Enhancing Adhesive Bonding in Cylindrical Stainless Steel Parts2025-09-09

The client, a manufacturer in the automotive assembly sector, required a batch of cylindrical stainless steel CNC machining parts with a central blind hole. These parts were integral to a sub-assembly where epoxy adhesive was used to bond them to mating components. During the initial inquiry and quoting process, the client specified two key modifications to the CNC machining:

・Increase friction on the outer diameter (OD)

The client requested surface treatments such as sandblasting or slightly rougher turning to enhance the mechanical interlocking with the adhesive, thereby improving bond strength and preventing slippage during curing and operation.

・Keep smooth end faces

Both ends of the cylinder must remain untreated by sandblasting, preserving a post-lathe machined finish with a surface roughness (Ra) of 0.8 μm or better. This was critical for aesthetic reasons and to ensure compatibility with downstream assembly processes, such as sealing or mating with flat surfaces.

The rationale behind the OD friction enhancement was straightforward: the adhesive's effectiveness relied on surface topography to create micro-mechanical anchors, distributing stress more evenly and reducing the risk of debonding under vibration or thermal cycling.

Problem Analysis

The engineering team conducted a thorough feasibility assessment, considering the part's geometry and the client's dual requirements. Key insights included:

・Challenges with Sandblasting

Sandblasting is an effective method for increasing surface roughness by creating a uniform, pitted texture that promotes adhesive wetting and mechanical grip. However, applying it selectively only to the OD while protecting the end faces posed significant production hurdles. Options like masking the ends with tape or fixtures could lead to inconsistencies, contamination, or additional setup time. Alternatively, oversanding the entire part and then re-machining the ends on a lathe for refinement would increase cycle times, material waste, and costs—potentially by 20-30% per part due to the need for secondary operations.

・Limitations of Rough Turning

Simply adjusting the lathe feed rate for a coarser OD finish (e.g., increasing from 0.1 mm/rev to 0.3 mm/rev) could achieve some roughness. However, this approach might not deliver the desired adhesive performance. Coarser turning often results in irregular peaks and valleys that are too large-scale for optimal glue penetration, potentially leading to weak bonding zones rather than the fine texture needed for capillary action and uniform stress distribution. Testing indicated a possible shortfall in bond strength compared to the client's benchmarks.

These considerations highlighted the need for a solution that balanced roughness enhancement with precision control, minimizing additional steps while ensuring repeatability across high-volume production.

Implementation of "Glue Grooves"

To address the challenges, the engineering team recommended a custom threading-inspired technique on the OD using standard lathe tooling. This method, internally dubbed "glue grooves," involved:

・Tooling and Parameters

Employing a 60-degree threading insert (commonly used for external threads) with minimal infeed (e.g., 0.05-0.1 mm depth) and a fixed pitch (e.g., 0.5-1.0 mm). The CNC lathe program was modified to traverse the OD longitudinally, creating a series of fine, helical grooves resembling miniature external threads.

・Surface Characteristics

The resulting texture provided a tactile roughness detectable by fingernail scraping, with groove depths and spacing optimized for adhesive flow. This micro-texture increases the effective surface area by 15-25% (based on preliminary profilometer measurements) without compromising the part's dimensional tolerances. Crucially, the end faces remained untouched during this operation, naturally retaining the smooth Ra 0.8 μm finish from the initial facing cuts.

・Production Integration

The glue grooves were incorporated into the primary lathe cycle, adding only few seconds per part—far less disruptive than sandblasting setups. No secondary masking or refinishing was required, reducing labor and scrap rates.

  Glue grooves on a cylindrical stainless steel parts  

Post-implementation trials, including pull-off adhesion tests per ASTM D4541 standards, demonstrated a 30-40% improvement in bond strength compared to smooth-turned samples, aligning closely with the client's performance goals. The client approved the approach during prototyping, leading to full-scale production of 5,000 units with zero defects related to surface finish.

Additional Recommendations

While the glue grooves proved highly effective, the engineering team suggests exploring complementary or alternative strategies for similar applications to further refine outcomes:

・Hybrid Surface Treatments

For parts with glue grooves requiring even higher friction, we recommend applying a strong degreasing process (e.g., nitric acid passivation for stainless steel, chemical conversion for aluminum alloy, acid pickling for steel). This could amplify roughness without affecting ends.

・Advanced Machining Alternatives

If volume justifies investment, integrate CNC knurling tools with diamond-pattern dies for the OD. This produces a cross-hatch texture that enhances grip in multiple directions, potentially increasing bond strength by another 10-20%. Learn more about Knurling (DIN 82).

  Knurling parts' OD to enhance adhesive bonding  

These techniques are adaptable for various cylindrical geometries and materials, making it a versatile addition to the company's process library. By the way, the solution promoted by Apporo met all specifications, with the client reporting enhanced assembly reliability in field tests. Future orders have incorporated glue grooves as a standard option. This case study underscores the value of innovative problem-solving in precision manufacturing, transforming potential constraints into competitive advantages. For inquiries on implementing similar solutions, contact our team.