power assisted tool grinding
bill tindall
>There are a variety of machines for rapid metal removal for tool sharpening and shaping- the traditional vertical wheel bench grinder with and without speed reduction, horizontal wheel grinders, extra slow speed grinders with water cooling, and coated abrasive disk and belt machines. Some ingenious arrangements have been recently posted for tool rests for belt grinding. Two of the stated goals of all these machines is to provide cooler grinding and improved finish over the traditional vertical wheel bench grinder. I will show below that the traditional bench grinder with the proper wheel is superior to all these alternatives for shaping tools as well as edge preparation prior to honeing. A more complete version of this information was submitted to WC Articles and awaiting posting.
Coolness of grinding- A sharp abrasive particle mills a sliver of metal from the tool and much of the heat resulting from the work to generate this sliver leaves with the sliver (the shower of sparks from a properly cutting wheel). A dull grit ploughs across the tool surface leaving much of the heat deposited in the tool. So, the key to cool grinding is sharp abrasive particles.
Three wheel properties are most important to achieve cool grinding.
Particle Friability- Different grinding procedures are done with more or less pressure of the tool on the wheel. Tool shaping could involve heavy pressure on the wheel. On the other hand, little metal needs to be removed for edge preparation prior to honeing, so this operation is done with very light pressure. This pressure must break small pieces from the abrasive particle to continuously expose fresh cutting edges. Obviously, a very friable particle is necessary to break under light grinding pressure. Most commercial grinding involves moderate to heavy pressure so it follows that the proper grit friability for finish grinding is uncommon, but such grits are readily available in surface grinder wheels.
Grit hardness- All common steel grinding wheels are aluminium oxide or a modification of this material. At Rc 60+, tool steels are the hardest steels commonly ground. Again, the common wheel grits, white for example, are not optimum for tool steels, but there are abrasive particles formulated expecially for steels like we use in tools, for example blue, pink and ruby.
Wheel bond- Fracture of the abrasive particles to expose new edges can't go on for ever. Eventually it is desirable to be rid of worn grits and expose new ones. Exposing new grits is accomplished by formulating the adhesive the holds the grits together to form the wheel such that grits break off at some rate as pressure is applied to the wheel. Softer wheels expose grits at a faster rate than harder wheels (here soft and hard refer to the wheel bonding material not the particle hardness).
Summary- With a friable particle, a hard particle and a soft wheel bond, sharp particles can be made continuously available for cutting the steel at light grinding pressure. As a result these wheels run cool. One can grind right to the edge of a chisel even at 3600 rpm and there will be no burning of the edge. Material removal is rapid, perhaps too rapid at 3600 rpm for inexperienced hands. A slower speed grinder provides more control. A proper wheel cuts so fast that there may be no practical advantage to a normal speed grinder.
Coated abrasives, for example belts and disks, are at a disadvantage over wheels. Coated abrasives are formulated for heavy grinding pressure so their particles don't easily fracture to expose new cutting edges, a problem with light pressure grinding. And, there is but one layer of particles. When this layer is dull there are no fresh, sharp particles underneath to take their place. Under equivalent conditions (speed and pressure) coated abrasives can not grind as cool or fast as a proper wheel.
Finish- There is no need to grind finer than 60 grit prior to honeing. The edge of a chisel, for example, can be ground to within a wisker of the edge. If the tool is ground to the edge but not beyond, the initial straightness of the edge remains. So there is no disadvantage of a 1" wheel over a wider abrasive format. A few strokes on the popular water stone, or other honeing format, quickly removes enough metal to establish a fresh cutting edge because the hollow grind left by the wheel means only a tiny amount of metal needs to be removed at the edge. In any case, most common honeing stones rapidly remove 60 grit grinder marks.
In summary, a bench grinder with a proper wheel is cheap, fast, cool, easy to master and it leaves an adequate finish prior to honeing. I would add that a good tool rest, homemade or the Wolverine, is essential for tool grinding. If your grinder doesn't grind cool, you have the wrong wheel for the job at hand.
A cheap Delta variable speed grinder, Wolverine tool rest, and a 1" Norton G 25A pink 60 grit wheel is but one of many suitable assemblies that will meet the need for tool sharpening and shaping. In any case the wheel should be described as suitable for light material removal (light grinding pressure) and hard tool steels. Manufacturers literature will provide this information.