Turning workpiece

51

INFORMATION SHEET NO. 5

10. 2 R P M CALCULATRIONS

A lathe operation is required to determine the number of revolutions per minute the
spindle must revolve at to procedure the required cutting speed, for a certain material,
of a particular diameter. The diameter of the work directory affects the cutting speed
since the cut is made on the circumference of the work.

Consider a bar of metal 25 mm diameter rotating at 1500 R P M. Replace this with a
bar 50 mm diameter. The metal passing the tool in one revolution would now be
doubles that of the first bar. To maintain the same half must reduce cutting speed the
revolutions per minute. That is form 1500 R P M to 750 R P M. A bar of 75 mm
diameter would required that the R P M be reduced one third that of the 25 mm bar,
which is 500 R P M.

52

To calculate the R P M of the lathe for a particular job, the following date is necessary:

Cutting speed. This is obtained from a table of speeds.
Diameter of work. This is required as cutting speeds are measured on the
circumference.

Circumference = n D mm, where D = diameter of work
= 3.142 x D

Cutting speed is expressed in meters per minute, to convert to millimeters per
minute multiply by 1000.

Cutting speed = speed from table x 1000

R P M = cutting speed ÷ circumference

= Cutting speed x 1000
= 3.142 x D

After calculation, should the correct speed lie between two settings of the machine, start
at the lower speed. Should the condition of the lathe, the tool and the work permit,
select the higher speed.

If the calculated R P M falls very close to the higher speed, select this speed.

When working with special costly tools always use the slower speed.

Example:

Calculate the lathe speed in revolutions per minute for rough turning a free
cutting mild steel bar of 25 mm diameter using a cemented carbide tool.

From the table, cutting speed for rough turning free cutting mild steel is
120 m/min.

R P M = cutting speed
Circumference

= 120 x 1000
25 x 3. 142

= 1528 R P M

53

A simple and quick method of calculating approximate R P M is to take n as equal to 3.

Then RPM = cutting speed x 1000
3xD

IF cutting speed is 120 m/min. and diameter of work 50 mm

RPM = 120 x 1000
3 x 50

This is reasonably close to the accurately calculated R P M of 764.

54

Use the monogram to obtain revolutions per minute by selecting data in the following
sequence:

55

• Type of machine operation  e. g. Rough turning
€ Cutting tool material  Cemented carbide
€ Metal to be cut  Free cutting mild steel
€ Cutting speed  120 m/min
€ Diameter of workpiece  75 mm
€ RPM  500 approximately.

From the inter section of the 75 mm diameter vertical line and the 120 m/min sloping
line, follow the horizontal line to the left hand scale.

10.3 FEED

Feed is the distance which the tool advances
along the face work for each revolution of the
spindle. Feed may be regarded as the thickness
of chips. A feed of 0.5 mm along the work
for each revolution of the spindle. It will therefore,

take 20 revolution for the tool to move 1 cm and
10 mm = 1 cm.

Take as few cuts as possible when machining a
workpiece. In most cases one roughing and one
finishing cut would be sufficient.

The finishing cut requires a fine feed to obtain the
necessary surface finish and accuracy of size.

A good practice is to use a medium depth, heavy
feed, and correct cutting speed for roughing out work.
Finishing is then done at an increased speed, with light
depth, and relatively fine feed.

It is better to have a deep cut with a fine feed than a
cut equal in depth to the feed, although the rate of metal
removal is the same in both cases.

With the deeper cut the work is distributed over a
longer cutting edge and the chips have to curl sharply,
thus requiring less power.

56

11 DEPTH OF CUT

The depth of cut is the amount of metal removed
from one side of the work by the tool.

A cut of 3 mm on a piece of work will reduce the
diameter of the piece by 6 mm.

When rough turning, after the dept of cut has been
decided, the feed may be made as heavy as the lathe,
the work and the tool will stand.

The following points should be considered as a guide
to deciding on depth of cut for a particular workpiece.

€ When selecting the rough bar for a job the
diameter of the bar should be approximately
3.18 mm larger than the largest finished
diameter of the workpiece.

 This is to ensure that the roughing
cut is deep enough to remove any faults
from the outside surface of the bar, and
leave sufficient material for the finishing cut.

€ If the calculated R P M falls between two
speeds, select the lower speed. Increase
the speed if the condition of the lathe, the
tool and the workpiece permits.

 If the calculated R P M falls very close to
the higher speed, select this speed.

€ Roughing cuts should be as deep and coarse as
possible. Leave about 0.76 mm on the
diameter for the finishing cut.

€ When turning cats iron or any other metal which
has scale on its outer surface, set the tool deep
enough to get under the scale at the first roughing cut.
Otherwise, the scale will wear away the cutting edge.

57

12 MACHINE TIME

To calculate the time required machining a workpiece many factors must be considered.
The feed in conjunction with other equally important factors such as the efficiency of the
machine, the method of holding the work, the speed and depth of cut will greatly affect
the time taken for a particular job. This forms the basis of production estimates.

Distance of tools traverse per min = Feed x R P M

Time to cut a given length =. Length of cut .

Distance of tool traverse per min

= Length of cut
Feed x R P M

Machine time = Time per cut x number of cuts on workpiece + setting up time.

Example

Calculate the time required to machine a 50 mm diameter medium steel bar that is 300
mm long to a finished diameter of 46.24 mm. A high-speed steel cutting tool is to be
used.

