Burnishing and Peening Machines Information
Burnishing and peening machines impart compressive residual stresses, which improves fatigue strength and corrosion resistance. These surface enhancement machines deform and strengthen a metal surface by hammering the surface with small beads, shot peen balls or other peening media. Peening and burnishing processes can dramatically improve fatigue strength and endurance limits of metal alloys by imparting a compressive residual stress on the surface of a metal alloy part. Rotating shafts and turbine blades are often shot blasted or peened for fatigue strength enhancement. The surface finish is also refined because the pounding of the peening media flattens high points in the surface profile. Burnishing and peening work-harden a part surface.
Burnishing and peening are surface enhancement processes proven to significantly extend the fatigue life of metal parts by providing enhanced resistance to common failure mechanisms, including:
- Foreign object damage
- Fretting fatigue
- High-cycle fatigue
- Corrosion fatigue
- Stress corrosion cracking
- Hydrogen embrittlement
- Creep deformation
Burnishing machines are similar to machine tools in construction and operation, while conventional peening systems are similar to abrasive blasters or sandblasters. In fact, peening can be performed in an abrasive blast machine by using metal shot or peening media in place of abrasive blasting grain. If sandblasting equipment is being used to surface enhance (i.e., improve surface integrity, generate compressive tensile stress, enhance fatigue life), then care should be taken to remove abrasive grit from the system or blasters dedicated for shot peening should be used. An abrasive grit particle could generate a scratch in a smooth polished shaft that is shot peened for surface enhancement purposes. Scratches and higher roughness will reduce fatigue properties.
Burnishing machines press rollers or balls against a surface to mechanicalky deform and refine surface finishes. Burnishing can also providing sizing or refine dimensional tolerances and geometry such as roundness. Deep rolling, ball burnishing, roller burnishing and low plasticity burnishing are variations of burnishing. Hand tools can also be used to manually burnish a surface, but without anywhere near the degree of uniformity that a burnishing machine can provide.
Roller burnishing is a controlled process for precision inner diameter dimensioning and surface enhancement of mission critical parts. Roller burnishing can typically improve surface finish to as fine as 0.10 to 0.15 microns (4 to 6 micro inch) roughness average (Ra). Roller burnishing controls size, providing tolerances within 0.000127 mm (.005 inch) on average and much finer for special applications. In addition, roller burnishing increases surface hardness 5 to 10%, improves fatigue life up to 300%, enhances corrosion resistance, and eliminates tool marks and minor surface imperfections. Burnishing is cleaner process than grinding, honing or lapping and can replaces these expensive secondary operations while providing superior surface engineering at lower costs and faster production rates.
- Conventional shot peening machines or shot peeners propel metal shot onto a surface using abrasive blasting equipment or similar products. Shot peening is one of the original surface modification technologies developed for improving fatigue life via induced compressive stress. The process of shot peening involves accelerating a hard media (metal, ceramics, etc.) toward a part surface at high velocities. Impacts of the media with the part surface induce plastic deformation and resulting compressive residual stresses. Shot peening continues to be a popular surface modification technology.
- Flap peeners or flap peening machines have media attached to a strip on a wheel that rotates at high speed. Parts are presented to the flap peen wheel. Alternatively, a flap peening wheel can be mounted on a hand held rotary tool for peening larger surfaces.
- Laser peening machines create a pressure wave to peen a surface by explosively vaporizing a layer of water or liquid.
- Ultrasonic shot peening machines use ultrasonic energy to activate the media and peen selective areas or internal surface within cavities, tube inner diamters, or holes.
- Waterjet or cavitation peening machines uses a blast of high pressure water to peen or deform a part surface.
Peening and burnishing machines may use a CNC controller for automated operation, with little-to-no operator intervention. Machines may also be system controlled through a PC interface.
In addition to feed options, other considerations for peening machines include the system or component type. Options commonly include a burnishing or peening cabinet or enclosures or rooms, dust collection or filtration system, gun or head, and media separator. Peening cabinets or enclosures typically consist of a chamber with glove ports, a viewing window, and internal lighting. When using this type of peening machine, the workpieces rest on an open grid that allows used metal shot or peening media to drop through for recycling purposes.
Other considerations for choosing peening machines include part handling and workpiece loading. Part handling refers to the direction a workpiece travels along a particular axis. Workpiece loading refers to how a piece moves through the peening machine. Common workpiece loading options include handheld options, or the use of belt or roller conveyors, overhead monorail conveyors, gantry or XY table, rotary table, tumbler belt, or a spinner hanger.