Natural diamond or MCD? Choosing single crystal diamond for cutting tools

Why single crystal diamond cuts a mirror finish

A single crystal diamond is one continuous crystal lattice, with no grain boundaries and no binder. That lets its cutting edge be polished far sharper and smoother than any tool made from grains. The edge copies itself onto the part, so a flawless edge leaves a mirror surface straight from the machine, with no polishing step.

Diamond is also the hardest known material, and it carries heat away faster than any metal. The edge stays sharp and cool at high cutting speeds, even on soft, sticky metals that tear under ordinary tools. That is why single crystal tools are the standard for high-gloss finishing of aluminum, brass, copper, precious metals and optical plastics.

Natural diamond and MCD

Natural diamond and MCD are both single crystals, so both can cut a mirror finish. They differ in how they are made, how consistent they are and what they cost.

Natural diamond (ND)

Natural diamond formed deep inside the Earth. For cutting tools, rough stones are sorted one by one for clarity, internal strain and shape, then oriented and polished into a tool tip. Most natural diamonds hold nitrogen in clustered form (type Ia), and no two stones are exactly alike.

• Strengths: a well-selected stone gives an extremely fine, durable edge. It is the traditional choice for the finest optical and decorative finishes.
• Limits: higher price, limited stone sizes and natural variation from stone to stone, so careful selection matters.
• Typical uses: ultra-precision optics, contact lenses, watch and jewelry finishing, small-radius and V tools.

MCD: HPHT synthetic single crystal

MCD (monocrystalline diamond) is grown in a press that recreates the conditions deep in the Earth. Carbon dissolves in molten iron, nickel or cobalt at 1,300–1,600 °C and about 6 GPa (roughly 60,000 times atmospheric pressure), then crystallizes on a diamond seed over days to weeks. Nitrogen spread through the crystal as single atoms gives MCD its yellow color (type Ib).

• Strengths: consistent quality from plate to plate, predictable tool life, and regular plate shapes that make tool geometry repeatable. It usually costs less than selected natural stones.
• Limits: like every single crystal, it can chip under impact or vibration.
• Typical uses: high-gloss turning and milling of aluminum, brass and copper; chamfer and engraving tools; acrylic and polycarbonate edges; most production finishing work.

CVD diamond, on request

CVD diamond is grown from gas in a vacuum chamber. Single crystal CVD can cut a mirror finish much like MCD, and we supply CVD tools on request.

Single crystal vs PCD

PCD (polycrystalline diamond) is made from many small diamond grains sintered together with a metal binder, usually cobalt, on a carbide base. The grains point in random directions, so PCD is tough and resists chipping. That makes it the right choice for roughing and for abrasive materials such as high-silicon aluminum and fiber composites. But an edge built from grains cannot be polished as sharp as a single crystal, so PCD cannot cut an optical mirror finish.

The two often work as a team: PCD removes most of the material, then a single crystal tool makes the final mirror cut. Brass valve balls show how this works: PCD inserts rough the ball, then a diamond finishing tool gives the final shine. Bulunmaz Tools focuses on the single crystal finishing side.

Comparison at a glance

Which diamond for which job

These are starting points; the final choice depends on the part, the machine and the volume.

What decides edge quality

Two tools with the same shape can perform very differently. Four things make the difference:

• Crystal orientation. Diamond is harder in some crystal directions than in others. The toolmaker orients each stone so the edge works in its most wear-resistant direction, which is a main factor in tool life.
• Nose radius and included angle. The radius (for example R0.5 mm) or the V angle (for example 60°) sets the shape the tool leaves on the part. Smaller radii and sharper angles reach finer detail but carry less load.
• Rake and clearance angles. A positive rake cuts more freely but weakens the edge; a negative rake is stronger and suits tougher alloys. The clearance under the edge can be conical, which is sharper, or cylindrical, which is more rigid and keeps its radius better after relapping.
• Edge waviness. Waviness is how far a radius edge strays from a perfect circle. For optics and lenses the edge shape is printed straight into the surface, so these tools are made to a controlled waviness, typically from 1 µm down to 50 nm.

What single crystal diamond can and can't cut

Good materials:

• Aluminum and aluminum alloys
• Copper, brass, bronze and beryllium copper
• Gold, silver, platinum and palladium alloys
• Electroless nickel (nickel-phosphorus) plating on molds
• Acrylic (PMMA), polycarbonate, nylon and other plastics
• Germanium, silicon and other infrared crystals

Not suitable:

• Steel, cast iron and other ferrous metals. At cutting temperatures the carbon in the diamond reacts with iron, and the edge dulls after a short cut.
• Very abrasive materials such as high-silicon aluminum or fiber composites. They wear a single crystal edge quickly; PCD is the better choice there.
• Heavy interrupted cuts or impact, which can chip any single crystal edge.