ATM-412 Machining M4

Machining

Module 4: Cutting Operation and Quality Parameters

PREPARED BY

Curriculum Development Unit

August 2013

© Applied Technology High Schools, 2013

ATM 412 – Machining

2 Module 4: Cutting Operation and Quality Parameters

Module 4: Cutting Operation and Quality Parameters

Module Objectives Upon the completion of this module, the student will be able to:

Describe how the cutting tools cut materials.

Select the correct cutting speed, feed, and depth of cut during machining.

Define the surface finish and its parameters.

Module Contents Topic Page No.

1 Chip formation 3

2 Lathe machine cutting tools 4

3 Surface finish 5

4 Cutting fluid 6

5 Cutting speed, feed and depth of cut 6

6 References 11


Module 4: Cutting Operation and Quality Parameters 3

1. Chip Formation:

In order for the machine tool to cut metal, a

sharp cutting tool made of special hard

metals must be used.

Fig. 4.1 shows a set of lathe cutting tools

with different shapes.

In order for a chip to be formed, a cutting

edge must penetrate the material, cutting

off a chip.

Fig. 4.1: Set of lathe cutting tools

The basic form of a cutting edge is a wedge.

Generally there are three angles that permit

the cutting action as illustrated in Fig. 4.2.

α Clearance angle

ɣ Rake angel

β Wedge angle

90°

γ

β α

Wedge Chip

Shaping tool

Fig. 4.2: Basic form of a cutting tool

The clearance angle is the angle required to make cutting easier and

minimize the friction and heat generation while the rake angle is the angle

responsible for chip removal.

The small wedge angle results in an easy penetration of the cutting tool but

would lead to early tool failure, while a cutting tool with large wedge angle

would need more power.



2. Lathe machine cutting tools:

To machine metal in a lathe, a cutting tool called tool bit is used. Tool bits

used in training are either High Speed Steel (HSS) or carbide-tipped tools.

HSS is probably the most popular type of tool steel; it is tough enough to

withstand most cutting shocks and retains its hardness at higher speeds. It

will cut most materials quite satisfactorily and is useful for general purpose

work.

Tool bits are made in a variety of sizes and shapes to be used for different

machines and different applications.

Left-hand tools have their cutting edge on the right-hand side. The tool

moves toward the tailstock while cutting. Right-hand tools have the cutting

edge on the left-hand side and move toward the headstock during the

turning operation as illustrated in Fig. 4.3

A lathe cutting tool is generally named by the operation it performs (facing,

turning …etc).

Right Hand

Left Hand

Head stock

Tailstock

Operator facing the lathe

Roughing Roughing Turning Facing Facing Turning

Right-hand tools

Left-hand tools

Round nose turning tool

Fig. 4.3 Standard shapes of cutting tools



To produce various surfaces, faces, and forms, the cutting edge of the tool

bit must be shaped to the required form and then relieved with clearance

angles to allow the edge to cut into the metal.

The standard terminology of cutting tool’s angles is shown below. Fig. 4.4

Front cutting edge angle

Nose angle

Side cutting edge angle

Back rake angle

Nose

Cutting edge

Face

Side rake angle

Side clearance

angle

Body

End clearance

Fig. 4.4: Standard cutting tool’s terminology

3. Surface finish

Surface finish is the degree of roughness of the machined surface. It depends on many factors such as, speed, feed, depth of cut, and the use of cutting fluids.

The roughness symbol in Fig. 4.5 means that the roughness required for this surface must be 3.2 µm (3.2 micrometer i.e 0.0032 mm); this information will help you to select the correct cutting operation that you should use to have the correct value of roughness. The degree of roughness obtained by any

machining operation and/or hand tool is

listed in the machining handbooks.

Fig. 4.5: Roughness symbol



4. Cutting fluids

Cutting fluids are very important to minimize or reduce the effects of friction and heat in machining operations. They affect the performance of the cutting tool and improve surface quality.

The cutting fluid must provide lubrication and cooling for the cutting tool, the chip and the workpiece. Generally soluble oils are used when cutting steels. Soluble oils are mineral oils that contain a soap-like material that makes them mix in water into a milky white solution. Fig. 4.6.

Fig. 4.6: Soluble oil

5. Cutting Speed, Feed and depth of cut:

In order to cut any material the machine must be adjusted to the correct

rotational speed of the part, the correct rate of tool travel and the cutting

depth.

5.1 Cutting Speed and Spindle Speed

Lathe work cutting speed (CS) may be defined as the rate at which a point

on the work circumference travels past the cutting tool. For instance, if a

metal has a CS of 30 m/min, the spindle speed must be set so that 30

meters of the work circumference will pass the cutting tool in 1 min i.e. the

length of chip produced in one minute = 30 m.

Cutting speed is always expressed in feet per minute (ft/min) or in meters

per minute (m/min).

The recommended CS for high-speed steel tools listed in table 4.1

These speeds may be varied slightly to suit factors such as the condition of

the machine, the type of work material, and sand or hard spots in the

metal.