First calculate machine time for ROUGH TURNING

From the monogram, cutting speed = 24 m/min

To convert to mm = 24 x 1000 = 24000 mm/min

R P M = cutting speed
circumference

= cutting speed
dia x n

= 24000 = 152.8

50 x 3.142 or 153 RPM

58

The same R P M would be obtained by referring to the nomogram.

Time to cut a given length = length of cut
Feed x R P M

A feed of 0.40 mm would be appropriate = 300 = 4.9

0.40 x 153

= 4.9 minutes

The rough cut of 1.5 mm would have reduced the work piece diameter to 47 mm.
Calculate machine time for FINISHED TURNING
From the nomogram, cutting speed = 30 m/min
= 3000 mm/min

= 3000 = 203.2
47 x 3.142

= 203 R P M

The same R P M would be obtained by referring to the nomogram.

A feed of 0.10 mm would be appropriate.

The time to cut the 300 mm length = 300 = 14.78

0.10 x 203

= 14.8 MINUTES

Machine Time = 4.9 + 14.8

= 19.7 minutes

59

SELF-CHECK: RPM CALCULATION

1. What is the R.P.M of a polishing wheel if the motor rpm of 1440 / min is
altered by two component transmission ratio of 1:3 and 5:4

a. 461 b. 463 c. 465

2. The spindle of a milling machine is driven via component transmission
with ratio of 2:3, 3:4, and 4:5. What is the total transmission ration?

a. 479 b. 480 c. 481

3. Calculate the chip section if the depth of cut is 3 mm and the feed is 0.6
mm.

a. 0.18 mm b. 0.16 mm c. 0.22 mm

4. R.P.M means

a. Revolution per minute

b. Revolution per mile

c. Revolution per move

5. If the cutting speed is 320 m/min and the diameter is 60 mm what is the
RPM?

a. 177.7

b. 177.8

c. 177.9

60

INFORMATION SHEET NO. 6

THREADS AND THREADING
I
NTRODUCTION

The lathe may be used to cut most
sizes and shapes of threads.
Cutting threads in a lathe is an
important and interesting operation.
Therefore the machinist should
have knowledge of the principles
involved.

This manual details the methods of
setting up, cutting and measuring
screw threads.

The information contained in ‘ The
lathe’ must be fully understood and
the skills mastered before
proceeding to use this manual.

Observe all the safety requirements
of your workshop and develop safe
working habits when operating the
lathe.

SCREW THREADS

A screw thread is a spiral ridge
formed on a straight cylindrical
surface.

The thread may be on an external
surface or an internal surface.

A bolt has an external thread.

A nut may be produced using hand
tools, such as taps and dies, or by
machine tools. The most common
thread is cut on a straight cylindrical
surface. It is called a straight
thread.

61

One of the most frequently used
machine tools for producing threads
is the center lathe. Cutting a spiral
groove in the surface of a rotating
workpiece produces the thread.

PARTS OF A SCREW THREAD

The parts of a screw thread
are identified by the following terms:

 Major diameter
 Minor diameter
 Pitch diameter
 Flank
 Root
 Crest
 Thread angle
 Depth
 Lead angle
 Lead
 And
 Clearance

MAJOR DIAMETER

The largest diameter of a straight
external or internal largest thread.

MINOR DIAMETER

The smallest diameter of a straight
external or internal thread.

PITCH DIAMETER

The distance between diametrically
opposite points on a straight thread.
It is equal to the major diameter
minus one depth.

62

The distance a threaded surface moves
parallel to the axis of the thread during one
complete revolution.

PITCH

The distance from a point on one
thread to a corresponding point on
the next thread, measured parallel
to the axis.

FLANK

The surface connecting crest and
root.

ROOT

The bottom surface joining the
flanks of adjacent threads. The root
diameter is the same ad the minor
diameter.

CREST

The top surface of the thread
bounded by the flanks.

THREAD ANGLE

The included angle between the
flanks of adjacent turns of the
threads.

DEPTH

The perpendicular distance between
the root and crest of the thread.

LEAD ANGLE

The angle which the spiral makes
with its axis. The lead angle is
determined by the lead and pitcher
diameter of the thread.

LEAD

63

The direction of rotation of the lead screw
determines the hand of the thread being cut.

HAND

The direction in which the thread is
turned to advance. A right hand
thread is turned clockwise to
advance. A left hand thread is
turned anti clockwise to advance.

CLEARANCE

A space left between the mating
external threads to facilitate easy
rotation
of threaded parts.

 THREADED CUTTING IN
THE LATHE

Threads are cut in a lathe by
rotating a cylindrical workpiece at a
constant speed and machining the
surface of the workpiece with a
cutting tool.

The cutting tool is moved at a
constant speed in a direction
parallel to the axis of the workpiece.

The workpiece is mounted in the
lathe using a suitable accessory and
rotated by the lathe spindle.

The cutting tool moves with the
lathe carriage by engagement of a
half nut with a lathe lead screw.

The shape of the cutting tool
determines the shape of the groove
cut in the workpiece and, therefore,
the profile of the thread.

64

nut following the thread of the rotating lead
screw.

The pitch of the lead screw thread
and the relative speeds of the lathe
spindle and lead screw determine
the number of threads cut per unit
length of the workpiece.

