Tech Talk
What is involved in "Honing of a Cutting Tool"?
There are many variables to consider when trying to optimize a metal-working process. In the simplest terms, you have to squeeze as much productivity as possible out of every aspect of the process. That includes the reconditioning cutting tools. When it comes to tools and optimizing a process, one question must be answered: Are the tools being fully utilized? If hand honing isn't part of your tool-reconditioning program, the answer is no. Tool life, which is intimately tied to part quality and productivity, can be increased through the proper selection and application of hones.
The first step in tool honing is stone selection. Matching the stone to the job is of paramount importance. A compatible match is as essential as selecting the correct wheel for a tool-grinding operation. In fact, the forethought required for both honing and tool grinding is similar. Consideration must be given to tool geometry and material and to the degree of abrasiveness needed for the job. Hones are available in a multitude of shapes, sizes and abrasive types. The vast assortment can seem overwhelming to the beginner. But once the selection process is understood, the best choice soon becomes obvious.
Cutting tools also come in many shapes and sizes. A tool's basic geometry usually consists of either planar or curved surfaces. Even special shapes are a combination of these two geometries. Stone shapes for planar surfaces include square, rectangle, triangle, diamond or any combination of these shapes. Curved surfaces require stones that are round, half-round or concave.
The geometry of the tool's cutting edge will determine the best stone for the job. A good stone for the 1/4" -radius form tool would be round, have a 1/2" diameter and be made of a medium-grit aluminum oxide. Generally, if the surface area of the cutting edge is large, select a large stone. Choosing the appropriate abrasive is the next task. The most common hone materials are AI2O3, silicon carbide, diamond and Arkansas (hard quartzide).
Most cutting tools are manufactured from either HSS or carbide. The old school of thought reserved diamond hones for carbide tools but the real determining factor in abrasive selection is the required rate of material removal. Expect a rapid rate of material removal when using a diamond hone on HSS cutting tools. The next step is choosing the grit size. The determining factor in grit-size selection is required surface finish. Finish improves as the grit number increases. The finer the surface finish required for the cutting edge, the longer it will take to hone the tool. Every tool doesn't require a superpolished cutting edge, so choose a stone with the appropriate grit.
After selecting the best stone for the job, the next step is applying it to the tool's cutting edge. Successful honing largely depends on two things: holding the hone at the proper angle and moving it in the correct direction. The angle of the stone must match the angle of the surface being honed. So, if you are honing a 7degree primary angle, the hone must be held at 7 degrees as well. The hone should always move in the direction of the cutting edge. That is, it should move from the relief surface to the cutting surface.
It is essential to keep the stone flat on the surface to be honed. Consistent pressure between the stone and the surface being honed is equally important. The original angle will begin to change if this pressure fluctuates. In the worst-case scenario, the cutting edge will be dulled. A fluid lubricant must be used during the honing process. Even an oil-filled stone requires an external lubricant. Oil is usually the best choice. It removes swarf from the work area and enhances the sharpening process. Without a lubricant, the honing stone will load. And, as with a loaded grinding wheel, it will degrade the operation.
Poor honing practices sabotage rather one stone to the cutting edge. It takes the honing process a step beyond deburring. A coarse-grit stone is used for roughing, followed by a fine-grit stone. The best guide for selecting stones is experience, which can be gained by testing different combinations on obsolete or scrapped tools.
Superfinishing reduces the microscopic irregularities found on all cutting edges. During the cutting action, the chip slides against the tool, and the tool slides against the workpiece. The friction caused by the sliding action is the tool-wear mechanism. Friciton is a function of surface finish, so improving the finish will decrease frictional resistance. That, in turn, will increase tool life and improve workpiece quality.
Honing can improve operations in other ways, too. For example, a 400-grit diamond hone can be used to add a special land to a carbide tool to strengthen its cutting edge. This would be done if the carbide grade were too hard for a specific job, if the workpiece surface were too rough (scale on the raw stock) or if the application called for an interrupted cut. The width of the land should not exceed 0.005" and must be at a 45 degree angle to the cutting edge.
Learning to hone correctly takes patience and practice. Begin with less aggressive stones, apply light pressure and inspect your work frequently with an optical comparator. After gaining experience, move on to more aggressive stones, increase the work pressure and visually inspect the tool. (A correctly prepared cutting edge will not reflect light). The learning process can also be shortened by talking to hone manufacturers. They have a wealth of knowledge that they will be glad to share. When executed correctly, the honing operation will enhance overall cutting performance. Additionally, tooling costs will decrease and the number of parts produced per tool can rise by as much as 20 percent.
