How to sharpen a spade bit
There are several methods that can be deployed when sharpening a spade bit, depending on the tools that you have available, whether or not your bit has spurs, and the material from which your bit is made.
Whichever method you use, it's important to remember that you should never remove material from the outside edges of the spade bit, or you will risk changing its diameter.
Method 1 - Using a file
Tool steel spade bits can be sharpened with a file, but if your bit is made of HSS (high speed steel) then you will need to use a diamond file for it to have any effect. If your bit has spurs, you will need to choose a file that will fit between centre point and spur - a flat Swiss file or an auger bit file should do the trick. (See.)
Step 1 - Sharpen lips
Lay your file so that it's flat against the bevelled (angled) edge on one of the lips of your spade bit. Keeping the file at the same angle, sharpen it. Three or four strokes with the file should be enough. Rotate your bit 180 degrees and repeat this process with the other lip.
If you use a file with a rectangular cross section then you will be able to sharpen the lip of your spade bit without touching the spur or centre point. This means you can avoid any accidental damage.
Step 2 - Sharpen centre point
One edge on each side of the triangular centre point of your spade bit should have a bevelled edge. Matching the angle of your file to the angle of this bevel, sharpen the cutting edge of the centre point, then repeat this process for the corresponding edge on the other side of the bit.
Step 3 - Sharpen spurs
If your spade bit has spurs, the inside edges of these should now be sharpened, once again matching the angle of your file to the angle of the bevel on each one.
The spurs should not need to be sharpened as often as the other parts of the bit. If they are razor sharp, there is a strong chance that they will 'grab' the workpiece. This means that they will become stuck, and cause the workpiece to try to spin free of the clamp.
Method 2 - Using a bench grinder
This method can be difficult if your bit has spurs, as the wheel on your bench grinder is likely to be too wide to sharpen the lip without grinding off the spur in the process.
Grinding off the spurs will not render your bit useless, although it will make it bore rougher holes. Deciding whether or not to use this method will depend on whether you will be using your spade bits on a project where neatness is important.
If your spurs do become a casualty, you can adjust the angle of your bit against the grinding wheel, shortening the centre point, but grinding the bit's edges so that they slope towards the centre. This will allow your bit to cut almost as neatly as if it had spurs. However, you'll need to be careful to make sure that you don't grind your centre point off centre.
Step 1 - Grind off spurs and sharpen lips
Adjust the tool rest on your grinder (the piece of metal in front of the grinding wheel) so that it matches the bevel on the side of the spade bit you're going to be sharpening. Grind off the spurs until you can touch the lip of the bit against the wheel a few times.
Step 2 - Sharpen centre point
Paying attention to the bevel on the edge of the centre point, and trying to match it, sharpen your centre point with a few touches against the grinding wheel.
Your bit is now ready to go!
Method 3 - Using a drill sharpening machine
Some automatic drill bit sharpening machines include a function for sharpening spade bits.
This is the quickest and easiest way to sharpen your spade bits, although more expensive than using a file. The machine will come with instructions that will guide you through the sharpening process, although this usually involves inserting the bit, activating the machine and then watching as the hard work is done for you!