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Countersinking spar nutplate holes

I would like to start this post buy providing a word of caution to anyone about to countersink holes on your nice, golden spars. My strong recommendation is to not touch a single tool until you know exactly what you need to do. I was very fortunate that my mistake was correctable but it could have resulted in me requiring to buy replacement spars. If you are reading this post as part of any research into what to do for your own kit, please, please, please read this entire post before starting work.

It was time to countersink the larger holes for the nutplate screws. I set my countersinking tool deep enough so that the AN5 flush head bolt would sit nicely. (MISTAKE 1: Do not do this. The countersink diameter should be 0.37″ or 9.4mm so set your countersinking tool to give you this. Fortunately for me, my first attempt gave me a countersink diameter of 8-8.5mm so I could countersink deeper to fix the next mistake).

The first few holes I went clow and careful, so the countersink were round and smooth. As I progressed, I sped things up a bit. (MISTAKE 2: Do not do this. Countersinking these large holes should be done as slow as possible. A variable speed cordless drill works well). Unfortunately, I started to encounter some vibrations every now and then. This effect is known as the bit “chattering”. This results in the hole not being countersunk perfectly round and smooth. Instead, the countersink circumference is waved and the the surface of the countersink is not smooth. I tried to rectify this by speeding up the drill bit and holding it in position as long as possible until the vibrations reduced. (MISTAKE 3: Do not do this. Alwyays use very slow speed settings. If things aren’t right – don’t push it. Stop and re-assess).

I eventually got all the holes countersunk and I sprayed self-etching primer over the holes. I chose to prime a strip rather spot priming as it looks neater.

I repeated this for each side on both spars, then sat back and reviewed my work. As I inspected each hole, I started getting uncomfortable. Whilst most of the holes were relatively round and smooth, some of them weren’t. The more I thought about it, the more worried I became. Here is an example of one of the worst examples.

OMG. How did I do that? As thoughts of replacing spars went through my head, I hit the VAF forums and asked for advice. As always, a lot of other builders posted useful advice and tips on how to avoid this (too latye for me) but also how to fix it. I needed to make myself a pilot guide. After looking at Brad Oliver’s website to look at his guide, I made one for myself.

First, I made these pieces out of some alclad stock. I made them taperd and round at the guide end so that it would not clash with the spar web for the inner nutplates that are at about 45 degrees to the spar web. The small holes were marked from one of the spar nutplate holes as a guide. I makred the centre hole based on measurement as the countersunk holes weren’t accurate to provide a good centre.

I then placed some flush rivets in the holes and sandwiched them between the two pieces. I then held the two pieces together with a set rivet.

Here is the pilot guide in position. I ended up sanding the rivet tails down a bit so that they were flush with the flange. This was required so that they did not clash with the countersinking tool.

I then set my countersinking tool to the correct depth that gives me a 0.37″ / 9.4mm diameter and very slowly and carefully began countersunking the holes to the correct depth.

Remember that horrible looking countersunk hole earlier? Here is the before and after shot.

It’s not perfect, but it’s much better than before. This is probably the worst hole out of the lot. I haven’t fixed all of them yet but all the other ones so far have completely removed any irregularities and they all look magic.

I just wanted to verify that I could fix this problem (which it appears I can) so that I wouldn’t stress about it anymore.

Build time to countersink the holes, make the pilot guide and repair a few of the holes – 3 hours.

About gstrack

Husband and father of 2. Control & Instrumentation Engineer. Flying nut. Gadget geek.


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