Finishing with multiple benefits
Essentials of the process
Roller burnishing is a chipless smoothing and compression process for metallic surfaces achieved by rolling elements. To understand what happens during this finishing process and to be able to use the full range of advantages, a basic knowledge about the requirements, impacts and possibilities is helpful. Here are the important subject areas:
|Surfaces created by geometrically defined cutting edges e.g. turning:
The results are very stable conditions for a roller burnishing process.
Typical surface created by geometrically
defined cutting edges
Surface created by geometrically undefined cutting edges e.g. grinding:
|The results are higher rolling forces and wear during rolling.||
Typical surface created by geometrically
undefined grinding edges
The surface measurement is mostly carried out with a so called profilometer. A diamond stylus (e.g. with a radius of 0.2 μm) is moved vertically in contact with the workpiece and then moved laterally across the surface over a defined measurement distance. Thus the surface profile is recorded. In reality every surface deviates more or less from the ideal surface (without deviation of dimension, form and shape). The different types of shape deviation can be displayed separately (e.g. in charts).
Surface roughness parameters
- Total height Wt :
The total height of the respective profile type is the maximum height between the highest peak and the deepest valley. It describes the waviness of the surfaces.
- Maximum roughness depth Rmax:
The maximum roughness is the largest single depth inside the evaluation length. It depends on individual criteria and therefore can vary widely.
- Average maximum height of the profile Rz:
Average value of the five Rz values from the five sampling lengths. It depends less on the individual criteria and therefore characterizes the „real“ roughness better.
- Arithmetical mean deviation of the assessed profile Ra:
Ra is the arithmetic mean roughness value from the amounts of all profile values. Ra does not differentiate between peaks and valleys and has a relativlely weak information character.
- Maximum profile peak height Rp, maximum profile valley depth Rv:
The quotient of Rp and Rv shows if peaks or valleys prevail in the profile.
- Material ratio of the profile Rmr:
Rmr indicates what ratio the totalled length in the material has assumed relative to the evaluation length (in %). The comparison is made in the specified section height (c) and the total evaluation length (lm). The material ratio curve indicates the material ratio as a function of the section height.
Strength in N/mm2 or MPa
Strength is the ability of material to resist applied force.
The strength of a metallic material is mainly determined by the crystal lattice and its structure (Lattice structure errors). The stress conditions also influence the material strength.
The tensile strength is detected by a tensile test. During this test material sample is exposed increasing stretching force and thereby the associated elastic and plastic deformations are recorded in the stress-strain diagram.
Hardness describes a material ability to resist indentations - that is, compressions in the surface of a material caused by impacts.
There are different testing methods (Rockwell, Vickers, Brinell) for determining hardness.
The increased surface hardness through roller burnishing is one positive result of the technology.
Surface layer hardening
In order to make components in technical applications durable and resistant, various methods of surface layer hardening can be applied. For example:
- thermal processes (hardening)
- thermochemical methods (nitriding or nitrocarburizing)
- mechanical methods (roller burnishing)
Strain hardening through mechanical methods is based on the following mechanisms:
- cold work hardening by increasing the dislocation density which is caused by the formation of new dislocations during the plastic deformation of the material.
- the generation of residual stresses in the surface layer. Internal compressive stresses, induced by the surface stretching which is compensated by the underlying material.
- the mechanically induced transformation of the microstructure.
- reducing the notch effect through improvement of the surface finish.
Stages in elastic and plastic deformation
The graph shows the typical extension behaviour of ductile materials in tensile testing where a sample bar is subjected to a progressively increasing tensile force.
|Point 0-1||The extension of the bar is proportional to the increase in tension. For example, when tension increases by 10 %, length increases by 10 %.|
|Point 1||The bar reaches the limit of proportionality. Beyond this point, length begins to increase at a slightly greater rate than tension|
|Point 2||The elastic limit is reached. Beyond this point, the bar will no longer return to its original length. In many materials, the elastic limit occurs almost immediately after the limit of proportionality.|
|Point 3||The bar reaches its yield point. Once it yields, it continues to increase in length, even without a further increase in tension.|
|Point 4||This is the ultimate tensile strength (UTS) of the material. Beyond this point, a waist (a narrower section) appears at a point along the length of the bar, signalling that it is about to fracture.|
|Point 5||This is the fracture point, where the bar breaks in two.|
The difference between static and dynamic strain has to be considered.
This is a constant force on a material by tension, pressure or torsion.
The load capacity of the material, beginning with plastic deformation until fracture, can be predicted from the material properties (stress strain diagram) and the load case.
Fmax = strength x cross-section-area
This is a recurring force on a material by tension, pressure or torsion.
