A flexible drill bit, also known as a bellhanger bit, will cut easily, provided it's kept sharp. In fact, a properly sharpened bit will practically pull itself through the wood.
A sharp drill bit cuts the wood fibers, while a dull one tears them.
One of the biggest errors I’ve seen even experienced alarm installers make is trying to drill with dull bits!
Using a dull bit means using extra pressure to get it to cut. This causes friction, heating the bit up. Metal wears faster when it is heated, which will make an already-dull bit even duller.
Pushing harder on the drill will also cause a long drill bit to bend, causing it to drill the hole off course. Despite its common name, a flexible drill bit isn’t meant to be bent. It should actually remain almost perfectly straight throughout any drilling operation. The shaft is thin and flexible because:
Drill using light pressure, and with the drill motor set to a higher speed. This will clear chips quickly, and also allow you to feel when you’ve gone through solid wood and entered a hollow space.
There are several ways to sharpen drill bits, but the two most common are a drill bit sharpener, and a bench grinder.
Drill bit sharpening tools come in lots of styles, but they all have some way to guide the bit into a sharpening stone at the correct angle. This regrinds the cutting edge as closely as possible to its original bevel, and will give the longest useful life for the bit
This is great, but sometimes not practical.
When drilling holes for alarm wiring, you’ll often hit nails. This will
usually chip off a big chunk of the cutting edge, making the bit fairly
useless until it’s sharpened. The ready-made drill bit sharpeners are
accurate, but they can be slow. It can take quite a while to grind off
enough steel to create a fresh cutting edge.
A regular 6- or 8-inch bench grinder is a quick method to sharpen drill bits. The idea is not to spend lots of time trying to restore the drill bit to 100% perfection. Instead, we want to get it back to around 85-90% of its original condition, in a much shorter time.
We just need to do two things:
First, we need to grind a new, sharp cutting edge. This will allow the bit to cut efficiently, with less effort and less heat generated.
Second, we must remove enough metal from the heel of each cutting face so that the cutting edge meets the material first. This is called the “relief”.
It takes some practice to sharpen a flexible drill bit free hand on a grinder. But, once you get the hang of it, it’s fast, especially if you have several bits to sharpen.
See more about How to Sharpen Drill Bits Using a Bench Grinder.
In the field, I’ve also used a 2-inch grinding wheel chucked into a cordless drill. You can touch-up the cutting edges of a drill bit, and then continue working. This is good if you only have a small selection of bits, and you’ve dulled one up that you really need.
Flexible drill bits with a threaded pilot are not easy to sharpen using a grinding wheel. For these, the best method is hand filing with a diamond file. For more information, see The Best Ways to Sharpen Drill Bits.
Drilling with gentle pressure as I’ve described is easier than forcing the drill. But, it also means the drill motor will be running a lot longer per hole drilled. A corded 110-volt drill is the only way to get this kind of sustained power.
I do recommend using a cordless drill and small drill bits to start holes. Then, switch to the corded drill and a long drill bit to finish.
The shorter bit is much easier to control than a long flexible drill bit. Drilling all of the holes to prewire an average-sized house requires more endurance than most battery-powered drills can offer. If you try to attempt this, what usually happens is that the battery begins to wear down after just a few holes.
You may switch to the second battery, placing the first in the charger. Soon though, the second battery is out of juice, and the first one still isn’t done charging. So, you try switching to the lower speed setting, or just end up taking a break. Either way, the battery-powered drill will usually fail to finish the job.
Meanwhile, a typical corded drill would have had the drilling mostly done in the same amount of time. For specific tips on drilling holes and prewiring, take a look at Using Long Drill Bits for Alarm Wiring in Doors
Don’t get me wrong, I love cordless drills, and own several. They are at their best for light drilling, and when portability is the most important consideration. I use them all the time for starting holes and driving screws. They are also good for drilling a few holes with smaller drill bit sizes, such as drilling holes for magnets.
As you drill, wood chips are created rapidly, and can get packed into the flutes of the drill bit. This will bind up the cutting head of the bit, causing increased friction. This added friction causes the bit to heat up, slows the drilling speed, and makes the drill motor work harder.
Avoid this by clearing the chips after every few inches of progress, by withdrawing the bit completely from the hole. This allows the cutting head to do its work in the most efficient way possible.
Once you’ve gone through the frame of a door or window and reached the hollow wall cavity, you’ll be drilling through the top plate next. You can clear chips from this second hole by just pulling the bit back far enough to clear the top plate. It’s not necessary to pull it completely out of the wall.
After drilling just a few holes, you’ll develop a feel for what the flexible drill bit is doing. Stay in tune with that, and your job will be much easier.
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