Jonathan McGargle, Abrasive Systems Division, 3M looks at specifying abrasive solutions for metalworking applications

In any metalworking application, selecting the right abrasive product saves time and money while delivering optimal finishing quality.

At the stage when a product is undergoing abrasive surface treatment, significant time and resources have been invested into it and any damage or loss could prove costly. Taking the time to select the most appropriate combination of machinery and consumables creates less work and offers the best chance of ‘right first time’ processing.

The importance of materials choice
Most abrasive consumables comprise three main elements: the backing, the mineral, and the resin coat.
All available in varying weights, backings are generally made from paper, fibre or cloth – cotton or polyester - with cloth backings usually the most durable and most suitable for aggressive, heavy applications. Backing material and weight govern the ‘feel’ the operator gets and also influence the product’s conformability, with greater conformability required when working with complex shapes.

The same mineral with different backings can produce different results in cutting quality and overall finish. Recent technological advances have seen cloth-backed discs undergo a laminating process increasing their durability and consistency without any major impact on flexibility.


Mineral selection
Four main mineral types are used in modern abrasives. Silicon carbide (SC) is typically very sharp and tough but also brittle, and is most commonly used with lighter backings on softer substrates.

For many years aluminium oxide (AO) was the most widely used mineral, but technological advances resulted in its reduced use in heavy applications, as AO is prone to break down in large fragments, leading to inconsistent performance and more frequent replacement.

These problems were addressed through the development of Aluminium Zirconia (AZ), with its smaller internal grain structure which breaks down more slowly. Suited to medium and heavy applications, AZ is very hard and tough, generating a deep, wide scratch pattern.

Further technological advances drove the development of Ceramic Aluminium Oxide, an extremely long-lasting, sharp and tough mineral for medium to heavy stock removal.

Each mineral comes in a variety of ‘grades’ or degrees of coarseness, but whatever the mineral, coarser grades usually last longer than finer grade products.

The issue of mineral wear affecting both cut and finish led to the development of Trizact technology. This contains multiple layers of abrasive, with each structure exposing a new layer as it wears to deliver consistent, predictable finishing. This technology delivers an even cut rate and longer belt life, and can even reduce process steps, save time and increase quality.
The resin coat is integral to product performance. It holds the mineral in place, but can also contain additives, such as a ‘Grinding Aid’ acting as both a lubricant and a coolant to the workpiece. This is especially important where excessive heat can damage the workpiece. Stainless steel, titanium and super-alloys, for example, are prone to ‘blueing’, with heat marks all but impossible to remove without affecting surface geometry.

Achieving the right finish
Metalwork finishing usually demands a cosmetic or a measured finish. In cosmetic applications, it is generally enough for the product to have a good aesthetic appearance unless further processes such as painting, powder coating or lacquering are planned, in which case the abrasive must deliver a sufficiently receptive surface.
A measured finish will require the finished workpiece to be within a pre-agreed tolerance in terms of any peaks or indentations in its surface. These applications generally require the finest abrasives at the end of the process, so when specifying it is often easier to ‘start at the finish’. A process with five steps currently may not require that many if different abrasives, for example Trizact discs, are used.

The role of equipment
Perhaps the largest issue affecting abrasive consumables is the type and performance of the equipment with which they are used. Even the best quality abrasives, if not used correctly, are unlikely to deliver the required results.

When it comes to power, more is generally better. A ceramic disc on a 750-Watt electric grinder generally performs better than an AO or AZ disc. However, using the same products on a 1400-Watt machine will deliver an exceptional improvement in the ceramic disc’s performance against a minimal improvement with AO and AZ. Processing is quicker, while the greater speed means less heat and less potential for damage, while operator effort and health risks caused by vibration exposure are significantly reduced.

The force applied by the machinery must also be considered. Typically, an abrasive disc and a backing pad are fixed to the machine. However, too soft a backing pad may not deliver a sufficiently aggressive action. Here, using a ribbed high-performance pad will assist in delivering the required impact pressure without increasing vibration. The same is true of contact wheels on belt machines which, if too soft, will not remove the required material. Ideally, a land/groove ratio of 1:2 or 1:3 should be the aim.

However, abrasives must always be used at the correct tool speeds. Quite apart from the performance issues, running at speeds higher than the recommended maximum RPM could lead to abrasive failure and compromise operator safety.

Understanding value and performance
The issue of value and performance is ever-present. Abrasive products must at least pay for themselves through the value they add, while damage from the process must be minimised.

In most instances, it is easy to calculate how many products can be processed by one disc or belt, but whole life costs and productivity also need to be factored in. It can be easy to dismiss an abrasive if it costs three times that of a competitive product. But if it can process five times as many parts, the investment is sound.

It is clear that ensuring a successful abrasive process involves far more than just fitting a disc or belt to a machine and hoping for the best. Properly planned and managed, it can add significant value to the production process.