Centre lathe-parts and facing operation

 Centre lathe-parts and facing operations

. the main parts of a lathe

. the size of a lathe

Turning and  centre lathe

Turning is a machining process to bring the raw material to the required shape and size by metal removal. This is done by feeding a cutting tool against a rotating work.

The machine tool on which turning is carried out is known as a lathe

Constructional features of a lathe

A lathe has provisions to

-hold the cutting tool and to feed it against the direction of rotation

 -have the parts fixed and sliding to get a relative movement of the cutting tool with respect to the rotation of the work

-fit accessories and attachments for performing the different operations

Main parts

The main parts of a lathe

Headstock

Tailstock

Carriage

Cross-slide

Compound slide

Bed

Quick change gear box

Legs

Feed shaft

Lead screw

The size of  a lathe based on

-the maximum diameter of a work that can be held

-the swing over the bed. This is the perpendicular distance from the lathe axis to the top of the bed

-the length of the bed

-the maximum length of work that can be turned between centres

Precautions in lathe work

Before starting

Before starting the work ensure that

-the lubricating system is functioning

-the meshing gears are in proper mesh and the power feed levers are in neutral position

-the work area is clean and tidy

-the safety guards are in proper condition and are correctly placed.

During working

Shift the levers to change speeds and feeds only when the rotating parts have fully stopped.

Wear an apron which is not loose, and have the sleeves folded

Avoid wearing rings or watches during working.

Remove the chips with a hook

Wear shoes

After work

Clean the machine with a brush and wipe cotton waste.

Oil the bed-ways and the lubricating points.

Clean the precision instruments and wipe them dry before storing.

Clean the cutting tools and place them in their respective places

Clean the surroundings of the lathe, wipe the split and coolant and remove the swarf.

Lathe operations -facing

Facing

This is an operation of removing metal from the work face by feeding the tool at right angles to the axis of the work.

Purpose of facing

To have a reference plane to mark and measure the step lengths of the work

To have a face at right angle to the axis of the work.

To remove the rough surface on the faces of the work and have finished faces instead.

To maintain the total length of the work

Facing may be rough or finish facing. Rough facing is done to remove the excess metal on the face of the work by coarse feeding with more depth of cut, leaving sufficient metal for finishing. Rough facing is done by feeding the tool from the periphery of work towards the centre of the work. Finish facing is the operation to have a smooth face by removing the rough surface produced by the rough facing

Finish facing is done by feeding the tool from the centre of the work towards the periphery.

Rough facing is done by choosing a spindile RPM according to the average diameter of the work, the recommended cutting speed with a coarse feed and more depth of cut.

Finish facing is done by choosing a cutting speed about twice that of the cutting speed for roughing, with a fine feed rate of 0.05mm approximately and with a depth of cut of not more than 0.1mm

The defects found in facing work

A  concave face

This is casued by the tool digging inti the work during the feeding as the tool is not clamped rigidly. By clampind the tool rigidly with minimum overhang, this defect can be avoided.

A convex face

This is caused by the blunt cutting edge of the tool and the carriage not being locked. To avoid this defect, resharpen the tool and use it :also lock the carriage to the bed of the lathe.

A pipe left in the centre

This is due to the tool not being set to the correct centre height. By placing the tool to the centre height, this defect can be avoided.