Edward Tatnall Canby - The Saturday Review

Cutting Heads - Cutting The Record

Cutting the DiskThere are a number of types of cutting head in use, but this would seem to be a topic of even less importance to us than the cutting principle itself. In usual transducer fashion, we have here a means of converting electric current fluctuations into actual motion or vibration, the side-to-side motion of a specially shaped jewel cutting stylus. The cutting head is a phonograph pickup in reverse. Other than in its greater size and complexity, the sharp edge of the cutting needle, and, of course, the feed screw mechanism which moves the head across the record to make the spiral (phonograph pickups follow this made spiral and don't need to be moved), it is not unlike a pickup in form as well as internal workings. An astonishingly extensive literature has been written concerning the minute details of stylus shape and the resulting groove that is cut; untold years of experiment have gone into the simple but enormously accurate jewel faces that do our groove cutting today. But with a nod of appreciation to such indefatigable workers in this special field as the lady engineer, Isabel Capps, we must move on.

Hot Stylus

But not before mention of one new device that has been of tremendous use in the cutting of fine-groove records: the "hot stylus," which is no more than a tiny wrapping of heating wire, glowing hot, around the point of the stylus to bring the jewel cutting end to a carefully controlled even heat. The hot stylus momentarily softens the lacquer surface of the recording disk into which the grooves are cut; the lacquer offers less resistance to the cutting process, the cut is smoother, and the distortion that comes from forcible plowing through tough material is considerably reduced. Altogether an excellent idea, and most professional disk recordings are now cut in this way.

The Lacquer Desk

In spite of that persistent term, "waxing," actual wax is as out of date for the so-called master record as are two-wheel brakes on the automobile. Modified versions of the old solid block of wax into which the original recording groove had always been cut were still used (if my information is correct) until not long before World War II. But the more satisfactory lacquer-cutting process came into use in the mid-thirties. Whatever the preceding steps may have been - and, as we've seen, they can be enormously diverse and complex these days - the final product before "processing" is now a single lacquered disk onto which the actual recording grooves are cut.

If you have had "private" or, as they are sometimes called, instantaneous recordings made for yourself, the one-of-a-kind records that numerous studios, department stores, booths and the like make for you on the spot, you have seen a lacquer disk. It is made of metal, usually aluminum, coated with an extremely carefully controlled layer of black, smooth, shiny lacquer. For professional work the lacquer surface must be perfectly even. (You can see yourself in it, as in a mirror, without distortion.) For less critical work, including home experimental recording and the like, the lacquer surface may show the slight "orange peel" look of the spraying process that coated the disk. Different grades of "blanks" are also distinguished by the thickness of the metal itself. Needless to say, for the making of commercial pressed records or radio transcriptions there is only one standard - the best that can be had for the purpose.

The Disk Cutter

The refinements on a professional cutting machine are most interesting. All motions, of course, are precision controlled and tremendously accurate, so that the grooves will lie perfectly evenly next to each other, so that the depth of cut will be uniform and exactly to specifications. The operator manages these things with hair-precision controls on the overhead cutting mechanism, which extends from the outside edge across the record to the center pin like an overhead bridge, and from which the actual cutting head with its stylus is lowered to the record surface. The spirals are cut by moving the head very slowly across this bridge, whether by a type of screw feed, or by one or another more ingenious systems now being used. You can understand that, where your own phonograph pickup follows the pre-set grooves in the record, a groove cutting bead, working upon a blank disk, must be mechanically moved in order to cut spiral grooves. Hence the fairly complex mechanisms used For record cutting. Extra mechanical motions must be provided in order to cut the "faster" spirals at the beginning and end of the record, and the operator usually does this by means of a small crane that he can turn to speed up the spiraling process.

Grooves are extremely tiny, even standard ones-not to mention the newer microgroove. In order that the operator can observe what is actually going on in this tiny area, recording machines come with attached microscopes that swing over the groove while the record is being cut. Through the microscope a good operator can tell instantly whether his cut is smooth, of the right depth, whether the grooves are far enough apart not to interfere with each other, whether the "land" between the grooves is undisturbed. (Sometimes the groove walls give under heavy pressure, and unpleasant humps may rear up; or the pattern of one groove may be forced through the wall into the adjacent groove, giving that strange "echo" effect we've run into in some LP records.)

