How to Drill Large Diameter Holes in Wood

You can drill clean, accurate holes larger than 3/8" with a twist drill bit under certain conditions. If the drill is sharp; if you start the hole with a countersunk hole to guide the drill or if you pre-drill a smaller hole first; and if you feed the drill bit slowly so that the variations in the grain structure won't pull your bit off center. You also need to clamp the work piece because of the larger twist drill diameter, the greater the torque while drilling.

Here are a few specialty bits you may want to know about, and a quick guide to how to use some of them.

Spade Bit
These chew holes fast in studs and joists when chucked into a portable drill. The holes are rough but that rarely matters in studs that are going to be hidden behind a wall anyway. When chucked into a drill press, they make clean holes all the way through, or holes with flat bottoms, if you don't mind the little hole in the middle of it from the guide point. However, these are uncontrollable in end-grain, so don't use them for drilling dowel holes for tenons.

Auger Bits
The feed screw of an auger bit accurately locates the hole and helps pull the sharp spurs into the wood. But once the feed screw breaks through; its usually muscle power that completes the hole. Straight-Shank augers in a drill press or hand drill require care because the feed screw can abruptly pull the bit out of control and into the work piece. You can lessen this problem by partially filing away the threads on the tapered feed screw. Solid Center auger bits forged from bar stock are the strongest and cheapest. The Russell Jennings type is twisted from strip steel and provides a cleaner more accurate hole with better chip removal. Use long Ship Auger bits for boring deep, clean holes.

Brad-Point Drills
The small 3/16" tip starts the hole accurately and keeps it going straight and the bit gives clean sides. It is the best all-around choice. It works best on drill press but can be used in portable drills, too.

Power Bore Bit
These bore clean, accurate holes and are roughly equivalent in performance to brad-point bits except that power bores have a longer drill guide tip, which can be as objectionable in a flat bottomed hole as the one on the spade bit.

Forstner Bit
This leaves an exceptionally clean hole with a flat bottom and only a very tiny center hole from the guide point. Flat bottomed holes are important for dowels in thin boards. Most drills are guided through the material by the center point, but Forstner bits are primarily guided by the spurs on the outside rim. They can also drill partial diameter or semi-circular holes, pocket holes (holes at an angle), partially overlapping holes or holes that are on the edge of the material. You must use a drill press and the work piece must be firmly clamped.

Multi-Diameter Bits
A plumber's tapered burring reamer is an interesting idea, if you have a bit brace, but maintaining accurate hole location and diameter is difficult because it cuts very fast through soft or thin material. It is a single flute tapered reamer and is more controllable when used in a drill press because diameter can be accurately controlled with depth-stop.

Expansive Bits
These can be used with a variable speed drill, or a bit brace. A pair of expansives can take the place of a set of augers when you need holes over 5/8 in diameter and can be used to bore holes up to 3 inches. When setting up an expansive bit's cutter, always test the hole size in a scrap first; don't trust the calibration. Always be sure the cutter locking screw is as tight as you can get it and use it with the slow speed of your power drill or drill press.

Hole Saws
There are two kinds of hole saws. One is a single mandrel with a set of seven blades that look like hack saw blades bent into a circle. Inexpensive, but you can't replace worn blades individually. The better kind looks like an inverted cup with teeth around the rim and it mounts on a mandrel that is sold separately, or you can buy a set. Both kinds have twist-drill center guides and can be used in either a drill press or with a portable. When drilling deep holes, back the drill out occasionally and chisel out the scrap wood that is in your way. The saw runs cooler with less chip clogging and you won't have to pry the plug out of the saw cup when you're done.

Lock-Set Bit
The most common need for a large diameter wood bit around the house is for installing a new lock-set on a door, such as adding a deadbolt. You can use a hole-saw or an expansive bit instead, but if you have a lot of doors to do, a door-lock bit or lockset bit is good choice. If you have a drill press, you can use a hole saw, lock-set bit or a multi-spur bit. Multi-spurs can drill semicircular holes just like a Forstner can.

Circle Cutter
This are also known as Fly Cutters and is used in a slow drill press for making large diameter holes. The work piece should be clamped and backed with a piece of scrap plywood. For a clean hole, cut partway from one side and finish from the other and feed the work piece very slowly. Variation of the fly cutter is the Compass cutter where the drill bit fits into a loop on the end. This can be used with a portable drill.

Deep Drilling bits
These are extra long twist drills that are often used for drilling the cord channel through a wooden lamp.

Mortising Bit
This is used to make square holes using a drill press. Use it to make mortises for tenons.

There may be many more choices for large bore holes, but these are just a few of the most common ones. If you have any doubts about how a hole will turn out, you can always practice on a piece of scrap wood similar to what you are using in your project.

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Comments

Wed, 18 Mar 2009 at 8:31 AM, by Roof Repair
thanks for telling me how we can do the big hole in the wood. firstly check the wood is strong or not. sometimes wood is not so strong and we want to do a big hole in it with drill. And then wood may be brake. so from your website i collect more information about doing hole in a wood with drill. which type of bits we used in the drill. which tools are used to doing the holes or cut the wood or anything.
i appreciate from your website.
thanks....... for the information.

Roof Repairs

Wed, 20 May 2009 at 12:38 PM, by Jay
Hi,

I googled large diameter holes in wood and your sire was the best choice for my seacrh. Just in case you'll check this out and are willing to reply w/ more feedback, here's my dilema; I need to cut two 120mm holes in an assembled enclosed wooden cart. The cart is finished and is quite large. I will not be able to take the side piece off and fix it to a bench or sawhorse of anykind. What would you do if you had this problem?

Thanks in advance!

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