Classification of Metal, Removal Processes & Machine Tools

Basic functional principles of machine tool operations

Machine Tools produce desired geometrical surfaces on solid bodies (preformed blanks) and for that they are basically comprised of;

  • Devices for firmly holding the tool and work
  • Drives for providing power and motions to the tool and work
  • Kinematic system to transmit motion and power from the sources to the tool-work
  • Automation and control systems
  • Structural body to support and accommodate those systems with sufficient strength and

For material removal by machining, the work and the tool need relative movements and those motions and required power are derived from the power source(s) and transmitted through the kinematic system(s) comprised of a number and type of mechanisms.

(i)    Concept of Generatrix and Directrix

  • Generation of flat surface

The principle is shown in Fig. Generation of flat surfaces by Generatrix and Directrix where on a flat plain a straight line called Generatrix (G) is traversed in a perpendicular direction called Directrix (D) resulting a flat surface.

  • Generation of cylindrical surfaces

The principles of production of various cylindrical surfaces (of revolution) are shown in Fig. Generation of cylindrical surfaces (of revolution), where,

  • A long straight cylindrical surface is obtained by a circle (G) being traversed in the direction (D) parallel to the axis as shown in Fig. Generation of cylindrical surfaces (of revolution) (a)
  • A cylindrical surface of short length is obtained by traversing a straight line (G) along a circular path (D) as indicated in Fig. Generation of cylindrical surfaces (of revolution) (b)
  • Form cylindrical surfaces by rotating a curved line (G) in a circular path (D) as indicated in Fig. Generation of cylindrical surfaces (of revolution) (c and d).



(ii)    Tool – work motions

The lines representing the Generatrix and Directrix are usually produced by the locus of a point moving in two different directions and are actually obtained by the motions of the tool-tip (point) relative to the work surface. Hence, for machining flat or curved surfaces the machine tools need relative tool work motions, which are categorized in following two groups:

  • Formative motions namely
    • Cutting motion (CM)
    • Feed motion (FM)
  • Auxiliary motions such as
    • Indexing motion
    • Additional feed motion
    • Relieving motion

The Generatrix and Directrix, tool and the work and their motions generally remain interconnected and in different way for different machining work. Such interconnections are typically shown in Fig. Principle of turning (cylindrical surface) for straight turning and in Fig. Principle of producing flat surface in shaping machine for shaping.


The connections in case of straight longitudinal turning shown in Fig. Principle of turning (cylindrical surface) (a) are:

Generatrix (G) – Cutting motion (CM) – Work (W) Directrix (D) – Feed motion (FM) –  Tool (T)


In case of making flat surface in a shaping machine as shown in Fig. Principle of producing flat surface in shaping machine the connections are:

G – CM – T

D – FM – W

which indicates that in shaping flat surfaces the Generatrix is provided by the cutting motion imparted to the cutting tool and the Directrix is provided by the feed motion of the work.

Flat surfaces are also produced by planning machines, mainly for large jobs, where the cutting motion is imparted to the work and feed motion to the tool and the connections will be:

G – CM – Work D – FM – Tool

The Genratrix and Directrix can be obtained in four ways:

  • Tracing (Tr) – where the continuous line is attained as a trace of path of a moving point as shown in Fig. Principle of turning (cylindrical surface) and Fig. Principle of producing flat surface in shaping machine.
  • Forming (F) – where the Generatrix is simply the profile of the cutting edge as indicated in Fig. Generation of cylindrical surfaces (of revolution) (c and d)
  • Tangent Tracing (TTr) – where the Directrix is taken as the tangent to the series of paths traced by the cutting edges as indicated in Fig. Directrix formed by tangent tracing in plain milling.
  • Generation (G): Here the G or D is obtained as an envelope being tangent to the instantaneous positions of a line or surface which is rolling on another surface. Gear teeth generation by hobbing or gear shaping is the example as can be seen in Fig. Generatrix (or Directrix) in gear teeth cutting by generation.

Fig. Directrix formed by tangent tracing in plain milling typically shows the tool-work motions and the corresponding Generatrix (G) and Directrix (D) while producing flat surface by a plain or slab milling cutter in a conventional horizontal arbour type milling machine. The G and D are connected here with the tool work motions as

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G – x – T – F

D – FM – W – T.Tr CM – T

Here G and D are independent of the cutting motion and the G is the line of contact between the milling cutter and the flat work surface. The present  cutter being of roller shape, G has been a straight line and the surface produced has also been flat. Form milling cutters will produce similar formed surfaces as shown in Fig. Tool-work motions and G & D in form milling where the ‘G’ is the tool-form.




