Someone emailed me a comment on this thread, because he was under the impression that I was rejecting all advice, and using my use of a twist drill as an example.
Well he was partially right -- these ribs are for a plane that, because of my level of flying skill, probably isn't going to have a long life. So I'm not going to throw away a bunch of already-made rib blanks if they'll be "good enough" for my purposes. But the reason I asked was so that when I build the next plane (which will hopefully have a longer life) I'll know more -- and I am listening, and appreciating -- the feedback.
He also criticized my use of a twist drill -- this is because I apparently did not communicate the fact that it isn't a twist drill any more, or at least not one designed for metal. The drill did, indeed, start out life as a twist drill, made in a twist drill factory for the purpose of making 9/32" diameter holes in metal and plastic and other suitable materials. But one day, while it was sitting in my drill index savoring the taste of the last bit of 6061 that it had gone through and joshing with its buddies about what it might be like to drill titanium or Rulon or some other exotic material, it was unceremoniously plucked from its warm, comfortable home and its tip was rudely ground into a completely different shape. In fact, what it is now is closest to a rather substandard Forstner bit, although given its size and price I'm not complaining.
A twist drill is designed to cut on the end of the drill, along as much of the face of the drill that can be managed. The very center of a twist drill (where the two faces come to an oblique point) can't do much more than mash material out of the way. But the ends of the flutes of a properly-sharpened twist drill make little chisels that cut strips of material away from the parent body. In most twist drills the flutes are twisted, which helps them to eject the shavings. This process works great in steel, aluminum, plastic, and similar uniform materials.
But (as my respondent correctly pointed out) wood -- and balsa wood in particular -- has entirely the wrong mechanical characteristics to work correctly with a twist drill. The materials that twist drills work well in are isotropic -- that is, no matter which way you slice them, they act the same. Wood is not isotropic. Wood has fibers that have great strength along their length, but which are not terribly well attached to each other. Attack a piece of wood with a twist drill, and instead of making a nice clean cut you end up crushing fibers with the tip, and ripping them out of the sides of the hole when the chisel end of the bit catches on.
I've posted a picture of the bit. If you look at drill bits that work well in wood you'll see that they all have some sort of knife blade (or pair thereof) around the edge of the bit. If you ignore the tit in the middle of the bit that's there to keep things centered, the very first interaction that the drill has with the wood is to cleanly scribe a groove down into the very outside diameter of the hole. After that, most bits are designed to chisel out pieces of wood and to (hopefully) eject them one way or another. If you look at the poor abomination that I made out of what was once a perfectly useful twist drill you'll see that I've cut the thing so that the first thing that meets wood is the outside diameter of the drill. If the picture were better you'd see that the ends of the 'knife' are angled so that it cuts cleanly into the wood.
I've posed the bit with a regular twist drill, and with the hole made by my modified bit. If you're careful about how you operate the thing, it makes really clean holes. I haven't been able to come close to that level of quality with a bit of sharpened brass tubing, and I suspect that if I could it wouldn't stay sharp for nearly as long as this thing will. In addition, this drill bit makes chips that can be ejected (with work) -- a sharpened bit of brass tubing (at least done the way I know how to do) is essentially a hole saw, and always fills up with little disks of wood that must be pried out periodically.
The downsides to the bit at this moment is that it doesn't really eject chips well, and it isn't self centering at all. It does the 'cut around the edge' perfectly, and the top hole in a stack of ribs is always as sweet as could be. But to eject the chips you have to peck at the wood very carefully, feeling for when the bit starts to load up, and then hope that you get some chips to come up with the bit. While that particular hole is a very good example of the breed, there are some that aren't as pretty -- but all of them make a very snug but not too tight fit to my 9/32" jig rods, so I'm not complaining! In addition to the chip ejection problem, the lack of a center 'tit' means that the only place that you can use this drill bit is in a drill press -- it will actively wander out of place if it's not held rigidly and fed slowly.
Other than that, it's a nice bit, far from being a regular twist drill that would make a disastrous mess of the balsa that you try to drill with it, and more than useful enough for the job I'm asking it to do now (although I did think I heard some gentle sobbing when I set it down next to a block of virgin aluminum, a bit earlier).
Topic: Cutting out wing ribs (Read 2110 times)