Grit Selection Guide: Choose the Right Abrasive
Choosing the Right Grit Size: SiC, AO & Diamond Compared
Which abrasive, which grit? The practical guide to targeted selection for Flex-Hone® tools – including a decision matrix and grit size chart.
At a Glance
- Silicon Carbide (SiC) is the standard choice for cast iron, steel, and most non-ferrous metals
- Aluminum Oxide (AO) is the right choice for soft metals such as aluminum, copper, and brass
- Diamond is the choice for hardened surfaces and hard chrome; for Nikasil, aluminum oxide in grit 240 is enough to deglaze
- The grit number determines the achievable surface roughness – higher number = finer finish
Why Grit Size Matters
A workshop foreman stands in front of the catalog: Flex-Hone® tools in six grit sizes from 80 to 800, plus three different abrasives. That’s over a dozen combinations for a single diameter. Which one is right? The answer depends on exactly two factors: the workpiece material and the desired surface finish.
Choose too coarse a grit and the surface will be rougher than specified – components wear faster, seals leak. Choose too fine and there is insufficient material removal: the tool “smears” across the surface without producing a clean cross-hatch pattern. In the worst case, you merely push burrs aside rather than removing them.
Basic rule: The grit size defines the roughness; the abrasive must match the workpiece material. Both factors must be correct – the wrong combination costs time, tools, and in the worst case, the workpiece itself.
This guide provides a systematic decision-making framework: First we explain the three abrasive types, then the grit scale with specific roughness values. Finally, you will find a decision matrix that maps material and target finish directly to a recommendation.
Abrasives Compared: SiC, AO & Diamond
Flex-Hone® tools are available with three different abrasive materials. Each has specific properties that make it ideal for certain workpiece materials and applications.
Silicon Carbide (SiC) – The Standard
SiC is the hardest of the three abrasives (Mohs hardness 9.5) and the most widely used. Its sharp-edged grains cut aggressively and produce a clean cross-hatch pattern. SiC is suitable for the majority of applications: cast iron, steel, stainless steel, and harder non-ferrous metals such as bronze.
- Best choice for cast iron cylinders, steel bushings, hydraulic cylinders
- High material removal with a consistent cutting pattern
- Approx. 80% of all Flex-Hone® orders use SiC
Aluminum Oxide (AO) – For Soft Metals
AO grains are less aggressive than SiC and produce a finer, more uniform surface on soft materials. Where SiC cuts too deeply and leaves deep scratches, AO works in a more controlled manner. Typical applications: uncoated aluminum cylinder bores, copper alloys, brass, and soft bronzes.
- Best choice for aluminum, copper, brass, zinc
- Gentler removal – prevents “digging in” on soft surfaces
- More uniform surface finish at lower Ra values
Diamond – For Hardened & Coated Surfaces
When SiC no longer achieves material removal, diamond comes into play. Hardened steel surfaces (> 50 HRC) and hard chrome coatings can only be processed with Diamond Flex-Hone® tools. With Nikasil cylinders it depends on the goal: for deglazing before a re-ring the manufacturer recommends aluminum oxide in grit 240. Diamond is only needed there when material genuinely has to come off.
- First choice for hard chrome, Galnikal and ceramic – on Nikasil only for real material removal
- Required for workpiece hardness > 50 HRC
- Higher initial cost, but longer tool life on hard materials
Caution with aluminum cylinders: Aluminum engine blocks are often coated with Nikasil. The coating looks externally similar to bare aluminum – but the right abrasive is a different one. Verify whether a coating is present before ordering. If in doubt, we are happy to advise you.
