Introduction

While robotic welding has been around for some time, and has only accelerated over the past 10 years with collaborative robots, the automation of downstream processes like material removal and surface finishing have lagged. That’s now changing with the emergence of AI-powered robots that use 3D vision and force feedback sensors to accurately and consistently process parts. However their integration into a manufacturing process requires a systems-level mindset to maximize the likelihood of success and sustained adoption in your operations. In this article, we highlight a few key areas to prioritize when considering the adoption of automation specifically in graining and blending processes, which are particularly difficult but also critically important to the final part. Grained finishes such as No. 3 and No. 4 finishes are ubiquitous in architectural and appliance applications, where the directional “grain” provides aesthetic sophistication while minimizing the effects of scratches from daily use.

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Upstream Processing Consistency

Graining and blending typically occurs after the fabrication processes including machining, bending and welding. If those processes are performed manually, part-to-part variation must be considered in the downstream finishing process. This can lead to additional system complexity and costs in the way of fixturing, sensors or AI capabilities. For example, a manual weld bead will have variability in the contours and shape, requiring characterization before a leveling material removal task can be performed.

Additionally, the nominal material surface may affect the graining operation. Pre-existing scratches will require a pre-processing step, either manually or with the proper selection of abrasive grits to prime the surface. Pre-polished material may vary from mill to mill, and the blending operation in a post-weld or post-bend area will require additional vision or AI capabilities to correctly blend the grain.

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Process, Tool and Media Selection

Graining is affected by the abrasive type, applied force, RPM and speed across the surface. A wide spectrum of abrasive materials is available for achieving specific surface finishes, each possessing unique characteristics that make them suitable for different materials and applications. Among the most common are aluminum oxide, ceramic alumina, silicon carbide, zirconia alumina, and diamond. Aluminum oxide is a versatile and economical choice, widely used for grinding and finishing ferrous metals and softer alloys. Ceramic alumina, a premium option, offers exceptional hardness and longevity, making it ideal for working with alloys and hardened steels, including aluminum castings, stainless steel, and titanium alloys. Silicon carbide, known for its sharpness, is particularly effective on nonferrous metals like aluminum and stainless steel, often producing a finer surface finish compared to aluminum oxide. Zirconia alumina stands out for its durability and is well-suited for heavy grinding and high-pressure applications on hard metals and high-strength alloys. Diamond abrasives, the hardest available, are reserved for grinding extremely hard materials and applications requiring the highest level of precision.

These abrasive materials are manufactured into various forms to suit different stages of finishing. Your robot solution provider can advise on the best options to replace a manual process, but may not be a direct 1:1 translation. For example, a stroke sander process might make better sense as a flap wheel abrasive form factor when automated. In other cases, a belt sander designed for robotic arms may be justified. Abrasive manufacturers with automation-specific solutions are a great resource for understanding options, including Walter, Norton and 3M. 

Graining applications must carefully consider the properties of the base material and the desired final surface condition to select the most appropriate abrasive grit and, potentially, sequence of grits. Different metals possess varying degrees of hardness, ductility, and thermal conductivity, necessitating abrasives with specific characteristics for effective processing. Using an unsuitable abrasive can lead to inefficient material removal, excessive heat generation, or a subpar surface finish. 

To achieve specific grained finishes, such as a brushed or satin appearance, a progression of abrasives is often necessary. Start with a coarser grit to reduce the weld bead and then move to finer grits to smooth the surface further, up to 180 or 220 grit. For softened grain, nonwoven abrasives (e.g. 3M’s Scotch Brite or Norton’s Bear-Tex) are typically used. It’s also possible to combine steps by selecting an interwoven combo wheel, that weaves coated abrasive sheets with alternating layers of a non-woven abrasive. 

The following additional tips are recommended for automating graining:

1. The repeatability of robot motion paths can exacerbate a “tiger striping” look where the passes are not blended seamlessly. Minimize the overlap of each pass to approximately 10% or less. Switch to a wider non-woven wheel for the final process step to then blend the prior passes.
2. Repeatability on similar parts can also lead to premature, uneven wear of the abrasive. Adjust motion paths to utilize the full surface of the abrasive. AI-based solutions can help readily randomize motion paths to account for this.
3. If cycle time allows, prefer higher grits with multiple passes instead of starting progressions at lower grits. Lower grits will create a deeper grain pattern that can make blending more difficult.
4. Leverage force control and/or compliance hardware to accurately apply consistent abrasive force to the part during the graining/blending process. 5-10 lbf is a good starting point, with a robot TCP speed of 2”/sec. 

Conclusion

Ultimately, the desired finish grain and blending needs dictate the selection of the abrasive material, its form, and the processing steps. Successful automation requires a systematic approach that considers the upstream processes. New technologies such as AI-powered robot programming and 3D scanning add enhanced capability and versatility to graining solutions.

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