Dan Conner, Business Development Manager and Dave Rogers, Director of Engineering, Sunnen Products Company
Honing is a precision manufacturing process that plays an increasingly vital role across industries, driven by the growing demand for cylindrical bores with ultra-tight tolerances and flawless surface finishes.
The honing process, along with its counterpart, lapping, can achieve tolerances measured in millionths of an inch—far surpassing the precision of most other machining methods by one or two orders of magnitude.
Success hinges on every element of the process working in perfect harmony, including tooling, abrasives, coolants or lubricants, and process control. When these components align seamlessly, the result is unmatched precision and accuracy.
Mastering Precision: Bridging the Knowledge Gap
Despite its essential role, honing remains a niche practice, representing less than 0.5% of the $120 billion global machine tool industry. As a highly specialized technique, it is rarely covered in engineering education, trade schools, and technical textbooks.
This knowledge gap often leaves businesses—ranging from top aerospace and medical device manufacturers to small engine repair shops—lacking the in-house expertise needed to meet stringent tolerances and consistently uphold them in a production setting.
Given the precision and complexity involved, collaborating with a solution provider that specializes in honing is a smart choice. These experts have spent years "honing their skills" and applying their expertise to solve practical challenges across diverse applications.
They provide integrated solutions for businesses of all sizes, offering turnkey systems that cater to a wide range of needs—from tailored solutions for smaller operations to fully customized automated systems designed for high-precision or high-volume production.
Precision Boring Techniques
Precision boring, honing, and lapping are essential machining processes aimed at achieving extremely precise geometries and exceptional surface finishes.
Boring is the process of enlarging a drilled cylindrical hole to create the desired alignment and diameter. While boring handles the creation of the hole, honing and lapping take over to achieve the final, exacting standards of refinement.
Honing is an ultra-precise bore machining process designed to perfect the geometry and surface finish of bores. Using a tool or mandrel equipped with engineered abrasive stones, the process combines rotational motion with reciprocal stroking inside the bore. As the tool moves in and out, the abrasive stones expand against the bore’s walls, gradually refining the interior surface.
This method excels at correcting minor imperfections while achieving exceptional dimensional accuracy. Honing is also valued for creating specific surface textures, such as the crosshatch pattern commonly found in engine cylinders and hydraulic components. This texture enhances lubrication retention and reduces friction, making it essential for applications where performance and durability are paramount.
No boring or grinding process is capable of delivering such precise levels of roundness, straightness and surface geometry at a lower cost per part.
Lapping is a precision finishing process that involves rubbing two surfaces together with an abrasive slurry to achieve exceptional flatness and an ultra-smooth finish. It is a highly specialized bore finishing technique, designed for applications demanding extremely tight tolerances that surpass the capabilities of advanced honing. Advanced lapping techniques deliver exceptional precision, achieving cylindricity up to 0.000060” (0.0015 mm), straightness up to 0.000040” (0.0010 mm), and roundness as precise as 0.000015” (0.0004 mm).
Once a labor-intensive and error-prone manual task, lapping has been revolutionized by automation. Automated bore lapping machines utilize specially engineered lap tools that distribute an abrasive compound evenly between the tool and the bore walls. State-of-the-art machines are equipped with automatic gauging systems that continuously monitor bore size and adjust the process to precise specifications, eliminating the need for manual intervention.
The benefits of automated lapping are significant: improved consistency, reduced reliance on manual labor, minimized risk of repetitive motion injuries, enhanced workplace safety, and tighter tolerances.
Bore Geometry
Bore geometry is defined by two critical factors: size and shape. Both are essential for ensuring precision and proper functionality in design and application.
Dimensions
The dimensions of a bore refer to its measurable physical size, specifically:
- Diameter: The width of the bore, measured across its cross-sections.
- Length: The total distance from one end of the cylinder to the other.
Shape
The shape of a bore is determined using Geometric Dimensioning and Tolerancing (GD&T) principles, which provide guidelines for consistent and precise design. Key elements include:
- Flatness: Represents a perfectly even surface, free of any irregularities like bumps or dips, existing within a single two-dimensional plane.
- Cylindricity: applies the concept of flatness to a cylindrical shape, resulting in a smooth, seamless tubular form.
- Circularity: Ensures the roundness of the cylinder by guaranteeing that every cross-section perpendicular to its central axis is a perfect circle. This precision maintains a uniform diameter throughout the bore, eliminating distortions or unevenness.
- Axial Straightness: Refers to the alignment of the bore’s central axis, ensuring it stays perfectly straight from end to end. Proper axial alignment is critical for structural integrity and accurate functionality.
Surface Finish
Surface finish plays a critical role in the performance and longevity of machined components. Honing often produces a smooth finish, with surfaces polished to within microns or nanometers of specified roughness standards. However, certain applications require textured finishes, such as cross-hatching or plateau honing, to optimize performance. Cross-hatched patterns, for instance, are widely used in the automotive industry for engine cylinders, where the texture helps retain lubrication, reducing wear and promoting efficiency.
Some applications demand uniquely tailored finishes. Medical devices, such as orthopedic implants, often require an ultra-smooth finish to minimize friction and discomfort while ensuring biocompatibility. Conversely, hydraulic systems may benefit from plateau honing, creating a balanced surface that reduces break-in periods and lowers the risk of contamination.
Mastering the interplay of geometry and surface finish across different materials and industries is what makes precision boring, honing, and lapping so impactful. By customizing these parameters to meet specific requirements, manufacturers can produce components that perform to the highest standards in even the most challenging environments.
- Too smooth:
- If the surface is too smooth, it may not adequately hold lubrication, leading to increased friction and seal wear.
- Too rough:
- A surface that is too rough can also damage seals by abrading them against the peaks and valleys.
Getting it Right Every Time
Ensuring the honing and lapping process is executed correctly is critical to maintaining overall product quality and cost efficiency. These processes occur at the final stages of production after significant time, effort, and resources have already been invested in each part.
Since honing and lapping are subtractive processes designed to refine and perfect surfaces, any errors at this stage are difficult, if not impossible, to correct. Not only are defects irreversible but diminished quality also compromises the performance and durability of the final product.
When honing is done correctly—with the right coolant, abrasive, and precisely controlled parameters such as pressure and speed—it eliminates the risk of surface stress known as "white layer," a common byproduct of other machining processes.
Think of the white layer as a crack in asphalt: once it forms, external factors, like freezing, can exacerbate the damage, eventually creating a pothole. Similarly, white layer deterioration can lead to premature component failure, even if the underlying material remains intact.
This problem is especially critical in high-performance industries like aerospace, where honed surfaces face extreme pressures, but it also occurs in areas like automotive transmissions and hydraulic systems.
Therefore, quality failures not only result in direct costs, such as material waste and rework expenses, but also carry indirect costs like production delays, reputational damage, and loss of customer trust.
Holistic Approach
Honing and lapping may seem like specialized processes, but with the right partner, anyone can achieve expert results. At Sunnen, we simplify the path to excellence by offering complete solutions that combine cutting-edge technology with decades of hands-on expertise. Our comprehensive approach ensures success at any scale, with every component—from tooling and abrasives to coolants, lubricants, and process control—engineered to work seamlessly together.
Whether you're a small business looking for a cost-effective turnkey solution or managing a large-scale operation in need of customized automation, a trusted honing partner can deliver the perfect setup. These integrated solutions are designed to boost precision and productivity, helping businesses streamline workflows, reduce downtime, and consistently achieve exceptional outcomes—no matter the complexity or scale of production.