If, during one revolution of the
workpiece, the cutting tool is moved
by one unit of length, then one unit
of length will be cut.

By varying the speeds of the lathe
spindle and lead screw a cut of any
desired number of threads per unit
length may be made.

The pitch of the thread is the
reciprocal of the number of threads
per unit of length.

Hence:

Pitch
=___________1____________

Number of threads per unit of
length

For example, a screw with 10 turns
per centimeter has a pitch of 1/10
cm.

SETTING UP THE LATHE
GEARBOX FOR THREAD
CUTTING

During a thread cutting operation on
a lathe the cutting tool is mounted
on the lathe carriage.

The carriage is moved along the
lathe bed by the action of the half

65

The position of the idler gears may also be
changed to reverse the direction of rotation
of the lead screw.

The lead screw is rotated by an
intermediate gearbox driven by the
lathe spindle. The gearbox provides
the necessary variations in speed
and direction of rotation of the lead
screw to enable threads of various
pitches to be cut.

There are two types of gearboxes:

 The standard change
gearbox

 The quick change gearbox

THE STANDARD CHANGE
GEARBOX

In standard change gearbox, motion
is transmitted from the spindle to the
lead screw via tumbler gears, a stud
gear, two idler gears and screw
gear.

The disposition of the tumbler gears
relative to the spindle gear
determines the direction of rotation
of the lead screw. A handle is
provided to permit selection of the
desired direction of lead screw
rotation, or a neutral position.

To change the speed of the lead
screw it is necessary to change the
size of the stud gear or the screw
gear.

This is done by removing the
appropriate gear from its shaft and
fitting a substitute gear from the set
of gears supplied with the machine.

66

A gear chart will usually be provided
with the machine to show the
gearing changes necessary to
obtain a desired pitch.

Lead screw pitches vary from
machine to machine. The
information given on the chart fitted
to a particular lathe is related to the
lead screw pitch and may not be
applicable to another machine.

USING THE STANDARD CHANGE
GEARBOX CHART

 Determine the number
of threads per unit of length
required in the thread cut.
 If, for example, the
requirement is for 16 threads
per inch, locate 16 in the
threads per inch column of
the chart.
 Note the
corresponding numbers in
the adjacent stud gear and
screw gear columns, i.e. stud
gear 24, screw gear 48.

67

CHART FOR THREADS AND
FEEDS
9 inch Workshop Lathe

68

3. Note that the top lever

must be moved to the

right and the sliding gear
moved to the “in”

position.

* Fit a 24 tooth gear and a 48 tooth 2. If, for example, the requirement is
lead for 16 threads per inch, locate 16

screw gear to the lathe.

The lathe is now set up to turn the
spindle at the speed necessary to
produce the required 16 threads per
inch.

THE QUICK CHANGE GEARBOX

The type of gearbox, which is fitted
to most modern lathes, simplified
setting up the desired lead screw
speed. Gear changing is effected
by movement of two levers and a
sliding gear.

This eliminates the need to remove
and change the gears as in the
standard change gearbox.

This relative positions of the levers
and the sliding gear for a given
number of threads per unit of length
is indicated on a chart fitted to the
machine.

USING A QUICK CHANGE
GEARBOX

The steps to take are:

1 .Determine the number of
threads per unit of length
to be cut.

on the threads per inch 69
chart on
The lathe is now set up to turn the lead
the machine. screw at the speed at the necessary to
produce 16 threads per inch.
4. Engage the tumbler lever
in the hole beneath the GEARING CALCULATIONS
column on the chart in which
16 appears. There are two situations where it will be
necessary to determine by calculation the
gearing necessary to produce a given thread
pitch, i.e. the particular number of threads
per unit of length:

70

 Where the range of
gears available does not
directly cover the ratio
required to produced the
desired thread.

 Where the lathe
gearbox is not

fitted with a chart.

OBTAINING A DESIRED RATIO FROM AVAILABLE GEARS

When it is necessary to turn an unusual thread it may be found that the range of gears
available is inadequate. For example, there is a requirement to cut a thread of 7 per
inch. Reference to the machine chart shows that a 16-tooth spindle gear and a 56 tooth
lead screw gear is required.

The range of gears available may be from 24 teeth increasing in increments of 4 teeth
to 100 teeth. In this example there is then no 16-tooth wheel available for the machine.

By multiplication of the basic gear teeth ratio it is possible to obtain a pair of
gears in the available range which will provide the required lead screw speed
The basic ratio in this instance is: 16: 58, which when simplified is:

2:7, i.e. both numbers divided by 8.

This may be multiplied by a factor to produce two gears with a tooth ratio in the
available range.

I.e. f (2: 7)

The factor f may have any value, but in this case must produce tooth numbers between
24 and 100.

For instance, if a value of 16 is chosen then

16 (2: 7) = 32: 112

There is no 112-tooth wheel available, so another value for f must be tried, and this
obviously must be less than 16.

If f is given the value of 12 then

12 (2: 7) =24: 84

Both these wheels are available and when fitted to the machine will turn the lead screw
at the appropriate speed to produce a thread of 7 turns per inch.

