Roller burnishing is a cold-working process which produces a fine surface finish by the planetary rotation of hardened rolls over a bored or turned metal surface. However, unlike cold rolling, which generally produces large sectional changes, roller burnishing involves cold-working on the surface of the workpiece to improve surface structure.In the burnishing process, the pressure generated by the rolls exceeds the yield point of the softer piece part surface at the point of contact. The result is a small plastic deformation of the surface structure of the piece part.Since all machined surfaces consist of a series of peaks and valleys of irregular height and spacing, the plastic deformation created by roller burnishing is a displacement of the material in the peaks which cold flows under pressure into the valleys. In the process, tool marks and irregularities are rolled out. The result is a mirror like finish with a tough, work-hardened, wear and corrosion resistant surface.
BENEFITS OF ROLLER BURNISHING
Improved metallurgical properties
Burnishing cold works the metal of a machined part. Tool marks are rolled out. Grain structure is condensed and refined, and the compacted surface is smoother, harder and longer wearing than ground or honed surfaces.The rolling action also greatly reduces surface porosity and scratches which could hold reactive substances or contaminants. As a result, the corrosion resistance of burnished surfaces is higher than the open surfaces produced by grinding or honing.Depending on the type of material being burnished, surface hardness can be increased by as much as 10 points Rockwell C. This increase often eliminates the need for heat treating of surface treatment as a means of improving wear resistance.Another metallurgical improvement stems from induced residual compressive stresses in the surface of the part as a result of plastic deformation in the roller burnishing operation. These compressive stresses greatly increase the strength properties and fatigue life of the part, because any forces on the part must overcome these residual stresses, as well as the tensile strength of the materials, before fatigue conditions occur.
Improved surface finish
Roller burnishing imparts a high finish to any machinable metal. Surfaces that are bored, reamed or turned to 125 micro inches or more can be finished to 4 micro inch CLA or less in one pass at feed rates of 125 to 300 mm/min.Roller burnishing replaces grinding, honing, lapping and other expensive secondary operations.
PREPARATION FOR ROLLER BURNISHING
Roller burnishing is a cold working process for obtaining fine finishings.We remind you some helpful considerations in order to obtain an excellent result:
•FINISHING: the ductility and hardness of the workpiece material and the surface preparation dictates the quality of the burnished finish.
•WORKABILITY: any ductile and malleable material up to RC40 can be roller burnished.
•WORKED SURFACE PROPERTIES: workpieces with an interrupted surface of no more than 10% of the circumference can be successfully burnished with a standard tool (see fig.1 & 2).
•TOLERANCE OF THE BURNISHED PIECE: the tolerance range achieved from the burnishing will be equal to that achieved from the pre machining as no material will be removed.
The ideal surface for the burnishing is a succession of peaks, which corresponds on regular feed of the preparation tool (fig.3). We suggest the ALVAN® expanding reamer for the pre-burnishing as it assures an uniform roughness and a tolerance in the range ISO6 – ISO7.Attention must be paid to pre machined tapers and surface irregularities caused by cutting tool failure as these conditions cannot be rectified by the roller burnishing operation (see fig.4 & 5).
ADJUSTMENT & HOW THE ROLLER BURNISHING TOOL WORKS
The roller burnishing tool incorporates a shank, a body and a planetary system of conical rolls which are evenly spaced by a retaining cage. Before use, the burnishing tool must be adjusted to the required working diameter using the following steps:
unscrew the lock nut (item 1)
pull the housing (item 2) towards the lock nut (item 1)
turn the housing (item 2) to increase or decrease the diameter
ensure that the micrometer nut and sleeve (item 4 and 5) are engaged
tighten the lock nut (item 1)
How it works
The roller burnishing tool must be aligned with the workpiece (maximum error 0,1); if there are doubts on the alignment error, a floating shank is needed. When the tool is in the bore, the cone pushes the rolls onto the surface to be burnished. The cage retains the rolls in an angled position to the axis of the tool, so the pressure of the burnishing generates a self feed that draws the tool into the bore. When self feed is not required, i.e. blind hole applications, you must use a cage which holds the rolls parallel with the axis of the tool. If the machine feed is stopped or slower than the natural rate, the rolls pull the cage, held in position by the release spring (item 3), and they then release from the surface of the bore. The release of the rolls permits the quick retraction from the bore (see table dimension "G"). The release spring will ensure the rolls are in the correct position to burnish the required diameter. The roller burnishing tool requires lubrication not cooling. A plentifull supply of lubricant must be fad directly to tool to provide a good flushing and cleaning action. It is recommended that a filter is fitted to the lubrication system to prevent the circulation of chips which may cause damage to the burnished surface and the tool.