IPE 401 (Machine Tools)

Lecture – 01
Introduction to Machine Tools
Machine tools
• Machine tools are devices for cutting materials (mostly metals),
to impart them the required shape.
• They have an in-built arrangement that facilitates the use of
various types of detachable cutting tools that can be changed to
suit the task at hand, and removed for replacement or
resharpening, after wear.
• The cut-off material obtained is usually in the form of chips. This
is the material peeled off the workpiece by the tool. It can be
continuous like a ribbon or in broken chips.
Classification of Machine Tools
According to size:
• light duty (weighing less than 1 tons),
• medium duty (1 to 10 tons), and
• heavy duty (above 10 tons).

According to the method of actuation:

• manually operated,
• semi-automatic,
• fully automated.
Classification of Machine Tools
According to purpose:
• general purpose machine tools
• used for a wide variety of operations, on a range of sizes
of workpieces
• special purpose machine tools,
• used for specific operations, on a limited range of
workpiece sizes and shapes.
Classification of Machine Tools
According to the types of motions used for removing material:
• Rotary cutting machines rotate workpiece (turning lathes,
turrets), or the cutting tool (drill, milling cutter, grinding
wheel), or both (cylindrical grinder).
• Linear cutting machines remove material by moving the tool
(shaping, slotting), or the workpiece (planning, surface
grinding), in a straight line.
In both the types of machine tools, linear motion is used for
moving (feeding) the workpiece or the tool, to mate (engage)
them for material removal.
Classification of Machine Tools
According to feed classification:
• The axial feed machines such as drilling machines that move
the workpiece/tool, parallel to the axis of the machine
spindle,
• The transverse feed machines, such as face milling
machines that move the workpiece or tool, in a direction that
is at right angle to the machine spindle axis.
• Combined feed machines, such as center lathes and boring
machines, can use both types of feeds.
Requirements of Machine Tools
A good machine tool should have-
• high productivity
• high precision commensurate with workpiece specifications,
• controlling levers, hand-wheels, push buttons, etc. at a position,
convenient for the operator.
It should-
• ensure operator and machine safety,
• its manufacturing and running costs should be reasonable
• breakdown frequency and duration should be minimal.
Productivity can be maximized by-
• Optimum speeds and feeds usage to suit the workpiece size
and material:
• General purpose machines call for a wider range of speeds
and feeds, to accommodate a variety of workpiece sizes and
materials.
• Special purpose machines, meant for either a single or few
specific workpieces and operations, do not warrant flexibility
in speeds and feeds.
• Multi-tool cutting that can help in overlapping of cutting times.
Productivity can be maximized by-
• Reduction of Non-productive (Non-cutting) time by-
• Jigs and fixtures: to reduce workpiece positioning (locating)
and clamping time.
• Minimizing maintenance: by reckoning fatigue stress on
parts, while designing running parts, providing easy access
for quick replacement of broken/worn parts, and adopting a
practice of preventive maintenance (replacing vulnerable
parts just before they are likely to break), on weekly holidays.
Precision depends upon-
• Geometrical manufacturing accuracy: straightness, squareness,
concentricity, etc. of guideways, carriages, and spindles.
• Adept assembly: minimizing clearances between guideway and carriage
(or bearing and spindle), for kinetic (motional) accuracy.
• Deflection: which can be minimized by reckoning cutting forces and
vibrations, while designing the structure and providing adequate stiffness
(strength).
• When a machine is subjected to high variation in temperature (as in
grinders), thermal stress and ensuing deformation should be minimized by
diffusing (conducting away) the heat to a solid structure, and/or by
providing for an arrangement for cooling the structure.
• Precision aids like fine division scales, dials, verniers, and optical means
for accurate readout, promote precise setting and operation.
Operator and machine safety
• Operator safety can be ensured by-
• Guards, which can prevent entanglement of operator’s clothes in
rotating/moving machine parts; and shield him from hot, sharp, flying
chips of the cut-off material
• Adequate clamping of workpiece, to withstand cutting forces, to prevent
the workpiece from flying off and injuring the operator
• Handling equipment for heavy workpieces, to avoid accidental fall and
injury.
• Electrical safety through earthing, to avoid shock to the operator
 Operator and machine safety
• Machine safety can be accomplished by-
• Stops (mechanical obstacles), and limit, and proximity switches, to
confine carriage travel within safe limits
• Interlocks through mechanical, hydraulic, and electrical elements, to
prevent engagement of conflicting and risky motions simultaneously, or in
a wrong order, e.g. ensuring that workpiece is clamped, before cutting
commences
• Overload prevention through cheap, easy-to-replace shear pins, electrical
fuses, etc. to eliminate the possibility of breakage of costly gears, burning
of electrical motor, etc.
• Appearance of machine tools is not as important as let’s say that of cars.
Accurate and trouble-free functioning is more important (as in a battle
tank). Simplicity of design is also a desirable feature of machine tools.
Thank You.