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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 supports Monica Nov 19 '18 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 supports Monica Nov 19 '18 at 8:25
  • @Graphus I disagree. When I make box joints on my cheap $200 table saw, I can reliably get the fit snug within 2-3 thousandths of an inch. The sled I built for it also reliably produces a 5 cut accuracy of 4 thousandths of an inch over something like 90". You can get very high accuracy out of cheap tools with a little TLC. I ended up with this prototype with sheer luck: youtube.com/watch?v=Z_1qt-Ox1ZU. The tolerance of that bushing is 9 thousandths of an inch wider than the 1/4" bolt, thus the 1/4" hole the bolt is in had to be nearly exactly on center. That wood is just poplar. – AgmLauncher Dec 27 '18 at 4:43
  • Notice the wording — "every time" and "sometimes". Also, the context was drilling holes. You can't drill into wood with this accuracy all the time because the material is variable, and even when you do the holes won't always stay accurate over time (round holes tend to become oval at some point in their lives due to differential shrinkage, which in effect means their position [position of one tangent] relative to one edge of the board, or spacing hole to hole, shifts). – Graphus supports Monica Dec 27 '18 at 9:28
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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.

  • Yep. I think this is the only way to guarantee re-registration of center short of a bigger drill press with more travel depth to allow changing of bits without moving the workpiece. – AgmLauncher Dec 27 '18 at 4:36
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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 '18 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 '18 at 18:28
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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 supports Monica Nov 20 '18 at 8:01
  • @Graphus that's why I said you said it! ;) – FreeMan Nov 20 '18 at 14:46
  • I meant the final final one :-) re. plastic or metal. – Graphus supports Monica Nov 21 '18 at 7:32
  • This does not work when one of the holes is too deep to see the pilot hole. One of my applications is to have a 3/8" hole 1" through a 1.25" piece of wood, with the remaining 1/4" thickness being drilled through via a 1/4" bit. So if I drill a small pilot first, then the shallower 3/8" hole to a depth of 1", there's just no way I can see the pilot hole to drill the remaining 1/4". I think the recommendation for a counterbore set in another answer is my only choice, short of spending a grand on a proper drill press with more than 2.5" of quill travel – AgmLauncher Dec 27 '18 at 4:34
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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.

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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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