Honing (metalworking)
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Honing is a manufacturing process that produces a precision surface on a workpiece by scrubbing an abrasive stone against it along a controlled path. Honing is primarily used to improve the geometric form of a surface, but may also improve surface texture.
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[edit] Honing Stones
Honing is classified as an abrasive machining manufacturing process. As with all abrasive machining processes, material is cut away from the workpiece using abrasive grains. In the case of honing, the grains are bound together with an adhesive to form a honing stone (or hone). Generally, honing grains are irregularly shaped and about 10 to 50 micrometers in diameter (300 to 1,500 mesh grit). Smaller grain sizes produce a smoother surface on the workpiece.
A honing stone is similar to a grinding wheel in many ways, but honing stones are usually more friable so that they conform to the shape of the workpiece as they wear in. To counteract their friability, honing stones may be treated with wax or sulfur to improve life; wax is usually preferred for environmental reasons.[1]
Any abrasive material may be used to create a honing stone, but the most commonly used are corundum, silicon carbide, CBN or diamond. The choice of abrasive material is usually driven by the characteristics of the workpiece material. In most cases, corundum or silicon carbide are acceptable, but extremely hard workpiece materials must be honed using superabrasives.[1]
[edit] Process Mechanics
Since honing stones look similar to grinding wheels, it is tempting to think of honing as a form of low-stock removal grinding. Instead, it is better to think of it as a self-truing grinding process.[2]
In grinding, the wheel follows a simple path. For example, in plunge grinding a shaft, the wheel moves in towards the axis of the part, grinds it, and then moves back out. Since each slice of the wheel repeatedly contacts the same slice of the workpiece, any inaccuracies in the geometric shape of the grinding wheel will be transferred onto the part. Therefore, the accuracy of the finished workpiece geometry is limited to the accuracy of the truing dresser. The accuracy becomes even worse as the grind wheel wears, so truing must occur periodically to reshape it.
The limitation on geometric accuracy is overcome in honing because the honing stone follows a complex path. In bore honing for example, the stone is moves along two paths simultaneously. The stones are pressed radially outward to enlarge the hole while they simultaneously oscillate axially. Due to the oscillation, each slice of the honing stones touch a large area of the workpiece. Therefore, imperfections in the honing stone's profile cannot transfer to the bore. Instead both the bore and the honing stones conform to the average shape of the honing stones' motion, which in the case of bore honing is a cylinder. This averaging effect occurs in all honing processes; both the workpiece and stones erode until they conform to the average shape of the stones' cutting surface. Since the honing stones tend to erode towards a desired geometric shape, there is no need to true them. As a result of the averaging effect, the accuracy of a honed component often exceeds the accuracy of the machine tool that created it.
The path of the stone is not the only difference between grinding and honing machines, they also differ in the stiffness of their construction. Honing machines are much more compliant than grinders. The purpose of grinding is to achieve a tight size tolerance. To do this, the grinding wheel must be moved to an exact position relative to the workpiece. Therefore a grinding machine must be very stiff and its axes must move with very high precision.
A honing machine, ironically, is relatively inaccurate and compliant. Instead of relying on the accuracy of the machine tool, it relies on the averaging effect between the stone and the workpiece. In fact, compliance is a requirement of a honing machine that is necessary for the averaging effect to occur. This leads to an obvious difference between the two machines: in a grinder the stone is rigidly attached to a slide, while in honing the stone is actuated with pneumatic or hydraulic pressure.
High-precision workpieces are usually ground and then honed. Grinding determines the size, and honing improves the shape.
The difference between honing and grinding is not always distinct. Some grinders have complex movements and are self-truing, and some honing machines are equipped with in-process gaging for size control. Many through-feed grinding operations rely on the same averaging effect as honing.
[edit] Honing Configurations
- Track/Raceway honing
- Spherical honing
- OD through-feed honing (taper and straight)
- Flat honing
- Bore honing
[edit] Comparisons to Grinding
Superfinishing is more expensive than grinding. Superfinishing has lower cutting efficiency because of smaller chips. Superfinishing has lower material removal rate. Superfinishing stones are softer and wear more quickly. Superfinishing stones don't need to be dressed. A superfinishing machine must move the stone in a compound or orbital motion relative to the part surface. In honing, the contact area between the abrasive and workpiece are larger than in grinding.[3]
[edit] Comparisons to Lapping
Unlike polishing, superfinishing can improve the geometric form of an object. The primary purpose of polishing is to improve surface finish without concern for form.
In many cases, the terms "honing" and "superfinishing" can be used interchangeably. Superfinishing is generally used more broadly, and may also be used to describe mass finishing, polishing, or lapping processes.
[edit] Economics
Although honing is a high precision process, it is also relatively expensive. Therefore it is only used in components that demand the highest level of precision. It is typically the last manufacturing operation before the part is shipped to a customer. The dimensional size of the object is established by preceding operations, the last of which is usually grinding. Then the part is honed to improve a form characteristic such as roundness, flatness, cylindricity, or sphericity.[3]
[edit] Performance Advantages of Honed Surfaces
Since honing is a relatively expensive manufacturing process, it can only be economically justified for applications that require very good form accuracy. The improved shape after honing may result in a quieter running or higher precision component.[3]
[edit] See Also
[edit] Notes
- ^ a b Schibisch, Dirk M.; Friedrich, Uwe (2002). Superfinishing Technology. Germany: verlag moderne industrie, 53-58.
- ^ King, Robert C.; Hahn, Robert (1986). Handbook of modern grinding technology. New York: Chapman and Hall, 301-336. ISBN 0-412-01081-X.
- ^ a b c Swigert Jr., Arthur M. (1940). The story of superfinish. Ann Arbor, MI: The Ann Arbor Press, 575-594. OCLC 568009.