71

CALCULATING GEAR RATIOS WHEN NO CHART IS AVAILABLE
There are three types of gear trains:

 The simple gear train
 The compound gear train
 The triple gear train

Simple gear train speed ratios are easy to calculate, but are limited by the largest and
smallest gears supplied with the machine.
Compound gear trains give a wider range of speed ratios, but are rarely used on most
lathes.
Triple gear trains give an even wider range of speed ratios, but are rarely used for
general thread cutting.
SIMPLE GEAR TRAIN CALCULATIONS

A simple gear train on a lathe consists of a driver gear turned by the lathe spindle, an
intermediate gear, and a driven gear.
The driven gear turns the lead screw.
The intermediate gear merely serves to transfer the drive from the driver gear to the
driven gear
The size of the intermediate gear does not affect the speed ratio calculations.
The lead of the lead screw thread and the ratio of the number of teeth of the driver gear
to the
To the number on the driven gear together determine the lead of the thread cut on the
workpiece.
For example, to cut a thread of 10 threads per inch from a 4 thread per inch lead screw,
the ratio of the teeth on the driver and driven gears is calculated as follows:
To produce a thread of 10 threads per inch the cutting tool must move 1/10 of an inch
for each revolution of the workpiece. The workpiece and driver gear are rotated by the

72

spindle, and at the same speed.
If the ratio of the driver and the driven gears

is 1: 1 the thread cut will be 4 threads
to the inch.
To cut required 10 threads per inch the gear
ratio must be 4:10, which can be simplified
as follows:

1/10 = Driver = Lead of workpiece
1 / 4 = Driven = Lead of lead screw

= 1/10 x 4/10 = 4/10

The ratio of the number of threads per inch of the lead screw and the number of threads
per inch to be cut on the workpiece is also 4:10, i.e. 2:5
Thus, to calculate the gear teeth ratio we may use the formula.

Driver = Lead of workpiece
Driven Lead of lead screw

To obtain a practical number of gear
teeth we may then multiply both sides
of the ratio by any convenient factor.
Note that this simply expresses the
same ratio in more practical terms.
It does not alter the ratio. If, for example,
the factor used is 10,

73

Then: Driver = 2 x 10 = 20
Driven 5x20 50

If gears are available with 20 and
50 teeth then the 20 tooth
gear is fitted as the driver gear
and the 50 tooth wheel fitted as
the driven gear.

These gears will produce the
desired lead screw speed to cut a thread
of 10 threads per inch on the workpiece.

COMPOUND GEAR
TRAIN CALCULATIONS

The formula derived for
calculating simple gear trains
is suitable also for compound
gear trains.

To obtain the necessary change
gears for a particular ratio the ratio
must be factorized.

For example, if the ratio

Lead of workpiece = 6 = 3
Lead of leadscrew 48 24

This may be factorized to give
two ratio groups:

3 = 25 x 3
24 3 8

Both top and bottom lines of each ratio group are then multiplied, as in the case of
simple gear trains, to obtain a wheel
with a number of teeth in the available
range.

Considering the 1/3 ratio group,

If = 25, then

1 x 25 = 25 = Driver
3x25 75 Driven

74

Similarly, for the 3/8 ratio group,
3 x f = Driver
8 x 10 Driven

Accordingly, a driver gear having
25 teeth and a driven gear having
75 teeth are fitted. The compound
chain will then produce the desired ratio.
To check that the calculations are
correct, multiplication of the calculated
wheel tooth ratios should produce the
original ratio.
25 x 30 = 1 = 6 , the original ratio
75 80 8 48

TRIPLE GEAR TRAIN
CALCULATIONS
The ratio is factorized as
before but into three groups
which are then enlarged to
suit the range of change gears.
For example, if the ratio
Lead of workpiece = 6
Lead of leadscrew 48
This may be factorized into three ratio groups
6/48 = ¾ x 2/6 x ½ = Driver/ Driven

75

= (3/4 x 10/10) x (2/6 x 10/10)
x (1/2 x25/25)

= 30/40 x 20/60 x 25/50
= Driver/ Driven

METRIC THREAD GEAR
CALCULATIONS

Metric threads are defined by
stating the lead of the thread in millimeters rather than the number of threads per inch.

The formula

Lead of workpiece is used.
Lead of leadscrew

There are 25.4 mm. in an inch

i.e. 1 millimeter = 1/ 25.4 inches.

For convenience in ratio calculations,
both sides of this ratio are multiplied
by 5 to obtain whole numbers.

1x5 =5

25.4 x 5 127

The formula then used is

Lead of workpiece x 5/ 127.
Lead of leadscrew

= Driver
Driven

76

For example, to calculate the gears
necessary to cut a thread of 1.75 mm.
lead with a lead screw of 6 threads
per inch:

Driver =1.75 x 5
Driven 1 127

65
=1. 75 x 1 x 127

=175 x 30
100 127

This may be readily separated to provide the
necessary ratios for a compound gear train:

175 is simplified 7 = 70
100 4 40

Thus the Driver ratio groups
Driven

Are 70 and 30
40 127

NOTE:

To simplify setting up for metric threads the
lathe will usually be provided with
the gear having 127 teeth.

6.2.5 Calculations For Multiple Threads

The most common threads are cut
with a single thread, consequently,
the lead and the pitch of the thread
are the same.

A double start thread is produced
by cutting two parallel helical grooves
n the workpiece

The second groove is cut in the center
of the land formed by the first groove.

The lead of the thread produced is twice the pitch.

