I wish to bore a 12mm hole in a 9x2 vertically. How do I keep the bore hole straight through 9 inches?
This is where a drill press comes in handy.
Drill guides are also available, but I've not had good luck with them.
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
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.
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.
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 piece would greatly help consistently start the bore in a consistent place.
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.
The 3 point jig described above is the best strategy, yet must be applied with consideration of several ratio issues:
- The drill bit selected should be as straight as possible and the tip very accurately on center; verify before using.
- 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.
- Given issues 1 & 2, the drill length must be over 3 times the depth of the intended hole, as measured from only one side.
- The drill bit utilized should be super sharp & a pilot bit might aid in the hole staying straight.
- 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.
- 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.
- 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.
A hack is to use an old optical disk like a CD or DVD.
Lay the disk down and line up the drill through the middle. The reflection of the bit /shank should be in-line with the bit/shank that you can see directly. Look from above and beside to make sure its in line in two directions.
This assumes you're drilling at 90 degrees into a flat surface.
Use a good quality wood auger selected for correct diam & of sufficient length. Then select the slowest drill you can find/borrow. It's easy to start exactly on center but even at slow RPM the bit will rapidly advance & provide little opportunity to correct if the auger wanders off course. The best way practically speaking to keep the auger true & square to the surface being drilled is to have two observers assist, each one checks the angle from 90 degrees apart & helps with alignment. If you mess up you will have to start again or correct with a chisel, so a practice run is almost essential.
I have the opposite idea. Move the wood to be drilled to the drill bit! Start with a straight piece of shutterboard and set up the drill on a block using the level to ensure the drill bit is level/horizontal. Then mark on the board two lines exactly parallel to the drill bit and the same size as the section you wish to drill. Now screw 2 fairly deep guides on the outside these lines, creating a channel so the section fits fairly tightly into the gap. Now switch the drill on and push the section towards the drill. With the guides preventing the section from moving to either side all one needs to do is hold the section down and at the same time pushing slowly to feed it into the drill