Abrasive Flow Machining: Definition, Types, Application, Advantages, and Disadvantage.

Abrasive Flow Machining (AFM) is used to deburr, polish, and radius complex passage and inaccessible surfaces.

It is a non-conventional finishing process. Unlike traditional machining, where rigid tools are used to remove material, AFM passes a viscous, abrasive material through or over the surface of a workpiece to abrade the surface.

The controllable flow of an abrasive medium can be used to abrade material and give a better surface finish.

What is Abrasive Flow Machining?

Abrasive Flow Machining (AFM) is a unique finishing operation that uses an abrasive-laden polymer that is semi-solid and serves as the tool.

The abrasive media flows under pressure through or across the workpieces passages and surfaces to deburr, polish, and create controlled radius.

AFM is held in high regard when it comes to addressing access challenges that cannot be made with typical machining processes, making it perfect for complex geometries and internal features.

AFM performs well in challenging conditions, such as internal geometries of aerospace, medical, and automotive components.

Working Principle of Abrasive Flow Machining

Abrasive Flow machining (AFM) works on the fundamental concepts of controlling the flow of an abrasive medium for purposes of material removal. The process begins with securing the workpiece between two media cylinders.

These cylinders contain the abrasive media, which is a viscoelastic polymer with a large loading of abrasive particles, such as silicon carbide, aluminum oxide, boron carbide, or diamond.

The heart of AFM is the extrusion and flow of this media. Hydraulic cylinders apply pressure at levels typically between 0.7 and 20 MPa (100 to 3000 psi), to move the abrasive media from one cylinder to the other through the workpiece, either through internal passages or across external surfaces.

The applied pressure indicates how much force the abrasive particles will contact the workpiece. As the media flow, the abrasive particles become pressed into the workpiece surface.

The viscoelasticity of the polymer allows the media to conform to the workpiece contours enabling the media to provide uniform contact during the abrasive flow, even on complex geometries.

The shearing and rubbing motion of the abrasive particles experiences on the workpiece removes microscopic levels of material, effectively performing deburring, edge radiusing, and surface polishing.

To provide even exposure and remove displaced material, the media flow us typically reversed at various points in the cycle and returned back through the workpiece.

The number of cycles, flow pressure, and media viscosity have a strong impact on how much material is removed, as well as, the final surface finish.

The most important aspect is the media rheology. The polymer is a viscoelastic material with just enough viscosity to hold the aggressiveness of the abrasive particles pressed against the workpiece but not so much it can’t flow through complex passages.

The type, size, and concentration of abrasive particles in the polymer is what effects how aggressive the material removal can be and what type of surface finish can be achieved. For example, polishing is done with fines abrasives and coarse abrasives are used for more aggressive deburring or material removal.

One of the main advantages of AFM is the self-deforming property of the abrasive media. Consequently, the abrasive can reach and process complex internal geometries that cannot be accessed any other way.

Additionally, the process is cold, generating little heat avoiding thermal distortion of the workpiece.

Types of Abrasive Flow Machining

While the fundamental principle remains the same, AFM can be categorized based on its application and the setup used:

#1. One-Way AFM.

As the name implies, One-Way AFM (also called Unidirectional AFM) – moves the abrasive media in a single direction through the workpiece. It is generally easier to set up than it’s multi-directional sibling.

The abrasive media is extruded from one media cylinder, into the workpiece, and then into a receiving cylinder. Media flow is unidirectional, meaning abrasive grains will abrade the surface only in that specific direction.

Advantages:

• Fairly simple set up and operation
• Less expensive for some applications
• Can be effective for simple geometries where one predominant alignment exists for deburring or surface finishing

Limitations:

• Less effective at achieving a uniform finish for complex geometries than two-way AFM
• May require several passes or different media formulations to achieve a desired result

Applications:

• Deburring of small holes or passages
• Light polishing of simple internal surfaces
• Removal of fine burrs from drilled holes

#2. Two-Way AFM (Most Common).

Two-Way AFM is the most versatile and commonly used form of Abrasive Flow Machining, and because of its oscillating nature, it can provide improved finishing on complex geometries.

