Re: Drill bits to drill main spar attach holes.
Posted: Wed Nov 19, 2014 6:12 pm
I used the Sonex method and recommended drill bit (McMaster-Carr) to do my main spar bolt/pin holes. I was actually quite successful, or so I thought. The holes look clean and round, and my bolts were nice and snug.... with everything assembled. But when I pulled the wings off and tested each hole with the AN6 bolt individually I was surprised how much play there was between the bolt and each hole. Keep in mind that a bolt is a couple of thousandths smaller than even a perfectly drilled 3/8" hole and, as someone else mentioned earlier, when the wings are on it can seem as if everything is snug due to any misalignment of the individual parts or normal forces on the bolts due to the weight of the wings. You have to under drill the hole and ream to the actual size of the bolt (0.372-0.373) for a true snug fit.
So, what did I do to fix my problem and get a snug fit on each individual hole? I purchased some 3/8" drill rod (drill rod is 0.375 plus or minus 0.0005), I ground a slight taper on one end, and found it had to be tapped lightly with a rubber mallet to get it to slip through each hole. Viola! A perfect snug fit on each individual hole.
BTW, if you've really screwed up and need to go to 7/16" (or just want larger spar bolts) I have two drills that I purchased on Ebay that are stepped (25/64 - 13/32) and "double margin". Google "double margin drill bit" if you've never heard of them. I also have two reams from Brown Tool that are 0.4219 x 0.4375 for getting a really good 7/16" hole. I'm willing to lend them to a poor soul in need (since it's probably bad etiquette to offer anything for sale on this forum). Keep in mind even a close tolerance bolt (AN177-XX for a 7/16" bolt) is still going to be a thousandth or so less than the hole but I figure if you're within a thousandth of an inch you're probably doing better than 90% of the aircraft that are built and flying.
So, what did I do to fix my problem and get a snug fit on each individual hole? I purchased some 3/8" drill rod (drill rod is 0.375 plus or minus 0.0005), I ground a slight taper on one end, and found it had to be tapped lightly with a rubber mallet to get it to slip through each hole. Viola! A perfect snug fit on each individual hole.
BTW, if you've really screwed up and need to go to 7/16" (or just want larger spar bolts) I have two drills that I purchased on Ebay that are stepped (25/64 - 13/32) and "double margin". Google "double margin drill bit" if you've never heard of them. I also have two reams from Brown Tool that are 0.4219 x 0.4375 for getting a really good 7/16" hole. I'm willing to lend them to a poor soul in need (since it's probably bad etiquette to offer anything for sale on this forum). Keep in mind even a close tolerance bolt (AN177-XX for a 7/16" bolt) is still going to be a thousandth or so less than the hole but I figure if you're within a thousandth of an inch you're probably doing better than 90% of the aircraft that are built and flying.