| Property | SiC | AO | Diamond |
|---|---|---|---|
| Hardness (Mohs) | 9.5 | 9.0 | 10 |
| Cutting behavior | Aggressive, sharp-edged | Gentle, controlled | Very aggressive on hard surfaces |
| Cast iron / Steel | ✓ Ideal | ✗ Too slow | ✓ Possible, but not cost-effective |
| Aluminum / Copper | ✗ Too aggressive | ✓ Ideal | ✗ Overkill |
| Nikasil | ✗ More aggressive than needed | ✓ Ideal for deglazing (grit 240) | ✗ Material removal only |
| Hard chrome | ✗ No effect | ✗ No effect | ✓ Only option |
| Hardened steel (> 50 HRC) | ✗ Greatly reduced removal | ✗ No effect | ✓ Required |
| Relative cost | ★ Low | ★ Low | ★★★ Higher |
Understanding the Grit Scale: From P80 to P800
The grit number indicates the fineness of the abrasive: the higher the number, the finer the grains, the smoother the result. Flex-Hone® tools use the internationally recognized FEPA/mesh scale (P-grit), which is also standard for sandpaper and grinding wheels.
What does the number mean? The grit number (e.g., P240) corresponds to the number of mesh openings per inch in the test sieve. P80 means the grains just pass through a sieve with 80 mesh openings per inch – relatively coarse. P800 requires 800 mesh openings – extremely fine. The resulting surface roughness (Ra/Rz) also depends on material, contact pressure, and processing time.
| Grit | Grain size (approx.) | Typ. Ra range | Typ. Rz range | Typical application |
|---|---|---|---|---|
| P80 | 200 µm | 1.6–3.2 µm | 8–16 µm | Heavy deburring, material preparation |
| P120 | 125 µm | 0.8–1.6 µm | 5–10 µm | Deburring, edge rounding, coarse cross-hatch |
| P180 | 82 µm | 0.4–0.8 µm | 3–6 µm | Standard plateau honing, cylinder finish (cast iron) |
| P240 | 58 µm | 0.3–0.6 µm | 2–4 µm | Fine plateau finish, hydraulic cylinders |
| P320 | 46 µm | 0.2–0.4 µm | 1.5–3 µm | Fine honing, pneumatic cylinders, valve guides |
| P600 | 26 µm | 0.1–0.2 µm | 0.8–1.5 µm | Super-fine finish, medical technology |
| P800 | 21 µm | < 0.1 µm | < 0.8 µm | Mirror/gloss finish, sealing surfaces |
Practical tip: The table values are guidelines for typical conditions (standard RPM, honing oil, 10–15 double strokes). On soft materials (aluminum), Ra values tend to be lower; on hard cast iron, somewhat higher. Detailed RPM and stroke parameters also influence the result.
Decision Matrix: Material × Finish → Recommendation
The following matrix combines workpiece material and desired result into a specific recommendation for abrasive and grit size. Find your material in the left column and your processing goal in the top row.
| Material | Deburring / Preparation | Standard cross-hatch | Fine plateau finish | Mirror finish |
|---|---|---|---|---|
| Gray cast iron (GJL/GJS) | SiC P80–P120 | SiC P180–P240 | SiC P320 | SiC P600–P800 |
| Steel (unhardened) | SiC P80–P120 | SiC P180–P240 | SiC P320 | SiC P600–P800 |
| Stainless steel | SiC P120 | SiC P240 | SiC P320–P600 | SiC P800 |
| Aluminum (uncoated) | AO P80–P120 | AO P180–P240 | AO P320 | AO P600–P800 |
| Copper / Brass / Bronze | AO P120 | AO P240 | AO P320–P600 | AO P800 |
| Hardened steel (> 50 HRC) | Diamond P120 | Diamond P240 | Diamond P320 | Diamond P600 |
| Nikasil / Hard chrome | – | Diamond P240 | Diamond P320 | Diamond P600 |
Further reading: For a comprehensive overview of all abrasive types – beyond Flex-Hone – see our Abrasive Guide.
Practical Tips: Grit Selection in Production vs. Workshop
Selecting the right grit size follows different rules in series production than in one-off manufacturing or the workshop. Both scenarios have typical pitfalls.
Series Production: Repeatability Comes First
- Measure first, then order Define the target roughness (Ra/Rz) precisely according to the drawing or standard. Select the grit size using the chart above – and order test samples in two adjacent grit sizes (e.g., P240 and P320) to validate the optimal variant in your actual process.
