Sharp, sharp!

Having the right cutting or drilling tools for a job is all very well, but if they are not sharp, the simplest of tasks will leave you frustrated.

 

By Roelof Strydom

 

If you’ve ever tried drilling a hole with a blunt drill bit or planing a tabletop with a blunt blade in the planer, you will know the frustration this leads to. As a woodworker, metalworker or hobbyist, your focus is usually on the work that needs to be done and it is often assumed that the tools required to do the work will perform perfectly every time. 

In fact, they can only withstand the battering for a certain period of time until they succumb to the inevitable.

 

If you’ve ever tried drilling a hole with a blunt drill bit or planing a tabletop with a blunt blade in the planer, you will know the frustration this leads to. As a woodworker, metalworker or hobbyist, your focus is usually on the work that needs to be done and it is often assumed that the tools required to do the work will perform perfectly every time. In fact, they can only withstand the battering for a certain period of time until they succumb to the inevitable.

 

Razor sharp tools are a joy to use, but it can be a bother to keep them that way. It is a good idea to set aside a day specifically for the sharpening of drill bits, plane blades, chisels, knives and router bits.

 

Sharpening in general

The list of sharpening methods are endless and includes oilstones, diamond stones, water stones, ceramic stones, sandpaper, grinders, wet and dry as well as slow and fast spinning wheel systems.

 

Each of these systems has its advantages and disadvantages; some are less messy than others, some require more skill, and then there is the price factor. You can buy the more affordable oilstones or diamond stones, but these require some skill on the operator’s side as a steady hand is needed. The other option is to spend a bit more money and buy something like the Tormek Water-Cooled Sharpening System, which is a quick and easy way to sharpen tools as everything is guided by jigs.

 

At the end of the day, the two most important qualities a sharpening system needs is speed and, of course, the ability to produce a really sharp edge.

 

What is a sharp edge?

In simple terms, a sharp edge is two flat, polished surfaces intersecting at an angle that will cut cleanly. Anything less than that is not sharp. Before sharpening a tool you need to consider whether it is a new factory-sharpened edge, an old rusted edge or a well maintained edge that merely needs some honing.

 

New tools have an edge that was created by coarsely grinding the surfaces, leaving behind a series of evenly spaced, coarse scratches in the cutting edge. Under magnification these scratches are seen as furrows in the metal and each intersects the cutting edge and creates a dip. This row of dips looks like the teeth on a saw. Even though this is considered sharp, more effort is required to cut with an edge like this and it will become blunt more quickly.

 

A worn edge has nicks and rust brought about by continued use. An edge like this requires a lot more force to work with, which is inefficient and leads to frustration. Nicks are similar to the furrows as described above, but are unevenly spaced and are usually bigger.

 

Sharpening is all about grinding and polishing the two intersecting cutting edges to as fine a point as possible.

 

For new as well as worn chisels, and plane blades that have never been sharpened before, there are three sharpening steps that need to be taken.

 

First, polish the flat backside of the tool, the side that doesn’t have the bevel. Rub the backside back and forth across a medium-grit stone or sandpaper. The backside should look like a mirror when you’re done. If you have a Tormek, you can use the flat side of the wheel.

 

Then grind the cutting bevel of the tool on an oilstone or diamond stone. If you have a Tormek, you’ll grind the entire area of the bevel on the 220-grit stone, and then change the stone to 1 000 grit by means of the stone grader and regrind the bevel if a smoother finish is required.

 

Finally, hone a small part of the cutting bevel to remove the burr. Honing can be done on a very fine stone or, in the case of the Tormek, on the leather honing wheel with some honing compound. These three steps are only for new and worn tools.

 

Once you’ve done these three things, maintaining the edge is much easier. In most cases you only need to regrind the cutting edge when you’ve hit a nail or dropped the tool.

 

Testing for sharpness

Pull your thumbnail across an edge at a 90 degree angle. If the edge catches and digs in immediately, it’s sharp, but if it skids across your nail, you have some more sharpening to do.

 

Bevel angles

* Wood chisels

These normally have a 25 degree bevel angle. When working with soft wood or if you require a finer finish, a bevel angle of 20 degrees can be used. For hard wood the bevel angle needs to be increased to 30 or 35 degrees. A smaller angle will cut more easily, but will become blunt much quicker.

 

* Hand plane blades

If the frog is at 45 degrees, the angle of approach is always 135 degrees. This means that the maximum angle you can sharpen the blade to is 44 degrees. If the blade is sharpened to 46 degrees or more, the heel of the bevel, rather than the sharp edge, will contact the wood and result in no shavings as you plane a surface.

 

Different angles will affect the performance of the plane. Sharpening the blade to 35 degrees is considered the normal angle and works well with most woods. The only time you need to sharpen a blade to a different angle is when you plane extra hard or extra soft wood.

 

For hard wood species like hard maple, a blade sharpened to an angle between 40 and 43 degrees will give you the best results.

 

For very soft wood species like white cedar, an angle between 25 and 30 degrees will produce a consistently smooth surface. A blade sharpened to an angle of less than 20 degrees is considered impractical.

 

Maintain the edge

After preparing and sharpening new or worn tools, they only need to be maintained. All tools will eventually become blunt and need to be sharpened again, but if you sharpened them correctly the first time around, maintaining them afterwards will be easy.

 

Tools dull because of friction and wear resulting from use. The sharp edge of the two intersecting surfaces rounds over and instead of shaving the wood, the rounded edge skates over the surface and greater force is required to engage the wood.

 

Test if an edge is blunt

Take the blade to a light source and examine it closely. Roll the edge in the light and you will see the rounded edge reflecting the light back to you. If the blade is sharp, the sharp edge will not be visible. When you see the rounded edge, it is time to sharpen again.

 

Tormek Water-Cooled Sharpening System

For a quick and simple way to satisfy all your sharpening needs, the Tormek system is ideal. This machine looks similar to a bench grinder.

 

On one side it has a ceramic bonded aluminium oxide wheel that spins at only 90rpm. The lower third of the stone is immersed in water and as it spins it pulls water up and over the stone for continuous cooling. T

 

he sharpening stone is 220 grit, but can be dressed to 1 000 grit in a matter of seconds by means of the grader. The grader has a fine and coarse side, so to change the grit from 220 to 1 000, hold the fine side of the grader against the stone for a few seconds. To reverse back to 220 grit, hold the coarse side of the grader against the stone.

 

The Tormek can sharpen knives (long and short), scissors, chisels, drill bits, hand and power plane blades, turning tools as well as axes. For each of these there is a specific jig, making sharpening easy.

 

Source

Pierre Grobbelaar, Cutting Edge Tools, 082-375-7386.