CNC stone cutting: how the tool actually removes rock

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  What CNC stone cutting removes, and with what
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  Which stones cut cleanly and which fight back
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  What tolerance CNC stone cutting can actually hold
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  Setting feed, speed and depth in CNC stone cutting
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  Chipping, burning and tool wear: causes and fixes
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  Step by step: from CAD model to finished stone part
• 7
  Choosing the cutting method for a stone part

Stone is brittle. That single property drives every parameter choice. A metal cutter shears a chip; a stone tool has to fracture a mineral grain and then carry the debris out before it grinds against the fresh surface. If the debris stays in the kerf, the tool rubs instead of cutting and the heat builds fast.

Most stone work uses diamond abrasive tooling, not fluted cutters. Diamond grains are bonded to a steel shank or a segmented rim, and each grain acts as a tiny cutting edge that ploughs through quartz, feldspar or calcite. The bond wears back as the grains dull, which exposes fresh diamond. That self-sharpening behavior is why diamond tooling lasts far longer than carbide on rock.

The machine itself is unremarkable. A three-axis router with a water-cooled spindle and a granite-capable work table does most of the work. The difference from a normal metal job is the coolant, the tool, and the feed rates. Water is not optional. It cools the diamond, clears the slurry, and keeps silica dust out of the air.

So the process is subtractive, but the cutting physics are closer to grinding than to milling. Every parameter you set on the controller is really a way of controlling how much load each diamond grain sees.

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  Brittle fractureMaterial leaves as small chips and dust, not a continuous curl.
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  Diamond bondTool life depends on bond hardness matching stone hardness.
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  Water floodCooling and debris removal are the same job.

Soft sedimentary stone such as limestone, sandstone and soapstone machines easily. Feed rates can be high, edge chipping is mild, and a standard diamond burr leaves a predictable surface. Sandstone is abrasive because of free silica, so tool wear is fast even though the cut is easy.

Marble and onyx sit in the middle. Calcite is soft, but the stone cleaves along grain boundaries, so thin sections chip at the exit edge. Onyx and travertine can be porous, and porosity means the tool grabs and releases. Reduce feed by 20 to 30 percent near a finished edge.

Granite is the hard case. Quartz content of 20 percent or more pushes diamond wear up sharply. Expect to slow the feed and accept more tool changes. Engineered quartz and sintered stone are harder still and more uniform, so the cutting is stable but the abrasive load is high.

A useful rule: the harder and more uniform the stone, the better the dimensional repeatability, and the worse the tool cost. Soft and veined stone is the opposite. There is no material that scores well on both.

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  Limestone, soapstoneFast feed, low chipping, low tool cost.
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  Marble, travertineWatch exit-edge chipping and hidden voids.
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  Granite, engineered quartzSlow feed, high diamond wear, tight repeatability.

Stone does not hold the same tolerances as metal. Thermal expansion and internal stress release after block cutting mean a slab can move after the first pass. A realistic shop tolerance for cut stone features is ±0.2 to ±0.5 mm on position, and ±0.1 mm on a well-supported flat surface.

Thickness control is easier than lateral control. Surface grinding with a diamond cup wheel on a rigid machine can hold ±0.05 mm across a 300 mm plate if the stone is homogeneous and the backing is flat. Veined or porous stone will not repeat at that level.

For reference, our metal work holds ±0.005 mm (±0.0002 in) and finishes down to Ra 0.2–0.8 μm, and our three-axis and five-axis centers handle parts up to 4,000 mm. Stone is a different envelope. Do not specify metal tolerances on a stone drawing. The inspection cost will exceed the part cost.

Design the assembly to absorb stone variation. Use slotted holes, shims, or a machined metal insert bonded into a stone pocket. That way the stone carries the look and the metal carries the fit.

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  Position, cut features±0.2 to ±0.5 mm is realistic.
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  Ground flat surface±0.05 mm on homogeneous stone, flat backing.
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  Better approachMetal insert for fits, stone for appearance.

Spindle speed for a 6 mm sintered diamond burr in granite typically lands between 6,000 and 9,000 rpm. In marble you can run toward the top of that range; in engineered quartz, stay lower and let the bond do the work rather than the speed.

Feed per tooth matters more than table feed. As a starting point, keep the chip load light, around 0.02 to 0.05 mm per tooth in granite, and raise it only if you hear the tool load up. A screaming tool means the grains are rubbing. A steady low tone means they are cutting.

Depth of cut should be shallow. Multiple passes of 1 to 3 mm beat one deep pass in every brittle material. The reason is lateral force. A deep pass pushes the stone sideways, and the exit edge chips before the cut is finished.

Finishing passes deserve their own settings. Leave 0.3 to 0.5 mm of stock, then run a finishing pass at 20 to 40 percent lower feed with a fresh or dressed tool. That single step removes most of the chipping complaints that come back from the floor.

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  Spindle speed6,000–9,000 rpm for a 6 mm diamond burr.
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  Chip load0.02–0.05 mm per tooth in granite.
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  Depth of cut1–3 mm per pass, never one deep pass.
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  Finish stock0.3–0.5 mm, cut at reduced feed.

Edge chipping is the most common defect. It comes from lateral tool force at the exit edge, so the fix is mechanical. Back the exit side with a sacrificial board or clamp, reduce depth of cut, and slow the feed as the tool leaves the material.

Burning or glazing shows up as a dark, glossy patch and a smell. It means the bond has smeared rather than worn, usually from too little water or too high a surface speed. Increase coolant flow, drop the rpm, and dress the tool before the next part.

Uneven tool wear on one side of the burr points to runout or a tilted spindle. Check with a dial indicator before blaming the tool. Runout above 0.05 mm will double the load on one side and shorten life noticeably.

Breakout on the bottom face is a fixturing problem more often than a cutting problem. Support the full underside, or cut from both sides. Stone has almost no tensile strength, so anything unsupported will fail.

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  ChippingBack the exit edge, cut shallower, slow at exit.
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  Glazing or burnMore water, lower rpm, dress the bond.
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  One-sided wearCheck runout; keep it under 0.05 mm.
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  Bottom breakoutSupport 100 percent of the underside.