What’s Roller Burnishing?

A skiving and roller burnishing tool is ready to go, installed on a deep hole drilling machine in a factory in Wisconsin. Skive burnishing tools are commonly used for mirror surface finish on the inside of tubes for hydraulic cylinders.
Photograph published at Wikipedia and licensed under the Creative Commons Attribution.

Roller burnishing can finish the surface of a metal part without putting it through an abrasive process such as grinding. It is essentially a method to polish a surface by rubbing against a harder object, smearing the texture of a rough surface and making it shinier. This process enables fast and repeatable finishing of metal surfaces to a mirror-like sheen without actually removing any metal from part.

Various Approaches to Finer Roughness Values

Roughness is the microscopic peaks and valleys that are left behind on a metal surface after machining. It is measured by averaging all the peaks and valleys relative to the profile. The roughness average (Ra) value is commonly adopted in engineering practice.

It is easy to see that the difference between abrading and burnishing a surface to reduce this peaks and valleys is that the former removes metal while the latter doesn’t. By cutting the peaks on the surface, abrasive finishing brings the average peak and valley distances nearer together. But this leaves sharp projections on the machined surface. Burnishing does not have this problem. To the casual eye, a burnished part looks as though the metal surface was smeared smooth. But that is erroneous. The burnishing tool’s polished and hardened rollers actually force a viscous flow of surface and subsurface material, deforming the part permanently with mechanical stresses.

Not What It Looks Like

Material can be deformed in two ways – elastic or plastic. Elastic deformation happens when stress is applied at a pressure below the material’s yield point; bend a piece of plastic a little, and it springs back again. Plastic deformation, on the other hand, is irreversible, a result of stress applied at a pressure above the yield point of the material. Crushing an aluminum can, for instance, is plastic deformation.

Applying the rollers of the rotary tool radially to the surface causes the peaks to flow into the valleys and the valleys to flow upward toward the peaks. This cold-flow effect creates a flatter profile. It is important to note that burnishing does not simply smears or pushes over the peaks to make the part smoother. Rather, it actually crushes the material both at the peaks and some distance beneath, cold flowing both into the valleys. Since the finish comes from subsurface effects, a burnished surface is not just smoother but also work-hardened for greater durability.

Which Parts Work for Roller Burnishing?

Roller burnishing is quick and repeatable. Some tools can size, finish and work-harden parts in seconds. This is the principal benefit over grinding, honing or lapping. However, not every part is suitable to be roller burnished. For optimum results the requirements are:

Material:

Although nearly all metal can be successfully roller burnished, ductile or malleable metals work better (for example, iron, steel, steel alloys, copper, brass, bronze, and aluminium).

Hardness:

The ideal hardness is less than 40 HRC. (Some materials as hard as 45 HRC can be effectively burnished as well.)

Uniformity:

The finish depends on a uniform and tear-free surface in order for the peaks and valleys to cold flow correctly.

Roughness:

An Ra of two- to three-microns (or 80- to 120-microinch) is ideal for roller burnishing because the rollers can apply pressure to more of the surface.

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