Schwinnsta

Maybe someone on the forum can explain to me why this is: When drilling holes along the center of cylindrical stock, why does spinning the work piece rather than the drill bit result in straighter holes with less drift. Why d

Duragrouch

I think you mean, the drill is spinning quickly to make the hole, while you periodically rotate the stock (which is sitting flat on its end); Assuming yes, this is to try to cancel out any error in perpindicularity between the flat end and the stock cylindrical axis.

However, if you mean a fixed drill and rotating the stock to drill it, like having the stock in a rotating lathe chuck and the drill in the fixed tailstock, that doesn't necessarily make the hole more true in axis versus the stock, you can still get drill drift; The relative motion between the two pieces is the same. However, grabbing the cylinder by the outside with a chuck, is inherently more precise in alignment, than standing it up on end, unless it is a very short cylinder, like a plate.

Schwinnsta

If the cylindrical stock is in the chuck of the drill press and the drill is in a clamp that is itself clamped to the table and the two are concentric, then spinning the stock results in perfect holes without drift, but not the other way around. The weight of stock does not matter. It could be alloy or wood. The fact that it is spinning, cancels out the drift. Your second paragraph above with the lathe is the same condition, but you're incorrect about the drift. If the stock spins, then no drift. If the drill spins and the stock is fixed, there will be drift. If you don't think so, try it.

Duragrouch

I've done my share of drilling, but seldom deep holes, I did more milling. I tend to believe the voice of experience, so I trust you for now, but the question is why? There's gotta be a reason. The biggest reason drills drift is if the two sharpened flutes are not perfectly even, like if it's been hand-resharpened. but that would have an effect whether you rotate the drill, or the stock. OK, here's a theory (1): The drill is smaller diameter than the stock, so perhaps an order of magnitude or more less laterally stiff; Rotating the drill causes any imbalance to pull the drill adrift, whereas rotating the stock, not. Theory number (2): Drill chucks can be notoriously imprecise, due to variation in the shank, and jaws mechanism. Which is why, when I had to drill really precise stuff, I held the drill in a collet. So spinning a drill in a chuck with any lateral runout would amplify the error, whereas holding it and spinning the stock would be less error, assuming the stock is held by something more precise than the chuck.

Schwinnsta

Somehow, by rotating, the stock corrects its axis. I am looking for an understanding of that, or at least a reason. I have tried googling and AI and while others noted this phenomenon, no one explained it. I came across this while looking for ways to drill concentric holes for making internally threaded rods. I used this on a bike project where I modified the rear rack on a Brompton to make it more the B when it is in shopping cart mode.

Duragrouch

OK, I think I may know. Picture this: You have a piece of round bar, faced perfectly flat and 90 degrees on both ends, stationary. You stand the stock on end, and start drilling into one end. You start the the rotating drill, but off-center on one end, and it goes right where you start it, doesn't correct. Now, instead, you fix the drill, and rotate the stock, but still contact the drill off-center; Forces will try to pull the drill to the center of rotation of the stock, because it has the least relative movement between the two parts, the drill will seek the center. I think that's it. I've seen it happen when drilling something on a lathe. Which explains, when I made myself a custom drafting lead holder decades ago, drilling a 3/16" hole about 6" deep on a lathe, the hole didn't drift.