In Two-Way AFM, the abrasive media moves back and forth in a bidirectional manner through the workpiece by means of two pressurized media cylinders.

The bidirectional nature of the flow of the abrasive media allows the abrasive grains to act on the cutting surface from multiple angles, producing more even removal of material and improved surface finish overall.

Stationary, pressure, and flow rate can be controlled to coordinate all parameters of the abrasive process.

Advantages:

• Outstanding material removal and surface finish due to bidirection flow.
• Very effective on complex internal geometries, intersecting holes, and contoured surfaces.
• Excellent deburring, radiusing, and polishing.
• The machine tool can reduce processing time in comparison to single direction or one-way AFM for roughly comparable results.

Limitations:

• Circumferences and costs of the equipment are a little bit more complicated compared to one-way AFM.

Applications:

• Polishing and deburring of turbine blades and impellers used in the aerospace industry.
• Finishing of medical implants and surgical instruments.
• Finishing hydraulic and pneumatic components by improving surface finish.
• Deburring of dies and molds, radiusing edges.
• Cleaning recast layers from components produced by EDM machining.

#3. Orbital AFM.

Orbital AFM (abrasive flow machining) incorporates movement via motion of the workpiece and/or tooling. This adds yet another element of abrasive action, as well as potentially better control of the process overall.

While the abrasive media is still traversed across the workpiece (often a two-part process), there is an additional orbital or rotary movement that is given to either the workpiece itself or a fixture holding the workpiece.

This motion imparts a non-linear nature to the path that the abrasive media follows over the surface, which increases repeatability and enables targeting of specific areas.

Advantages:

• Improved control of the surface finishing process, and therefore, more discriminating material removal.
• Improved overall uniformity of surface finishing when dealing with complex parts.
• As an added advantage, orbital AFM is often especially helpful in finishing parts that have non-uniform cross-sections.

Limitations:

• Equipment and setup tend to be more complicated.
• May require custom or specialized fixtures.

Applications:

• Precision finishing of parts that have critical radius demands.
• Polishing of molds and dies that have complex patterns.
• Applications where material removal must be on a local or targeted basis.

#4. Multiple-Passage AFM.

Multiple-Passage AFM is used whenever a single large workpiece or multiple small workpieces needs to be processed through many passages of abrasive media in parallel or sequentially.

This variation includes designing the tooling that allows the abrasive media to flow through multiple passages or multiple workpieces simultaneously.

The tooling can be designed to process identical parts simultaneously or to address different portions of a single large part in a sequential manner.

Advantages:

• Higher production throughput for high volume applications.
• Higher efficiencies for processing multiple similar components.
• Can be customized for larger or irregularly shaped workpieces.

Disadvantages:

• Tooling design will be more complex and expensive.
• Media flow distribution must be considered thoughtfully to achieve comparable results throughout multiple passages.

Application:

• High-volume production processing providing AFM finishing to small components.
• Processing of manifold blocks featuring multiple passages within the internal area.
• Finishing of large industrial components featuring numerous internal features.

#5. External AFM.

Contrary to the previously discussed types which have an internal focus, External AFM focuses on finishing the external surface of workpieces.

In External AFM, the workpiece is placed in a controlled environment where abrasive media is flowed over its external surfaces. Tooling is designed to make a flow path around the outside of the part which provides proper contact and abrade for the abrasive media.

Advantages:

• Effectively deburr, polish, and radius external contours.
• Access to more complex external geometries that are very challenging to finish with traditional methods.
• Good for components with elaborate external features.

Limitations:

• Setup requires specific tooling to contain flow around the exterior of the part.
• Setup is sometimes more complicated than AFM for certain geometries, especially if External AFM is not an intended route for these parts.

Applications:

• Finishing orthopedic implants, e.g., knee or hip joints.
• Polishing aerospace components with external airfoil.
• Debur and radius gears and splines.
• Surface finishing intricately shaped jewelry or decorative features.

#6. Extrusion Honing.

Extrusion Honing can be thought of as a specialized and controlled form of AFM, specifically focused on bore finishing and bore sizing. The media is used to hone internal cylindrical surfaces.

With Extrusion Honing, a highly viscous, bilateral, and abrasive polymer media is extruded through a bore or passage under high pressure.

