Wood Boring: Spade Bits, Hole Saws, and Self Feed Bits
We all know that drill bits have their limits when it comes to the size hole you can drill. But what happens when you need to do some larger wood boring?
As a teenager, I decided that I would replace the soft, rotted plywood back deck of our old bass boat. Being relatively ignorant in the ways of tools and working on getting my junior man card, I went to work ripping out the carpet and unscrewing the now very rusty fasteners. Cutting the new board and installing it wasn’t too much of a challenge. The problem came when I needed to reinstall the pedestal seat. It was secured by bolts but also sat several inches down through a 2-1/2 inch hole.
Looking back at trying to cut that #$%&*@! hole… what was I thinking? I started with the largest drill bit I had in the center, then tried to work outward with a hand saw. When that didn’t work, I tried drilling holes in a circle and connecting the dots with that same dull hand saw. I did eventually get it in there, but it took a heck of a lot longer than it needed to and man, was it UGLY!
Had I known what to look for with spade bits, hole saws, and self feed bits, I might have spent the afternoon fishing instead of cussing and sweating. Fortunately, I’m wiser now, both in my use of tools and the English language. I’m here to pass on that wisdom along with some help from Milwaukee Tool and their accessories.
Wood Boring with Spade Bits
Hole Diameter Range
Spade bits are going to cover the smallest of your wood boring needs. In fact, the smallest spade bits can be in the same range as a standard wood drill bit. Often called flat boring bits (or variations on that), spade bits can be found in diameters of 1/4″ to 1-1/2″
Spade bits like Milwaukee Flat Boring Bits have a shape that resembles a paddle. A brad point or threaded tip guides the bit into the wood and through the cut. The sharpened edges of the bit take small slices of wood out and eject them out of the hole. Spade bits can be used in high speed mode up to 1″ in diameter and sometimes larger, depending on the design. The largest spade bits need to be used in low speed mode to deliver more torque to the bit.
A good, sharp spade bit will allow you to apply minimal down force on your drill to make the cut. You can put additional pressure on it to drill faster, but it isn’t necessary. Particularly with larger bits, you’ll want to use your drill’s side handle if it has one. Today’s drills deliver a lot of torque and the bit can bind up causing the drill to twist your wrist or elbow painfully. Some spade bits have a 1/4″ hex shank that can be attached to your impact driver. Spade bits are less expensive than other wood boring choices.
When to Use Spade Bits
Spade bits like Milwaukee Flat Boring Bits are a great option for cutting holes up to 1″ in wood that doesn’t require a fine finish. The breakthrough with a spade bit can be rather unsightly though. If you’re going to be leaving one side as the finished surface, drill into that side and let the breakthrough happen on the side that will be hidden. Once you need to drill holes over an inch (and no longer in high speed with a spade bit), it’s time to look at hole saws.
Wood Boring with Hole Saws
Hole Diameter Range
Hole saws come in three basic designs. Diamond grit hole saws are typically used in masonry and glass applications. Bi-metal are used for metal and often find their way into wood applications. Carbide tipped hole cutters are the most efficient for cutting wood and are where I am going to focus. This style of wood boring accessory can typically be found as small as 1″ and run up to about 6″.
The hole saw consists of two parts – a mandrel and the hole saw. Collectively, they’re usually just referred to as a hole saw. The mandrel is has a threaded and sometime locking coupling built around a drill bit that holds the saw in place. The pilot bit guides the cut. Milwaukee’s Big Hawg Hole Cutters actually employ a replaceable spade bit. This reduces the amount of friction against the core to improve the efficiency of the cut.
Because the hole saw as a system is much more complex than a spade bit, they are designed to be able to change hole saws around the same mandrel which saves us money. Hole saws require more torque than spade bits, so the shaft has to be thicker to avoid damage. You’ll need a 1/2″ drill for most hole saw systems.
A hole saw works by shredding away just the circumference of the hole rather than chewing out the entire hole like a spade bit or Forstner bit. This results in a solid core or plug that has to be removed from the saw at completion.
Look at the teeth of the hole saw to see the difference between carbide tipped wood boring and bi-metal general purpose designs. Wood boring hole saws have 2, 3, or 4 carbide teeth depending on the diameter. These are easily sharpened as they dull. To give you an idea of sharpening cycles, Milwaukee Big Hawg Hole Cutters can bore up to 600 holes before needing a sharpening. Bi-metal hole saws… well, I wouldn’t sharpen all those teeth and it’s recommended that you don’t try.
While bi-metal hole saws can be used, they’re up to 10 times slower than carbide tipped hole saws designed for wood boring. If you’re letting the hole saw and drill do the work, you’ll be able to drill in high speed most of the time. Be prepared to drop down the RPM’s and go to high torque mode with larger diameters.
When to Use Hole Saws
Hole saws like Milwaukee’s Big Hawg Hole Cutters are perfect for wood boring 1″ – 6″ rough cut holes. It is possible to get bigger hole saws (Milwaukee Big Hawg goes up to 6-1/4″), but you’re really straining a typical 1/2″ drill at that point and you’ll start needing more specialized equipment.
Like spade bits, hole saws are a rough cut wood boring accessory. The breakthrough will leave a rough edge that will need to be sanded and/or covered for finished work. Like spade bits, bore in from the side that is to be your finished surface so that the breakthrough side is hidden.
Wood Boring with Self Feed Bits
Hole Diameter Range
Self feed bits typically run from 1″ to 4-5/8″, not to be confused with Forstner bits that can start as small as a 1/4″.
Self feed bits have outer teeth that cut a circumference like hole saws and a radial blade that slices out the core like a spade bit. The result is a cleaner finish that neither style can accomplish on its own. Self feed bits also have a threaded tip that pulls the bit through the wood. The threaded tips like those on Milwaukee Switchblade Self Feed Bits stick out further than that of a Forstner bit to act a guide and pull the bit ahead. They are usually favored by contractors who aren’t concerned with simply creating a recessed hole.
Most self feed bits are constructed in a way that allows the radial blade to be sharpened. Milwaukee Switchblade Self Feed Bits take it one step further. By adding a removable blade edge, the bits offer just as fine a finish while saving you time. Those removable blades are made of hardened steel, so you’re still getting long life out them. You just switch them out instead of sharpening them.
Like hole saws, self feed bits require a lot of torque to chew out a hole. You’ll need a 1/2″ drill to accommodate the larger diameter shaft. You’ll also want a spare battery nearby if you’re using a cordless model. Only with the smallest diameters will you be able to drill in high speed mode with self feed bits. The majority of cuts will be made in high torque mode.
When to Use Self Feed Bits
Because self feed bits fall into a range that is covered by spade bits and hole, it can be confusing when to use them. Forstner bits are finish bits that woodworkers and carpenters rely on to make cleaner through holes or recessed holes in material. However, self feed bits are purely for powering through wood as fast as possible. For applications like undercabinet lighting installation, using a jig and hand held drill are fine with a Forstner bit. When precision is your goal, you’ll find most woodworkers turning to a drill press with a Forstner bit and avoiding that self feed tip. However, general contractors, remodelers, and plumbers will be looking for the threaded tip found on Milwaukee’s Switchblade Self Feed Bits.