The Murphy Builders Site

Had a slip yesterday when I thought the backing block was under the drill and it wasn't. Hole is good but left a nice sharp crescent in the rear fin spar surface, well past the "snag your thumbnail depth". I know the fix is to smooth it out to prevent it from being a stress concentrator but how deep is repairable? Is there a rule of thumb where the damage is deep enough that it is better to replace than repair?

Photo?

I forgot to take a photo before polishing it out. I measured the thickness at the narrowest point this evening and it is .0443 where the base sheet is .0483. That's almost 10% of the metal gone, I'll check my measurements but I suspect it is toast.
Here it is after polishing:

Hi 'cgroves',
In your photo, it is difficult to actually see the area of concern. Is it a long scratch, or just an indentation same size as the drill you were using?

Regardless, the following advice is based on my reading of AC 43.13-1B section 4. Your situation is not explicitly described there, but a 'repair' or 'patch' appears to be a reasonable solution rather than throwing out the piece altogether. Same thickness and alloy, slightly larger than the affected area, with a good number of rivets around the periphery. See figures 4-13 and 4-14 for examples. This would be the most conservative approach. I'd also encourage you can also ask a local EAA Technical Councilor, just to give you some more comfort at these early stages of the build.

In my experience with my Rebel project (I'm not providing a recommendation here, just saying what I have done) a simple indentation of this type (if sufficiently far away from another rivet hole) would have seen me drill through the indentation with the smallest possible bit size, to prevent any stress risers. In my view, one extra #40 hole is not going to significantly diminish the strength of the material, and drilling it out and deburring prevents cracks.

Alternatively, if the damage is very close to an existing rivet hole, you have the option of enlarging the hole to next size rivet holes.

Again..this is my take on your situation...I would like to hear if others agree or disagree. I certainly wouldn't be throwing out the part because of this.

Rob

I also cannot see the problem area in this picture and suggest that you take another picture and use a marker to point out the damage. If possible zoom in or crop the location of the damaged area.

Unfortunately the only picture I have is after polishing out the gouge so it isn't visible, just the polished area. I was drilling the third hole from the right on the top row. The gouge was left by the outer edge of the chuck teeth on the drill, not the bit. On the Sioux air dills the chuck teeth aren't very tapered so it left a mark well out from the drill hole. I was foolishly not using a drill stop since the backing block had been working so well. It was an arc about 7/16th" long .0040" deep. Because of its length and the doubler edge right below it on the other side of the spar drilling it out or patching wasn't going to work.

Hi 'cgroves',
Many of us have long forgotten our work on the flying surfaces, so you may want to take a few shots of how the parts fit together (including a pen mark around the area of concern, as Mike suggested).
You say there's a doubler to be installed in this area....that may be a way to add the needed thickness for a 'patch'?
Rob