In case of dynamic strain the load limit is much lower compared to static strain. The material performance is defined under such strain. It is displayed in a S-N curve. It shows the tolerable strain depending on the number of load cycles till fracture. Depending on the number of load cycles we distinguish between static, temporary and permanent strength. The area of fracture is often at a change of diameters because there a peak of tension occures in the material. Also areas of high surface roughness are the reason for fractures caused by the notch effect.
The benefit of roller burnishing is the economical, simple and reliable manufacturing of maximum surface quality while increasing the strength and hardness of the workpiece.
What is roller burnishing
- Roller burnishing is a non-cutting method for smoothing and strain hardening metallic surfaces with forming elements.
- During roller burnishing, the forming elements are loaded with a vertically directed force to the surface (roller burnishing force). Thereby the roughness profile is plastically deformed and levelled.
- Roller burnishing changes the stress condition in the surface layer of the material.
- Roller burnishing is a method of microfinishing.
Roller burnishing for smoothing
The roller burnishing force produces a surface pressing (Hertzian stress) in the contact zone of the burnishing elements. Thereby the flow limit of the material is reached in the contact area and thus the surface profile is plastically deformed and levelled. The material volume of the elevated areas of profile peaks is pressed out into the levelling profile valleys.
Thus the surface roughness is significantly reduced. The resulting dimensional difference between the preworked and the roller burnished workpiece depends on the original roughness. Here the rolling force is kept as low as possible. The preferred aim of the process is the surface quality, not so much the strain hardening.
- Mirror like surfaces with roughness below 1μm
- High material ratio of the profile creating optimized wear characteristics
- Reduced risk of crack formation caused by micro notches
- Improved corrosion resistance
Strain hardening by deep rolling
During deep rolling the same kinematic is carried out as with roller burnishing. The aim is the strain hardening of the material. The rolling pressure is higher in this case. Thus the following effects occur:
- Work hardening caused by dislocation movements within the crystal structure of the material.
- The occurance of a stress state in the surface layer. This appears due to the interaction of plastic surface stretching, which is compensated by elastic deformation of the boundary layer. This stress state typically takes place in a depth up to 0.8 mm.
- The mechanically induced micro structural transformation.
- The improvement of the surface quality and reduced notch effect.
The level of the strain hardening depends on different parameters:
- The rolling pressure and -speed
- The geometry of the roll and the workpiece
- The material properties
- The number of revolutions in a certain section
Properties of roller burnished surfaces
Roller-burnished surfaces are characterized by the following properties:
- very low roughness values, up to Rz <1μm, results in reduced crack formation and corrosion.
- very high material ratio of the profile caused by plateau formation.
- reduced profile peaks.
- „Rounded “ profile, with lower abrasiveness compared to a ground surface.
- Increased dynamic resilience caused by signifi cant strain hardening.
- Increased surface hardness, reducing abrasive wear.
Which materials can be roller burnished
- Every plastically deformable metal can be roller burnished.
- Standard roller burnishing tools with steel rollers can be used at hardness of up to 45 HRC.
- When using diamond burnishing tools material hardness can exceed 60 HRC.
- The rollability is defined by the ability of the material to be plastically deformed. An indication is the break elongation, which should be higher than 5 %. A higher break elongation improves the rollability.
What results can be achieved ?
Due to variety of the materials only rough numbers are shown
Average roughness Rz
|Process conditions||Steel (1.4104)||Cast Ison (GG40)||Steel ca. 60 HRC|
|Optimal||0.5 - 1||1.5 - 2.5||0.5 - 1|
|Normal||0.8 - 1.5||2.5 - 4||0.8 - 1.5|
|Difficult||1.5 - 3||4 - 6||1.5 - 3|
- Hard machining over 60 HRC:
In the machining of materials with a hardness of more than 60 HRC the surface should be preprocessed in a range of Rz 2-5 μm. Then the achievable surface finish is approximately Rz 1 μm.
- Material ratio
Roller burnishing increases the material ratio. In a height C of 0.2-0.4 μm the values should reach more then 70 %.
- Dynamic resilience
The vibration resistance generally can be increased by 20 - 60 %. Under certain conditions more than 100 % can be achieved.
- Surface hardness
The increase of hardness in steel material can be more than 20 HV and up to 50 HV.
Which geometries can be roller burnished?
Roller burnishing can be applied on external and internal surfaces of almost all rotationally symmetric workpieces.
For roller burnishing holes and shafts we have a comprehensive range of standard tools available.
Based on over 40 years of experience we are also able to provide tailor made solutions for nearly all other geometries.
The development of diamond burnishing technology enables us to work in new areas such as the burnishing of free form surfaces, e.g. in moldmaking.