77

In a triple thread, formed by the three grooves, the lead is three times the pitch.

The gear ratios are calculated using
the formula:

Lead of workpiece
Lead of leadscrew

The gears selected must have a
number of teeth which is a multiple
of the number of threads to be cut.

Thus, to cut a triple thread, the
number of teeth on the driver gear
and the driven gear must each
be divisible by 3.

This method is employed when using
a divided driving gear to obtain the
required number of starts.

Multiple start threads may be cut by
indexing the cuts. This is done by the
moving the top slide a distance equal
to the pitch of the thread.

7 THE THREAD CHASING DIAL

To produce a smooth accurately
profiled thread the cut is made with
several passes of the cutting tool.
The depth of the thread is increased

78

with each successive cut.

At the end of the pass, it is
necessary to withdraw the tool from
the cut and accurately position it to
ensure that the path of the first cut is
followed by the successive cuts.

In practice this means that the
carriage half nut is disengaged
from the lead screw at the end
of each pass. The carriage is then
returned to the starting position and
the lead screw re-engaged with the
half cut.

Even though the tool may be
correctly positioned, if the lead screw
is engaged with the half nut at the
wrong instant, the tool will not follow
the original cut and an unwanted
second groove will then be cut in the
workpiece.

To eliminate this possibility a thread
chasing dial mechanism is used.

The mechanism, which is fixed to the
carriage, consists of a rotating graduated
dial. The dial is rotated by a worm gear, which
meshes with the lead screw.

79

By nothing the movement of the graduated
dial with the respect to an index line it is
possible to determine the correct instant
for engaging the lead screw with the half nut.

Thread chasing dials are available for
cutting metric threads, in which case the
lathe is fitted with a metric lead screw.

The dial can be fitted in three positions on
the lathe to provide for each of the worm gears
meshing with the lead screw.

80

THREAD CHASING DIAL CHART

THREADS PERINCH WHEN TO ENGAGE READING ON DIAL
TO BE CUT HALP NUT

EVEN NUMBER ENGAGE AT ANY 1
OF ½
THREADS GRADUATION ON 2
THE DIAL 2
ODD NUMBER ½
OF
THREADS 3

3½

82414EN½GAGE AT ANYpositions
MAIN DIVISION
3

4
4 positions

HALF FRACTIONAL ENGAGE AT
NUMBER OF 1&3
THREADS EVERY OTHER or
MAIN DIVISION
QUARTER 2&4
FRACTIONAL
NUMBER OF 21position
THREADS ENGAGE AT or
THE SAME 2
MIAN DIVISION or
3 or 1 positions
4

THREADS WHICH ARE ENGAGE AT ANY TIME USE OF DIAL
UNNECESSARY.
A THAT HALF

MULTIPLE OF THE NUT MESHES.

NUMBER OF THREADS

PER INCH OF THE

LEAD SCREW.

7.1 CUTTING AN EVEN NUMBER OF

81

THREADS

EXAMPLE:

It is required to cut threads per inch on a
lathe fitted with a lead screw of 4 turns per inch.

The dial gauge worn gear will be provided
with 16 teeth so that one major division on the
dial is equivalent to one inch of carriage movement.

The gearbox will be set for a ratio of 6:4

At the end of each ½ inch of tool movement.
The spindle and lead screw will be engaged at
any main division or half division of the dial.

7.2 CUTTING AN ODD NUMBER OF THREADS

If, in the previous example, a thread of three
turns per inch is required, the work would be
required to turn three revolutions and the lead
screw four revolutions for each of carriage movement.

The lead screw may be engaged by the half nut when
any of the main dial divisions reaches the index line.

7.3 CUTTING FRACTIONAL THREADS

When cutting fractional threads such as 3.25
threads per inch, the lead screw must, as a general
rule, be engaged with the half nut at the same dial
reading on each pass.

However, to save time when cutting half threads
such as 3.5 threads per inch, the lead screw may be
engaged with the half nut at every second main
division of the dial, i.e. at 1 and 3 or at 2 and 4.

82

INFORMATION SHEET 7
RECOGNIZING THE EFFECT OF HEAT ON MEASUREMENTS

Effects

1. When heat is applied to a material it can
affect it in a number of ways, some of which
concern us when measuring in the workshop.

2. Heating can cause a change of state.
For example, when ice is heated it becomes
water and when further heated changes in to
steam. Thus the ice changes to a liquid
(water) and the liquid changes into a
gas (steam).

3. When solder or welding rods are heated
they melt to from a liquid metal.

4. There are other effects in addition no
the above, but the effect that is of
interest to us when measuring is
change in dimension. Liquids, gases
and metal expand when heated and
contract (shrink) when cooled.

83

Temperature

5. Temperature is the degree of
hotness or coldness of a
material. A material has a high
temperature when hot and a
low temperature when cold.

Change in Dimension

6. If a bar of metals is heated it
becomes longer.

7. The amount that dimension
increase when a workpiece is
heated depends on the type of
metals and the length of its
dimensions. It is know that
metals increase their dimensions
by a constant amount for each
degree of temperature rise.

84

8. The amount that a unit length of
material expands a one degree
rise in temperature is called the:

Coefficient of Linear
Expansion

9. This table shows the coefficients of
of linear expansion ( ) of common
engineering materials.

10. By using the information in the above
table, we can calculate the expansion
of a given piece of metal for a given
rise in temperature, by using the
formula:

Expansion in mm =

 x length in mm x temperature
 rise in oC or K

11. Example: A steel bar 100mm long is
heated so that its’ temperature rises
60oC. By how much does the
bar increase in length?

