Free Abrasives Flow for Automated Finishing
Exploring new methods of surface finishing that go beyond deburring to specific isotropic surface finishes that can increase service life
By Dr. Michael L. Massarsky
By David A. Davidson
Deburring/Finishing Tech Group
Turbo-Abrasive Machining (also referred to as TAM or Turbo-Finish) is a mechanical deburring and finishing method originally developed to automate edge finishing procedures on complex rotationally oriented and symmetrical aerospace engine components. Aerospace parts such as turbine and compressor disks, fan disks and impellers pose serious edge finishing problems. Manual methods used in edge finishing for these parts were costly and time-consuming. What’s more, human intervention, no matter how skillful at this final stage of manufacturing, was bound to introduce some measure of non-uniformity in both effects and stresses in critical areas of certain features on the part.
Since its inception, turbo-abrasive machining, a method that utilizes fluidized abrasive materials, has facilitated significant reductions in the amount of manual intervention required to deburr large components. Additionally, the process has also proved to be useful in edge and surface finishing a wide variety of other nonrotational components by incorporating these components into fixturing systems.
The advantages of this method go beyond the simple removal or attenuation of burrs. The method is also capable of producing surface conditions at these critical edge areas that contribute to increased service life and functionality of parts that are severely stressed in service. Among these advantages are (1) the creation of isotropic surfaces, (2) the replacement of positively skewed surface profiles with negative or neutral skews and (3) the development of beneficial compressive stress.
Deburring, Finishing, Part Performance and Productivity
Deburring and surface conditioning of complex machined parts is one of the most troublesome problems faced by the metalworking industry. In many cases, parts with complex geometric forms that are machined, or manufactured with very sophisticated computer-controlled equipment, are then deburred, edge finished, and surface conditioned with manual or hand-held power tools. This labor-intensive manual handling often has a considerable negative impact on manufacturing process flow, productivity, and uniformity of features as well as part-to-part and lot-to-lot uniformity.
The workflow interruption and production bottlenecks that can result are frequently one of the most significant headaches that manufacturing managers must confront. The total costs involved in performing manual finishing often defy quantification. As these types of processes are seldom capital intensive, they frequently escape the budget scrutiny they deserve. Additionally, it is becoming increasingly clear that edge and surface finish effects can now be produced on parts that contribute substantially to their performance as well as wear and fatigue resistance values.
TAM processes were developed primarily for automating deburring and surface conditioning procedures for complex rotating components. As an automated machining/finishing process, TAM is designed to address the uniformity and productivity concerns noted above. Repetitive motion injury problems can be minimized or eliminated as manual methods are replaced with automated machining procedures. Substantial quality and uniformity improvements can be made in precision parts as the art in manual deburring is removed and replaced with the science of a controllable and repeatable machining sequence. The time and cost of having substantial work-in-progress delays, production bottlenecks, nonconforming product reviews, rework and scrap can be reduced dramatically. Manual processes consuming many hours are reduced to automated machining cycles of only a few minutes.
Fluidized Bed Technology in Action
TAM machines could be likened to free abrasive turning centers. They utilize fluidized bed technology to suspend abrasive materials in a specially designed chamber. Parts interface with granular abrasive material on a continuous basis by having part surfaces exposed and interacted with the fluidized abrasive bed by high-speed rotational or oscillational movement. This combination of abrasive envelopment and high-speed rotational contact can produce important functional surface conditioning effects and deburring and radius formation very rapidly.
Unlike buff, brush, belt and polish methods or even robotic deburring, abrasive operations on rotating components are performed on all features of the part simultaneously. This produces a feature-to-feature and part-to-part uniformity that is almost impossible to duplicate by any other method. Surface finishes and effects can be generated on the entire exterior of complex parts, and also fixtured nonrotational components. Various surface-finish effects can be obtained by controlling variables of the process such as rotational part speed, part positioning, cycle times, abrasive particle size and characteristics, and others.