Some examples of burnishable shapes
Due to the different requirements roller burnishing tools are divided into different types:
- multi-roller tools and machines
- single-roller tools
- diamond burnishing tools
- forming tools
The classic design of roller burnishing tools are the multi-roller tools. They are offered in a broad range of standard and special forms.
They are normally used to work cylindrical holes, shafts, tapers and plane surfaces.
The advantages of multiple rollers working simultaneously is a fast and economical machining without cross force to the rotation axis.
These type of tools are used on all established types of machines.
Kinematics of multi roller tools
The workpiece, the tool or both rotate during roller burnishing. During roller burnishing the rolling motion is similar to the kinematics of a planetary gear. The taper (1) is firmly connected to the tool fixture (4). The ball beared cage (3) carrying the rollers (2) can freely rotate. The taper supports the rollers and it adjusts the pressure required for forming the surface. The axial position of the taper defines the tool diameter and the rolling pressure.
Single roller burnishing tools
- Here only one roll is in operation.
- Single roller tools are offered in different designs: Variable, Modular and Slim systems.
- Single roller tools are used to process various diameters.
- Single roller tools are spring loaded to compensate prework tolerances.
- Single roller tools can be fitted with standard or specially designed rollers according to process requirements.
- Single roller tools are suitable for processing cylindrical parts and profiles like radii, tapers or recesses.
- Single roller tools are perfectly suitable for strain hardening.
Diamond burnishing tools
- Here the burnishing process is not carried out by a rotating roller but by a spherical, fixed diamond. The diamond slides over the surface and forms a profile, comparable to a ball rolling over the surface.
- This process of smoothening and strain hardening is similar to the process with conventional rolling tools.
- The possibilities in design and the outstanding material characteristics
of the diamond generate a significant extension for the applications of roller burnishing.
- With the point-shaped contact area and the slim design of the diamond numerous contours can now be processed. For example, thin walled parts can be smoothened with the diamond.
- The enormous hardness of the diamond enables the machining of materials with a hardness of more than 60 HRC.
- The design of the tools exclusively contains mechanical components, therefore the tools can be used on almost every machine tool. There is no additional expensive equipment such as driven tools or hydraulic pumps required.
- The slim design enables application of the tools in small spaced machines such as swiss type lathe machines.
- According to the requirements of the workpiece the shape of the diamond can be adapted from variable radii to cones and pyramides.
- The combination of the diamond burnishing tool with cutting tools is possible.
- Forming tools are a special feature in the Baublies product range. The design of the tools is related to roller burnishing tools.
- The most important aim of the forming process is not the improvement of the surface roughness but the specific transformation of the workpiece geometry.
- Forming tools are normally used on standard (CNC-) machine tools or on special machines which are integrated in assembly lines.
Roller burnishing tools are suitable on all common machine tools as…
- Lathes, both conventional and CNC
- Machining centers
- Transfer lines
- Revolving transfer machines
- Drilling machines
- Milling machines etc…
Roller burnishing tools are in use in nearly all metalworking branches,e.g. automotive industries, hydraulic and pneumatic components, aircraft industries, medical industries, machine building industries, jewellery making …
Fixtures and clamping possibilities
The standard tool fixture for multi roller burnishing tools is:
- Cylindrical according to DIN 1835
- Morse taper according to DIN 228
All common clamping systems are available, e.g.:
VDI - DIN 69880,
SK - DIN 69871, DIN 2080
HSK - DIN 69893
Also producer specific systems are available: Capto®, MVS®, KM®, ABS®
For roller burnishing in a manual process (e.g. with a drilling machine)
lubricatiion with a small amount of oil is adequate.
At high burnishing speed or pressure a continuous cooling with emulsion or cutting oil is very useful to increase the lifetime of the tool.
The coolant/lubrication fluid is also used for removal of dirt from the surface and should be kept as clean as possible to avoid the dirt particles being pressed into the surface. (filtration of the coolant is recommend)
Diamond burnishing tools must be used with coolant because the frictional heat of the diamond sliding on the surface would damage the diamond quickly.
As a result of the complete machining using one machine, no special machines are required. The handling of the workpieces in the production is simplified thus the costs of transport, storage and machine setup are reduced.
The process of roller burnishing is extremely reliable, quick and easy to execute. It can easily be adapted to an established manufacturing process.
Compared to chipping processes there is a signifcant increase of surface quality, strength, hardness and wear resistance of the material.
In the roller burnishing process no material is removed, therefore there is no waste products to be disposed of.
Roller burnishing is economically efficient due to short cycle times and results in high product quality. The result is good value for money manufacturing with fast amortisation.