Increase in length =
0.000 0124 x 100 x 60

= 0.0744mm

85

12. The example just given shows that
errors can occur if measurements are taken
when a work piece is very hot, such as
after a heavy machining operation. It is
therefore necessary to have a standard
temperature at which measurements should
be taken.

The standard temperature for talking
measurements is +20 oC
( the so-called “room temperature”)

13. Always allow a how work piece to
cool down therefore, before you take
a final measurement.

86

14. We can make use of this change of
dimension caused by changes in temperature.
For example, when shafts and bores have to
fit very tightly together (the so-called “shrinkage fit”).
The part having a bore is heated to a temperature
calculated to increase the diameter of the bore so
that it will fit easily onto the shaft. As the bore
cools down, it shrinks and thus grips the shaft
very tightly.

87

Self Check

Each question in this check is shown with several possible answers.
Mark your choice in the appropriate box. When you have completed the check,
CALL THE INSTRUCTIOR.

88

1. Metals change their dimensions (a) They increase l mm for each 10o
when change in temperature…………
heated. The way the dimensions
(b) They expand when heated and
change is contract when cooled…………..

(c) They contract when heated and
expand when cooled…………..

(d) They increase in length by l mm for
each lo change in
temperature……………………...

2. The degree of hotness or coldness (a) Temperature……………………..

of a

material is its: (b) Heat……………………………...

(c) Expansion………………………..

3. The Coefficient of Linear (d) Coefficient of Linear
Expansion of Expansion………………………..

Cast Iron is: (a) 0.000 023……………………………

(b) 0.000 0124……………………….

(c) 0.000 0106……………………….

(d) 0.000 0169……………………….

Continue to next Element Instructor
Repeat Element and Check

89

INFORMATION SHEET NO.8

CUTTING FLUIDS

Cutting fluids are used in connection with most machining operations,
especially on steel, to prevent friction and heat generation

Functions of cutting fluids chips produced by heavy cuts at
high speeds, thereby preventing
€ Cooling the tool and the wokpiece from
- During the cutting process, becoming fouled by chips.
energy appears in the form of
heat. Most of this heat is - Deep drilling or boring operations
generated at the point of the tool cause more difficulty from fouling
by friction between the tool and than any other operation.
the workpiece, the deformation of Fouling may be prevented by
the chip and the chip sliding directing a stream of cutting fluid,
across the tool. often at high pressure, to the
cutting edges of the tool so that
- A stream of coolant is directed to the chips will be forced back
the point of cutting to carry away through the bore or along the
this heat, which otherwise may flutes of the drill.
break down the cutting edge of
the tool or may distort the - Special drills with holes through
workpiece. them for fluid flow can be used to
ensure that the fluid reaches the
€ Lubricating cutting edges (see figure 1)

- Some of the cutting fluid get Figure 1. oil hole drill
under the chip and onto the nose
of the tool. This lubricates the € Preventing Corrosion
contacting faces and helps the - Cutting fluids must be no
flow of chips. The amount of corrosive so that the machines
friction is reduced so that a sharp and the workpiece will not rust.
cutting edge on the tool is
maintained and a better finish on - Some types of cutting fluids react
the workpiece results. Also, the slightly with copper and its alloys,
energy that would be necessary and subsequent difficulty is
to overcome friction is lower, and experienced if the part has to be
generally the whole operation is tinned electroplated.
more effective.

€ Chip Removing

- The stream of cutting fluid helps
to remove the large amount of

90

and silicate of soda are the two
most commonly used.

Additional Properties - This type of fluid is generally
- As well as the four primary used for grinding operations,
functions described above, a where surface cooling and
cutting fluid described above, a washing away of abrasive
cutting fluid must be: particles are not important.

* Non-injurious to the operator; € Soluble Mineral Cutting Oils
* Non-flammable at normal - Soluble oils is the “maid of all
work” in metal cutting. They are
working temperatures; adaptable, law in cost and
* Reasonably cheap and in effective. They are composed of
minerals oils, plus emulsifying
plentiful supply; agents, make the oils miscible
* Disposable. with water. Full use is made of
the superior cooling properties of
Types of Cutting Fluid an emulsion, in addition, they
During the past few years, oil have some lubricating action.
companies have produced many Additives of a suitable type are
varieties of cutting fluid to meet the introduced to heavy duty oils to
ever increasing demands of usefully increase the ability to
industry, and their products have lubricate and so extend their field
taken the place of other fluids. of operation.

