CNC Glass Cutting Necessities

• 1
  Why glass behaves differently under a CNC spindle
• 2
  Tooling that survives contact with glass
• 3
  Parameters that keep the edge chip-free
• 4
  Coolant, fixturing and workholding for brittle parts
• 5
  Where it works and where it does not
• 6
  Matching the process to the glass part

Glass has no plastic zone. Load it past its elastic limit and it fractures instead of deforming, so a standard two-flute end mill drives a crack ahead of the cutting edge. The first of the CNC glass cutting necessities is therefore the tool: material is removed by grinding, not by shearing.

Brittle fracture travels at roughly the speed of sound in the material, far faster than any feed rate a machine can command. Once a crack starts at the tool tip, the spindle cannot outrun it. The practical result is that every parameter on this page exists to keep the tool tip from loading the glass beyond its elastic limit.

Hardness sets the tool material, not the machine. Soda-lime glass runs around 5.5 on the Mohs scale, fused silica closer to 7. Tungsten carbide scratches it but wears quickly; polycrystalline diamond and diamond-coated carbide hold an edge long enough to be economical on production runs.

Thermal shock is the second failure mode. Grinding concentrates heat in a small contact zone, and glass conducts heat poorly. Localized heating followed by coolant contact creates a steep gradient, and the resulting tensile stress can flake the edge even when the mechanical load was fine.

• 1
  Grinding, not shearingDiamond abrasive removes material in fine chips.
• 2
  Crack speedFracture outruns any commanded feed rate.
• 3
  Hardness mattersPCD or diamond-coated carbide for production.
• 4
  Thermal gradientCoolant strategy must control, not shock, the edge.

Diamond is the only abrasive that stays sharp on glass for more than a few minutes. Electroplated diamond burrs cut aggressively and cost little, but the single layer of grit wears through and the tool is done. They suit prototypes and one-off repair work where tool life is not the constraint.

Brazed diamond tools hold grit in a metal bond and tolerate higher cutting temperatures. They are the workhorse for edge profiling and slotting. Polycrystalline diamond (PCD) inserts are the production answer: a solid diamond layer on a carbide body, resharpenable several times, with a geometry that can be ground to a specific edge radius.

Geometry matters as much as the abrasive. A negative rake angle scrapes rather than lifts, which suits brittle material. Small edge radii, typically 5–20 μm, reduce the point load at the contact line. Relief angles stay generous so the tool does not rub the finished wall on the way out.

Tool diameter follows feature size, not machine capacity. A Ø0.5 mm diamond burr will cut a 0.6 mm slot but deflects easily and needs light depth of cut. A Ø6 mm tool removes material far faster but cannot enter a tight inside corner. Most glass work lands between Ø1 mm and Ø3 mm.

• 1
  ElectroplatedCheap, sharp, short life. Prototypes only.
• 2
  BrazedBetter heat tolerance for profiling and slotting.
• 3
  PCDResharpenable, consistent edge radius, production grade.

Spindle speed for diamond on glass usually sits between 8,000 and 24,000 rpm, set by tool diameter rather than by a single number. The target is surface speed, typically 100–300 m/min at the contact point. Too slow and the diamond plows; too fast and the bond erodes before the grit dulls.

Feed per tooth is small. On a Ø1 mm diamond burr, 0.005–0.02 mm per tooth keeps the chip thin enough that the crack tip stays inside the removal zone. Feed rates that work on aluminum, 0.1 mm per tooth and up, will chip glass on the first pass.

Depth of cut stays shallow, often 0.02–0.1 mm per pass, with multiple passes to reach final depth. This spreads the load and gives the coolant time to clear heat between passes. Plunging full depth in one move is the single most common cause of a cracked part.

Stepover on profiling passes should be 20–40 percent of tool diameter. Wider stepover leaves uncut ridges that a finishing pass then has to remove under load, which is exactly where edge chipping shows up. A light finishing pass at 0.01–0.02 mm radial engagement cleans the wall without stressing it.

• 1
  Surface speed100–300 m/min at the contact point.
• 2
  Feed per tooth0.005–0.02 mm on small diamond burrs.
• 3
  Depth of cut0.02–0.1 mm per pass, never full depth.
• 4
  Stepover20–40 percent of diameter, plus a light finish pass.

Flood coolant is the default on glass. It carries away grinding heat and flushes diamond and glass fines out of the cut, where they would otherwise recut the surface and scratch it. Water-based coolant with a rust inhibitor works for most work; some shops run plain filtered water for optical parts to avoid residue.

Coolant direction matters more than volume. Aim the stream at the exit side of the cut so it reaches the contact zone, not just the entry. On deep pockets, through-spindle coolant outperforms external nozzles because the stream arrives where the heat is generated.

Workholding has to support the part without point loads. Vacuum chucks, wax mounting and dedicated fixtures distribute clamping force across a face. Metal clamps on glass concentrate stress and crack parts during the cut, not during clamping, which makes the failure hard to trace.

For thin sheet, a sacrificial backing plate of acrylic or MDF under the glass keeps the exit edge supported. Without it, the tool breaks through into air, the last few microns of material tear instead of grind, and the bottom edge chips. This backing is one of the least glamorous but most reliable of the CNC glass cutting necessities.

• 1
  Flood, not mistMist cannot clear fines or carry heat.
• 2
  Aim at the exitCoolant must reach the contact zone.
• 3
  Distribute clampingVacuum, wax or full-face fixtures.
• 4
  Back the exitSacrificial plate stops bottom-edge chipping.

Glass types split into two groups for machining. Fused silica, borosilicate and quartz handle grinding well because of low thermal expansion and good chemical durability. Soda-lime float glass machines acceptably but is more prone to edge chipping and thermal shock. Tempered glass cannot be machined at all after tempering, because any cut releases the surface compression and the panel shatters.

Feature size sets the practical floor. Holes below Ø1 mm become difficult because the tool is thin and deflects, and coolant cannot reach the bottom. Aspect ratios beyond about 3:1 depth to diameter need pecking cycles and often a smaller tool than the drawing suggests.

Thickness cuts both ways. Thin sheet, under 1 mm, is hard to fixture without flexing it into the tool. Thick plate, over 20 mm, is stable but slow, and the coolant has to travel further to reach the cut. Most production work falls between 2 mm and 12 mm.

Finishing expectations should be set before quoting. An as-ground edge at Ra 1.6–3.2 μm is fine for a mounting face. An optical edge at Ra 0.2–0.8 μm needs additional polishing passes and inspection, and it changes the cost structure of the part. Decide which one the drawing actually calls for.

• 1
  Good candidatesFused silica, borosilicate, quartz.
• 2
  AvoidTempered glass, and any post-temper cutting.
• 3
  Feature floorHoles below Ø1 mm and aspect ratios over 3:1.
• 4
  Set finish earlyAs-ground versus polished changes cost.