Principles of Cutting Granite: From Mineral Structure to Cutting Parameters
Why granite behaves differently under the blade and how to tune speed, feed and cooling parameters.
What granite is and why it cuts hard
Granite is a crystalline interlock of quartz (Mohs 7), feldspar and mica. Quartz is harder than steel, which makes granite exclusively a diamond-cutting problem. Its heterogeneous grain also makes cutting forces fluctuate constantly; unstable speed and feed settings then chip the edge.
Choosing the diamond blade
- Segment height: typically 20 mm for granite with a ~35% diamond concentration band at the rim.
- Bond grade: very hard black granites (e.g. Na'in) need a softer bond so dulled diamonds shed quickly and expose fresh ones; lighter granites tolerate harder bonds.
- Diameter: a 1200 mm blade at 950–1100 rpm gives near-optimal peripheral speed of about 60 m/s for granite.
Feed rate and listening
Feed in granite should hover around 0.5–1 m/min for plunging cuts, slower for full-depth passes. Your ear is the best sensor: a steady low hum means healthy cutting; a scream means the blade is rubbing without cutting (too slow) and hammering means overload. Automatic bridge saws implement exactly this logic through blade-motor amperage control.
Cooling
Starved water is granite's worst enemy: the contact point exceeds 700 °C and the diamonds graphitise — the blade burns. Symmetric water delivery on both sides of the segment, roughly 120 L/min for a 1200 mm blade, and slurry filtration to protect pumps are the three simple rules.
Safety
Granite's silica dust causes silicosis. Take local extraction, minimum N95/FFP2 masks and wet hall cleaning seriously. Never remove blade guards or emergency stops.
Defects from wrong cutting
- Wavy cut face: loose ways or warped flange.
- Edge chipping at exit: feed too fast — decelerate before blade break-through.
- Burn smell and discolouration: insufficient cooling.
Conclusion
Successful granite cutting is the sum of four variables: blade matched to the stone's grade, correct rpm, feed tuned by sound/current, and complete cooling. Write these four into every operator's checklist.