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Quick answer: G-ratio is the volume of workpiece removed divided by the volume of wheel worn: G = Vworkpiece / Vwheel. A higher G-ratio means less wheel wear per part. Cost per part depends on G-ratio, wheel price and dressing cost — not on wheel price alone.
Related reading: Metal Bond Diamond Grinding Wheel · Diamond Wheel Concentration Selection · Diamond Grinding Wheel Troubleshooting.
What G-ratio tells you
G-ratio (also called the grinding ratio) is a measure of wheel efficiency. If you remove 1000 mm³ of workpiece and wear away 10 mm³ of wheel, the G-ratio is 100. Harder bonds (metal) and suitable grit/concentration give higher G-ratios; softer bonds (resin) give lower ones but finish better.
| Wheel / material | Typical G-ratio | Comment |
|---|---|---|
| Metal bond diamond | High (often 100–1000+) | Best wheel economy |
| Vitrified diamond | Medium–high | Good balance with finish |
| Resin bond diamond | Low–medium | Finer finish, more wear |
| Electroplated diamond | Low (single layer) | Cost is per-layer, not G-ratio |
Ranges vary widely by material and process; measure on your own parts for decisions.
Cost per part, not wheel price
A cheaper wheel is not necessarily cheaper to use. The real metric is cost per good part:
- Cost per part ≈ (wheel cost / parts per wheel) + (dressing cost) + (machine time cost).
- A longer-life metal bond wheel usually lowers cost per part at high volume, even though it costs more.
- A short-life resin wheel is often cheaper for low volume and fine finish.
- Reduce cost by improving G-ratio (right grit/concentration/bond), cutting dressing frequency, and minimising scrap.
How to improve G-ratio
Use a harder bond or higher concentration for the material, avoid excessive infeed, keep the cut cool, and dress little and often. Do not chase G-ratio at the expense of the finish the part needs.
A simple worked example
| Wheel | Price | Parts per wheel | Cost per part |
|---|---|---|---|
| Resin bond | Low | 1,000 | Low |
| Metal bond | High | 8,000 | Usually lower |
| Electroplated | Low–medium | Single layer, replaced | Varies with load |
Fill in your own numbers: price and parts per wheel are all you need to compare.
Balancing G-ratio with quality
G-ratio is a tool, not a target in itself. A wheel that lasts forever but leaves a poor surface costs more in scrap and rework than it saves in wheel life. Choose the specification that minimises cost per good part at the finish the customer requires.
Sub-table 2 — by grinding process
| Process | Typical bond | Grit (FEPA D) | Concentration | Common shapes | Notes |
|---|---|---|---|---|---|
| Tool & cutter grinding (flute, gash) | Resin / Metal | D91–D181 | 100–150% | 11V9, 12V9, 1A1 | Cool cutting protects carbide edges |
| Surface / flat grinding | Resin / Vitrified | D64–D151 | 75–125% | 1A1, 6A2 | Balance finish vs. removal |
| OD cylindrical | Metal / Vitrified | D91–D181 | 100–150% | 1A1, 1F1 | Even coolant coverage |
| ID / internal | Resin / Metal | D46–D126 | 75–125% | Cup, mounted points | Small wheels, high speed |
| Form / profile / creep-feed | Metal / Vitrified | D64–D151 | 100–150% | 6A2, 11V9, 12V9 | Profile retention is critical |
| Centerless | Metal / Resin | D91–D181 | 100–150% | 1A1, 6A2 | High volume, tight roundness |
| Back-grinding / wafer thinning | Electroplated / Resin | D126–D400+ | 100–200% | Cup, 1A1 | Ultra-fine finish, low damage |
| Glass edge / bevel / CNC | Metal / Resin | D64–D126 | 75–125% | 1A1, 1F1, 12A2 | Prevent edge chipping |
| Cut-off / slot / groove | Electroplated / Metal | D181–D400 | — | 1A1R, saw | Thin wheels, rigid setup |
| Dressing & truing (of other wheels) | — | D46–D181 | — | Pen, roller, block | Diamond tools, not diamond wheels |
Measuring G-ratio on the shop floor
- Fix the batch. Record the number of parts and the stock removed per part.
- Measure the wheel before. Diameter or weight, with the same method you will use afterwards.
- Run to a defined end point. Stop at a fixed part count or when the finish limit is reached, not when the wheel feels dull.
- Measure the wheel after. Convert diameter loss to volume, or use weight loss directly.
- Calculate. G-ratio = volume of workpiece removed ÷ volume of wheel worn.
- Repeat once. A single measurement is noise; two consistent runs are a baseline.
Levers that improve G-ratio
- Coolant: better delivery lowers temperature and slows bond wear.
- Dressing: a properly opened wheel cuts instead of rubbing.
- Bond: a harder or more wear-resistant bond holds grains longer.
- Concentration: more diamond spreads the load per grain.
- Parameters: excessive speed or infeed burns life without adding output.
- Machine rigidity: vibration chips grains and destroys the layer early.
Browse the matching Diamond & CBN Grinding Wheels range for this application.
When G-ratio misleads
A high G-ratio is not automatically cheaper. If the wheel holds form by glazing, the part may need extra polishing, or scrap may rise, and the cost per good part goes up even though wheel wear went down. Always pair G-ratio with finish, edge quality and cycle time before declaring a winner.
G-ratio also depends on how the end point is defined. A wheel measured after a gentle finishing batch will look far better than the same wheel measured after roughing. Compare only measurements taken under the same operation and the same end point.
A second example: when the cheaper wheel wins
Suppose wheel A costs 100 and grinds 2,000 parts, while wheel B costs 260 and grinds 4,000. Per part the wheel cost is 0.05 for A and 0.065 for B, so the cheaper wheel wins on abrasive alone. Add dressing downtime, though, and the picture can flip: if A needs a dress every 250 parts and B every 1,000, B removes three downtime events per thousand parts.
That is the whole discipline of G-ratio economics. Convert every difference to cost per good part, including dressing time, labour, scrap and rework, then compare. The wheel with the better G-ratio is only the better choice when the saved downtime is worth more than the price difference.
Boundary rule — when not to use diamond
Diamond is the hardest abrasive and the correct choice for hard, brittle, non-ferrous materials such as cemented carbide, ceramics, glass, silicon, sapphire, ferrite, graphite and composites. It must not be used on ferrous metals — steel, cast iron, and their alloys — because carbon in diamond reacts with iron at grinding temperatures (roughly above 700 °C / 1300 °F). This chemical affinity causes rapid, abnormal diamond wear (graphitisation) and destroys the wheel. For steel, hardened steel, cast iron and other ferrous workpieces, use CBN (cubic boron nitride) instead.
