The Evolution of Vitrified Grinding Wheels (Part 1)
The vitrified bond grinding wheel first appeared 150 years ago, and continues to become ever more valuable. While some products reach a kind of developmental plateau where there's no potential for further improvement, research continues to uncover ways of making vitrified wheels cut faster and last longer.
When it comes to shaping hard materials quickly, accurately, and cheaply the vitrified superabrasive grinding wheel has few rivals. Low wear, high heat stability, combined with a free-cutting nature and excellent ‘dressability’ mean very high material removal rates and less downtime.
Understanding how this tool evolved yields some fascinating insights into its use in manufacturing. This begins with an overview of grinding wheels and abrasive materials, covers the production process, and explores where the technology might be heading.
HISTORY OF GRINDING WHEELS
Any review of grinding wheel development needs to cover the two primary components: the abrasive materials and the bond medium.
ABRASIVE MATERIALS
It is thought that sandstone was the first abrasive material. Probably used for putting a sharp edge on axes, compacted quartz embedded in the rock grains proved an effective way of removing material, (much like sandpaper today.) However, as a natural material sandstone has the disadvantage that the quartz particles vary in size and shape, resulting in unpredictable performance.
An alternative material, emery, was known to the Greeks and Romans as an abrasive and is still mined on what is today the Greek island of Naxos. Emery is a form of corundum, the second hardest naturally occurring material, is the crystalline form of Aluminum Oxide containing traces of Iron, Titanium, and Chromium.
As an abrasive emery had two problems: it was expensive to extract and ship to the manufacturing centers in the UK and USA, and its performance was unpredictable due to variance in raw materials that are mined. Spotting an opportunity, entrepreneurs set about developing alternatives. The results included synthetic or manmade Silicon Carbide and synthetic Aluminum Oxide (Corundum). These materials are generally known as “Conventional Abrasives."
INTRODUCING SUPERABRASIVES
Natural Diamond, has been used for grinding since at least the seventeenth century when Belgian gem-polishers used Diamond powder embedded in cast iron. Cost and variability of natural Diamond held back wider use until the creation of synthetic Diamond changed the equation in the 1950s.
As the hardest known substance, Diamond makes an excellent abrasive in most instances. There are certain Ferrous, Cast Iron, and Aerospace alloy materials, however, that produce a chemical reaction with Diamond causing it to wear rapidly and to alter the properties of the material. To address this problem, in 1969 General Electric introduced a crystalline material they had developed with a hardness approaching that of Diamond: Cubic Boron Nitride (CBN).
Despite having a lower hardness than Diamond, CBN doesn’t react with Ferrous and Aerospace alloys when grinding and has an even higher temperature stability. Therefore, it offers better grinding performance when grinding these particularly tricky materials.
Diamond and CBN are generally known as “superabrasives" for their unique, highly effective, grinding capabilities.
In part 2 & part 3 of this series, we will look specifically at the vitrified bond medium and its advantages and limitations in various grinding applications.