For purposes not demanding any great degree of accuracy, a point tool, or the corner of a flat tool, or upon wood, a lead pencil, may be employed upon the tee of the hand rest, to scribe the divisions upon the work. A circular line or lines, to determine the commencement or termination of the length of the lines to be scribed, or to mark the position for the centers of a series of apertures, being first struck upon the work with a point tool or with the pencil.
The tee is adjusted in its pedestal with its top surface exactly to the height of center of the lathe, and is placed and fixed with its edge almost in contact with the work. To avoid variations in the width of the spaces marked out, the tool or scriber is held horizontally, that is radially to the work, with its face flat upon the surface of the tee; but, the narrow width of the flat upon the top of the ordinary tee, renders this position rather difficult to maintain with accuracy, and errors are still more liable to occur, when from the form of the work, the edge of the tee cannot be placed close to the part to be divided. The method however is often sufficiently good for marking the terminal lines in first reducing the circular to the square or other form, and for finding the position to bore holes, for the insertion of other parts for construction or for ornament, as will be seen in the practical sections.
A tee with a wide flat top, is occasionally used as a means for preventing the tool losing its horizontal position. From this, the dividing tee fig. 126, used for more careful dividing, has been derived. The flat under surface of the scribing bar of fig. 126, lies upon raised surfaces at either end of a flat tee five or six inches wide, carried by a stem and adjustable for height in the pedestal of the handrest. The scribing bar works by a slot on a pivot at the back end of the tee, which permits it to advance or recede when scribing a long line, and in following the outline of curved forms. A bridle placed towards the front, prevents excessive lateral motion, and retains the bar fairly under the guidance of the front surface. The scribing point of steel, can be removed for sharpening, and is fixed in a socket in front by a side screw.
The bar is held near the point end, between the thumb and two fore-fingers of the right hand, which at the same time keep it firmly pressed down upon the tee. The point is placed in the line traced on the work to give the termination of length, and is then pushed towards the right to scribe the division. The slot and pivot behind, allow the bar to work freely in respect of its length, and the point is made to cut to an equal depth by the sense of touch; care being principally required to keep the bar always equally pressed down upon the tee. Variation in this latter respect, even a momentary lapse of attention, will produce errors, arising from a trifling variation in the height of center of the point while the bar is in use; and such errors are very visible upon finely divided work.
The dividing gate fig. 127, used for micrometers and other works demanding moderate accuracy, is a superior tool. A rectangular block to be held between the fingers, which carries the point, is hung by three pairs of centers to a vertical support fixing upon the lathe bearers. Adjustment for height, is given by the intermediate pair of centers and all the center screws are provided with fixing nuts to prevent their displacement. The mobility given to the point by the construction of the dividing gate, very greatly assists the touch in obtaining equality in depth and width of cut; and the height of center being assured, entire attention can be devoted to the length and quality of the lines, that may be scribed upon either straight or curved work.