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I punch metals before I start a bit in them, but with small-diameter bits I am finding that they wander inside the material. E.g., drilling a 1/8" hole in 1/2" of aluminum the (titanium-coated HSS) bit will come out the other end a measurable distance to a side (orthogonal to the drill feed direction) from where it entered.

To be clear, the bit starts where I want it to since I punched the metal. But the bit appears to be flexing within the material, causing the resulting hole to be angled.

How can I prevent this? Is there something about the material that I need to understand in order to prevent the bit from wandering and causing the hole to be skewed? I'd like to know the mechanics behind the materials that is causing these skewed holes.

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It's common to start with a shorter, stiffer tool such as a center drill or a spotting drill. In addition, using the shortest drill bit that drills the hole you need will increase stiffness. Because of the flutes in a drill, the stiffness goes down geometrically as the length increases. The other variable you have control over is how you are holding the tool. A collet will keep the tool in better alignment than a chuck.

If you're doing this often, it's worth making sure you have a good grasp of feeds and speeds, and that you're using the best type of drill for the job. Both of those could have some affect on hole perpendicularity.

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    $\begingroup$ Feeding too hard (for the given speed) can be a problem. It can cause the bit to bend or buckle. I got a little aggressive on a deep drilling operation this morning and trashed a part. $\endgroup$ – Dan Jan 22 '15 at 20:57
  • $\begingroup$ If you are just mucking about at home a center punch and hammer will work fine, (though not as accurate) and you don't have to change bits. (I've always called center drills, starter drills) $\endgroup$ – George Herold Jan 23 '15 at 1:44
  • $\begingroup$ How the drill is sharpened also has an effect. If one flute bites better than the other, some wander is likely. $\endgroup$ – Brian Drummond Apr 14 '15 at 17:08

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