MCD vs PCD: which diamond cutting tool do you need?

The short answer

Use PCD when the tool has to last through long runs, heavy cuts, interrupted cuts or abrasive materials, and a good bright finish is enough. Use MCD (or natural diamond) when the surface has to come off the machine as a mirror, for example a bright chamfer, a reflective face or an optical surface. Many factories use both on the same part: PCD to rough and shape, a single crystal tool for the last pass.

What PCD is

PCD, polycrystalline diamond, is made of many small synthetic diamond grains. The grains are sintered together under high pressure with a metal binder, usually cobalt, and bonded to a carbide base. The cutting tip is then brazed onto the tool and cut to shape by grinding or erosion.

Because the grains point in every direction and the binder holds them together, PCD is tough. A crack cannot run straight through the tip, so it survives impact and vibration well. It also resists wear in abrasive materials, such as high-silicon aluminum alloys, carbon fiber and other composites. That makes PCD the standard tool for high-volume machining in the automotive and aerospace industries.

The limit is the edge. However fine the grains are, the cutting edge is made of many small grains with tiny gaps between them. It can be made sharp, but never perfectly smooth. The surface it cuts is bright, but not a mirror.

What MCD is

MCD, monocrystalline diamond, is one single diamond crystal. It is grown in a press under high pressure and high temperature, cut into plates and brazed onto the tool. Natural diamond is also a single crystal and works the same way; CVD single crystal diamond is a newer, lab-grown option.

A single crystal has no grains and no binder, so its edge can be polished almost perfectly straight and sharp. That edge is copied directly onto the part. On aluminum, brass, copper, gold and optical plastics, an MCD tool on a stable machine leaves a mirror finish in one pass, with no polishing afterwards.

The trade-off is toughness. A single crystal has cleavage planes, directions along which it splits more easily. Heavy impact or vibration can chip the edge. MCD tools are therefore used for light finishing cuts on smooth, steady machines, and the crystal is mounted so that its strongest direction faces the cutting load.

MCD and PCD side by side

How to choose for your part

• What finish do you need? If the surface must reflect like a mirror, choose a single crystal tool. If a clean, bright machined surface is enough, PCD is usually the more economical choice.
• How is the tool loaded? Interrupted cuts, such as spokes, holes and slots crossing the cutting path, and deep cuts favor PCD. A steady, light finishing pass suits MCD.
• Which material? Both cut aluminum, copper, brass and plastics. Very abrasive materials, such as high-silicon cast aluminum and composites, wear a single crystal edge faster; PCD handles them better. Neither is for steel.
• How stable is the machine? A mirror finish needs a rigid machine and a spindle that runs true. On a machine with vibration, an MCD tool chips and loses its advantage.
• How many parts? PCD often wins on cost per part in very long runs. Where the finish is what sells the part, the single crystal tool pays for itself by removing the polishing step.

Using both: PCD first, MCD last

On many parts the best answer is both. A phone or laptop housing, for example, is milled to shape with carbide or PCD tools, and only the visible chamfer is cut last with an MCD cutter. Each tool does the job it is best at, and the expensive mirror edge is used only where it shows.

Where Bulunmaz fits

Bulunmaz makes single crystal diamond tools, in natural diamond and MCD, for the finishing pass: turning tools, end mills, chamfer cutters, inserts and fly cutters, as standard tools or to your drawing. We do not make PCD tools. If you are not sure which diamond your part needs, send us the material, a drawing and the finish you want, and we'll tell you plainly. For more on natural diamond, MCD and CVD, see our diamond types guide.