Spade Drill Bits (aka Paddle or Flat Bits)
Spade bits are the drill bits that we turn to to drill larger-than-average holes. They are flat (like spade shovels), and always feature a center point. The best spades also feature spurs on the edges to assist with cutting. Spade bits are commonly found in sizes ranging from 1/4″ through 1-1/2″ and are available at just about every hardware and home improvement store. There is no limit to the size a spade bit can be; however, speed becomes an issue as the bit gets larger, making sizes larger than 1.5 inches uncommon. A typical 1/2 inch electric drill would struggle to effectively power a 2 inch+ spade bit through framing lumber.
Because of the way spade bits cut and scrape the surface of wood to drill a hole, they require more force than their standard twist drill bit counterparts. Spade bits also cause tend to cause splintering (a.k.a. tearout) on the face opposite the drill start side. Near the end of the hole, it is easier for the wood to push out of the way than to be cut. (Techniques described below can limit this effect). Spade bits featuring a spur and reamer design, like the the Bosch version shown at that Popular Woodworking link, help to limit this effect by cutting the the edges first.
Drilling a Hole with a Spade Bit
Before you drill: If you’re trying to drill a very precise hole (say, for furniture or other wood working), you should drill a pilot hole first. The pilot hole will serve as the guide for the spade’s center point and will eliminate inaccuracies in your placement. For less precise work, like electrical work where spade bits are frequently used to pierce framing members for Romex, no pilot hole is needed. Some spade bits, like the one shown in the Popular Woodworking article, are better at self-piloting than traditional style bits like the one on the right. Also, as Experts Village suggests in their video on the topic, you should place another piece of wood behind the wood you are drilling to reduce splintering.
Drilling the Hole: Set the center point on your pilot hole and start drilling slowly until the full spade is engaged. Steadily increase speed until your drill is operating at full power. Note that spade bits have a tendency to grab during drilling. This is especially true if you are forced to work in a tight area (for instance, between two framing members where the drill cannot be properly held straight). When this happens, the drill will whip around violently if not held tightly. It is very easy to get bruised and battered. Highly sharpened blades or those with excessively tall (a.k.a. aggressive) spurs will tend to grab more. and require more care.
Follow Through: Don’t stop drilling until you make it all the way through the wood. As you get closer to the end of the hole, the drill may vibrate significantly more, especially if a dull bit is being used. A spade bit is particularly prone to grabbing at the end of the hole, so care should be taken to keep the drill at a 90 degree angle to the surface, with force applied and the drill moving at full speed. If the speed is dropped, the bit will be more likely to grab.
Sharpening a Spade Bit
There’s just something about resharpening a bit that makes you feel really good about life. It saves money and reduces waste. All that’s needed for this job is a simple bench grinder that can be used for all kinds of sharpening jobs. Sharpen both sides of the middle of the blade and the protrusions on the edges. Answerbag.com has a great video showing how to sharpen a spade bid using this method, so we won’t rehash it fully here.
Hole Saws (When Spade Bits Aren’t Enough)
When a hole larger than 1.5 inches is needed, the tool of choice is the hole saw. This article isn’t about hole saws, though. If you’re interested in learning more about hole saws, take a trip over to Wikipedia. (Their article is complete with some great pictures from WikiCommons).
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Image courtesy of Luigizanasi on WikiCommons. Cropped and description text added by us.
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