Frequently asked questions
What is roller burnishing?
Roller burnishing is chipless smoothing and compression of metallic surfaces by rolling elements.
Are chips produced during roller burnishing?
No. Roller burnishing is a microfinishing process that plastically shapes the boundary layers of the component. Roller burnishing is the only chipless microfinishing process.
Are the terms smoothing, burnishing, roller smoothing and roller burnishing identical?
Yes. During burnishing a certain roughness is produced on the workpiece surface. The objective is a high surface quality for optimum running, sealing and joining properties. Carbide or steel rollers are guided over the workpiece with pressure and displace the surface peaks into the boundary layer.
Is "roller compression" a different method than "roller burnishing"?
No. During roller compression the same kinematics is carried out as with burnishing. Hardened rollers are guided over the workpiece with pressure and displace the surface peaks into the boundary layer. The displaced material volume flows from below into the profile valleys rising up. Surface plateaus then result from the surface peaks, and these form the high contact area ratio of roller burnished surfaces.
The objective of roller compression is to improve the strength properties. The rolling force produces internal stress in the micro-structure, which increase the dynamic strength of the material. The material state is formed up to a depth of 0.5 to 0.7 µm.
The increase in strength is dependent on the rolling force, the geometry of the roller and workpiece, the material properties and the number of times the material is rolled over.
What surface quality is produced during roller burnishing?
This leads to the formation of surfaces smooth as a mirror with a low surface roughness (< 1 µm) and reduced coefficients of sliding friction.
The measuring variables mean surface roughness Rz and material contact area ratio Mr are used as a measure of the quality of a surface.
Roller burnishing compresses the workpiece surface in the affected zone, improving the resistance to wear, the corrosion resistance and the material fatigue limit.
As no volume change takes place, approximately half of the surface roughness is flattened and the same volume is raised.
The achievable final surface roughness is directly proportional to the roughness of the premachined surface.
Are there definitions of surface quality?
Yes. The terms roughness, material contact area, roughness profile, measuring distance and cut depth are shown in profile graphs and material material contact area curves, and describe the surface quality.
Must the workpiece rotate during roller burnishing?
The workpiece, the tool or both rotate during roller burnishing.
Must a driven tool rotate clockwise?
Yes. During roller burnishing, the tool pulls into the workpiece in the clockwise direction due to the design; counterclockwise running would prevent the roller burnishing process.
Can all materials be roller burnished?
All plastically formable metals can be roller-burnished. Here the material hardness should be below 45 HRC (Rockwell hardness) when using conventional roller burnishing tools. Diamond finishing tools permit the machining of considerably harder workpieces (< 60 HRC).
The rollability of a material is determined with the ductile yield; if the ductile yield is above 5 %, the rollability is generally given. The greater the ductile yield, the better the materials can be plastically formed.
Plastics can generally also be roller burnished, however show clear tendencies to return to their original shape, so that the effects of smoothing tend to be minor.
Are cooling and lubrication necessary during roller burnishing?
A small amount of lubrication with a low-viscosity oil is generally sufficient during roller burnishing, e.g. by spraying the workpiece prior to machining.
At high roller burnishing speeds or high rolling pressures, it is advisable to use continuous cooling/lubrication with an emulsion or oil.
A cooling/lubricating emulsion also serves to transport away dirt particles, and must therefore be sufficiently filtered itself in order to avoid tool wear due to an abrasive effect. Recommendation for filtration: < 40 µm.
What speeds and feed rates can be used for roller burnishing?
The roller burnishing speed can be up to 200 m/min, however values of 100-150 m/min are recommended. The feed rate is dependent on the tool principle used and on the tool diameter. Common values for one-roller tools are: 0.05 mm/rev. to 0.3 mm/rev., and for multi-roller tools: 0.20 mm/rev. to 3.0 mm/rev.
What geometries can be roller burnished?
Roller burnishing can be used on the external and internal surfaces of virtually all rotationally symmetrical workpieces.
The decision on the suitability of interrupted sections or asymmetrical contours for roller burnishing must be made in conjunction with the roller burnishing tool.