The Chip

A major problem is the "chip," the continuous thread of cut material that the stylus throws off as the groove is made. Most home recording enthusiasts have run afoul of this exasperating by product. On occasion, it will throw itself nicely to one side and lie flat on the disk in neat rows, out of the way. But far more often it entangles itself on each turn with the thread of earlier turns; it catches under the stylus and quickly builds a rat's nest of tangled material which ruins the cut since you cannot lift the stylus away from the record in order to clear it without interrupting the recorded sound and the groove itself. "Chip" is highly charged with static electricity, and sticks obstinately to anything and everything in sight. Moreover, it is dangerously inflammable, an can, when a big ball of it catches fire, virtually explode in your face.

The professional answer to the problem is simple, on a professional scale: a suction device with a pipe mounted immediately in front of the cutting needle, which draws in the chip by main force and swallows it whole. It is hauled into big bottles of water, and there neatly drowned. Complications are that there must be no extra noise from the suction pump and, above all, no vibrations. These problems have long since been fairly well solved in professional record making, but the chip nevertheless can still make agonizing trouble. Many a record master has been spoiled by chip at the last minute when all else was perfect.

Cutting the Disk

So much for assorted incidentals. How about the actual cutting? A tense business, even for the hardened operator! Once started on its spiral course, a cutting stylus cannot be stopped without ruining the disk, nor can its speed be varied. Everything must be pre-set and double-checked beforehand, from the flatness of the disks to the condition of the sapphire or diamond cutting edge and, of course, the electrical circuits. Usually a test run is made first, with blank grooves which are examined under the microscope, and sometimes played back to see that the noise level is appropriately low. Sometimes disk after disk is rejected, stylus after stylus tried, before things roll smoothly.

The vacuum pumps are started, the tape players readied, the table set rolling, and, with the go-ahead signal, the stylus is carefully lowered to the record surface, the fast beginning spiral (lead-in) is cut, and the record is on its way. The chip must be constantly watched, in spite of the vacuum that is supposed to swallow it up. Time and again it will whip on around the circle and tangle itself on the stylus. A camels hair paint brush or a blast of breath may save the day, but sometimes it's already too late: a sudden hiss in the record when it is played back shows up the damage. Depth of cut must be checked all the time, and of course the volume levels must be watched constantly, even though one may hope that the recorded tape original will have been checked for anything drastically unusual in the way of sudden loud music. Margin control, if manually done, is an additional task that is almost insupportable. Hence the desirability of an automatic system.

At the end of the record a crank is turned which makes the final spiral lead-away groove. Add an eccentric repeat circle-and the master is done, except that it must be examined very closely for flaws, and is sometimes played with extreme care, to be certain that the sound is acceptable. For moving to the processing plant, masters are bolted by their center holes into an elaborate box with separations that keep the actual surfaces untouched; lacquer is extremely delicate and easily scratched. (Masters are conveniently marked for identification in this manner with a scratching stylus, the marks appearing on every finished disk.)

Speeds and Grooves

This description of cutting applies to records of any speed, and to the 16-inch transcriptions that are used by radio stations and educational institutions. There are only minor differences between the sizes. The greatest differences, needless to say, have to do with groove size. Radio transcriptions use a groove slightly narrower and slightly more closely packed than the average 78 "standard" groove, but it is played at 33s' rpm. The groove used for LPs and 45s is far smaller and much more closely packed. The problems of accuracy with LP are far greater all along the line than with the old larger groove, and for a while after 1948 and LP's debut there were dire mutterings and groanings throughout the industry; smallgroove cutting was all but impossible. We may well sympathize with the hapless recording engineers who were given the early LP records to cut! A half-hour side instead of a five-minute one - a half-hour ruined at every mishap! And a groove so delicate that trucks passing miles away could jar the cutter an spoil a master; grooves so shallow that an almost invisible ripple on the smooth surface of the lacquer would act as a miniature mountain range, the stylus plowing too deeply, then losing contact altogether with the surface.

But after interminable and unheralded agonies of experiment and slow experience, the cutting of small grooves began to seem a bit less terrifying, and by now most professional operators have no more trouble in this new area than the old, though extra care in every respect is implicit. Three-speed tables are commonplace, and with minor adjustments as to stylus size and the cutting pressure (weight), any sort of record may be cut at will.

Excerps from the Saturday Review Home Book of Recorded Music and Sound Reproduction
by Edward Tatnall Canby, 1952 - Prentice-Hall, Inc. New York