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 rigidity.

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 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.

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

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.

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

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 duty.

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 desired.

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 G.P. (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 feeds.

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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Author: Aliva Tripathy

Taking out time from a housewife life and contributing to AxiBook is a passion for me. I love doing this and gets mind filled with huge satisfaction with thoughtful feedbacks from you all. Do love caring for others and love sharing knowledge more than this.

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