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How to sharpen an expansive bit?

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Diagram showing some of the locations on an expansive bit that may need to be sharpened There are several different parts of an expansive bit that must be kept sharp for the tool to work efficiently: the guide screw, the lips and the cutters. Each requires a different approach.

Guide screw

A tub of valve grinding paste which is used to sharpen the thread of the guide screw on an expansive bit When sharpening a guide screw, the best results can be achieved using grinding paste, a compound containing abrasive particles that are designed to wear down imperfections in metal surfaces that are difficult to reach.
DIYer making a hole in a piece of wood with the guide screw of the expansive bit

Step 1 – Create reservoir

Using the screw of the bit you would like to sharpen, bore just far enough into a piece of softwood for the guide screw to fully enter the wood, then remove the bit.

DIYer collecting grinding paste on a toothbrush so that it can be dabbed into the hole that was made with the guide screw

Step 2 – Apply grinding paste

Apply some fine grain grinding paste into the screw hole with the implement of your choice (you might use a toothbrush, a scoop or even a lollipop stick).

Turning a guide screw in a hole containing grinding paste to sharpen the thread

Step 3 – Grind screw sharp

Reinsert the guide screw into the bore hole and twist it backwards and forwards. This will cause the abrasive particles in the grinding paste to remove any dirt and sharpen the edges of the thread.

Rinse off the grinding paste using water if the packaging says it is water soluble

Step 4 – Clean off excess paste

Remove the bit from the bore hole and clean off any paste left on the screw. Check the packaging for the grinding compound to see if it’s water soluble. If it is, you can rinse it with water.

A can of water displacing oil that can be used to clean non-water soluble grinding paste off an expansive bit If not, you will need to spray water displacing oil onto the guide screw before wiping it with a cloth.

Body

An auger bit file which is used for sharpening the lips and spurs on an expansive bit Sharpening the tip and cutting edges of an expansive bit is a similar process to sharpening the tip of an auger bit. As such, you will need an auger bit file. These tools are specifically designed for sharpening right into the corners of the narrow lips and spurs of wood bits without wearing down any other part of the tool.
Sharpening the upper primary lip of an expansive bit with an auger bit file

Step 1 – Sharpen lip

Sharpen the top of the primary lip with the file. A few passes should be enough to revitalise the edge. Be sure not to sharpen the bottom of the lip, or this will lift the cutting edge of the lip too high for it to be of any use.

Labelled diagram showing the location of the primary spur on an expansive bit

Step 2 – Sharpen spur

Repeat this process on the inside of the primary spur. Be sure not to sharpen the outside of the spur, as this will change the profile of your tool and prevent it from boring even holes.

Cutter

A custom wet stone for sharpening the lip and spur of an expansive bit The adjustable cutter is a little more complicated to sharpen than the rest of the tool, and requires a specially prepared wet stone.
Example of a wet stone that can be customised to sharpen an expansive bit

Step 1 – Prepare wet stone

In order to sharpen your outrigger cutter properly, you will need to customise a wet stone by rounding off one of the edges.

A bench grinder which is used to round the corner off one side of a wet stone to create a custom sharpening tool for expansive bits This can be done on a bench grinder, a machine that rotates abrasive wheels at high speed to shape and smooth down objects made from a wide range of different materials.
Check your wetstone for lumps and bumps by gently running it across the curved surface of your adjustable cutter Once you have rounded one of the edges of the stone, run it across the concave surface of the cutter to find any bumps. If there are, return them to the grinding wheel and grind them down.
Image to illustrate the motion of drilling complete circles with a spoon bit once the seating process is complete Repeat this process until there are no remaining bumps.
Sharpening the outrigger cutter lip on an expansive bit with a custom shaped wet stone

Step 2 – Sharpen cutter

With your wet stone secure on a solid surface, locate your cutter so that the concave surface is resting against its curved edge. Push your blade forwards and backwards against the stone, applying light pressure and wetting it occasionally by dipping it into a small container of water and shaking off the excess. The grit of the stone will grind the edge of your cutter sharp.

Check your wetstone for lumps and bumps by gently running it across the curved surface of your adjustable cutter After sliding your blade along the wet stone around twenty to thirty times, your blade should be sharp. Check it regularly during the sharpening process just in case you need to sharpen it a little more.

Do not sharpen the bottom (flat surface) of the cutter, as this will impair its ability to evacuate waste wood by lifting the cutting blade out of contact with the material.

Sharpening the inside of the primary spur on an expansive bit with an auger bit file

Step 3 – Sharpen spur

Just as you did for the primary spur on the body of the bit, sharpen the inside of the cutter spur using an auger bit file.