GlossaryKind of term: Manufacturing termRoller burnishing
What is roller burnishing?
Roller burnishing is a chipless finishing operation in which hardened rollers press the peaks of a turned or bored surface down to a low roughness.
The tool carries hardened rollers, or a single ball, and rolls under pressure over the surface, which is cold-worked in the process. The peaks of the roughness profile are pressed plastically into the valleys; no chips are produced. The result is lower roughness, a slightly harder surface layer from work hardening and compressive stress in the surface layer. It is applied to bores, shafts, flat sealing faces and radii, often in the same setup as the turning or boring.
What does roller burnishing depend on?
The result depends on the preceding cut, the material and the process settings. Roughness and size before burnishing have to lie within the window the tool supplier gives for that tool and material, because burnishing only moves material that is already there. The material has to deform plastically: tough steels, stainless steel, aluminium and brass usually respond well; grey cast iron gives a more limited result because of its graphite flakes, and brittle materials are unsuitable. Roller tools have a hardness limit; for harder materials there are tools with a hydrostatically supported ball. Burnishing force, feed and speed also shape the outcome. Form and location errors from the preceding cut, such as ovality or a misaligned bore, largely remain.
How does roller burnishing work in practice?
Roller burnishing can replace a separate grinding, honing or polishing step where roughness is the issue and form and size are already right from the preceding cut. That removes a machine, a setup and sometimes a subcontractor; the price is a dedicated tool, a tighter preceding cut and time to set the process up. A bore grows slightly and a shaft shrinks slightly; by how much depends on the peak-to-valley height before burnishing, the material and the force, so you machine to a pre-burnish size from the tool supplier's guidance. A well-known example is the hydraulic cylinder tube, which is often skived and roller burnished. Whether a burnished surface is acceptable as a running surface for a dynamic seal depends on the seal maker's requirement, often a material ratio (Rmr) beside Ra or Rz, and belongs on the drawing.
What is roller burnishing often confused with?
Roller burnishing is not honing or abrasive polishing: those remove material, and honing can also correct form; burnishing only displaces material. Nor is it deep rolling: deep rolling uses a higher force to put compressive stress deep into the surface layer for fatigue strength, whereas roller burnishing is aimed mainly at roughness. And it is not knurling, which deliberately presses a pattern into the surface.
Frequently asked questions
Can roller burnishing replace grinding?
For roughness, sometimes, on material that deforms plastically and is not too hard. For size and form correction, no: burnishing does not improve roundness, straightness or position. Those have to be right from the preceding cut.
Does roller burnishing change the size of a bore?
Yes, slightly. The bore grows because the peaks flow into the valleys; a shaft shrinks. How much depends on the roughness before burnishing, the material and the burnishing force. That is why you machine to a pre-burnish size.
Which materials does roller burnishing not suit?
Brittle materials, and material harder than the roller tool can handle. Grey cast iron gives a more limited result because of its graphite flakes. For hardened parts there are ball tools with hydrostatic support; where the limit lies differs per tool.