Examples of possible contours: cylindrical, conical, spherical, various diameters without and with a shoulder, stepped shoulders, slopes, through-holes, stepped holes, blind holes, threads, internal contours, external contours, flat surfaces, flared contours, grooving, undercuts, recesses, radii, offset contours, splines, free-form surfaces etc.
Must the roller burnishing diameter be pre-machined with an oversize?
Yes. Roller burnishing is a process that utilizes cold working. To determine the required pre-machining, the elasticity and toughness of the material to be machined must be taken into account.
The following must be observed: Surface roughnesses of pre-machining and the desired surface roughness through roller burnishing, the oversize before roller burnishing and the type of pre-machining.
A uniform roughness profile (turning profile) is ideal pre-machining. Surfaces with major fluctuations in the roughness (e.g. reamed surfaces with retraction grooves).
Surface roughnesses between Rz 6 -15 µm form a good surface pre-machining.
For materials with a low ductile yield, like gray cast iron, magnesium and highly quenched and tempered steels, the surface roughness Rz of the prepared surface should be less than Rz 6 -15 µm.
The production tolerances are to match the quality of the finished part or the quality IT 7 for multi-roller tools.
How is the oversize for roller burnishing pre-machining determined?
The required oversize is dependent on the influencing factors: type of preparation, surface roughness of the preparation and material.
The most reliable determination for specifying the oversize is a preliminary test, i.e. a workpiece is pre-machined for roller burnishing, the produced diameter is measured and then roller burnished.
Following roller burnishing, the finished diameter is compared to the pre-machined diameter. The difference is the required oversize.
For turned surfaces, the oversize can be determined as follows: Oversize in the diameter matches the Rz value of the turned surface.
For ground surfaces, the oversize can be determined as follows: Oversize in the diameter is equivalent to the Rz value of the ground surface minus 25-50%, as the contact area ratio for grinding is higher due to the process used.
The infeed or preclamping of the tool must be determined on the workpiece in the individual case.
Can workpieces be cutted und smoothened with one single tool?
Yes. The Baublies combination diamond burnishing tool is cutting and smoothening internal contours in one cycle: at first the geometry is produced by the cutting edge and then this surface gets burnished by the diamond. This rotating tool is usually used together with an U-axis.
What advantages are characteristic of roller burnishing?
Roller-burnished surfaces are characterized by a low surface roughness, up to Rz < 1 µm, by a high contact area ratio due to favorable plateau formation and by the fact that no protruding material peaks remain. As a result, the abrasiveness is extremely low. The very low depth of surface smoothness Rz results in a very high contact area ratio.
The clear material compression and increase in the surface hardness results in increased resistance to wearing, a reduced tendency to crack and a low susceptibility to corrosion. The surfaces are suitable for use as sliding surfaces, sealing surfaces and joining surfaces.
Do the tolerances of the pre-machined surfaces change?
Normally, the dimensional tolerances of the pre-machining are maintained through roller burnishing, i.e. do not become worse or better, as roller burnishing is a forming and not a cutting process. Therefore, the required dimensional tolerance must already be met by pre-machining. The shape and position tolerances, like concentricity and cylindricity of holes and pins, are generally not improved by roller burnishing.
During pre-machining, dimensional fluctuations above IT 7 (for multi-roller tools) and IT 9 (for spring-mounted one-roller tools) are to be avoided. Excessive oversize produces extremely high rolling forces, and therefore reduces the tool life.
What advantages does roller burnishing offer compared to other processes?
Thanks to finish-machining in one clamping, no special machines are required, handling of production is extremely simple, and transport, storage and setup costs are reduced.
The process is extremely rugged and reliable and produces no noise.
Compared to cutting processes, a considerable improvement results in the material characteristics, such as roughness, hardness and resistance to wear.
Roller burnishing processes are characterized by high economy, as inexpensive production and very short machining times with consistently high quality are possible.
No material removal No material is removed with roller burnishing, there's no grinding dust or sludge, and therefore no disposal costs result for these waste products.