Honing Explained: Process, Tools & Applications
Honing Explained: Process, Applications & How It Differs from Grinding
From the principle of superimposed cutting motion to selecting the right tool: What honing can do, when it pays off and how it differs from grinding.
At a Glance
- Honing is a finishing process using geometrically undefined cutting edges to achieve the tightest tolerances
- Typical results: roundness < 1 µm, surface roughness Ra 0.1–0.8 µm, diameter tolerance IT5–IT7
- The characteristic cross-hatch finish improves oil retention and sealing performance in cylinders
- Flexible honing tools (Flex-Hone®) enable plateau finishing without expensive honing machines
What Is Honing? Definition and Principle
You've machined a cylinder bore on your CNC – the diameter is spot on, the surface looks good. But when you measure the roughness, the Rz values come in at 8–12 µm. For a hydraulic seal, you need less than 4 µm with a defined cross-hatch finish. This is exactly where honing comes in.
Definition: Honing (DIN 8589-14) is a material-removing finishing process using geometrically undefined cutting edges. The superimposition of rotation and linear stroke motion produces a characteristic cross-hatch pattern – the so-called plateau finish.
What makes honing special: Unlike grinding or turning, the goal is not just material removal but the deliberate creation of a functionally optimized surface. The cross-hatch grooves generated during honing form channels that retain lubricant – crucial for the service life of seals, piston rings and bearings.
The Principle of Superimposed Motion
Honing involves two simultaneous motions:
- Rotation: The tool rotates inside the bore (slowly, typically 60–200 RPM)
- Stroke motion: Simultaneously, it moves axially up and down
- Expansion: The honing stones or balls are pressed radially against the bore wall
The ratio of rotation to stroke determines the cross-hatch angle – typically 30–60°. This angle defines the oil-carrying capacity of the surface.
Honing Processes Overview
Different honing processes are used depending on the objective and workpiece. The three most important in day-to-day industrial practice:
- Long-Stroke Honing The classic process for cylinder bores. The tool performs long axial strokes, systematically removing material. Result: the tightest dimensional and form tolerances (roundness < 1 µm). Typical for engine blocks, hydraulic cylinders and pneumatic components. Requires a dedicated honing machine with precise stroke control.
- Short-Stroke Honing (Superfinishing) Here the tool oscillates with short, rapid strokes (amplitude 1–6 mm) at high frequency. The goal is not dimensional correction but surface improvement. Shafts, cylinder liners and bearing seats are brought to Ra < 0.1 µm. Also known as superfinishing or microfinishing.
- Plateau Honing (Flex-Hone®) A simplified, flexible honing process: Ball-tipped nylon filaments conform to the bore geometry and produce a plateau finish – the peaks of the roughness profile are removed while the valleys remain as oil reservoirs. No stock removal required, usable in any drill press or CNC spindle. Ideal for post-processing after conventional honing or grinding.
Practical Tip: In many workshops, conventional honing (long-stroke) is combined with a subsequent plateau honing pass using a Flex-Hone®. The first step achieves dimensional accuracy, the second produces the optimal surface finish – with a tool that delivers results in seconds.
Important – Honing Oil: Honing only works with a suitable metalworking fluid (honing oil). Without adequate lubrication and cooling, the abrasive grains load up and the tool "smears" instead of cutting. With Flex-Hone, the requirements are lower – a light honing oil or WD-40 is often sufficient.
Honing vs. Grinding: What's the Difference?
Both processes work with bonded abrasive – but the results differ fundamentally. In grinding, the tool (grinding wheel) rotates at high speed while the workpiece is fed slowly. In honing, it's the opposite: the tool moves slowly, maintains constant surface contact and removes material uniformly.
| Criterion | Honing | Internal Grinding |
|---|---|---|
| Cutting Speed | 10–80 m/min (slow) | 1,500–3,000 m/min (fast) |
| Tool Contact | Multiple stones, surface contact | Grinding wheel, line contact |
| Surface Structure | ✓ Cross-hatch (plateau) | ✗ Unidirectional grooves |
| Dimensional Accuracy | IT5–IT7 (very high) | IT6–IT8 (high) |
| Roundness Correction | ✓ Actively corrected | ✗ Limited |
| Heat Generation | Minimal (no microstructure impact) | Higher (risk of thermal damage) |
| Oil Retention of Surface | ✓ Excellent (cross-hatch) | ✗ Low |
| Typical Application | Cylinders, hydraulics, bearings | Bores, bushings |
Key Difference: Grinding produces a smooth but "sealed" surface. Honing produces a surface with defined oil pockets – which is why cylinder bore surfaces are always honed, never just ground. The surface roughness values Ra and Rz alone are not sufficient – the material ratio (Abbott curve) is what matters.