The media, along with a controlled flow can create a flexible hone tool that can remove material and create a better surface finish and dimensionally accurate internal diameter. Extrusion honing is typically carried out in a two-way tooling arrangement to maximize results.

Advantages:

• Excellent for bore sizing and improved bore roundness.
• Great way to provide inner diameters with superior surface finish.
• Can remove micro-burrs and enhance oil retention characteristics.
• A cost-effective means of finishing and honing small to large diameter bores.

Limitations:

• Primarily a performance measure for cylindrical inner geometries.
• Requires special purpose fixtures to hold the workpiece and media direct.

Applications:

• Finishing of hydraulic valve bodies and sleeves.
• Honing of certain engine components (e.g. connecting rods and cylinder liners).
• Improving surface finish and sizing of gun barrels.
• Finishing of fuel injectors and pump components.
• The process type of AFM can be determined based on the geometry of the workpiece, desired surface finish, material to be processed and production quantity.

Applications of Abrasive Flow Machining

AFM’s unique capabilities make it indispensable across a wide range of industries and applications, particularly where precision finishing of complex geometries is critical:

#1. Aerospace Industry.

• Turbine Blades and Vanes: Remove burrs, radius edges, and polish internal cooling passages and complex aerodynamic surfaces to achieve maximum airflow, reduced drag, increased engine efficiency, and engine life.
• Fuel System Components: Finish fuel nozzles and manifolds to – maximize flow and atomization.
• Hydraulic and Pneumatic Manifolds: Remove burrs and polish internal passages to minimize pressure drop and maximize fluidic efficiency.

#2. Medical Devices.

• Surgical Instruments: Polishing and deburring cutting edges, internal lumens, and cosmetic engineering of complicated details to reduce friction during an operation, facilitate biocompatibility, and ease of cleaning. e.g., endoscope, catheter, biopsy.
• Implants: All polishing and finishing of implantable orthopedic devices (e.g., knee and hip) to produce a smooth area to reduce friction and promote body tissue integration.
• Drug Delivery Devices: Deburring and polishing of complex micro-fluidic channels and nozzles to calibrate precise drug delivery.

#3. Automotive Industry.

• Engine Components: Deburring and polishing of intake and exhaust ports, fuel injector bodies and cylinder heads improves airflow and fuel economy and reduces emissions.
• Transmission Components: Stripper and deburr of valve bodies and hydraulic passages provide freedom of motion for smooth shifting and reduced wear.
• Die and Mold Polishing: Polishing of intricate molds for plastic injection molding and die casting improves part release, reduces cycle times, and enhances the surface quality of the molded parts.

#4. Tool and Die Industry.

• Cutting Tools: Honing and polishing cutting edges on drills, reamers, and end mills to enhance tool life and cutting efficiency.
• Extrusion Dies: Polishing the internal surfaces of plastic and metal extrusion dies to create smoother extrudates and lower friction.
• Stamping Dies: Deburring and polishing of complex features in stamping dies to minimize galling and enhance part quality.

#5. Additive Manufacturing (3D Printing).

• Post-Processing 3D Print Parts: AFM is used more frequently as a means for improving finish and removes support structures from multi-component 3D-printed metal and plastic parts, as they are generally left having rough surfaces and internal porosity.

#6. General Manufacturing.

• Hydraulic and Pneumatic Valves: Finishing of spools and bodies for tight tolerances and leak-proof operation.
• Precision Gears: Deburring and edge radiusing gear teeth for reduced noise and longer life.
• Jewelry and Decorative Items: Polishing complex shapes and details.

The versatility of AFM to handle a wide range of materials, from soft plastics to hardened steels and ceramics, further expands its applicability.

Advantages of Abrasive Flow Machining

AFM has a lot of advantages over traditional finishing methods and is preferred in some applications.

