On the Lathe

Cutting speedCutting speed for turning is the speed at

which the work rotates. This is also known as surface speed

Rate at which point on work circumference travels past cutting tool

expressed in meters per minute (m/min)Important to use correct speed for material

Too high: cutting-tool breaks down rapidlyToo low: time lost, low production rates

Cutting speed selectionThe softer the work material, the faster the

recommended cutting speed

The hardness of the cutting tool material has a great deal to do with the recommended cutting speed. The harder the cutting tool material, the faster the cutting speed. The softer the cutting tool material, the slower the recommended cutting speed

The depth of the cut and the feed rate will also affect the cutting speed, but not to as great an extent as the work hardness

Factors that Determine cutting speedThe material being cutThe rigidity and condition of the

machineThe material of which the tool is

made fromThe depth of cut and the feed rateAvailability of coolant (cutting fluid)

Setting Speeds on a LatheSpeeds measured in revolutions per minute

Changed by stepped pulleys or gear levers

Belt-driven latheVarious speeds obtained by changing flat

belt and back gear driveGeared-head lathe

Speeds changed by moving speed levers into proper positions according to r/min chart fastened to headstock

Safety Note!! NEVER change speeds when lathe is running.

Setting Speeds on a Lathe

Speeds measured in revolutions per minuteChanged by stepped pulleys or gear levers

Belt-driven latheVarious speeds obtained by changing flat

belt and back gear driveGeared-head lathe

Speeds changed by moving speed levers into proper positions according to r/min chart fastened to headstock

9

Lathe Cutting SpeedsMaterial being cut Cutting speed (metres/minute

Mild steel 20 to 28

Cast iron 18 to 25

High carbon steel 12 to 18

Brass 45 to 90

Bronze 15 to 21

Aluminium Upto 30047-10

Calculating Lathe cutting speedGiven in metres per minutespindle speed of machine (N) and

diameter of work must be known

S (m/min)= πDN/1000Where π= 22/7 or 3.142 D= diameter of material N= Spindle speed(rev/min)

47-11

Calculating Lathe Spindle speed (rev/min)

D xS1000 rev/min

S (m/min)= cutting speedWhere π= 22/7 or 3.142 D= diameter of material N= Spindle speed(rev/min

ExerciseCalculate the spindle speed required to

turn 200mm diameter piece of high speed steel, if the cutting speed is 28 m/min.

Find the cutting speed of a 15mm diameter bar being turned with a spindle of 955 rev/min.

Determine the lathe speed to cut a 40mm diameter at 30 m/min