Cutting fluids may be a straight Care must be taken when
oil, compounded oils or may contain mixing an emulsion as over
varying proportions of oil and water. dilution may cause rusting, while
This last one is an excellent coolant, under dilution involves
but a poor lubricant and can cause greater expense, will reduce the
rusting. The two former are cooling effect of the fluid and
excellent lubricants and non- may cause skin reactions. The
corrosive, but have fess than half amount of dilution varies
the cooling power of water. accordingly the nature of cutting
operations of soluble oil used.
There is no perfect cutting The best advise to follow is the
fluid, with all the required properties correct practice is to pour the
for all types of work, so production oil gradually into the agitated
engineers must know the particular water.
properties of a variety of fluids to be
able to select the most effective type € Oil – less Cutting Fluids
for a particular job. - There are some transparent
soluble cutting fluids that do not
€ Alkaline Solutions contain any mineral oil. These
- These solutions consist of a mild
alkaline compound mixed with
water, carbonate of soda

91

are especially suited for light center, capstan and turret lathes; and
machining operations such as die- head screwing. They are also
grinding where the operator useful for general purpose duties on
requires an obstructed view of medium capacity bar automatics.
the work. The fat content can vary between
- Typical of the chemical solutions 5 and 40%

oil-less cutting fluids are no 3. Sulphurized and Chlorinated
longer a novelty, and consist of
carefully chosen chemicals in Base Oils
solution, which when further If highly chlorinated or sulpho-
diluted with water, possess a chlorinated additives are used, the
good flushing action, cool well resultant product also has extreme
and maintain all contact areas in pressure (EP) properties and
a rust free conditions. Using this enables severe tooling to be carried
type of fluid for grinding out successfully on a high duty
operations, mixture as lean as nickel alloys and on stainless steels.
1:100 are permissible. Broaching is a typical application.
€ ‘Straight’ Cutting Oils
- These oils are normally used The sulphurized oils are probably the
undiluted with water. They come most useful group of straight cutting
in many forms. oils. Their sulfur content can be
a. Straight mineral oils incorporated in many different
b . Mixtures of mineral and fatty ways. Provided there is no free”
acids sulphur present, the product is
c. Sulphurized and chlorinated equally suitable for machining steel
and cupreous metals. High nickel
base oils. alloys are also susceptible to
1. Straight mineral oils are used for sulphur tarnish under certain
conditions but as these materials
non exacting work as they have are difficult to machine
quite a low performance factor,
but are suitable for light satisfactorily with alternative oil
repetitions work on single fluids, the benefits of
capstan and turret lathe work sulphurization should not be
where free machining brass and disregarded entirely. The answer
steels are the main work lies in careful choice of the most
materials. Their lubricating films appropriate sulphurized grade
are unable to accept or, if this proves unsatisfactory from
considerable tool loading. the tarnish aspect, a suitable grade
of straight oil containing inactive
2. Mineral and fatty oil mixtures these only should be chosen.
mixture are more versatile since
they possess better file
properties. They may be used
over quite a wide range of work,
including turning on

92

€ Flow and temperature When high pressure is used, or when
the nozzle is too far from the work,
The flow quality of cutting fluids is an excessive splashing may result. This
important factor. The flow should not is wasteful and reduces the cooling
only be copious but the fluid should action.
flow gently onto the workpiece.
(see Figure 3)

Large operations involving a Sometimes, the swarf or chip prevents
broad tool or more than one tool may the cutting fluid from reaching the
require several streams of cutting fluid, cutting edge as is the case in
while for high speed machining, conventional turning operations when
special forms of jets may be needed for the flow is from above the tool. The
use. The main function of the multiple jets should then be arranged below
jets is to deliver the cutting fluid to the the tool, an if necessary point from
cutting edge of the tool in sufficient other directions to be certain that the
quantities to dissipate heat, provide required functions are
lubrication, and in some cases to help
remove swarf. performed.

93

The supply reservoir must WARNING: Magnesium must not be
contain enough fluid to dissipate all the machined using any fluid
heat overheating of fluid may cause a containing water,
breakdown in its lubricating qualities otherwise fire will result.
and lead to subsequent and sizing Use only mineral oil.
troubles.
€ Method of Supply
€ Choosing a Cutting Fluid
A central supply system, with
Expert advice is readily available many machines operating from the one
from manufacture’s agents. Table 1 cutting fluid source, is the most
will be useful for selecting fluids efficient. It has the advantage that
suitable for many jobs. Soluble oil can careful control of cleanliness and
be used for most work expert with soft proportion can be maintained. Figure 2
cast brass and cast iron, which are shows a schematic arrangement for a
machined dry (grinding excepted). central supply system.

€ Application

Where possible the flow of cutting
fluid should be directed to the point
where heat is to the point where is
greatest and with minimum splashing
(see figure 3, page 4)

94

Supply
tank

Machine Shop

Drain

Receiving
tank

Filter Sterilizer

Drain

Figure 2. Layout for central coolant supply

95

96

Filtering, Sterilizing and Reclaiming

Because cutting fluids are now extensively used, two major considerations
are cost and cleanliness.

The cost can be kept down by reclaiming the fluid that would normally be
thrown out with the swarf; this may amount to several gallons per machine each
day. If it is decided to reclaim the fluid, cleanliness must be considered, as
reclaimed fluid has certainly been contaminated by contact with machines,
operations and work and this would be detrimental to its use as a lubricant.

Contaminated fluid is also a major cause of dermatitis. Cutting fluids are
sterilized to remove any bacterial contamination. Sterilizing units are normally
fitted into the system as shown in figure 2.

A set for reclaiming cutting fluids is shown in figure 4

CUTTING FLUID

Swarrf in

Heating coil

Swarf Screen Magnetic Drain Clean
centrifuge filter filter fluid
Oil
Centrifuge

Figure 4 Schematic layout for cutting fluid recovery

97

Swarf is placed in a perforated container in the swarf centrifuge separator
and is then rotated rapidly to throw off the fluid. Metal particles are separated by
screens and magnetic filters. The cutting fluid is further cleaned in the oil
centrifuge. A heating coil in the settling tank makes it easier for any remaining
particles to drop from suspension in the fluid.