Surface-finish effects in TAM are generated by the high peripheral speed of rotating parts and the large number and intensity of abrasive particle-to-part surface contacts or impacts in a given unit of time (200–500 per mm²/sec). It should be noted that surface-finish effects developed from this process depart significantly from those obtained from air or wheel blasting. TAM processes can produce much more refined surfaces by virtue of the fact that the rotational movement of parts processed develop a very fine finish pattern and a much more level surface profile than is possible from pressure and impact methods.
A very important functional aspect of TAM technology is its ability to develop needed surface finishes in a low-temperature operation (in contrast with conventional wheel and belt grinding methods), with no phase shift or structural changes in the surface layer of the metal. A further feature of the process is that it produces a more random pattern of surface tracks than the more linear abrasive methods such as wheel grinding or belt grinding. The nonlinear finish pattern that results often enhances the surface in such a way as to make it much more receptive as a bonding substrate for subsequent coating and even plating operations.
TAM provides a method whereby final deburring, radius formation and blending in of machining irregularities could be performed in a single machining operation. This operation can accomplish in a few minutes what in many cases took hours to perform manually. It has become obvious that the technology could address edge-finishing needs of other types of rotationally oriented components such as gears, turbocharger rotors, bearing cages, pump impellers, propellers, and many other rotational parts. Nonrotational parts can also be processed by fixturing them to the periphery of disk-like fixtures. Many larger and more complex rotationally oriented parts, which can pose a severe challenge for conventional mechanical finishing methods, can easily be processed.
TAM as a surface-conditioning method is a blend of current machining and surface-finishing technologies. Like machining processes the energy used to remove material from the part is concentrated in the part itself, not the abrasive material interfacing with part surfaces, and like many surface-finishing processes material removal is not accomplished by a cutting tool with a single point of contact, but by complete envelopment of the exterior areas of the part with abrasive materials. As a result deburring, edge finishing, surface blending and smoothing, and surface conditioning are performed on all exterior exposed surfaces, edges, and features of the part simultaneously. Many metal parts that are machined by being held in a rotational workholding device (for example: chucks, between centers, rotary tables, etc.) are potential candidates for TAM processes, and in many cases these final deburring and surface conditioning operations can be performed in minutes, if not in seconds.
TAM Processing Characteristics
TAM produces an entirely different and unique surface condition. One of the reasons for this is the multidirectional and rolling nature of abrasive particle contact with part surfaces. Unlike surface effects created with pressure or impact methods such as air or wheel blasting, TAM surfaces are characterized by a homogeneous, finely blended, abrasive pattern developed by the nonperpendicular nature of abrasive attack. Unlike wheel or belt grinding, surface finishes are generated without any perceptible temperature shift at the area of contact and the micro-textured random abrasive pattern is a much more attractive substrate for subsequent coating operations than linear wheel or belt grinding patterns. TAM processes have strong application on certain types of parts that have critical metal surface improvement requirements of a functional nature. Significant metal surface integrity and improvement has been realized in processes with both abrasive and nonabrasive media. As a result of intense abrasive particle contact with exposed features, it has been observed that residual compressive stresses of up to 400–600 MPa can be created in selected critical areas. Tests performed on rotating parts for the aerospace industry that were processed with this method demonstrated a 40–200% increase in metal fatigue resistance tested under working conditions, when compared with parts that had been deburred and edge finished with less sophisticated manual treatment protocols.
Significant process characteristics to keep in mind include (1) very rapid cycle times; (2) a high-intensity, small media operation that allows for access into intricate part geometries; (3) a completely dry operation; (4) metal surface improvement effects: including isotropic, negatively skewed surfaces with improved bearing load ratio and contact rigidity (5) no part-on-part contact; (6) modest tooling requirements; (7) primarily an external surface preparation method some simpler interior channels can also be processed, and (8) many types of rotating components can be processed and non-rotational components can also be processed when attached to disk like fixtures. ME
This article was first published in the October 2013 edition of Manufacturing Engineering magazine. Click here for PDF.