Reality check. Reamers mostly aren't necessary (100% not necessary for the designs I've built). I would agree, reamers would be necessary if the application calls for a bolt loaded in shear - you want the hole very close to the bolt size as the intent is for the sides of the holes in each piece being joined to bear up against the bolt shank. Well, AN3 bolts are NOT close-tolerance bolts. As in, the bolt size itself is allowed to "wander around" enough such that it just isn't a close-tolerance fit. If one was to try using AN3 bolts in a close-tolerance shear-loaded condition, one would need to measure each bolt individually, determine where in the allowed tolerance band it fell out, then have a selection of reamers throughout the tolerance band and individually ream each hole for each individual bolt. Well, homebuilt designs (that I know of) don't call for this.
AN3 bolts are loaded in TENSION. As such, where the sides of the hole are doesn't matter as much - there is intended to be a little gap there. The way a non-precision-fit bolted joint works is the preload on the bolt (generated via the torque applied), puts the items attached together into compression - yes, they actually deflect a little into the direction the bolt force is pressing on them. Failure of this joint is defined as applied load exceeding the preload. There is separation of the materials at this point.
So how are there so many joints in aircraft, made with AN3 bolts that are loaded in tension, but it looks like the force on the parts is sideways through the bolts? How does that work? Well, the friction generated by the preload keeps the parts from sliding relative to each other. The bolts aren't loaded in shear. Of course, in reality, there's forces and loads and combinations of things going on all over the place, but that's the general theory.
Back to reaming - I wouldn't worry about it unless your plans/kit designer has specifically called for it. First airplane I built, the designer recommended #11 drill (.191) for all AN3 holes. This I did. I sold it before I flew it, but it had well over 1000 hours on it before I lost track of it. So I'd say it works. Second airplane, which I built and completed (same design), I again used the #11 drill across the board for AN3 bolts. 150 hours and 3 annuals so far, and no problems.
To the point that a drill won't make a round hole - true, a 2-fluted drill bit will make a 3-lobed hole - in thin material. Generally, if it's going through a stack of parts, approximately the drill bit diameter's thickness or greater, there's enough material there to support the drill bit (around the helix of the flutes) to stabilize it and produce a round hole. A reamer could be useful in achieving round holes through thinner stacks. But if you're starting to think, I've got 10,000 rivet holes (meaning at least 20,000 holes through material), and most of these material stacks are 1/2 of the drill bit thickness, so maybe I should ream 20,000 holes - no, probably not. A rivet shank expands in the hole, so perfect circularity isn't even required. A rivet IS a shear fastener (generally, there are some tension rivets, but this is not common in a small/simple airplane design). As such, the SIDES of the rivet are loaded against the SIDES of the hole. So the fact that a rivet expands to perfectly fill a less-than-perfectly circular hole is just what is needed here.
Another issue to consider - if the idea is to ream such that the bolts "tap fit" or "hammer into" the holes - this is a BAD idea for at least two reasons.
1. In hammering a bolt into a hole, the protective cadmium plating on the AN bolt will be scraped off. So you've just lost the corrosion protection built into the bolt.
2. This will introduce HUGE stresses around the perimeter of the hole. Aluminum can not handle this. Cracks will start at the hole.
So be careful about this reaming subject. I would contact the designer / kit / plans supplier and see what they require, and not necessarily follow along with advice presented here and start to believe that reaming is necessary for a "better" product. The holes may be rounder, but maybe they don't have to be. The fit may be tighter, but that may be introducing stresses that wouldn't be there otherwise. See my signature line.
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Originally Posted by Neil
