I-Strut Drill Jig

4/21/2006

As per plan, I cladded all around the spars with 1/4 birch ply between st. 68 1/4 and st. 79 1/4. I used T-88 glue for this. This cladding is to add strength around the holes that will be drilled through the spar.

I then decided to make a drill jig since a vertical hole through the spar would be difficult / impossible to be anything other than inaccurate if done freehand. The jig was made out of a few drop-offs [aren't all jigs?] I had floating around the shop. I tried to find some steel tube locally, but to no avail. So I ended up buying aluminium tubing from the Hobby Lobby. The size was 9/32 x .27, this gave a nice 1/4 internal fit for the drill bit. I was concerned regarding the thickness of the tubing [or should I say thin-ness] due to potential wear from the bit, so I tried a test piece. All seemed to work well, with only a microscopic amount of alum powder after the test run. The tube was glued in the dado full length, after curing I just cut out the middle section.

After checking about 1,203 times that the jig was in position, was flat against the spar, and was sitting squarely, I drilled using a 12" bit from the top to about half way through the spar, then started again from the bottom and met in the middle. The drill I used was an air drill [small and very light] - this gave me more control to focus on keeping the drill bit lined up so as to avoid a slight bend during drilling...which would have started to wear the inner alum tube, and worse, cause the hole to veer of course...