The IBM SELECTRIC Composer: Multiple Index Mechanism

When high-quality printed copy is desired, the ability to use variable styles of type in one printing system is a fundamental requirement. This feature has existed to a limited extent for some time in low-cost, cold-type printing equipment and typewriters, and has been notably realized in the changeable typehead of the IBM SELECTRIC Typewriter. The ability to vary type size, however, is rarely found in such systems and has been restricted for the most part to commercial hot-type equipment or to manual methods. To remove this restriction in the IBM SELECTRIC Composer, it was necessary to design an index mechanism which would provide variable line-to-line increments. Increment selection was to be flexible enough to accommodate several sizes of type, but it had also to be reliable and easily controlled by the operator. The design which has resulted and is described in this paper is a dual-ratchet, planetary gear system. Platen increments of from 5 to 20 “points” can be selected, and the platen can be indexed manually one point at a time. Other features are also provided, including a “carriage-return, no-index” which permits changing styles on the same line of type without manual “rollback."