Industries as diverse as aerospace, automotive, industrial heavy equipment and medical devices depend on precision grinding applications (surface, cylindrical, gear, creepfeed, centerless) to produce increasingly challenging geometric design parameters and surface finish requirements. Products including camshafts, crankshafts, medical pins, vanes, blades, blisks, bearings and gears are using materials that are more difficult to grind. In addition, the parts are becoming smaller and larger, the tolerances are more exact and the finishes required are getting finer. Grinding machine and wheel manufacturers are being asked to provide new technology and expertise in order to meet these needs.
The primary products for all of these precision grinding applications are vitrified grinding wheels (many with intricate face geometry) and segments (Figure 2.1).
Vitrified abrasive products have two major components, abrasives and bond (Figure 2.2). The abrasive is the cutting tool portion of the wheel while the bond holds the abrasive in the matrix to form a wheel or segment. The space between the abrasive and the bond is referred to as the structure or porosity of the product.
Abrasives Grain Overview
A substantial amount of work has been done over the last 20 years on the cutting tool portion of the wheel, the abrasive grain. Standard white aluminum oxide (Figure 3.1) (with selected color variations for off-white, pink, red) was the product of choice for over 50 years due to its very friable (sharp/easily broken down) nature. In 1990, Norton Abrasives introduced the first ceramic alumina abrasive, Norton SG (Figure 3.2). This new grain offered much higher performance in all areas of precision grinding as a result of a combination of the hardness of each ceramic alumina grain and the new science of “controlled micro-fracturing”. The ceramic alumina grain fractures at a controlled rate which provides two distinct advantages over aluminum oxide
- a constant and consistent supply of sharp cutting edges
- an increase in the utilization of each grain from 25% to 80%
The resulting benefits were a continuously sharp abrasive that cut up to 100% faster without burning very hard and difficult to grind materials and provided up to 300% life improvement due to the superior utilization of each grain.
Improved iterations of ceramic alumina grain were produced from 1990-2006. In 2007, Norton Abrasives introduced Norton Quantum ceramic alumina abrasive which was a substantial leap forward in grain technology and is the standard in the precision grinding industry for over six years.
Vitrified Bond Overview
The vitrified or glass bond is the material that holds or bonds the abrasive grain together. The process of firing precision wheels was invented by Norton Abrasives in the 1880’s as part of the industrial revolution. Process improvements and materials have changed to improve the performance and consistency of vitrified bonds, but the basic technology has remained the same. The image (4.1) shows a representation of the grain being held in a matrix by “bond posts”. The heavier the bond-to-grain ratio, the harder or stronger the wheel is and the more pressure needed to break down the bond to release new abrasive grain for cutting. The correct balancing of bond-to-grain ratio is necessary to provide the enough holding power for the grain to perform the stops cutting and starts to burn or damage the work. For some materials or in some wide contact areas, artificial media is introduced into the matrix which burns out during firing to leave a large porous structure (4.2) which promotes better coolant flow which retards the dulling of the grain and resulting burn.
Norton Vitrium³ - Revolutionary Grain Adhesion Science
This revolutionary bond platform features an exclusive chemistry that delivers an entirely new grain adhesion science, resulting in improved product versatility across a wide range of precision grinding applications.
The chemistry of holding the abrasive grain in the bond matrix for the precise amount of time is referred to as “grain adhesion science”. Norton Vitirum3 is a revolutionary new product formulation that substantially increases the module of elasticity or strength of the bond. This allows for less bond to be used to provide the same holding power on the individual abrasive grain. Figure 5.1 shows bond posts on a standard vitrified wheel. Figure 5.2 shows how a stronger bond can result in thinner bond posts.
Benefit - Cooler Cutting
An improved holding power utilizing less bond-to-abrasive ratio exposes a larger grain surface area which improves the standard bond-part interaction (Figure 6.1). The thinner bond posts provide reduced bond-part interaction and improved coolant flow for better chip clearance (Figure 6.2) which enables the wheel to provide:
- increased cut rate
- minimized heat build-up, reduced metallurgical damage
- lower power consumption
- reduced grinding forces on the part
- improved part quality
Benefit - Precise Profile Holding
A higher strength bond provides superior grain holding properties significantly improving wheel form and corner holding. This is especially important in applications requiring intricate part profiles (Figure 7.1) where the less frequent wheel dressing to regenerate profiles provides the following benefits are:
- better part geometry and integrity
- less downtime for dressing, increases productivity
- lower abrasive cost
- reduced dresser wear and dresser replacement cost
Benefit - Higher Operating Speeds
Precision machinery is increasing being produced with higher operating speeds to improve productivity. These higher speeds mandate manufacturers provide wheels tested and rated for the higher speeds in order to take advantage of the higher machine speeds. The Norton Vitirum3 bond provides the maximum wheel strength allowing softer grades to be qualified for high speed grinding. At the same grade, a Norton Vitrium3 wheel is 20% stronger (Figure 8.1). Higher operating speeds provide:
- faster the stock removal
- increased feed rates
- significantly increased throughput and production
Benefit - Improving Your Carbon Footprint
Norton Vitrium3 allows work with higher feed rates, speed and pressure, to significantly increase production while using fewer wheels. Norton Vitrium3 technology also requires a lower firing temperature than standard vitrified bonds. This reduces energy consumption in the following ways:
- reduced energy consumed operating existing machinery for the same production
- fewer wheels, less transportation, less fuel
- optimal firing temperatures, reduced energy costs
By choosing Norton Vitrium3 technology for your grinding operation, you help to preserve the environment.
Total cost reduction is the goal of all manufacturing organizations. While on average, abrasives consumables only account for about 3%, machinery and labor account for 58%of total manufacturing budgets (Figure 9.1). With a 20% decrease in cycle time per part, there will be a reduced total cost per part of more than 15%.