Spar countersinking, part the first

Started out today by doing some rib deburring.  Josie got about halfway through the main right wing ribs last weekend – just touching up the little nooks and crannies – and we’d left the stack sitting on the workbench.  I was planning on getting rolling on spar countersinking, which would require the bench, so I decided to just sit down and finish that stack up so I could get them back on the shelf for storage.  Man, if I thought the tedious repetitive stuff was done when I got the last rib straightened, I was wrong. (for the record, I thought no such thing, and I only mention it here for the purposes of rhetoric)  Just sitting there with a couple of needle files, filing, checking, filing, checking, filing… whew.

That done, I brought over the first wing spar and clamped it down on the bench.  Like other parts, the holes for the nutplates are undersize, so I started by reaming the rivet holes out to #40.  That was easy, but then I wasn’t sure about what size the screw holes needed to be.  I got out a #27 bit and drilled a test hole in some scrap, but found that wasn’t big enough.  On the other hand, the #21 bit looked too big.  I sure didn’t want to go hacking up this beautiful spar.  Then it occurred to me that I might not even need to drill those holes to final size anyway.  The procedure specified in the instructions is to rivet the nutplates in place, then use a #30 countersink for the screw hole; that bit will center nicely inside the threads of the nutplate.  Using this method, in turn, means that it’s not necessary to get the hole to final size for the countersink bit to center, since I’m not actually using a #8 screw countersink.

Of course, the screw still has to go through there, but it’s possible that by the time the countersink is deep enough to accept the tank skin, it’ll be deep enough that the hole is large enough anyway.  It was time to put together a test rig.  First problem: there’s no raw material in this kit that’s the same thickness as the main spar channel. But it turned out that the piece of angle aluminum I bought for aligning the trim tab was the perfect thickness…nice!  So I sawed off a section and proceeded to rivet a nutplate in place.  Then I got a piece of scrap sheet the same thickness as the tank skin and drilled and dimpled it for a #8 screw.  This is necessary to get the countersink right, since it accepts the dimpled tank skin, not the actual screw.  With a bit of trial-and-error, I had the countersink right, and wouldn’t you know…it was deep enough that the hole was bigger than the screw.  So after about an hour or so of work, I’d convinced myself that I didn’t actually need to find the right drill size for the #8 screw.

Here’s my little test setup.  Still need to get the countersink a little deeper; the skin piece isn’t sitting flush…

IMG 3240

(I did, however, note that the screw holes in the tank skin are undersize…so I’m going to have to figure this out/get the right bit before too much longer…)

Finally, I went to work with the #40 countersink, working on the nutplate rivet holes.  I got through about half of those on the bottom of the spar before dinner arrived in the form of pizza delivery, which seemed like a good time to stop for the evening.  Tomorrow I should be able to get into a good rhythm, knock out the rest of the rivet countersinks, and then start riveting nutplates on.  Even though I went through those complex testing machinations, I think I’ll start by riveting a single nutplate and then countersinking, just to make sure that there’s not a hole size issue in the real thing.  Worst case, I have to drill out a single nutplate…no big deal.

Some of tonight’s countersunk rivet holes:

IMG 3241

At this rate, it won’t be long before I’m riveting the skeletons together and hanging them on the yet-to-be-constructed wing stand!

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