For making holes in drilling machines both the cutting motion and the feed motion are imparted to the cutting tool i.e., the drill bit whereas the workpiece remains stationary. This is shown in Fig. Tool-work motions and G & D in drilling. The G and D are linked with the tool-work in the way:

G – CM – T – Tr

D – FM – W – Tr


Boring machines are mostly used for enlargement and finishing of existing cylindrical holes. Boring machines are of two types:

  • Vertical boring machine – low or medium duty and high precision, e.g., Jig boring machine
  • Horizontal axis boring machine – medium or heavy

In respect of tool-work motions and G and D, vertical boring and drilling are same. In horizontal boring machine the feed motion is imparted to the work to provide the Directrix by Tracing.

(iii)     Machine tool drives

For the desired tool-work motions with power, machine tools are driven by electric motors and use of some mechanisms like belt-pulley, gears etc. In some machine tools, the tool-work motions are provided by hydraulic drive also.

Machine tools essentially need wide ranges of cutting speed and feed rate to enable

  • Machining different jobs (material and size)
  • Using different cutting tools (material, geometry and size)
  • Various machining operations like high speed turning to low speed thread cutting in lathes
  • Degree of surface finish

Machine tool drives may be

  • Stepped drive
  • Stepless drive

Stepped drives are very common in conventional machine tools where a discrete number of speeds   and feeds   are available   and   preferably   in

  • (Geometric Progression) series. Whereas the modern CNC machine tools are provided with stepless drives enabling optimum selection and flexibly automatic control of the speeds and

Stepped drive is attained by using gear boxes or cone pulley (old method) along with the power source. Stepless drive is accomplished usually by

  • Variable speed AC or DC motors
  • Stepper or servomotors
  • Hydraulic power pack
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Configuration of Basic Machine Tools and their use

· Centre lathes

–     configuration

Fig. Schematic view of a center lathe shows the general configuration of center lathe. Its major parts are:

  • Head stock: it holds the blank and through that power and rotation are transmitted to the job at different speeds
  • tailstock: supports longer blanks and often accommodates tools like drills, reamers etc for hole
  • carriage: accommodates the tool holder which in turn holds the moving tools
  • bed: Δ headstock is fixed and tailstock is clamped on it. Tailstock has a provision to slide and facilitate operations at different locations Δ   carriage travels on the bed
  • columns: on which the bed is fixed
  • work-tool holding devices

–     uses of center lathes

Centre lathes are quite versatile being used for various operations:

  • facing, centering, drilling, recessing and parting
  • thread cutting; external and internal
  • knurling

Some of those common operations are shown in Fig. Some common machining operations done in center lathes. Several other operations can also be done in center lathes using suitable attachments.

· Shaping machine

Fig. Schematic view of a shaping machine shows the general configuration of shaping machine. Its major parts are:

  • Ram: it holds and imparts cutting motion to the tool through reciprocation
  • Bed: it holds and imparts feed motions to the job (blank)
  • Housing with base: the basic structure and also accommodate the drive mechanisms

o    Power drive with speed and feed change mechanisms.

Shaping machines are generally used for producing flat surfaces, grooving, splitting etc. Because of poor productivity and process capability these machine tools are not widely used now-a-days for production.




· Planing machine

The general configuration is schematically shown in Fig. Schematic view of a planning machine. This machine tool also does the same operations like shaping machine but the major differences are:

  • In planing the job reciprocates for cutting motion and the tool moves slowly for the feed motions unlike in shaping
  • Planing machines are usually very large in size and used for large jobs and heavy duty

· Drilling machine

Fig. Schematic view of a drilling machine shows general configuration of drilling machine, column drill in particular. The salient parts are

  • Column with base: it is the basic structure to hold the other parts
  • Drilling head: this box type structure accommodates the power drive and the speed and feed gear
  • Spindle: holds the drill and transmits rotation and axial translation to the tool for providing cutting motion and feed motion – both to the

Drilling machines are available in varying size and configuration such as pillar drill, column drill, radial drill, micro-drill etc. but in working principle all are more or less the same.

