Wheel bond systems can be divided into two types: those holding a single layer of abrasive grain to a solid steel core, and those providing a consumable layer many grains thick with the abrasive held within the bond. The latter may be mounted on a resilient core or produced as a solid monolithic structure from the bore to the outer diameter. This chapter deals with the different types of bonding structures employed in grinding wheel design and the effects on wheel performance.
Single-layer wheels are generally limited to superabrasives because of the economics of wheel life. They can be subdivided into electroplated wheels fabricated at essentially room temperatures, and brazed wheels fabricated at temperatures as high as 1,000°C. The following discussion applies in general to plated cubic boron nitrides (CBN) and to plated diamond wheels, although in practice CBN dominates the precision grinding market and is the central focus below.
6.3 ELECTROPLATED (EP) SINGLE-LAYER WHEELS
6.3.1 Structure of an EP Layer
Electroplated wheels consist of a single layer of superabrasive grains bonded to a precision- machined steel blank using nickel deposited by an electroplating or occasionally electroless plating process. The plating depth is controlled to leave about 50% of the abrasive exposed (Figure 6.1).
6.3.2 Product Accuracy
The accuracy and repeatability of the process is dependent on many factors. The blank must be machined to a high accuracy, and the surface prepared appropriately and balanced; ideally, blank-profile tolerances are maintained to within 2 pm and wheel runout maintained to within 5 pm [McClew 1999]. The abrasive is generally resized to provide a tighter size distribution than that used in other bond systems. This is to avoid any high spots and better control grain aspect ratio. The abrasive is applied to the blank by various proprietary methods to produce an even and controlled-density distribution. For tight tolerance applications or reduced surface roughness the wheel may also be postconditioned (also termed “dressing,” “truing,” or “shaving”) where an amount equivalent to approximately 5 to 7% of the grit size is removed to produce a well-defined grit protrusion height above the plating. With good control of plate thickness this helps to control and/or define a usable layer depth.