€ Care of electrical Equipment

Cutting fluids can adversely affect the insulation and operation of electrical
equipment and thereby create a dangerous situation. The machine operator
should take care to see that cutting fluids do not flow or mist over electrical
cables, switches or motors. This cannot always be avoided by regular cleaning
of exposed surfaces and periodic inspection by a qualified electrician will lessen
dangers and avoid delays.

€ Cutting lubricant. Cutting speeds for any materials may be increased by the
use of proper lubricant. This keeps the tool from overheating at higher
speeds.

€ The rigidity of the work. Short work supported closed to the chuck may be
turned faster than a slender piece mounted between centers.

€ The capacity and condition of the lathe. A rugged machine with a powerful
motor will be able to drive work at a higher speed on heavy cuts. Overheating
of the cutting tools should be prevented.

13 CUTTING FLUIDS

13.1 THE FUNCTIONS OF CUTTING FLUIDS

There are a large number of cutting fluids in general use. They are designed to
increase the rate of production and prolong the life of cutting tools.

The function of these fluids is any or all of the following:
€ Cooling
€ Lubricating
• Chip removing
• Corrosion prevention
• Improved surface finish
• Operator protection.

98

13.1.1 Cooling

The cooling action is the important of these functions. During any machining
operation heat is generated between the tool and the work. Provision must be
made for the removal of this heat, if not the temperature may become excessive
and result in the cutting edge of the tool breaking down.

13.1.2 Lubricating

The fluid flows onto the nose of the tool and cuttings, thus lubricating the contact
surfaces. The amount of friction is reduced and the keen cutting edge of the tool
is maintained.

13.1.3 Chip Removing

In certain operations such as deep hole drilling, the ability of the cutting fluid to
wash away chips is most important. The fluid should be supplied in sufficient
volume and pressure to wash away the chips.

13.1.4 Corrosion Prevention

Cutting fluids must be non-corrosive. They must not cause rust in the workpiece
or parts of the lathe.

13.1.5 Improved Surface Finish

The lubricating action of the fluid causes a reduction in friction between the tool
and workpiece. This helps retain the cutting edge of the tool which results in an
improved surface finish on the work.

13.1.6 Operator Protection

The fluids used must be on-inflammable at normal working temperatures. It
should carry away extra fine chips and when reclaimed, must be sterilized.

99

SELF-CHECK: CUTTING FLUID

1. The “ maid of all work “ in metal cutting is

a. Alkaline solution
b. Sulphurized
c. Soluble oil
d. Oil and water

2. Which of these is not a function of cutting fluid?

a. Cooling, lubricating, chip removing
b. Corrosion prevention, improve surface finish, operate protection
c. Improved finish, cooling, lubricating
d. Facilitate production, cleaning heating

3. Cutting fluids lubricates contacting faces in turning. Friction is

a. increases
b. decreased
c. hastened
d. doubled

4. In high speed turning, high carbon steel which is the best cutting fluid?

a. mineral oil and lard
b. soluble oil
c. mineral oil
d. lard oil

100

OPERATION SHEET NO.1

OPERATION TITLE: TURNING WORKPIECE

PURPOSE: Turning operation is necessary in order to produce a component.

CONDITION:
Given a blueprint, tools, and materials the trainee should be able to

turn workpiece in one hour.

EQUIPMENT: Lathe machine

PROCEDURE:

1. Mount the work in the lathe.
2. Obtain or prepare a sharp cutting tool of the shape & size needed.
3. Insert the tool in the holder, with the point projecting about 13 mm (1/2”).
4. For turning steel or cast iron, set the tool holder in the tool post so that the

point of the tool is between 0o and 5o above the center of the workpiece.
Position the tool holder so that it raised project only slightly beyond the
edge of the compound rest.
5. Saving the tool holder so that the cutting edge of the tool ids more than
90o preferably 8ofrom the surface of the workpiece.
6. With the cross-feed crank, withdraw the tool until its clear the work. Start
the lathe and carefully advance the tool until it just touches the work. If
possible set the micrometer collar on the cross feed screw at zero.
Advance the tool to desire cuts.
7. Set the rate and direction of feed desire. Advance the tool to the end of
the workpiece with the carriage hand wheel, and engaged automatic feed.
8. When about 6.35 mm (1/4”) of the piece has been machine, disengage the
power feed, stop the machine and move the tool back to the end of the
workpiece with the carriage hand wheel. Test the piece for size with a
micrometer.
9. If the piece is under size or oversize make necessary adjustment. Be sure
to allow about 0.25-0.76 mm (0.010”-0.030”) for the finish cut.
10. After making adjustment, proceed the rough cuts using a feed rate of
about 0.13-0.25 mm (0.005”-0.0010”) per revolution.
11. Continue until the tool or the side of the compound rest is within about
6.35 mm (1/4”0 of the dog. Stop the lathe and run the carriage back until
the point of the tool is past the end of the workpiece.
12. If the entire length of the workpiece must be machined, remove the piece
from the dog, and place the turned end. Again mount the workpiece in the
lathe and proceed turning.