Tips on grinding plane irons and chisels
by Bill Clark & Larry Williams
Recently, many woodworkers have turned to white or pink grinding wheels for general purpose grinding. We believe this is a result of the influence of turners who, looking to go from the grinder to the lathe, prefer fine grinding wheels for their finished edge. Grinding plane irons and chisels, however, is or should be a rough shaping operation. We suggest you try a coarse grinding wheel similar to what likely was provided with your grinder before absorbing the expense of these specialty wheels. Chisels and plane irons should be finished with whatever honing techniques the woodworker is comfortable.
A visit to your local machine shop where grinding is a constant activity will reveal that machinists use coarse aluminum oxide grinding wheels for shaping normal tool steel. The white and pink wheels will generally be reserved for surface grinders. Surface grinding is the application for which these specialty wheels were developed and at which they excel. You'll also notice that there are special highly accurate techniques for dressing these specialty wheels.
dressing a grinding wheel
Well dressed coarse grinding wheels actually generate less heat than even the most expensive fine grinding wheels. A critically important accessory to any grinding system is a wheel dresser. A number of types of wheel dressers are available. Diamond dressers, one of the more expensive options, perform best where some type of accurate feed and contact adjustment are available. It can be easy to peel diamonds from the mountings while free-hand dressing coarse wheels. We advise the woodworker with limited experience to choose the less expensive star wheel dresser. The star wheel dresser is fast, aggressive, and will only cost in the ten dollar range. Carborundum dressing sticks are available but these tend to dull the freshly exposed abrasive.
We feel that a course grinding wheel in the 36 to 46 grit range is best for shaping the bevel on chisels and plane irons. Dressing will both true these wheels and keep the freshly exposed surface in good condition. A wheel that's been trued with the arbor will cut longer because the entire surface is used and the grinder will run smoother. The advantage of a smooth running grinder is that the operator feels more in control and confident.
Features of better grinders
High speed grinders have the advantage of ease of dressing and truing the grinding wheel. A dresser will tend to follow a wheel's irregularities at slow speeds. High speed grinders also remove more steel and make grinding easier.
One of the most important features of a grinder is the quality of the tool rest. Large tool rests give better support and are more versatile. The tool rest should be two-piece and made of cast iron or machined steel. This allows for more versatile double adjustments. A single angle adjustment can limit both angles and access to the wheel for shorter chisels and tools. Aluminum tool rests scar easily in the abrasive environment and can actually allow abrasives to be imbedded in their surface which will mar the surface of the tools they support.
Another important feature of a quality grinder is that the motor housing should be of a smaller diameter than the grinding wheels. This allows much better access to the wheels when grinding long tools such the side edge of a plane iron. Once some competence is gained in grinding this type of tool adjustment can become more frequent.
There are also some simple techniques which can further reduce the risk of overheating tool steel while grinding. Rough grinding should be done in two steps. The cutting edge of a tool should be established with the tool ground at 90 degrees to the tangent of the wheel or with the tool pointed directly at the grinder's arbor.
establishing an edge
This is done because thin steel can't transfer the heat generated in grinding to the body of the tool as fast as thicker steel. If you try to both shape the edge and shape the bevel, your tool's edge will likely be exposed to more heat than it can transfer and the excess heat will draw the hardness from the thin sections of steel. Establishing the edge by grinding at a right angle to the tangent of the wheel quickly and safely works the thin edge.
grinding the bevel
With the edge roughly established it is easier to shape the bevel or bezel of the cutting edge with less worry about heat at the edge. Finish grinding the bevel of the tool with a freshly dressed wheel and work more carefully to the edge after the initial shape is formed. The tool being ground shouldn't get hotter than a person can comfortably hold in their hand. A large metal surface near the grinder can act as a heat sink to draw excess heat from tools if several are ground in each grinding session.
Grinding wheels are available in a variety of densities. That's a topic that would best be addressed after the woodworker gains some experience with the above. Our main purpose here is to help woodworkers move in what we feel is the right direction with basic grinding.