Table 4.1: Lathe cutting speeds in meters per minute using

a high-speed steel cutting tools

Turning and Boring Threading

Rough cut Finish cut

Material m/min m/min m/min

Machine steel 27 30 11

Tool steel 21 27 9

Cast iron 18 24 8

Bronze 27 30 8

Aluminium 61 93 18

After the correct cutting speed (CS) is selected from the table above (when

we use high speed steel cutting tool), the spindle speed (n) of the lathe

machine in revolutions per minute (RPM) must be calculated and then set

on the machine.

Note: Never change the speed while the lathe machine is running.

The formula used to calculate the lathe spindle speed is shown below:

Where

n: Spindle speed in rev/min (RPM)

: Cutting speed in m/min

d: Diameter of the workpiece in (m)

: Constant = 3.14



Example:

Calculate the rev/min required to rough turn a 40 mm diameter piece of

machine steel?

Solution

Given diameter (d) = 40 mm

d = 40/1000 = 0.04 m

CS from table no. 4.1, For machine steel, under rough cutting = 27 m/min

= 215 r/min

Note:

The calculated spindle speed is not necessary to be found on your machine

headstock selection plate, so you should use the nearest lower speed

available

5.2 Feed The feed of a lathe may be defined as the distance the cutting tool

advances along the length of the work for every revolution of the spindle.

For example, if the lathe is set for a 0.4 mm feed, the cutting tool will travel

along the length of the work 0.4 mm for every complete turn that the work

makes (mm/rev). The feed of a lathe machine is dependent on the speed of

the lead screw or feed rod. Feed is controlled by changing the gears setup

as shown. Fig. 4.7.



Fig. 4.7: Feed tables and gearbox. Whenever possible, only two cuts should be taken to bring a diameter to

size: a roughing cut and a finishing cut.

Since the purpose of a roughing cut is to remove excess material quickly, a

coarse feed should be used. The finishing cut is used to bring the diameter

to size and produce a good surface finish, and therefore a fine feed should

be used. For general-purpose machining, a 0.25 to 0.4 mm feed for

roughing and a 0.07- to 0.012-mm feed for finishing is recommended.

Table 4.2 lists the recommended feeds for cutting various materials when a

high-speed steel cutting tool is used.

Material Rough cuts in (mm) Finish cuts in (mm)

Machine steel 0.25–0.5 0.07–0.25

Cast iron 0.4–0.65 0.13–0.3

Bronze 0.4–0.65 0.07–0.25

Aluminium 0.4–0.75 0.13–0.25

Table 4.2: Feeds for various materials, using a high-speed steel cutting tool

5.3 Depth of Cut, Rough and Finishing cuts:

The depth of cut may be defined as the depth of the chip taken by the

cutting tool and is one-half the total amount removed from the workpiece in



one cut. Fig. 4.8 shows a 1.25 mm depth of cut being taken on a 40 mm

diameter workpiece. Note that the diameter has been reduced by 2.50 mm

to 37.5 mm.

Diameter reduced 2.5 mm

Original diameter = 40 mm

Depth of cut = 1.25 mm

New diameter = 37.5 mm

Fig. 4.8: Cutting depth

When machining a part, you need to do roughing and finishing cuts. The

roughing cuts are the cuts that are taken to reduce the diameter to

approximate size; the work is left around 0.5 mm oversize for finishing

(final) cut. The depth of a rough-turning cut will depend on the following

factors:

The condition of the machine

The type and shape of the cutting tool used

The rigidity of the workpiece, the machine, and the cutting tool

The rate of feed

The material being cut



The depth of a finish-turning cut will

depend on the type of work and the

finish required. In any case, it should

not be less than 0.13 mm. The

micrometer graduated collars are used

to set the required depth of cut

accurately. Fig. 4.9.

Fig. 4.9: Micrometer Graduated

collars.

References

1. Technology of Machine Tools. Seventh Edition, McGraw-Hill

Companies,

2. Machine shop operations and setups, 4th edition, Lascoe nelson

Porter.

3. Machine tool and Manufacturing technology, Steve F. Krar, Mario

Rapisarda, Albert F. Check., Delmar Publishers.

4. www.en.wikipedia.org/wiki/Machining

5. http://www.mini-lathe.com

6. http://mdmetric.com/tech/surfruff.htm



Student’s notes

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Worksheet

1. Complete the following

a) The ……………………………………. angle is the angle responsible for chip

removal in lathe cutting tools.

b) In cutting tools, the ……………………………….. angle is the angle

required to make cutting easier and minimize the friction and heat

generation.

c) The ………………………………….….. provides lubrication and cooling for

the cutting tool, the chip and the workpiece.

d) …………………….. is the distance the cutting tool advances along the

length of the work for every revolution of the spindle.

e) ……………..………… is the degree of roughness of the machined

surface.

2. Calculate the spindle speeds for roughing and finishing cuts, if an

aluminum workpiece of 30 mm diameter required to be cut by a lathe

machine using High Speed Steel cutting tool?

(Use tables provided in the module).

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Documents

ATM-412 Machining M4