Preparation and Setting of Lathe | Machine Tools | Industrial Engineering

Setting is the preparation of the lathe for a specific operation according to the manufacturing process in order to ensure the required production rate, accuracy, and surface finish. After setting, two or three work parts are processed and checked, and if their size deviates from the values stated by the drawing, the tool position is corrected to obtain the required size.

Adjustment of the lathe is done to provide for the necessary speeds and feeds. Adjustment involves a kinematic preparation of the machine for operation under specified cutting conditions according to the manufacturing process.

Before operation, the lathe should be checked for proper working condition. Make sure that the machine responds to all commands, the carriage saddle moves smoothly, without any stick-slip when driven both by hand and by power, and the chuck is firmly fixed on the spindle.

Then start and stop the motor, engage and disengage the spindle rotation and the carriage feed motion to check whether the machine functions as it should. Then start setting the machine for operation.

First, determine whether the work is to be held in a chuck or between centres.

To mount the chuck, wipe the spindle nose, its thread, and taper bore with a cotton cloth wetted in kerosene; clean the inside thread or the taper bore in the chuck’s back plate; insert a guide mandrel [Fig. 12.44 (a)] into the spindle taper bore; take the chuck with both hands [Fig. 12.44 (b)], carefully put it on the mandrel, move it towards the spindle nose, align the first threads of the chuck and the spindle, and, supporting the chuck from beneath with the left hand, screw it with the right hand on the spindle as far as it goes; insert the key into one of the square sockets in the chuck, turn the chuck towards yourself a little and then jerk it forcefully in the opposite direction [Fig. 12.44 (c)]; to prevent the chuck from loosening on the spindle insert lock strips into the spindle slots and secure them with screws; remove the mandrel by pushing it out of the spindle bore with a brass rod through the hole in the spindle.

To set the work into a self-centring three-jaw chuck [Fig. 12.44 (d)], operate the chuck key with your left hand so that there is room for the work between the jaws ; place the work in between the jaws with your right hand and tighten the chuck first with the left hand, then finally with both hands.

When machining is to be done between centres and the chuck should be removed, move the jaws apart [Fig. 12.44 (d)] and insert the guide mandrel into the spindle bore; remove the lock strips; insert the chuck key into one of the sockets in the chuck and jerk it towards yourself; supporting the chuck from beneath with the left hand and turning it with the right hand, carefully screw it out of the spindle onto the mandrel and then take it out of the machine.

Remove the mandrel, wipe the taper bore in the spindle and the taper shank of the centre. Then insert the centre with your right hand into the spindle bore [Fig. 12.45 (a)] and push it inside as far as it will go. Engage rotation of the spindle and check the centre for run out. If the centre runs out of true, remove it from the spindle with a brass rod, turn 30 to 45° and insert it again. Then set the back centre into the tailstock spindle.

To check the centres for alignment, push the tailstock up to the headstock so that the centres come to each other as close as 0.3 to 0.5 mm between them, lock the tailstock spindle and see whether the centres coincide in a horizontal plane. If they do not, adjust the tailstock to bring them into coincidence. Then mount a faceplate [Fig. 12.45 (b)] in the same way as you mounted the chuck.

The next step is to select and set up the cutting tool. Move the tool holder close to the tailstock centre (Fig. 12.46), place the tool so that its point overhangs no more than 1 to 1.5 times the height of its shank and coincides with the centre, for which purpose shims are placed under the tool shank. The shims must have well-finished parallel surfaces and should not protrude beyond the base of the tool holder. Do not use more than two shims.

After setting over, begin adjustment of the machine. Set the spindle-speed selection lever to the neutral (intermediate) position, and the feed control levers into disengaged positions; move the carriage away from headstock and stop it at a distance of 100 to 150 mm from the tailstock.

First, adjust the speed and feed gear trains and then bring the controls on the speed and feed gear boxes into the positions that provide for the specified speed and feed rate. These are selected to suit specific operating conditions.

A rational choice of cutting speed, feed, and depth of cut is done with a view to fully utilize the capabilities of the cutting tool and the machine.

To select the cutting condition, proceed as follows: determine the depth of cut from the machining allowance so that as few cuts are taken as possible; select the rate of feed having in mind the stiffness of the feed mechanism (for roughing cuts) and the surface finish required, tool geometry, and workpiece material (for finishing cuts); select the cutting speed with regard to the established depth of cut and feed rate, machine horsepower and workpiece material, and also the geometry, material, and life of the cutting tool.