Sunday, April 20, 2014

21-17, (- # 9) Step attach angles

The philosophy behind the EAA’s existence is to learn about aviation via the process of assembling or designing your own airplane.  Today was a great day to learn things.  Specifically, I learned how hard it is to fabricate even a simple piece.  I’m not being cynical or sarcastic, just very matter-of-fact.  

I didn’t pay attention and countersunk the wrong side of the F-1251 Nutplate Attach Brackets.   After I discovered my mistake (and de-riveted the K1000-428 nut plates) I realized that I couldn’t just countersink the correct side as that would leave no material to hold the rivet.  I tried rotating the nut plate and drilled another hole about 30° from the first one, but again, that didn’t leave enough space to have good material grip. (see photo below, bottom right). Turning the Nutplate Attach Bracket upside down and putting in my own 1/4” wouldn’t work because the existing 1/4” hole was right where a rivet would be. 

Rather than waiting a few days for a piece to arrive, I decided to fabricate my own replacement.  I scrounged around the garage and noted some spare extruded angle material.  Some basic measuring, drilling, de-burring and buffing created some parts that looked pretty good.  I test fitted them without really looking at the diagram and discovered that the corner of the extruded material didn’t quite permit a good fit into the filleted grove of the Step Attach Angles, so I cut off the ‘vertical’ portions of my new creations.  Then I noted that those were actually supposed to *away* from the ‘vertical’ portions of the Step Attach Angles.  I conclude that the reason the original parts had that bend in them was to add additional bending strength to the part, so I proceeded to cannibalize a longeron (I’ve ordered the replacement) and make another batch of four. 

I drilled the 1/4” hole, then held a K1000-428 in place with an AN4 bolt while I drilled the mounting holes using the K1000 as a template.  Even with this technique, there was some filing and re-drilling required to actually get a rivet to go into place.  Because this material was thicker than the original stock, I used 426D3-4.5 rivets (vs -3.5’s) and got good looking rivets.  One Nut Plate started to cross thread and I noted that the lip of the fabricated part wasn’t quite centered and thus caused the bolt to enter at an angle.  That got filed out and an AN4

All in all, I spent about 3 hours on this project, but it felt good to do a bit of basic engineering and shop work and I saved about 3  or 4 days.  As it turned out, I got a lot done in those 2 days, so my efforts were well spent.  What a wonderful day!

In the photo below, you can see the original ruined parts and their fabricated replacements (primed and ready for installation.)

IMG 3112

Original & Replacement F-1251 Nut Plate Brackets

IMG 3116

Step Attach Angles and Nut Plates

IMG 3108

temporarily attached Boarding Step

BTW, step #9 calls for sealing the opening on the boarding step with ProSeal, which I don’t happen to have on hand.  Because the step can be attached at any later time (as per 21-18 #10), I’m deferring that for now and marking this page otherwise complete. 

No comments:

Post a Comment