Precious Stones for Cutting Precious Records

There is a long history of use of precious stones as industrial materials, from precision bearings to cutting tools, lapping compounds to grinding tools and several other specialised applications.

The recording of phonograph records is essentially a machining operation, performed by means of a machine tool called a disk recording lathe. A special cutting tool is used to cut a continuous spiral groove on the surface of a blank disk. This tool is called a disk recording stylus, or simply cutting stylus.
The big difference to other forms of machining is that there is sonic information stored as modulation of the cut groove, which can be reproduced on another special machine, often called turntable or record player, or disk reproduction system.

Many different stylus materials and geometries have been used through the years, along with different blank recording disk materials. The superiority of precious stones as cutting stylii was already evident from the early days of disk recording, so they are now the only type of stylus used for cutting masters for vinyl record manufacturing.

One such example, shown in the picture above, is ruby. This is very commonly used for cutting lacquer blanks, which are used as masters for subsequent processing, in the industrial manufacturing of vinyl records. Lacquer disks are also frequently cut with sapphire stylii.

While lacquer disks are mostly used as masters, they can also be used for instantaneous recordings, which can be instantly played back on a reproduction system. However, due to the material being soft, the recording quickly deteriorates.

Long lasting instantaneous recordings are possible on harder materials, such polycarbonate plastics, polystyrene and PVC-based disks.

These materials require a diamond stylus, such as the one shown in the picture above. It is not just the material of the stylus being different, but also the geometry.
Diamond stylii are also used for cutting copper, for master disks in the Direct Metal Mastering (DMM) process. These diamonds again have a different geometry to the ones used for cutting plastics.

Lacquer and plastics benefit from being cut with a heated stylus. A coil of nichrome wire is wound around the stone, acting as a heating element. The hair-thin wires visible on the pictures are the two ends of the coil, which are attached to special terminals on the cutter head and connected to a heating current supply.

After being formed into cutting tools of the desired geometry, the stones are usually mounted on an aluminum shank, which is fitted to the cutter head. There are many different shank formats, as discussed in “Lab Report: Making cutting stylus shank adapters“. The heating coil is then wound around the stone and cemented in place.
This complete stylus assembly is fitted to a cutter head, which is mounted on a disk recording lathe.

The blank disk is placed on the platter, the motor is started, the cutter head is lowered and the journey begins…!

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