CNC stone carving skills for granite, marble and quartz

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  Key takeaways
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  Why CNC stone carving skills differ from metal cutting
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  Tool selection for CNC stone carving skills
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  Feeds, speeds and depth of cut on stone
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  Workholding and dust control as core CNC stone carving skills
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  Step by step: setting up a stone carving job
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  Stone type vs practical cutting approach

Stone does not bend or smear the way metal does. It fractures along grain boundaries, so the cutter is always trading between removing material and starting a crack. That single fact drives every parameter choice on the machine. Hardness is the second factor: granite sits around 6 to 7 on the Mohs scale, marble around 3, and quartz engineered slabs land near 7 because of the resin and silica mix.

That spread means one recipe cannot cover a shop's whole stone catalog. A feed window that leaves a clean edge in marble will glaze the diamond in granite within a few passes. The safe approach is to treat each stone family as its own material group and keep a short parameter sheet at the machine, the same way you would for 6061 aluminium versus 17-4PH stainless.

Thermal behavior matters too. Diamond does not cut by sharpness alone; it grinds. Rubbing instead of cutting raises tip temperature fast, and the bond around the grit softens. When you see the tool stop throwing chips and start throwing fine powder, the cut has turned into rubbing. Back off the feed or reduce step-down before the segment glazes over.

Dust is not a side issue. Respirable crystalline silica from dry cutting is a health hazard and it also wears guideways, ball screws and spindle tapers. Any shop that plans to run stone regularly should treat extraction as part of the machine setup, not an accessory added later.

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  Brittle, not ductileChip load must stay low enough that the stone fractures cleanly at the edge of the pass.
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  Abrasive, not gummyTool wear shows up as glazing and powder, not built-up edge.
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  Porosity variesMarble and limestone absorb fluids; granite and quartz are denser and more stable.

Use diamond tooling. Brazed diamond burrs, segmented shank tools and electroplated diamond end mills all work, but they behave differently. Electroplated tools have a single layer of grit and hold form well for finishing passes; once the plating wears through, the tool is done. Segmented and brazed tools expose fresh grit as the bond wears, so they last longer in roughing but cut less freely.

Tool diameter drives both surface finish and reachable detail. On a five-axis center with a Ø400 mm rotary table, a 6 mm to 12 mm diamond burr is a practical middle ground for lettering and relief work, while 20 mm and up is reserved for bulk stock removal. Going below 3 mm is possible but the tool deflects and the risk of snapping inside a narrow pocket rises sharply.

Keep overhang short. A diamond tool hanging 60 mm out of the holder will chatter on granite no matter how good the parameters look on paper. Set the tool as deep in the collet as the geometry allows, and where reach is unavoidable, reduce step-down proportionally rather than pushing the feed down alone. Rigidity comes from the holder and the setup before it comes from the recipe.

Cooling is a decision, not a default. Most stone work runs dry with high-volume dust extraction at the shroud. Water cooling helps on dense granite where tip temperature climbs, but it will stain or darken porous marble and can carry grit into the machine ways. If you do run wet, plan for a dedicated machine or thorough cleaning between jobs.

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  RoughingSegmented or brazed diamond, 12-20 mm, higher step-down, lower spindle speed.
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  FinishingElectroplated diamond, 6-10 mm, light step-over, higher rpm for a cleaner edge.
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  Deep reliefLong-reach tools with reduced step-down; avoid side loading in narrow slots.

Start conservative and read the chips. For a 10 mm brazed diamond tool in granite, a spindle speed around 4,000 to 6,000 rpm with a feed of 400 to 800 mm/min and a 1 to 3 mm axial step-down is a reasonable starting block. Marble accepts faster feeds and deeper passes because it is softer, but the same brittleness means edge chipping appears before the tool complains.

Quartz and engineered stone sit at the other end. The resin binder holds abrasive silica that dulls diamond quickly, so lower the step-down to 0.5 to 2 mm and keep the feed steady. Varying the feed mid-pass is worse than a slightly slow constant feed, because the tool alternates between cutting and rubbing at every change.

Radial engagement matters more than raw feed on contours. A 40% radial step-over at a moderate feed usually beats a 10% step-over at high feed, because the tool stays in cut and the chip load stays even. On tight internal corners, slow the feed and let the machine arc the corner rather than stopping and reversing, which is where most chipping starts.

Watch spindle load and sound. A clean cut in granite has a steady, slightly gritty tone. A rising pitch with a drop in load usually means the tool has glazed and is polishing the surface instead of cutting it. Stop, dress or replace the tool, and re-check the step-down before resuming.

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  Granite4,000-6,000 rpm, 400-800 mm/min, 1-3 mm step-down for a 10 mm tool.
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  MarbleHigher feed and step-down are tolerable, but edge chipping still limits finishing passes.
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  Quartz / engineered0.5-2 mm step-down, constant feed, more frequent tool changes.

Stone blanks are heavy and irregular, and they do not clamp like a billet of aluminium. Cast-in lifting points or a sling under the block are standard practice before the part goes near the table. Once positioned, support the full underside with a bed of timber or a machined sacrificial plate so the load is spread and the block cannot rock under cutting forces.

Clamp on the sides, not the top face, wherever the geometry allows. Toe clamps and low-profile side clamps keep the top surface clear for the tool and reduce the chance of the cutter hitting a clamp during a five-axis move. Torque the clamps evenly; uneven pressure is a common cause of hairline cracks that only show up in the finishing pass.

Dust extraction should pull from the cutting zone, not from the far side of the enclosure. A shroud around the spindle with a high-flow extractor keeps the visible cloud down and stops fine grit from settling on the ways and in the taper. Empty or service the collector before it fills; a saturated filter pushes dust back into the work zone.

If the part has thin sections or unsupported overhangs, add support before the final pass. Machining the detail first and removing the support last keeps the stone from vibrating. On a rotary table setup, index slowly between positions and re-check clamp torque after the first pass, because stone settles into its support under load.

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  Support the baseDistribute load over the full underside to stop rocking and edge chipping.
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  Clamp lowKeep the top face open for tool access on five-axis paths.
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  Extract at the cutShroud-mounted extraction protects both the operator and the machine.