Micro-Point Cutting the Groove

A recording or cutting stylus performs two basic functions; it cuts the groove in the master record, and it smooths or burnishes the resulting groove walls. While cutting the groove the stylus is driven by the cutter head in a 45/45 stereo mode to form the complex mechanical waveforms in the master record which are a precise analog of the electrical waveforms fed to the recording amplifier.

The cutting edge is the intersection of the stylus face and the burnishing facets, and as the master record moves past the stylus, a groove is cut. The cutting action leaves a certain amount of roughness on the groove walls. The burnishing facets, which trail the cutting edges, protrude slightly beyond the cut groove. As they pass by, the new walls are forced to rub against the facets, and in this way are smoothed or burnished. Heat generated by the cutting and burnishing friction, combined with heat supplied by the stylus heater raises the temperature of the lacquer sufficiently to allow a minute amount to flow and thus improve burnishing action. Any roughness remaining in the groove walls after cutting and burnishing is heard as noise during playback.

Low groove noise depends on three factors; recording stylus dimensions, stylus temperature, and lacquer composition. Recording stylus dimensions that influence recorded noise and program levels are; burnishing facet width and angle, cutting edge accuracy, tip radius, and back angle. Optimum dimensions of the stylus depend upon the specific lacquer composition.
Burnishing facets are specified by width and angle dimensions. Changes of facet dimensions cause variations in a number of recording quality criteria in opposing directions. Some become better; some become poorer, this is a complex relationship between facet dimensions and the three recording qualities they influence; noise level, high frequency performance, and stylus life.

Burnishing action increases as facet width and angle are increased and noise levels are reduced. Changes in facet width can influence groove noise by more than 6 dB! However, large facet dimensions degrade high frequency performance.
High frequency performance improves as facet width is reduced and/or facet angle is increased. Facet dimensions influence two high frequency factors; loading on the cutter head, and waveform distortion. Output decreases as facet width becomes larger because of increasing interference between stylus and groove walls which increases cutter loading. As the facet width becomes large and approaches modulation wavelength dimensions at high frequencies and at inner disc diameters, the groove walls will become distorted. Thus an optimum width and angle exist for good high frequency performance at low noise.

An increase in facet angle also means an increase in cutting angle. The larger facet angle will improve burnishing action and reduce noise, but the increase in cutting angle means a less sharp cutting edge and an increase in noise. Thus an optimum facet angle exists for minimum noise.
If the facet angle and width are reduced the cutting edge will wear faster. As the cutting-edge wears, noise levels increase. To put stylus wear in perspective, each hour of recording produces a groove nearly one mile(1.6km) in length. Again, in determining optimum facet dimensions, wear is an important consideration.

A combination of precision recording measurements and studio and processing experience involving many brands of recording equipment and master records has led to standard, general purpose facet dimensions of 4.2 microns (0.000165 inch) wide at an angle of 22.5 degrees. Because recording quality does vary with facet dimensions, production tolerances are + 0.2 micron to assure consistent recording performance.

The cutting edge is formed by the intersection of the front face and the burnishing facets. The finish of these surfaces determines the accuracy of the cutting edge itself. The finer the polish, the sharper the edge. Production holds these surfaces to a finish specification of better than 0.05 micron. In addition, the edge must be flawless to produce a smooth groove without streaking.

The stylus tip radius provides a smooth, defined groove bottom at the intersection of the two groove walls. This permits clean metal-plastic parts separation during processing. To ensure bottom radius smoothness, the burnishing facets continue around the stylus tip from one side to the other. The stylus tip radius at 3.0 microns (0.00012 inch) maintains clearance between playback stylus and groove bottom.
The back angle imposes a limit on recording levels due to an interference called slope overload. When the slope of the modulated groove exceeds the slope of the trailing faces of recording stylus, overload occurs, and distortion must result. Figure 6 illustrates this condition. The stylus shown by the dashed lines could not have cut the groove modulation as shown. The maximum slope that can be cut is that part of the groove that remains after the shaded section is removed. Older styli designs were made with a back angle of 45 degrees. As compared to the specified angle of 40 degrees, maximum slope has been increased 5 degrees. This translates to an increase in recording level limits of over 1.5 dB.

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