Drilling machines are used:

o    Mainly for drilling (originating or enlarging cylindrical holes)

  • Occasionally for boring, counter boring, counter sinking
  • Also for cutting internal threads in parts like nuts using suitable



· Milling machine

The general configuration of knee type conventional  milling machine with horizontal arbour is shown in Fig. Schematic view of a milling machine. Its major parts are

  • Milling arbour: to hold and rotate the cutter
  • Ram: to support the arbour
  • Machine table: on which job and job holding devices are mounted to provide the feed motions to the
  • Power drive with Speed and gear boxes: to provide power and motions to the tool-work
  • Bed: which moves vertically upward and downward and accommodates the various drive mechanisms
  • Column with base: main structural body to support other parts.
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Milling machines are also quite versatile and can do several operations like o    making flat surfaces

  • grooving, slitting and parting
  • helical grooving
  • forming 2-D and 3-D contoured surfaces

Fig. Some common milling operation shows some of the aforesaid milling operations.


Specification of Machine Tools.

A machine tool may have a large number of various features and characteristics. But only some specific salient features are used for specifying a machine tool. All the manufacturers, traders and users must know how are machine tools specified.

The methods of specification of some basic machine tools are as follows:

o    Centre lathe

  • Maximum diameter   and   length   of   the   jobs    that   can   be accommodated
  • Power of the main drive (motor)
  • Range of spindle speeds
  • Range of feeds
  • Space occupied by the

o       Shaping machine

  • Length, breadth and depth of the bed
  • Maximum axial travel of the bed and vertical travel of the bed / tool
  • Maximum length of the stroke (of the ram / tool)
  • Range of number of strokes per minute
  • Range of table feed
  • Power of the main drive
  • Space occupied by the machine

o       Drilling machine (column type)

  • Maximum drill size (diameter) that can be used
  • Size and taper of the hole in the spindle
  • Range of spindle speeds
  • Range of feeds
  • Power of the main drive
  • Range of the axial travel of the spindle / bed
  • Floor space occupied by the machine

o      Milling machine (knee type and with arbour)

  • Type; ordinary or swiveling bed type
  • Size of the work table
  • Range of travels of the table in X-Y-Z directions
  • Arbour size (diameter)
  • Power of the main drive
  • Range of spindle speed
  • Range of table feeds in X-Y-Z directions
  • Floor space

Broad classification of Machine Tools

Number of types of machine tools gradually increased till mid 20th century and after that started decreasing based on Group Technology.

However, machine tools are broadly classified as follows:

  • According to direction of major axis :
    • horizontal center lathe, horizontal boring machine
    • vertical – vertical lathe, vertical axis milling machine
    • inclined – special ( e.g. for transfer machines).
  • According to purpose of use :
    • general purpose – e.g. center lathes, milling machines, drilling machines etc.
    • single purpose – e.g. facing lathe, roll turning lathe
    • special purpose – for mass
  • According to degree of automation
    • non-automatic – e.g. center lathes, drilling machines
    • semi-automatic – capstan lathe, turret lathe, hobbinh machine
    • automatic – e.g., single spindle automatic lathe, swiss type automatic lathe, CNC milling machine
  • According to size :
    • heavy duty – e.g., heavy duty lathes (e.g. ³ 55 kW), boring mills, planning machine, horizontal boring machine
    • medium duty – e.g., lathes – 3.7 ~ 11 kW, column drilling machines, milling machines
    • small duty – e.g., table top lathes, drilling machines, milling machines.
    • micro duty – e.g., micro-drilling machine
  • According to precision :
    • ordinary – e.g., automatic lathes
    • high precision – e.g., Swiss type automatic lathes
  • According to number of spindles :
    • single spindle – center lathes, capstan lathes, milling machines
    • multi-spindle – multispindle (2 to 8) lathes, gang drilling machines
  • According to blank type : o bar type (lathes)
    • chucking type (lathes)
    • housing type
  • According to type of automation :
    • fixed automation – g., single spindle and multispindle lathes
    • flexible automation – e.g., CNC milling machine
  • According to configuration :
    • stand alone type – most of the conventional machine
    • machining system (more versatile) – e.g., transfer machine, machining center, FMS

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