Thursday, December 31, 2009

When easy steps aren't...

It should have been an easy day in the shop. Absent on more rivet that needs to be driven, all that was left to do on the aft bulkhead was to add a couple of "wings" that hold the bearings upon which the horizontal stab will pivot.

The two parts get clipped apart and cleaned up as we've done many times before, but not before labeling them as -R and -L respectively so as to remember which side of the plane they each belong on. There's a somewhat dubious drawing that shows how the parts are to be clecoed onto the bulkhead. What it comes down to is that you cleco each part into the flanges on the bulkhead. There's nothing that specifically tells you whether they should be oriented with the angle of the "wings" facing towards or away from the bulkhead, but it's one of those situations where you'd have to go out of your way on some sort of masochistic mission of abject stubbornness to get it wrong. Just in case that actually appeals to you, here's how it should look:



Except, well... that didn't work. The point of this exercise is to rivet only the holes on the tab; the holes on the flanges will get done later when the tail cone skins are all riveted together. Once it's all clecoed together, we have to final drill the six holes on each side, drilling through both tabs and the bearing itself. With the clecos where I originally had them, they interfered with the drill. I moved them to the other side:



Once the holes get final drilled, everything gets taken back apart for deburring, then it gets put back together again. Final drilling works best if the parts are held together as tightly as possible, so I try to only remove one cleco at a time. It's hard to tell if any given hole has already been drilled once you start moving clecos around, so I marked each one just after drilling it:



Because this isn't my first rodeo, I know that disassembling and reassembling can cause issues with the alignment of the holes later if I don't get everything back where it was. Just to make sure the bearing got put back in the same position, I marked each one before removing it:



With it all put back together again, it was easy to squeeze in the twelve #4 rivets. The next step sounded like it would take all of five seconds: press the SB437-4 into the hole shown. That turned out to be a bit more difficult that it seemed it would be. Here you can see the bushing (whose sole function in life will be to prevent the electrical wires running from the instrument panel back to the trim motor from chafing) and the hole where it is intended to live out the remainder of its days:



See the problem? No? Well, here it is. The '437' in the SB437-4 part number indicates .437, which is the diameter of the barrel of the bushing. A consultation of a drill bit size chart (thanks, Wikipedia!) tells me that .4375 is the diameter of a 7/16" drill bit. That's all well and good, you say, but so what? Well, just take a look at this:



Yep, 7/16" diameter. Now, compare and contrast with this:



Count 'em yourself if you wanna, but trust me - it's a 6/16" diameter hole. Or 3/8" for you folks that prefer your fractions reduced. Either way, it's pretty clear why the bushing wouldn't fit. The hole pretty clearly needed to be enlarged to 7/16" since I could think of no easy way of shrinking the bushing to 3/8".

That would require a 7/16" drill bit which, thankfully, I have. But... I have a 3/8" drill, and as we learned just moments ago, 3/8" is less than 7/16". That meant that I'd have to use the drill press. Which, well, is not so easy once the part to be drilled is adorned with all sorts of inconvenient protuberances. It took a bit of trial and error to finally get the bulkhead secured in a useful and accurate position underneath the bit on the drill press:



I have to confess to be a little put out that the plans hadn't had me drill that hole out back when it would have been easy, and I was also fairly sure that a 7/16" drill bit hadn't been called out in the required tools list. Sensing a potential gotcha, I went back and looked:



I was correct in the specific, but incorrect when you consider the requirement for a Unibit. That said, I would not have wanted to use a Unibit for this. My experience with Unibits is that they are too easy to use. By that I mean that it is just as easy to drill to 8/16" as it is to drill to 7/16". The hard part is ensuring that you don't go a step to far. But, with the utmost caution and a steady hand, it could have been done. That doesn't excuse making us figure it out for ourselves, though. There's nothing more frustrating that finding out that something that should have been easy is anything but, especially when the resolution to the problem is something that could have been done with far less difficulty earlier on.

Which, interestingly enough, brings us to the next step.

Rather than build an entire airplane only to find out that the horizontal stab won't fit during final assembly, we do a test fit. This, according to the directions, is intended to have us choose the right set of washers to provide the best fit at the pivot points. The first problem was that it is very, very hard to get the washers between the brackets and the bearing. Both my fingers and my needle-nosed pliers were too thick and bulky. After a dozen or so attempts, all of which ended up with the dropping of a washer and the incumbent risk of losing it, I decided I'd have to make a run to Harbor Freight for more appropriate tools. Here's what I came back with:



Confident that it would be a cake walk now that I had a way to hold the washers, you can probably imagine the emotional letdown when I got a bolt all washered up only to find that the shank was too large to fit through the hole in the bracket:



I went back to the section of the plans where those brackets are prepared for installation and could not find a directive to "Final Drill 1/4," but it's pretty clear at this point that there should have been one. Unless, of course, I'm just missing it.

It's New Years Eve, and while I'm steadfastly avoiding leaving the house because I don't like to be out when I know the rest of the world is tanked up on booze and good cheer, I decided that it might, yet again, be a good time to just walk away from the airplane for a little while.

3 comments:

Jim Thornton said...

I just got to the bushing installation in my build. My plans call for a SB375-4 bushing which is 3/8", so it looks like this issue has been corrected in later kits.

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