Cutting granite on a CNC bridge saw

Granite properties that matter for cutting

Commercial “granite” covers a wide range of igneous and metamorphic stones. For the saw, the useful split is soft granite and hard granite. Soft granites cut fast with little blade wear. Hard granites, often with high quartz content or very fine grain, cut slowly and wear segments quickly. Blade makers group them into classes: Alpha, for example, separates soft granite (class 1–3) from hard granite (class 4–5) and gives them different peripheral speeds.

Countertop slabs are usually 20 mm or 30 mm (3/4 in or 1 1/4 in) thick. Granite is less brittle than porcelain and has no resin, so the main defects are edge chipping on exit, blade wander in thick or hard material, and burn marks when water is short. Some slabs arrive with resin-filled fissures and a mesh backing; the resin softens if the cut runs dry.

Blade selection

Use a segmented blade made for granite. Segment height of 15–25 mm is common: taller segments last longer, while shorter, thinner segments cut with less load on the motor. For hard granite, choose a softer bond so worn diamonds release and fresh diamonds keep cutting. For soft, abrasive granite, a harder bond resists fast segment wear. A blade that stops cutting and starts to polish (glazing) usually has a bond too hard for the stone.

Blade diameter sets cutting depth. On a bridge saw with a Ø625 mm (24.6 in) blade, 90° depth is about 200 mm, far more than countertop work needs. Most fabricators run Ø350–450 mm (14–18 in) blades for 20–30 mm slabs because smaller blades are stiffer, cheaper and run truer in miter cuts.

Blade speed and feed rate

Blade speed follows peripheral speed, so rpm drops as diameter goes up. Amastone gives 25–30 m/s for hard granite and 32–40 m/s for soft granite. Converted to rpm:

Blade diameter	Hard granite rpm	Soft granite rpm
350 mm (14 in)	1,300–1,700	1,750–2,000
400 mm (16 in)	1,200–1,500	1,550–1,900
450 mm (18 in)	1,100–1,300	1,400–1,700
500 mm (20 in)	1,000–1,200	1,250–1,500

Published feed rates vary a lot because they depend on blade design, motor power and machine rigidity:

• Spider II (20 mm segment): 1.8–3.7 m/min (6–12 ft/min) on granite.
• ZENESIS Z-Pattern: 1.5–3.0 m/min (5–10 ft/min) on granite and engineered stone.
• ZENESIS II (25 mm segment): 3.0–5.2 m/min (10–17 ft/min).
• DIABU Red Eagle: 2.5–4.1 m/min (100–160 in/min) on granite.
• Alpha chart, 20 mm slab: 2.5 m/min straight for soft granite; hard granite is listed much lower.

Take 1.5–4 m/min as the working range for 20–30 mm countertop granite and set the final value by test cut. Slow down for hard or thick material and whenever motor load climbs. A CNC saw that holds a set feed rate and reports spindle load makes this easier to repeat than a manual saw.

Entry, exit and water

Most chipping on granite happens where the blade enters and leaves the slab. Reduce feed for the first and last part of each cut, then run full feed through the middle. The reduction needed depends on the stone: brittle, coarse-grained granites need more than dense, fine-grained ones. The CNC can program this ramp per cut, so the operator does not have to do it by hand.

Water cools the segments and carries slurry out of the kerf. Short water causes burn marks, glazing and early segment loss. Check flow at the blade, not only at the pump, and clean the nozzles when the recycled water carries fines.

Miter cuts

A mitered edge in granite needs a straight, square cut without blade deflection. Feed for a 45° cut is typically about half of straight-cut feed: Alpha lists 1.25 m/min against 2.5 m/min for soft granite in 20 mm; the Patriot blade lists 1.65 m/min (65 in/min) on miters. On a 5-axis CNC bridge saw the A-axis head tilt sets the angle and the slab stays flat on the worktable, which removes the setup error of tilting the table by hand. See mitered edges for joint preparation.

Sink cutouts and core drilling

Sink and cooktop cutouts start with core drilling at the corners, then saw cuts between the holes. The saw can plunge-cut straight sides; the radius corners need a core bit or a finger bit on the electrospindle option. Leave the cutout supported until the last cut so the drop-out does not snap the narrow rails.

How machine features reduce defects

• Slab thickness reader: Natural slabs vary in thickness across their length. The reader sets blade depth so the blade cuts through but does not cut deep into the worktable.
• Vacuum manipulator: Lifts cut pieces off the worktable and repositions them for the next cut without a forklift or extra handling, which cuts handling time and edge damage on finished parts.
• Rigid structure: A heavy monoblock frame and backlash-free drives keep the blade on line through hard granite, where a light frame flexes and the blade wanders.

Silica and wet cutting

Granite contains crystalline silica. OSHA and NIOSH list granite at 10–50%, and the CDC reports less than 45%, against more than 90% in engineered quartz. Cut wet. Under Cal/OSHA Title 8, Section 5204, cutting natural stone with more than 10% crystalline silica is a high-exposure trigger task: dry cutting is prohibited and water must cover the work surface where the tool contacts the stone. California also requires air monitoring and respirators for these tasks, and the federal OSHA PEL is 50 µg/m³.

Next step

Send us a slab of your hardest granite and we will run a cutting test on it, recording feed and blade data. Request it through request a configuration or watch it live on a factory visit.