Typical Applications and Industries
Honing is indispensable wherever bores must be both dimensionally accurate and functionally optimized. The most important fields of application:
- Engine Manufacturing: Cylinder bore surfaces in combustion engines – the classic application. The cross-hatch finish ensures oil supply to the piston rings and minimizes blow-by
- Hydraulics & Pneumatics: Cylinder liners, valve bores, spool valves – tolerances in the µm range for leak-free operation
- Transmission Manufacturing: Bearing bores and gear bores with press fit – here, roundness determines noise and service life
- Medical Technology: Syringe barrels, implant bores – the highest surface quality for biocompatible applications
- Printing Industry: Ink rollers and anilox rollers with a defined plateau finish for uniform ink transfer
- Tool & Mold Making: Guide bores, ejector bushings – dimensionally accurate and wear-resistant
Special Case – Nikasil Cylinders: Coated cylinders (Nikasil, Galnikal) may only be machined with diamond honing tools. Standard SiC grit has no effect on the hard nickel-silicon carbide layer. Learn more in our guide Honing Nikasil Cylinders.
Honing Tools: From Honing Stones to the Flex-Hone®
The range of honing tools is broad – from manual honing stones to fully automated CNC tools. For most applications in workshops and series production, three tool types are relevant:
- Conventional Honing Stones Bonded abrasive (SiC, CBN or diamond) in stone form, clamped in an expanding mandrel. Mounted in dedicated honing machines. Highest precision, but high investment costs and setup times. Typical for series production with tight tolerances.
- Flex-Hone® (Flexible Honing Tools) Ball-tipped nylon filaments on a flexible shaft. The tool is self-centering and conforms to any bore geometry. No stock removal needed, no honing machine required – a drill press or CNC spindle is sufficient. Ideal for deburring, plateau honing and surface finishing in a single operation.
- Honing Brushes & Special Tools For special cases: Miniature brushes for bores as small as 1 mm diameter, tube brushes for through-bores, and NamPower® abrasive disc brushes for flat surfaces and external geometries.
The Right Honing Tool for Your Application
| Application | Diameter | Recommendation | |
|---|---|---|---|
| Plateau honing, deburring, cross-hatch finish | 4–200 mm | Standard Flex-Hone® Volume Pricing | Select |
| Nikasil cylinders, hardened bores | 20–150 mm | Diamond Flex-Hone® | Select |
| Precision mechanics, watchmaking, medical technology | 1–12 mm | Miniature Brushes | Select |
| Flat surfaces, threads, external machining | 50–150 mm | NamPower® Abrasive Disc Brushes Volume Pricing | Select |
| Tubes & through-bores | 5–200 mm | Tube Brushes | Select |
Conclusion: When Is Honing Worth It?
Honing is the right choice whenever tightly toleranced bores require a functionally optimized surface – meaning a defined roughness, high roundness and ideally a cross-hatch pattern for oil retention. For series production with µm-level tolerances, there is no way around conventional honing machines. But for plateau finishing after drilling, grinding or turning – and for all workshop applications – a Flex-Hone® delivers reproducible results in seconds. No special machine, no setup time, using existing infrastructure.
FAQ
What does honing cost per part?
Per-part costs depend heavily on the process. Conventional long-stroke honing on a honing machine typically costs €2–15 per bore (including tool wear and machine time). A Flex-Hone® for plateau honing starts at approx. €25 and processes 50–200 bores depending on the material – resulting in per-part costs of €0.15–0.50. Volume purchasing further reduces tool costs.
Can you hone without a honing machine?
Yes. Flexible honing tools such as the Flex-Hone® can be used in any standard drill press, bench drill or CNC spindle. They are self-centering and require neither a dedicated honing machine nor fixtures. The result is a defined plateau finish with cross-hatch pattern – however, without the dimensional correction capability of a honing machine.
Which grit size for which surface?
The grit size determines the achievable roughness: Coarse grits (80–120) leave a rougher surface and are suited to deburring and fast material removal. Medium grits (180–240) produce a standard plateau finish. Fine grits (320–600) achieve a mirror finish with Ra < 0.2 µm. A detailed overview is available in the guide Choosing the Right Grit Size.
What RPM should I use when honing with a Flex-Hone?
The recommended RPM depends on the diameter: The larger the bore, the lower the RPM. As a rule of thumb, aim for a cutting speed of 30–60 m/min. For a 20 mm Flex-Hone this means approx. 500–1,000 RPM; for an 80 mm diameter, only 120–240 RPM. Excessive RPM leads to overheating and premature wear of the filaments.
Find the Right Honing Tools
Over 400 Flex-Hone® variants for diameters from 4 to 200 mm – in Silicon Carbide, Aluminum Oxide and Diamond.
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