- Document parameters Record RPM, stroke speed, and the number of double strokes. Only with consistent parameters will you achieve reproducible results across hundreds of workpieces.
- Monitor tool life Flex-Hone® tools wear evenly, but after 50–200 bores (depending on material), material removal decreases. Define a replacement interval and integrate it into your production plan.
- Take advantage of volume pricing At higher quantities, volume discounts make a significant difference. The cost per tool drops considerably – ask specifically about volume pricing for your production run size.
Workshop & Repair: Flexibility Comes First
- Basic kit: One SiC P180 and one SiC P320 cover approx. 80% of all workshop applications (deburring to fine plateau finish)
- Aluminum jobs: Add at least one AO P240 for aluminum and brass parts
- Multi-step approach: For rough starting conditions, pre-work with P120 first, then finish with P240 or P320 – this saves the finer tool and reduces costs
- Test run before the workpiece: For unknown materials, always test on a non-critical area or a scrap part first
Common mistake: Using too fine a tool on a rough starting surface. If the prior machining operation (drilling, turning) has left Rz values above 10 µm, a P320 will not eliminate those scratches. Start with P120 or P180, then switch to the fine grit.
Found your grit? Select the right tool
| Abrasive | Available grit sizes | Recommended for | |
|---|---|---|---|
| SiC (Standard) | P80–P800 | Cast iron, steel, stainless steel Volume pricing | Select |
| AO (Aluminum Oxide) | P80–P800 | Aluminum, copper, brass, bronze | Select |
| Diamond | P120–P600 | Nikasil, hard chrome, hardened steel | Select |
| NamPower® abrasive disc brushes | Various | Flat surfaces, threads, external processing Volume pricing | Select |
Conclusion: Choosing the Right Grit Is Not a Guessing Game
Selecting the abrasive and grit size follows clear rules: SiC for ferrous metals, AO for soft non-ferrous metals, Diamond for coated and hardened surfaces. The grit number then determines the achievable roughness – from heavy deburring (P80) to mirror finish (P800). Use the decision matrix above as a starting point and, when in doubt, validate with test samples in two adjacent grit sizes. Our application engineers are happy to assist you with the selection for your specific application.
FAQ
Can I both deburr and finish with a single grit size?
To some extent. Medium grit sizes such as P180 or P240 can handle both in a single operation, provided the starting condition is not too rough (Rz < 8 µm). For coarser pre-machined surfaces, a two-step process is recommended: first P120 for deburring and leveling, then P240 or P320 for the desired plateau finish. This protects the finer tool and delivers better results.
Which grit size do I need for a motorcycle cylinder?
That depends on the cylinder material. Cast iron cylinders: SiC P240 for the standard cross-hatch finish that shortens the piston ring break-in period. Uncoated aluminum cylinders: AO P240. Nikasil-coated cylinders (common on KTM, BMW, Porsche): AO P240 for deglazing. You can find the matching diameter in our Flex-Hone range – or read our guide to cylinder honing.
What happens if I use the wrong grit size?
Too coarse a grit produces a rougher surface than specified – leading to increased wear, leaking connections, or increased oil consumption in engines. Too fine a grit cannot eliminate existing scratches on rough starting material: the tool slides over the peaks without producing a clean cross-hatch pattern. Regarding the abrasive: SiC on soft aluminum digs in and leaves deep scratches; on Nikasil, SiC bites more than necessary – AO P240 is the better choice there.
How many bores can a Flex-Hone process before it needs to be replaced?
The tool life of a Flex-Hone® depends primarily on material and grit size. As guidelines: In cast iron with SiC, a tool typically processes 50–200 bores; in aluminum with AO, 100–300 bores. Diamond Flex-Hone on Nikasil often achieves 30–80 bores. Wear is recognizable by decreasing material removal and the visual condition of the abrasive globules. For series applications, we recommend establishing a replacement interval based on roughness measurements.
Find the Right Flex-Hone® Grit Size
Over 400 variants in SiC, Aluminum Oxide, and Diamond – for diameters from 4 to 200 mm, all grit sizes in stock.
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