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I wish to bore a 12mm hole in a 9x2 vertically. How do I keep the bore hole straight through 9 inches?

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See also: How can I drill a hole in a small stick of wood? – BMitch Nov 15 '13 at 13:48
Does the bore hole have to be perfectly straight or "straight enough"? What is the final use of this hole and project? – Freiheit Nov 15 '13 at 17:49
Nothing is ever perfectly straight, flat, square, or immobile. Perhaps you could drill a larger hole and let whatever it is that you're passing through it be straight that way? – Andrew Beals Nov 15 '13 at 19:55

This is where a drill press comes in handy.

Drill Press

Drill guides are also available, but I've not had good luck with them.

Drill Guide

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Most of the time in home repair this sort of thing has to be done freehand, since the house framing won't walk over to the drill press. – Ecnerwal Nov 15 '13 at 13:29
Every time I don't use my drill press, I kick myself. – Evil Elf Nov 15 '13 at 13:35
If this is for a finished surface you can avoid chip-out by starting with a forstner bit, but it will likely clog up before you get through so you might just want to do that on the surface and switch to something else. – Incognito Nov 15 '13 at 14:49
+1 for "have not had good luck" with so-called drill guides. I can generally do better freehand than with one of those things. Steer clear. – mac Nov 19 '13 at 21:02

Nine inches is quite a distance. Even w a 1/4" pilot drill you'll end up getting some wandering of the hole at increasing depth. Going straight to a half inch is likely to bog down your drill press or lathe. When you need precision, for example when making long rollers that must be symmetric around a central drive shaft, the usual practice is to start w two pieces of wood, route out a centered channel through each, glue the pieces together, drill hole to final size, and turn the result on a lathe until everything is nice and even. This book may be of use in your particular case: Deep Hole Drilling

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While a drill press (the most popular suggestion so far) might seem to be the ideal tool for this, it is quite difficult to find one which has a stroke of more than 3.5 inches. Even by reversing the work piece, that leaves more than 2 inches not bored with little assurance that the holes from each side are co-linear.

At a minimum, a long shaft drill bit is needed. Frys has 18, 36, and 72 inch long 3/8" (9.525 mm) and 18 by 1/2 inch (12.7 mm) drill bits intended for stereo/security/network/hvac system installers.

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For your problem, I would obtain one of these at least 36 inches long and build a jig (a drill guide on steroids) to hold it. The shaft on my 72 by 3/8 inch bit is 3/16 inches (4.76 mm) diameter and while it is not firmly rigid, it is not highly flexible either.

enter image description here

At least 3 guides are needed to counteract sideways forces you or the drill would tend to make. As for technique, fast rotation and slow drill movement with low forces is the best way to make the bit not wander. Adding an extra guide with a bore that is bit tip width which rests against the work pice would greatly help consistently start the bore in the same place.

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While a drill press may only have a 3.5 inch stroke, the work platform is adjustable letting you start low and move it upward toward the drill head as you complete each 3.5 inch bore. You do need a long bit though. – bib Nov 15 '13 at 22:27
@bib: Interesting. I have never seen a drill press with a precision adjustable work platform. Tester101's photo shows what I am used to. Maybe some rigid guides could be added to make it wander less when moved. – wallyk Nov 15 '13 at 22:46
The press shown does have an adjustable table. The locking lever is on the right side of the shaft and the table moves up and down that shaft. – bib Nov 15 '13 at 23:02
@bib: But there is nothing to prevent the table from moving left/right (rotating around the post) once the lever is loosened. – wallyk Nov 15 '13 at 23:10
True. Your idea about guides is a good one and alignment rods could be rigged. The bit itself, left in the hole already drilled could be used to guide the platform up. This would be far from precision, but might be okay, depending on the exactness required. – bib Nov 15 '13 at 23:13

If you need high precision, use a lathe with a center boring attachment.

A drill press is probably good enough. However, I would drill from both ends with a pilot bit and then finish with the size required.

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The 3 point jig described above is the best strategy, yet must be applied with consideration of several ratio issues:

  1. The drill bit selected should be as straight as possible and the tip very accurately on center; verify before using.
  2. The distance between the 2 end drill guides should be over twice the intended drill hole depth, as measured from only one side with the one nearest the target kept close to the target material.
  3. Given issues 1 & 2, the drill length must be over 3 times the depth of the intended hole, as measured from only one side.
  4. The drill bit utilized should be super sharp & a pilot bit might aid in the hole staying straight.
  5. It will be very important to run the drill bit at a high speed & low pressure, thus a slow speed of advance since wood has many variations in relative hardness which will inherently "steer" the drill tip away from a straight line.
  6. Given issue 4, it will also be important to keep the drill bit well lubricated to minimize both binding and heat which dulls the tip & can induce warping of the drill shaft. A custom fabricated bell hanger bit would be useful; the bulk of the drill shaft behind the bell hanger style bits is of slightly smaller diameter.
  7. If a very long hole is wanted, a laser pointer, carefully jigged, could be used to intermittently check that the bottom of the hole is still within tolerances. If off, use a drill rod with an off center point, carefully tapped/hammered into the center point base, so as to shift the drill tip back towards the desired path.
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I drilled a bunch of dog holes in a workbench recently using a wooden guide block that I made in a drill press, and it worked really well.

This method can be handy if the wood is too big to easily balance on the drill press, or if you don't have a drill press, but know a friend with one who could make you a guide block. Use a thick scrap chunk of hardwood with the true face (the one you choose to be your reference) face down on the drill press, then drill a guide hole with the same diameter as your final hole. The thicker the wood, the more accurate it will be as a guide.

Then you put the reference face against the surface to drill, and put a long auger through it to get the hole started. In your case, with such a deep hole to drill, you can remove the guide once you are a few inches in. I like using an auger bit with an impact driver, even though it is loud and requires adapters, because the combination is easy to control and makes steady progress.

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Tape or tack a couple of long, straight sticks (sometimes a framing square serves as one) in place so that you can locate the body of the drill in relation to them to keep it straight. You would not normally actually touch the sticks (that would make affixing them a major job) you just keep a constant distance from them.

If you have helpers, you can also sometimes use two helpers at 90 degrees to each other spotting the drill bit.

They make a level bubble that you can attach to the back of the drill, or drills with those built in, but I don't find those to be much more use than the drill guides, which are pretty useless in practice. Both look nice in theory.

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Just thinking out loud here.

You could duct tape a level to the side of your drill. Use a plumb to confirm that the level is parallel to the bit. Drill slow with a thin bit first; focus on keeping the drill level instead of getting through the wood.

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Do it by eye , I did through a brush pole with a 19mm bit all the way through .

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