The traversing mandrel produces both the external and internal screw with ease and certainty, it is therefore a desirable addition to the powers of the lathe, when the habit of striking screws by hand has either not been acquired, or has been partially lost by disuse; it is also frequently necessary and to the most practised turners, to cut screws of difficult proportions, when the material is intractable, or when it only just suffices for the screw and a failure would be therefore irretrievable. The traversing mandrel, its screw guides and conducting apparatus, have been made in various forms, some of which have been mentioned; the modern, fig. 112, will suffice for explaining the manipulation, which is alike with all, and for cutting either wood or metal.

The piece for the external screw is prepared as usual, fig. 511: the cap at the back of the mandrel is then withdrawn and replaced by the screw guide, the corresponding segment of the conducting piece is placed uppermost, and the eccentric carrying it, turned round to bring the two into close contact. The eccentric is then slightly turned the reverse way, to slacken the contact and permit the mandrel to revolve easily but without end shake; upon which, the mandrel when revolving, travels at the rate communicated by the screw guide, advancing or receding as the lathe is turned in either direction. The screw guide and conducting piece are also sparingly supplied with oil, which completes the preparation of the apparatus. The cutting edge of the tool, and the tee of the rest, are both always parallel with the mandrel; and while cutting, the shaft of the tool remains at one exact spot on the tee without any lateral movement.

The fly wheel of the lathe is never permitted to make a complete revolution, but is only allowed to swing backwards and forwards, until the bend of the crank arrives a little above the horizontal plane, pointing alternately towards and away from the operator; necessary for the purpose of arresting the advance of the mandrel, to prevent the shoulder of the screw cut from striking against the left side of the tool and displacing it on the rest. The partial revolution of the fly wheel, being checked and determined, by the pressure of the foot on the treadle, at the termination of the upward swing of the crank in either direction. The action is regular and without jerk, the foot never leaves the treadle, but after giving its impulse for descent, its pressure is relieved until the crank completing its partial revolution has risen nearly to the highest point on the opposite side. The foot then checks the further ascent of the treadle by gentle increasing pressure, almost instantly converted into an equally gentle impulse, which starts the flywheel back in the opposite direction. The easy, regular, backward and forward swing of the fly wheel, causes the mandrel to advance towards the tool, when the bend of the crank rises and points from the operator, and to retire from the tool, when it rises towards him.

The partial revolution of the fly wheel, effects several complete revolutions of the mandrel, which, measured on the work by any one point of the screw tool, produce two or three complete turns or threads, copies of those of the screw guide employed ; and, as the screw tool remains quiescent laterally, the screw cut may be of any length within its width, plus the advance of the mandrel. This generally suffices for the length of the screw required, as the screw tools, which are the same as those used for chasing screws by hand, but matching the thread of the screw guides, vary with the coarseness of the thread, from about one quarter to about three quarters of an inch in width. The length of screw resulting from the advance of the mandrel and the width of the tool, may also then be increased if required, by lateral replacement of the latter.

The position at which the hand tool is to be held upon the rest for cutting the external screw, is first ascertained by trial.

The bend of the crank having arrived at its highest point away from the operator, and thus determined the extent of the advancing traverse of the mandrel, the tool firmly held in the horizontal manner, is placed on the rest with its left side just out of contact with the shoulder of the work; which latter, retiring by the receding traverse, does not strike against nor displace it. The tool is then made to cut by pressure, or by slightly lowering the handle, it remains always in contact with the thread during its formation, the pressure only, being slightly reduced to relieve the friction during the backward traverse of the mandrel. After the thread has been distinctly cut by one or two traverses of the mandrel, the tool may be withdrawn to examine the progress, and in replacing it, it is only necessary to guide the points into the threads previously cut, again observing that the shoulder of the screw cannot reach the side of the tool. Screws of greater length than the width of the tool commands at one operation with the traversing mandrel, are commenced at the end in the manner described, the tool is then shifted about half its own width along the rest towards the shoulder, and re-applied to the work; the thread being then continued in length by about half the teeth, the remainder dropping into that already cut, and guiding the position of the tool, to exactly connect the new to the finished portion. The length of the screw may thus be gradually continued to many times the width of the tool without break or irregularity; and when necessary, the thread may then be improved or the screw rendered quite parallel, - the traverse of the mandrel being exchanged for its continuous revolution, - by the correctional action of the numerous points of the screw tool, traversed from one end to the other, in the ordinary manner of chasing screws by hand.

The piece for the internal screw, is prepared to the form of fig. 515. The inside screw tool is held upon the armrest, the face about radial, and the line of its teeth parallel with the mandrel; the tool remains in one place without endlong motion, and is made to cut by pressure or by slightly twisting the handle. The manipulation is essentially the same as for the external screw, except in the support of the tool and the lateral advance of its shaft, differences due to form.