather under the snap ring inside the cage which will not allow the cutter to fully go to the set depth.

 

[alpinelakespilot2000]

Myron-

I would argue, if anything, that it is more likely that you over-countersunk the spar rather than under-countersunk. If you countersink too far, the skin will get pulled down too far into the dimple and subsequently forcing the skin edge up. Additionally, if it was under-countersunk, you would have seen that gap when it was clecoed, and you reported it as laying nice and flat when clecoed.

Before you start drilling any rivets out, I'd definitely recommend trying to post a picture. It might be an issue, but you may be worrying about nothing. Don't drill out a lot of rivets until you know for sure.

Good luck and hang in there. Building gets a lot easier.

 

[Rick S.]

Oversized is also a good point, without a photo it's a best guess I suppose. I hadn't seen what was described by oversized countersinks mainly because the two parts would lay flat with the dimples hiding inside larger countersinks. I find Clecos hold parts more in hole alignment but once rivets start to set the pieces are pulled together much tighter than the cleco held them. I still stand by the comment about some, not all of the countersinks may be smaller than required. It's easy to get cruising with the microstop and forget about keeping the chips cleared. It only takes a little bit to throw the adjustment out of spec. by a thousandth or so. I have made that mistake myself early on when I thought it was the cutter but turned out to be technique. Since Myron is on the HS, which is the same place I discovered this, I thought I would pass along this tip, which has worked on my -10, which is just waiting for the cabin top to be riveted. All the metal work has been completed.