A sewing frame is designed to hold tapes or cords while the signatures are stitched onto them. It keeps them taut and vertical, so that the book block is straight. Very few book styles can be created without some sort of sewing frame. Thread-only styles such as the Coptic style can be done without one, as can sewing on any sort of stiff tape such as vellum. But sewing on cords, or on any kind of fabric tape, really requires a frame.
The simplest structure for a sewing frame is a rigid rectangle perpendicular to a flat surface. The tapes or cords are then stretched vertically from the top bar of the rectangle to the flat surface. When the signatures are laid on the flat surface, folds facing toward the tapes or cords, they can be easily and neatly sewn to create the book block. Cords can be tied around the top bar, while tapes are looped over and secured with a single stich. Both cords and tapes can then be fastened to the front of the flat surface with masking tape or thumbtacks.
The main refinement on the basic structure is to make the height of the top bar adjustable. This reduces waste and improves tensioning, but it also introduces additional design problems. The best solution for adjusting the top bar is to use a threaded rod for the uprights. However, threaded rods are generally set back from the edge of the sewing surface. To preserve a straight drop for the tapes or cords, these sorts of sewing frames generally have a slot cut in the surface, in between the threaded rods. Tape and cord keys are then used on the bottom.
My sewing frames have generally been of the adjustable type. I use aluminium threaded rod for the uprights. My crossbar is a hollow aluminium tube, into which I slip the shanks of two eyebolts. The eyes of the bolts then go onto the threaded rods, and rest on upside down wing nuts. Both the threaded rod and the tubing is sold by the meter in my local hardware store, and can be cut with a hacksaw or a hacksaw blade on a jigsaw. (Wear eye protection when doing this.) Click on the close-up for a larger image.
These are the types of sewing frame I have tried, or read about.