How to sharpen a square hollow chisel for mortising?

Are your mortising bits getting dull?

Aaron Barnett of BangingToolbox suggests: Whether you own a drill press or a mortiser, your machine becomes useless in cutting square holes when the hollow chisel and auger bits get dull.

These bits are so efficient in cutting square holes in the wood.

However, the bits face extreme pressure when cutting, which wears their cutting edge over time.

So if you do not have spare mortising bits at hand, the only way you can bring them back to life is to sharpen the edges. But with the extraordinary mechanism, the idea of sharpening the bits may at first seem impossible.

Actually, you have many options to sharpen a hollow chisel and an auger bit.

Signs that the hollow chisel and auger bits are dull

The first sign your hollow chisel and auger bit needs sharpening is when it cannot make clean and fast cuts.

You’ll have trouble cutting through when the edges are burred. And if it successfully cuts through, you’ll surely get jagged cuts.

So you need to check your hollow chisel and if you find burnt areas on the sides of the tips, keep reading.

This event happened because the chisel tried hard to make a cut, increasing the temperature of the chisel bits and making the edge more blunt.

The same happens when wood chips cannot eject from the chisel slot. The mechanism tries hard to push them out. So the tips’ heat increases, burning the sides of the chisel bit.

If you experience any of this when cutting mortises, your mortising bits are dull, and the sooner you sharpen, the better!

Step 1. Hone the bevel of the hollow chisel.

The bevel on the cutting edge makes the cut.  While you might not know the exact angle measurement, you can still find a few sharpeners that can sharpen hollow chisels from the inside. The most important step is to see which cones match the bevel angle.

Good thing most manufacturers of sharpening cones know this figure.

Diamond sharpening cone

One of the most common ways to hone the bevel is using coarse and fine-grit diamond cones. Apply lubricant on the cone before sharpening. 

If your force is not enough to remove material when sharpening manually, you can attach the sharpening cone to your drill press and set it to a slow speed of around 250 RPM.

Make sure to secure your hollow chisel to a vice grip.

Start with the coarse grit and finish with the fine grit. Make sure both are lubricated. Sharpen until you find the burrs raising on four sides inside the hollow chisel.

Cone-shaped grinding attachment

You can also use a cone-shaped abrasive grinding wheel attachment to hone the bevel of a hollow chisel. Apply lubricant or water.

Simply attach the grinding wheel to your drill press and set it to low speed, 250 RPM or more but less than 500 RPM.

Let it run on the bevel until you see a burr coming off the edge.

A triangle file

If you do not have a drill press, you manually sharpen the bevel of a hollow chisel with a handy file.

Apply only light pressure at gliding motion, following the shape of the bevel.

Maintain a light and controlled motion until you feel burrs coming out from each edge.

Step 2. Flatten the edges of the hollow chisel.

Since hollow chisels are crucial in cutting square holes, you must ensure that the chisel has uniform cutting edges.

But if the hollow chisel is already blunt, the edges it used to make uniform square cuts may already be out of shape.

Of course, you need equal lengths on all four sides when you’re done and attach them to your drill press or mortiser.

Flattening the cutting edge enhances the efficiency of the hollow chisel.

Also, this measure ensures you have a stable and consistent starting point for sharpening and honing.

There are many tools you can use for flattening the edges.

Fine diamond sharpening plate

A diamond sharpening plate smoothens and flattens the sides of the cutting edge.

Pair it with water to lubricate the surface.

And using one finger to push the chisel and the other hand to hold the chisel, sharpen all sides by pushing away and pulling in the chisel one by one in one direction.

Ultra sharp Diamond Whetstone

Adhesive backed Sandpaper

If you do not have a sharpening plate, you probably have adhesive-backed sandpaper lying around aye!

Look for a fine 220-grit sandpaper and secure it on your working table.

Rub all the sides against the sandpaper equally and in one direction for about 10 to 15 strokes. Apply light and controlled pressure and see the progress from each side. 

Remember that the hollow chisel will become smaller after flattening the sides, which may affect its structural integrity. You can use this step only once with your hollow chisel.

Whetstone or oil-stone

Whetstone or oil stone is another option to flatten the sides of a hollow chisel.

Use honing oil for oilstone as a lubricant and apply the same honing directions for diamond sharpening plates.

Step 3. Sharpen the auger bit.

Now that you’re done with the hollow chisel, you can turn to sharpen the auger bit.

Needle file, triangle file, or auger bit file

You will only sharpen the tip that pierces first on the wood.

Start sharpening with an auger bit file, then sharpen the tips around with any of these files. Avoid altering the auger’s shape so it conforms to the original way it cuts.

MAGBIT Auger bit file

Bonus tip!

You can polish the flutes of an auger bit with a buffing wheel attachment. 

And for the hollow chisel, you can use a cylinder grinding or sandpaper attachment to polish it inside the hollow chisel.

This additional measure helps the mortising bits eject the wood chips smoothly!

How many times can you sharpen your mortising bits?

The amount of times you can sharpen a square chisel depends on different factors: the chisel’s condition, the amount of material that gets removed, and the thickness of the cutting edge.

You can flatten the edges a few times, but it may affect the structural integrity of the chisel and alter the results of the chisel.

You don’t want your work to shatter into pieces when you use the bits on a mortising machine.

You can minimize this by honing the bevels frequently as part of maintenance.

This way, the hollow chisel is constantly in good sharped condition. You do not have to push harder and cause burnt marks on the edges.

If sharpening is not possible, you can always mortise with a hand chisel. There are many tool options to cut square holes in wood.

Check your toolbox and see if you have something else while you wait for a new set of durable mortising bits.

You need mortising bit sets that cut the square smoothly, and you might find it easier to buy a new set.