Often times I need to drill maybe a 1/4" hole, all the way through the piece, and then a 3/8" or 1/2" hole half way through the piece in the same spot. It is often imperative that these holes are exactly concentric.

With a smaller drill press, you cannot just keep the workpiece clamped in place because to remove the smaller bit and replace it with a larger bit (or vice-verse) will end up breaking the alignment and it's nearly impossible to get it back again (within a few thousandths of an inch) by eye.

Are there special drill bits or guides for this purpose?

  • The woodworking guides suggest this is actually very simple, you are supposed to be able to replace a smaller bit with one or more larger bits without moving the workpiece and breaking alignment. But this ability obviously depends on the design of the drill press and on the length of some or all of the bits being used and how they relate to the workpiece height. Now that said, saying you need a bigger drill press isn't really a viable answer! And there are other means to ensure you get near-perfect concentricity no matter which hole you drill first BUT.... – Graphus Nov 19 at 8:22
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    ......you're asking too much of the material to achieve accuracy within a few thousandths of an inch every time. Especially with softer woods, especially including most common softwoods, the wood itself just won't allow this (even with the best equipment in the world). Accuracy in the range of a couple hundredths is all you can realistically expect sometimes, because wood is inherently variable (and occasionally contrary) stuff! – Graphus Nov 19 at 8:25

Forstner bits have a small point on the end which leaves a slight indentation in the wood in the center of the hole. Use this to align your drill bits after changing them. But, to do this, you'll have to drill them the opposite way you're thinking - drill the biggest (but shallowest) hole first, followed by the next smallest / deepest. This keeps that slight indentation from the point on the drill bit present in the wood, allowing you to align the next drill bit.

  • I've found this does not work with softwood, or plywood because it's very easy for the point of the forster acting as a probe to alter the existing point in the shallower hole. And because the point of the forstner is not conical (but rather, a two-edge blade), it can be quite easy for the alignment to be off by a few thousands of an inch. This can have an adverse effect on the precision of say, a threaded insert or a bushing, often resulting in binding of the screw. Generally the point of a forster is NOT a reliable register for exact concentricity. – AgmLauncher Nov 19 at 2:45
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    @AgmLauncher For what it's worth Even though the tip of the Forstner is not conical, the divot that it produces is conical. – Ast Pace Nov 19 at 18:28

You can drill the smaller through-hole first, then use a counterbore tool. The end of the counterbore will use the through-hole as a guide and will cut a flat-bottomed hole of a larger size exactly centered on the smaller hole.

Similar to the other answers, but for any drill bit type:

  1. Drill a very small pilot hole deeper than the shallowest, big diameter hole. Use a standard twist bit. Limit depth if this is to be a stopped (not through) hole.
  2. Use your small pilot hole to center the new bit while you drill the largest diameter, shallowest hole.
  3. Use your small pilot hole to center the new bit while you drill the next smaller diameter, slightly deeper hole.
  4. Lather, rinse, repeat until you have the smallest, deepest hole.

If you use, say a 1/16" bit for your pilot hole, your 1/2", 3/8", and 1/4" bits of any sort should line themselves up quite nicely on that 1/16" hole.

Additionally, as Graphus pointed out in his comments on the OP accuracy within a few thousandths is highly unlikely with home- or even commercial-grade woodworking drill presses. You'd likely need an expensive CNC drill press to achieve that accuracy and a much harder, or at least much more consistent material (plastic or metal) to achieve that kind of precision.

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    Your final comment was basically what I was going to say to the OP if/when he came back. And even if you can achieve this accuracy in wood, without the wood itself causing a problem, there's little chance that subsequent seasonal movement won't cause an issue at some point down the line (and likely sooner rather than later) if the thing is built to tolerances this high. – Graphus Nov 20 at 8:01
  • @Graphus that's why I said you said it! ;) – FreeMan Nov 20 at 14:46
  • I meant the final final one :-) re. plastic or metal. – Graphus Nov 21 at 7:32

If you use brad point drill bits drill the larger one first then carefully center the smaller one over the point (left in the hole) of the larger one then drill through. OR Drill a very small hole as a center point through the board use this as a center for the 1/4" and the 3/8" hole.

Are you able to drill the bigger hole first?

That way there will still be some wood there to hold the smaller drill bit, and the pointy pilot tip in the middle will act as a center.

If the workpiece is too thin, try clamping a scrap piece below the planned hole to give the pilot tip somewhere to indicate onto.

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