• Access to Reach In Areas: This is by far AFM’s biggest advantage. AFM can effectively deburr, polish, and radius internal passages, cross-drilled holes, and complex geometries that cannot be accessed using conventional tools.
• Completely Uniform Removal of Material: The fact that the thickness of the deburring or polishing is self-deforming, there is uniform contact with the workpiece surface and uniform removal of material. This will yield a very uniform finish even on wide variety of angles and irregular geometries.
• Controlled Edge Radiusing: AFM can produce safe, predictable controlled-radius on the edges, that are essential in reducing stress concentration, thereby improving fatigue life and performance of the part.
• Improved Surface Finish: Achievable surface roughness values can be extremely low (down to Ra 0.05 µm and finer) which will aid in the aesthetic appearance of the part, will improve efficiency by reducing friction and in functionality improvement.
• Burr Removal and Stress Risers: removes burrs left over from the machining process to eliminate any premature part failure and increased life span. Removing stress risers also improves durability of the part.
• No Thermal Distortion: The fact that AFM is a cold process and produces little to no heat, there is no fear of thermal distortion, microcracking, or altering material properties.
• Processing Versatility: can work with nearly all alloyed metals (steel, aluminum, titanium, superalloys), ceramic-like materials, composites, and even plastics.
• Part Performance: Improved surface finish, radius, and deburring will contribute to better flow characteristics, less wear, increased fatigue life, and overall performance of the component.
• Automation Potential: AFM systems can be automated to be a high-volume process, allowing increased efficiency in production and reduced labor costs.
• Environmentally Friendly: Compared to some chemical finishing processes, creating less chemical waste because it is a contained abrasive medium. The media is also wash and recycled after filtration.

Disadvantages of Abrasive Flow Machining

Despite its numerous advantages, AFM also has certain limitations:

• High Initial Capital Cost: AFM machinery and tooling can be costly, creating a significant cash outlay.
• Consumable Media Cost: The abrasive media can be used a limited number of times because it is a consumable; depending on the volume of work produced, this can make this a significant contribution to the operating costs.
• Slow Material Removal Rate: AFM is primarily a finishing process and has a low rate of material removal associated with it, particularly compared to many traditional machining operations and thus is not appropriate for the removal of significant amounts of material.
• Need for Optimal Process Parameters: It typically takes some time, and on some occasions will require a fair amount of optimization, with parameters such as viscous media, abrasive type and size, flow pressure, and the number of cycles, before the desired finishing results are achieved.
• Specialized Tooling Design: Special tooling design for the purpose of precisely directing the abrasive media through complex internal passages can be overwhelming or too complex without an experienced tooling engineer.
• Not the Best Solution for All Geometries: AFM is generally understood to be a great option for finishing internal passages, but it is not always the most efficient or effective method for finishing larger flat external surfaces or extremely simple geometries, which are more easily suited for traditional processes.
• Contamination of Media: Extraction of material from workpiece contaminates the abrasive media, thereby reducing its effectiveness and length of service. To ensure a significant working life of the abrasive media regular and ongoing filtration should take place, along with refilling or replacing the media when required.
• Limited Edge Definition: While it makes controlled radius, AFM is not appropriate for the maintenance of sharp precise edges that require a zero-radius.
• Abrasive Particle Embedment: Incidents of an abrasive particle becoming lodged in the workpiece surface can occur with soft materials and aggressive processing, and on occasion it is possible to have embedment of the abrasive media, and this may be unacceptable for some applications as this contaminates the finish.
• Process Monitoring: Real-time monitoring throughout the finishing process can be difficult to accomplish, resulting in reliance on post-process inspection to ensure that the desired results were actually achieved.

It is important to understand both the strengths and the weaknesses of AFM as a process in relation to deciding it is suitable for any application.

AFM is best used when there are unique characteristics offered by the process and they align with a specific workpiece needs, for example, careful finishing of an inaccessible geometry or passage in a workpiece.

Conclusion

Abrasive Flow Machining is an effective and versatile non-traditional finishing process, filling the gaps where traditional processes do not work as effectively.

AFM is used to uniformly deburr, polish, and radius complex and hard-to-reach passageways, making it an essential production process in critical industries such as aerospace, medical, and automotive.

Utilizing the flow of viscoelastic abrasive media allows AFM to create excellent surface finishes, increase part performance, and allow components to last longer.

Although the initial cost and optimization of the process presents challenges, the significant advantages of high-quality surface integrity for complex geometries secures Abrasive Flow Machining as a key differentiator in advanced manufacturing.